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In the recent years, lithium-ion batteries have become the battery technology of choice for portable devices, electric vehicles and grid storage. While increasing numbers of car manufacturers are introducing electrified models into their offering, range anxiety and the length of time required to recharge the batteries are still a common concern. The high currents needed to accelerate the charging process have been known to reduce energy efficiency and cause accelerated capacity and power fade. Fast charging is a multiscale problem, therefore insights from atomic to system level are required to understand and improve fast charging performance. The present paper reviews the literature on the physical phenomena that limit battery charging speeds, the degradation mechanisms that commonly result from charging at high currents, and the approaches that have been proposed to address these issues. Special attention is paid to low temperature charging. Alternative fast charging protocols are presented and critically assessed. Safety implications are explored, including the potential influence of fast charging on thermal runaway characteristics. Finally, knowledge gaps are identified and recommendations are made for the direction of future research. The need to develop reliable in operando methods to detect lithium plating and mechanical degradation is highlighted. Robust model-based charging optimisation strategies are identified as key to enabling fast charging in all conditions. Thermal management strategies to both cool batteries during charging and preheat them in cold weather are acknowledged as critical, with a particular focus on techniques capable of achieving high speeds and good temperature homogeneities.
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Lithium-ion battery fast charging: A review
Anna Tomaszewska
a
,
*
, Zhengyu Chu
b
, Xuning Feng
b
,
**
, Simon O'Kane
c
,
d
, Xinhua Liu
a
,
Jingyi Chen
a
, Chenzhen Ji
a
, Elizabeth Endler
e
, Ruihe Li
b
, Lishuo Liu
b
, Yalun Li
b
,
Siqi Zheng
b
, Sebastian Vetterlein
f
, Ming Gao
g
, Jiuyu Du
b
, Michael Parkes
f
,
Minggao Ouyang
b
, Monica Marinescu
c
,
d
, Gregory Offer
c
,
d
, Billy Wu
a
,
d
,
***
a
Dyson School of Design Engineering, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
b
State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, 100084, PR China
c
Department of Mechanical Engineering, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
d
The Faraday Institution, Quad One, Harwell Science and Innovation Campus, Didcot, OX11 0RA, UK
e
Shell International Exploration &Production Inc., 3333 Hwy 6 South, Houston, TX, 77082, USA
f
Shell Global Solutions UK, 40 Bank Street, Canary Wharf, London, E14 5NR, UK
g
Shell (Shanghai) Technology Limited, Building 4, 4560 Jinke Road, Pudong District, Shanghai, 201210, PR China
article info
Article history:
Received 26 June 2019
Received in revised form
9 August 2019
Accepted 10 August 2019
Available online 16 August 2019
Keywords:
Lithium-ion battery
Fast charging
Lithium plating
Charging protocols
Electric vehicles
abstract
In the recent years, lithium-ion batteries have become the battery technology of choice for portable
devices, electric vehicles and grid storage. While increasing numbers of car manufacturers are intro-
ducing electried models into their offering, range anxiety and the length of time required to recharge
the batteries are still a common concern. The high currents needed to accelerate the charging process
have been known to reduce energy efciency and cause accelerated capacity and power fade. Fast
charging is a multiscale problem, therefore insights from atomic to system level are required to un-
derstand and improve fast charging performance. The present paper reviews the literature on the
physical phenomena that limit battery charging speeds, the degradation mechanisms that commonly
result from charging at high currents, and the approaches that have been proposed to address these
issues. Special attention is paid to low temperature charging. Alternative fast charging protocols are
presented and critically assessed. Safety implications are explored, including the potential inuence of
fast charging on thermal runaway characteristics. Finally, knowledge gaps are identied and recom-
mendations are made for the direction of future research. The need to develop reliable onboard methods
to detect lithium plating and mechanical degradation is highlighted. Robust model-based charging
optimisation strategies are identied as key to enabling fast charging in all conditions. Thermal man-
agement strategies to both cool batteries during charging and preheat them in cold weather are
acknowledged as critical, with a particular focus on techniques capable of achieving high speeds and
good temperature homogeneities.
©2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
1. Introduction
In recent years, efforts to limit the effects of climate change and
local air pollution have driven rapid progress in the development of
lithium-ion (Li-ion) battery powered electric vehicles (EVs). While
manufacturers race to introduce electried options in their ranges,
customer acceptance of EVs, and particularly battery electric ve-
hicles (BEVs) that are not hybridised with internal combusion en-
gines (ICEs), is still limited. Range anxiety and long charging times
compared to the refuelling of petrol vehicles are often quoted
among the main issues hindering wider adoption of EVs [1]. Fast
charging capability has therefore become one of the key features
targeted by battery and EV industries. However, charging at high
rates has been shown to accelerate degradation, causing both the
capacity and power capability of batteries to deteriorate. Low
ambient temperatures, prevalent in many key EV markets,
*Corresponding author.
** Corresponding author.
*** Corresponding author. Dyson School of Design Engineering, Imperial College
London, Exhibition Road, London, SW7 2AZ, UK.
E-mail addresses: a.tomaszewska18@imperial.ac.uk (A. Tomaszewska), fxn17@
tsinghua.edu.cn (X. Feng), billy.wu@imperial.ac.uk (B. Wu).
Contents lists available at ScienceDirect
eTransportation
journal homepage: www.journals.elsevier.com/etransportation
https://doi.org/10.1016/j.etran.2019.100011
2590-1168/©2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
eTransportation 1 (2019) 100011
exacerbate the problem even further due to the slower diffusion of
Li
þ
ions in the electrodes and the electrolyte and the sluggish ki-
netics of intercalation. On the other hand, the heat generated
during fast charging due to resistive heating is often difcult to
remove in a uniform and efcient manner, leading to accelerated
degradation and safety concerns. The behaviour of cells and packs
subjected to fast charging depends on a multitude of factors
spanning multiple scales from atomic to system level, as illustrated
in Fig. 1. This paper looks to review the existing literature and
identify some of the key knowledge gaps at each of these length
scales.
EVs can be charged using either alternating current (AC) or
direct current (DC) infrastructure. Out of these, DC offers signi-
cantly higher charging speeds. The most common DC fast charging
(DCFC) posts can charge at a power of 50 kW using CHArge de
MOve (CHAdeMO), Combined Charging System (CCS) or GB/T
standard connectors. Tesla were the rst to introduce 120 kW
charging posts (Tesla Superchargers) equipped with custom con-
nectors. CCS has since followed suit, developing 150kW chargers.
In 2017, Porsche pioneered 350 kW charging by unveiling two CCS
charging posts rated to this power at the company's ofce in Berlin
[2,3]; however, the rst light duty EV capable of accepting the full
350 kW is only scheduled to become available in 2019, when
Porsche's debut electric model, the Taycan, is released. This is
because today's EV battery packs are normally rated at 400 V.
350 kW and higher power charging necessitates higher voltage
packs to avoid extremely high charging currents and to limit
resistive heat generation. The Porsche Taycan as well as the recently
unveiled Audi e-tron GT concept (also expected to charge at up to
350 kW) will both be equipped with 800 V Li-ion battery packs
[4,5]. In December 2018, a prototype 450 kW CCS charger was
tested on two research vehicles in Bavaria, Germany [6]. Both the
charger and the vehicles were developed as a result of the Fast-
Chargeresearch project, conducted by a consortium led by the
Abbreviations
AC Alternating current
BMS Battery Management System
CC-CV Constant current - constant voltage
CCS Combined Charging System
CEI Cathode electrolyte interphase
CP-CV Constant power - constant voltage
DC Direct current
DCFC DC fast charging
DEC Diethyl carbonate
DMC Dimethyl carbonate
DVA Differential voltage analysis
EC Ethylene carbonate
ECM Equivalent circuit model
EIS Electrochemical impedance spectroscopy
EV Electric vehicle
FOM Full order model
ICA Incremental capacity analysis
LAM Loss of active material
LCO Lithium cobalt oxide
LFP Lithium iron phosphate
LLI Loss of lithium inventory
LMO Lithium manganese oxide
LTO Lithium titanium oxide
MCC Multistage constant current
N/P Negative-to-positive electrode
NCA Lithium nickel cobalt aluminium oxide
NFRA Nonlinear frequency response analysis
NMC Lithium nickel manganese cobalt oxide
NMR Nuclear magnetic resonance
P2D Pseudo-two-dimensional
PCM Phase change material
PDE Partial differential equation
ROM Reduced order model
SEI Solid electrolyte interphase
SEM Scanning electron microscopy
SOC State-of-Charge
SP Single particle
UVP Universal Voltage Protocol
Fig. 1. Key factors affecting Li-ion battery fast charging at different length scales.
A. Tomaszewska et al. / eTransportation 1 (2019) 1000112
BMW Group and involving Porsche and Siemens.
While impressive progress has been made in increasing the
power capability of EV chargers, these improvements do not always
translate directly into fast charging in all circumstances. Contin-
uous charging power is normally derated compared to the power
rating of the charger and depends on the specications of the EV as
well as the environmental conditions. In particular, charging speeds
are signicantly decreased at low ambient temperatures: according
to the Nissan LEAF Owner's Manual [7], charging up to 80% State-of-
Charge (SOC) with a 50 kW charger can take from 30 min to over
90 min depending on the temperature. Furthermore, fast charging
rates are typically only achievable up to about 80% SOC level [8,9]
due to safety limitations. At high SOCs, the current needs to be
gradually decreased to avoid exceeding the maximum cell voltage
limits, resulting in much longer times required to charge to full
capacity. The maximum charging power is limited not only by the
charger but also by the Battery Management System (BMS) on the
vehicle. Typically, EVs with smaller battery packs such as the Nissan
LEAF (40e62 kWh) or BMW i3 (22e42 kWh) can only charge at a
maximum power of 50 kW [7,10], whereas vehicles equipped with
larger packs accept higher power levels. This is due to the fact that
the charging C-rate that can be safely accepted by the batteries used
in EVs is still limited to about 1e1.5C in most cases. However, ac-
cording to the current specications, the aforementioned Porsche
Taycan is expected to defy this trend with a maximum charging rate
of around 3C. Given the increasing industrial interest in battery
fast-charging, there is a need to understand rate limiting processes
and lifetime implications of different charging approaches.
The progress in understanding various aspects of fast charging
has recently been analysed and reviewed in a number of publica-
tions, with notable works highlighted here. Zhu et al. [11] discussed
some of the key strategies to improve electrode rate capabilities
and electrolyte conductivities in both traditional Li-ion and solid
state systems, with a thorough consideration of the issues related to
solid electrolyte/electrode interfaces. Liu et al. [12] reviewed the
recent developments in battery materials to tackle challenges with
the mass transport in electrolytes and charge transfer in electrodes,
while also describing useful characterisation techniques for mate-
rial research. Some of the common charging algorithms were dis-
cussed from a control and implementation perspective by Shen
et al. [13], but with little consideration of the implications on cycle
life. Gao et al. [14] recently published a review of charging algo-
rithms with a focus on model-based optimisation. Chen et al. [15]
published a technological review on EV fast charging, discussing
the thermal management issues and charger design. The authors
also listed some of the available charging algorithms, however little
critical commentary was provided and no links were made to the
electrochemical aspects of battery charging. This work aims to
highlight the multiscale and multidisciplinary nature of fast
charging, establishing the links between microscale processes,
material characteristics, cell and pack design, and charging strategy
optimisation. Recent progress in each of these aspects is analysed in
a critical way to identify the key questions that remain unan-
swered, to understand the limitations of the current technologies,
and to highlight possible directions for future research.
The discussion of key aspects of Li-ion battery fast charging is
arranged according to scale, starting from atomic to pack and sys-
tem level. Section 2describes the rate limiting processes that
restrict fast charging capability in Li-ion batteries. Degradation
phenomena that result from these processes, and which are exac-
erbated by fast charging, are discussed along with detection and
mitigation methods in Section 3. Approaches to material, cell and
pack design to reduce degradation are discussed in Section 4.
Section 5evaluates the existing charging protocol optimisation
strategies, while the implications of fast charging capability on
battery thermal management are outlined in Section 6. Particular
attention is paid to the effects of extreme temperatures on fast
charging performance. Section 7deals with the safety implications
of fast charging, including the effect on thermal runaway charac-
teristics. Finally, recommendations are made on the existing
knowledge gaps and directions for future research in Section 8.
2. Principles of battery fast charging
An ideal battery would exhibit a long lifetime along with high
energy and power densities, enabling both long range travel on a
single charge and quick recharge anywhere in any weather. Such
characteristics would support broad deployment of EVs for a vari-
ety of applications. Unfortunately, the physics of each of these re-
quirements results in tradeoffs [16]; for example, thicker electrodes
needed for high energy density suffer more acutely from the con-
centration and potential gradients resulting from fast charging
[17,18]. The combination of physical properties of materials and
devices with temperature dependent behaviour denes the oper-
ating envelope for batteries. As ambient temperatures decrease,
charge rates and recommended maximum voltages are typically
kept low to improve safety and performance, making operating
temperature a key barrier to fast charging [16]. The risk of lithium
plating during charging signicantly increases with decreasing
temperatures, impacting capacity retention. The temperature
threshold below which lithium plating becomes likely depends on
many factors, including the cell parameters, age and C-rate.
Although many authors have reported lithium plating at temper-
atures below 25
+
C[19e21], it can also occur at higher tempera-
tures, particularly when high C-rates or high energy density cells
are used [22,23]. Additionally, the efciency of fast charging
equipment is often strongly dependent on temperature, with po-
wer conversion efciencies of 50 kW chargers reported at up to 93%
and as low as 39% for operation at 25
+
C and -25
+
C, respectively,
primarily due to the derating of power levels requested by BMSs at
lower temperatures [24]. Efciency in this case is dened as the
ratio between the instantaneous DC power delivered by the charger
to the vehicle and the instantaneous AC power supplied to the
charger by the grid. The level of this temperature-dependency
varies between charger models, with some still able to maintain
about 70% efciencies at -25
+
C and others failing to function at all
[24]. This section looks to provide insights into the fundamental
phenomena involved in fast charging and their resulting impacts.
2.1. Rate-limiting processes
A Li-ion battery typically includes a graphite anode, a lithium
metal oxide cathode with a layered, spinel, or olivine structure, a
liquid electrolyte containing a mixture of organic carbonates, salts,
and additives, as well as copper/aluminium current collectors and a
porous polymer separator. Processes that take place within the
battery, whether within electrodes or at key interfaces, are central
to enabling reliable operation and fast charging [16] and are
dependent on factors such as ion transport and temperature. As
shown in Fig. 2, when a Li-ion battery is charged, ions move from
the cathode, through the electrolyte, to the anode. Key mechanisms
that inuence this journey are ion transport 1) through the solid
electrodes, 2) across the electrode/electrolyte interface for both
anode and cathode, and 3) through the electrolyte, including Li
þ
solvation and desolvation [25]. In an ideal situation, these primary
phenomena involved in battery charging would be favoured.
However, battery operating conditions can lead to a range of side
reactions that compromise performance and lifetime. In addition,
the thermal behaviour of the battery is strongly dependent on the
conditions: factors such as high charging/discharging currents,
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 3
high cell resistance or high concentration polarisation increase the
heat generation rates, affecting the efciency and safety of the
process.
The anode receives a lot of the attention during the charging
process, since multiple studies have explored the role of cathode
degradation and cathode electrolyte interphase (CEI) layer growth
and concluded that these processes, while important, are generally
not rate-limiting for traditional Li-ion systems [22,25,26]. At the
anode, lithium metal can begin to deposit on the surface of the
graphite under certain conditions during charging. This can result
from a lack of accessible intercalation sites within the electrode if
lithium reaches the surface when the surface concentration is
already high. Understanding the site accesibility is a complex pro-
cess, with studies showing that lithium diffusion into the electrode
along the edge plane of graphite is signicantly more likely
compared to the basal plane of graphite [27,28]. Under certain
circumstances, the lithium may continue to deposit preferentially
where deposits already exist, forming needle-like structures known
as dendrites which pose a risk of piercing the separator and causing
a short circuit. As the cell is repeatedly cycled, the dendrites may
also break and become electrically disconnected from the anode,
reducing the amount of cyclable lithium. However, even relatively
uniform lithium plating removes signicantly more lithium from
active availability than typical solid electrolyte interphase (SEI)
layer formation on graphite, leading to dramatic capacity losses.
The reactivity of lithium metal and the chemical instability of
electrolyte components near the Li/Li
þ
potential result in the for-
mation of gas by-products and new SEI material, removing active
materials from the system [28].
Factors that inuence lithium deposition and resulting struc-
tures include lithium diffusion rates within the anode [18], limited
ion transport in the electrolyte leading to concentration gradients
across the anode and salt depletion at the current collector [29],
and reactions at electrode/electrolyte interfaces [30]. Both mate-
rials and cell designs have been studied in detail to manage the
impacts of fast charging on these phenomena [23]. Managing
internal resistances, including those associated with the transport
through the electrolyte and electrodes as well as charge transfer at
interfaces, is important to good charging characteristics, perfor-
mance, and lifetime [31], with low temperatures increasing these
transport resistances and requiring more sophisticated control
strategies [24]. In studies of electrolyte additives to mitigate lithium
plating under fast charging conditions, Liu et al. [32] found signif-
icant differences in cell capacity losses between C/2 and 1C
charging rates at 20
+
C which were attributed to lithium plating.
They concluded that many of the phenomena occurring at the
anode could be combined into a single guideline relating the area
specic resistance for the anode to the onset current of lithium
plating for the conditions tested. Additionally, work on charge
transfer by Jow et al. demonstrated that reducing the anode resis-
tance, which dominates overall cell resistance, is important for
improving fast charging performance [25].
Regarding temperature, work by Waldmann et al. [33] explored
the main aging mechanisms at both low (0
+
C) and high tempera-
tures (45
+
C) at 0.5C in lithium nickel cobalt aluminium oxide
(NCA)/graphite systems to better understand implications related
to different aging mechanisms. Cells that experienced aging at
higher temperatures by consumption of active materials through
SEI growth show higher temperature onsets for thermal runaway in
accelerated rate calorimetry testing than cells that have experi-
enced low temperature aging even at relatively mild charging rates.
While dendrites and the potential for short circuits are viewed as
the typical hazard resulting from lithium plating, the reactivity of
lithium metal is also a signicant consideration. Reactions between
lithium and the electrolyte not only contribute to the growth of the
SEI layer but can also be highly exothermic and may play a part in
the evolution of thermal runaway. Historically, high temperatures,
such as those above 30
+
C were avoided for battery operation, due to
the increased kinetics of SEI formation and other side reactions
relative to functional reactions. However, when batteries are sub-
jected to extreme fast charging, higher temperature has a benecial
effect in spite of the increase in kinetic rates of SEI formation, most
likely by avoiding operating conditions that would induce lithium
plating [23]. Finally, recent work by Yang et al. [23] explored the
dependence of the optimal cell temperature on the charging C-rate
and cell energy density. The results of their modelling study
showed that while both extremely low and extremely high tem-
peratures were generally damaging, fast charging shifted the bal-
ance in favour of higher temperatures, particularly for high energy
cells.
While the intersection of phenomena, materials chemistry and
component design are described in the following sections, anode
thickness and its inuence on transport provides a useful lens to
explore their linkages. While thin electrodes are generally consid-
ered to represent ideal transport [26], when electrodes are suf-
ciently thick, it becomes critical to ensure sufcient Li
þ
concentration at the electrode/electrolyte interface throughout the
anode to keep the overpotential stable and reduce the chances for
lithium plating [28,34]. Strong heterogeneities, whether in elec-
trode microstructures [35] or in lithium concentration gradients
observed during the charging process [36], have recently been
shown to have signicant inuence on lithium deposition re-
actions. Non-uniform SOCs within graphite anodes have also been
shown to inuence lithium plating [25], with reduced pore tortu-
osity highlighted as a key lever to avoid electrolyte phase transport
limitations and the resulting overpotential increase [17,37]. In
relatively thick anodes subjected to fast charging conditions, the
lithium salt may become depleted near the current collector,
leading to non-uniform electrode utilisation and an increase in the
local current densities near the separator [17,29,31]. Local salt
depletion at the anode has also been shown to inuence the
Fig. 2. A schematic view of Li
þ
ion transfer during a) charge, b) discharge.
A. Tomaszewska et al. / eTransportation 1 (2019) 1000114
morphology of lithium deposition, inducing the formation of den-
dritic structures in place of the more uniform mossy deposits
formed when sufcient amounts of lithium are available in the
electrolyte [29,38]. To avoid the phenomena that result in plating or
dendrite growth, Gallagher et al. recommended operating at cur-
rent densities below 4 mA cm
-2
but acknowledged the complex
interactions of operating parameters in providing such guidelines
[17]. Due to the difference in mass loadings and other characteris-
tics between cells, the general applicability of this recommendation
may be questioned.
Alternative anode materials are also the subject of signicant
research, to assist in addressing the challenges related with lithium
plating on graphite anodes during fast charging described above, as
well as to increase energy density. Since most studies have been
conducted on graphite, much remains to be studied for other sys-
tems [39], including the effects of localised current densities on
lithium metal anodes [40].
3. Degradation effects
3.1. Thermal effects
In a Li-ion battery, the heat generation occurs as a result of both
reversible and irreversible processes [41]. The irreversible heat
generation Q
irr
is given by Ref. [42]:
Q
irr
¼ðV
bat
UÞI(1)
where Uis the open circuit potential, V
bat
is the cell voltage, and Iis
the current (I>0 during charging). The difference between V
bat
and
Urepresents the total overpotential of a battery induced by pro-
cesses such as the charge transfer reactions at the electrode/elec-
trolyte interfaces [43], the diffusion and migration of Li
þ
ions across
the electrolyte [44], diffusion and migration of Li
þ
ions in the
electrodes [45] and Ohmic losses [46]. A large contribution to the
irreversible heat generation is resistive (joule) heating:
Q
joule
¼I
2
R(2)
where Ris cell resistance. As fast charging requires higher charging
currents, more heat is generated due to the quadratic dependency
of irreversible heat generation rate Q
irr
on the current. The heat
generated/consumed in the reversible process, also known as
entropic heat, originates from the reversible entropy change
D
S
during electrochemical reactions [47]. Once the entropy change
D
S
is measured [48,49], the reversible heat Q
rev
can be calculated ac-
cording to [50]:
Q
rev
¼IT
D
S
nF (3)
where Tis the absolute temperature, nis the stoichiometric num-
ber of electrons involved in the electrochemical reaction and Fis
Faraday's constant. To understand heat generation in batteries,
Nazari et al. [51] employed a mathematical model to simulate the
heat generation in lithium iron phosphate (LFP), lithium manga-
nese oxide (LMO) and lithium cobalt oxide (LCO) batteries with
graphite anodes. The results revealed that the total heat generation
in all cells investigated is of the same order of magnitude. At low C-
rates the reversible heat generation is dominant and at high C-rates
the irreversible heat is dominant. Li-ion battery lifetimes vary
greatly with cell temperature. To understand this correlation, the
US National Renewable Energy Laboratory has developed aging
models [52,53] of Li-ion cells that consider the impact of temper-
ature and charge/discharge cycle on battery life. It was found that
battery lifetime roughly doubles when the average battery tem-
perature (during storage and cycling) is reduced from 35
+
Cto20
+
C.
The effects of lower temperatures were not explored.
3.1.1. Temperature distribution during cycling
In Li-ion batteries, whether pouch, cylindrical or prismatic, heat
can dissipate from some locations more easily than from others: for
example, the poor through-plane conductivity of battery materials
such as polymer separators causes higher heat accumulation in the
core compared to the regions closer to the surface. Furthermore,
the current densities and the heat generation rates are not equal at
different locations. These inhomogeneities are aggravated for large
format cells. For cylindrical cells in particular, the temperature in
the battery core is signicantly higher than at the surface [54]as
shown in Fig. 3. For pouch [55] or prismatic [56] cells, the tem-
perature is higher in the regions close to the tabs [57] as shown in
Figs. 4 and 5 due to the higher current densities in those locatons.
Furthermore, the temperature near the positive tab is often higher
than near the negative tab due to the higher ohmic resistivity of the
aluminium current collector on the cathode side compared to
copper on the anode side. The heat generation rate among the
different regions is not uniform due to the non-uniformly distrib-
uted current during cycling. The non-uniformity of current density
can be caused by a temperature gradient and the location of tabs
[58,59]. High current density near the tabs may lead to local
overcharge or overdischarge, potentially resulting in localised
failure [60]. The inhomogeneities in the temperature and current
distribution can also lead to different local rates of side reactions
and therefore different local degradation rates [60]. Zhu et al. [61]
recently used micro-Raman spectroscopy as a temperature-sensing
platform to investigate the effects of temperature hot spots on
lithium dendrite growth in Cu/Li and Cu/LCO optical cells, showing
that high local temperature gradients dramatically increased local
current densities, leading to dendrite growth and eventual short-
circuiting during charging. In Li-ion cells with graphite anodes
charged at low temperatures the relationship between hot spots
and dendrite growth rates may be more complex due to the
interaction between temperature and the local potential of the
graphite anode.
Uneven heat generation is not solely a cell-level effect. Design
decisions on the pack level, particularly those concerning the pack
layout and the thermal management system design, have a strong
inuence on the temperature variations within a pack. Wu et al.
[62] used a coupled electrochemical-thermal model to investigate
heat generation in battery packs containing cells connected in
parallel. Finite interconnect resistances were shown to result in
unequal interconnect overpotentials and, in consequence, lead to
load imbalances and heterogeneous heat generation between cells.
These effects were aggravated during pulse loading due to the
additional heat generation caused by rebalancing between pulses.
Over time, different degradation behaviours between individual
cells can also be expected to stronglyaffect the homogeneityof heat
generation in an aged pack due to the unequal rise in cell re-
sistances. The resulting temperature gradients may, in turn, further
aggravate the differences in the aging behaviours between cells.
The design of the thermal management system to minimise tem-
perature inhomogeneity is discussed in Section 6.
3.1.2. Temperature-induced degradation
Many degradation mechanisms that take place in Li-ion batte-
ries show temperature dependence. The temperature effect is
correlated with Arrhenius equations [20,63] or modelled with
empirical equations tted to experimental data [64] in the past
studies. At high temperatures, the SEI layer on the anode grows
faster, becoming more porous and unstable [65]. At the other end of
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 5
the spectrum, low temperatures lead to slower diffusion and
intercalation with the possibility of lithium plating and subsequent
lithium dendritic growth [20]. On the cathode, the temperature
increases during cycling may lead to binder decomposition [66],
phase transitions [67], metal dissolution and CEI growth. The
electrolyte, typically ethylene carbonate (EC)/dimethyl carbonate
(DMC) (1:1 wt ratio)-LiPF
6
(1 M), can decompose to release gaseous
species, for example CO
2
[68], when operated at elevated temper-
atures. In general, most degradation mechanisms are accelerated at
high temperatures. Lowering the temperature can suppress the
degradation rate but low temperatures also undesirably slow down
the diffusion of active species and change the reaction chemistry,
leading to accelerated degradation if metallic lithium starts
depositing on the anode. Furthermore, operation at low tempera-
tures leads to lower energy efciency [69] and more heat genera-
tion due to the higher overpotential resulting from the increased
transport limitations and slower kinetics.
The SEI growth at the anode/electrolyte interface is one of the
main degradation mechanisms in Li-ion cells under most cycling
conditions. The passive surface layer adds onto the cell impedance
and diminishes the power. The lithium consumed by the SEI is not
recoverable, resulting in capacity fade. The electrochemical
reduction of the electrolyte which generates the complex mixture
of SEI with lithium species such as LiF, Li
2
CO
3
, (CH
2
OCO
2
Li)
2
,is
highly temperature dependent [70]. The SEI layer formed initially
may serve a protective function, but upon further cycling becomes
unstable and grows to block pores of the electrode or even pene-
trate the separator, which causes an impedance rise from the
decrease in active surface area and safety issues. At elevated tem-
peratures (60
+
C or above), the dissolution and decomposition of SEI
species, Li
2
CO
3
for example [71], has been found to disrupt the
integrity of the protective thin lm to form separate islands.
Fig. 3. Simulated temperature and current density distribution in a cylindrical 4.4 Ah cell subjected to SoC-neutral pulse cycling with a pulse amplitude of 70 A [59].
Fig. 4. Surface temperature evolution of a pouch cell during 5C constant current discharge obtained by a) simulation and b) measurement at t ¼250 s; c) simulation and d)
measurement at the end of discharge/t ¼667 s; e) 3D representation of internal temperature distribution [55].
A. Tomaszewska et al. / eTransportation 1 (2019) 1000116
Besides forming the SEI on the graphite anode, thermal
decomposition of the carbonate-based electrolyte yields many
other possible products, including carbon monoxide, carbon diox-
ide, hydrogen, ethylene, alkyluorides, uorophosporic acids, etc.
Gachot et al. [72] recovered lithium methyl carbonate from the
separators in Li/chromium-based-oxide (Li/CBO) cells cycled at
55
+
C. LiPF
6
is not stable at 60
+
C, and its dissociation into PF
5
dis-
solves the protective SEI and exposes fresh graphite surface [73].
The decomposition reactions are thermally activated and were
observed to become signicant at 85
+
C by comparison of nuclear
magnetic resonance (NMR) spectra of 1.0 M LiPF
6
in EC/DMC/
diethyl carbonate (DEC) [74]. The release of gaseous by-product
during SEI formation and electrolyte decomposition also compro-
mises the mechanical properties of the anode and the cathode.
As side-products from parasitic reactions accumulate, electrode
delamination and particle cracking are also detrimental to a battery
after prolonged exposure to elevated temperatures. Delamination
was observed by Pieczonka et al. for a polyvinylidene uoride
(PVDF) bound LiNi
0.5
Mn
1.5
O
4
cathode from the Al current collector
after storage in electrolyte at 60
+
C for three weeks and during
cycling [75]. Volume changes in the electrode material, exacerbated
by fast charging/discharging and temperature variation, induce
stress which generates cracks and eventually delamination when
the binders cannot hold the electrode fragments together [76]. The
carbon additives in the cathode have been found to be electro-
chemically active towards PF
6
-
intercalation which leads to struc-
tural changes in the cathode, with more severe effects observed at
45
+
C compared to 25
+
C[77]. Other mechanisms which cause
structural instability, for example the interaction of electrolyte
oxidation products and corrosion from hydrouoric acid (HF), are
mostly accelerated by higher temperatures.
In the extreme cases when the temperature rises beyond the
safety threshold, thermal runaway may occur. Thermal runaway is a
process whereby an exothermic reaction rate and the resulting
temperature increase run into an uncontrolled positive feedback.
The series of reactions during thermal runaway include SEI
decomposition, anode/electrolyte and cathode/electrolyte reaction,
electrolyte decomposition and reaction with the binder [78]. The
breakdown of the SEI layer due to either overheating, overcharging
or physical abuse is associated with exothermic decomposition of
metastable components such as (CH
2
OCO
2
Li)
2
, followed by further
exothermic reactions between the anode and the electrolyte [79].
Common polymer separators melt at above 130
+
C, although some
separators are designed to maintain structural integrity and pre-
vent a short circuit. For example, trilayer polypropylene-
polyethylene-polypropylene (PP-PE-PP) separators can be engi-
neered to exhibit a shut-down process when temperature rises
beyond a threshold. As the PE layer melts down, the separator
becomes nonporous and is no longer ionically conductive, stopping
the electrochemical function of the cell. At the same time, the PP
layers maintain their mechanical properties due to their higher
melting point, preventing separator shrinkage and a short circuit
[80]. Alternatively, a method of inducing thermal shut-down by
introducing an extra material layer between the cathode and the
current collector has been proposed by Chen et al. [81] The material
consisted of graphene-coated spiky nickel nanoparticles in a
polymer matrix and exhibited high electrical conductivity at
moderate temperatures. However, at high temperatures (70
+
Cin
the study, but tunable depending on the choice of polymer matrix
and nickel-to-polymer ratio) its electrical conductivity dropped by
seven to eight orders of magnitude within a second, effectively
shutting down the cell and preventing thermal runaway. If thermal
runaway is not successfully stopped before the separator meltdown
occurs, the cell is internally short-circuited, and consequently the
cathode breaks down. During thermal runaway, electrolyte
decomposition at 250e350
+
C is severe with rapid release of
gaseous species, building up the pressure of the cell and eventually
venting ammable vapour into the environment [82].
3.2. Lithium plating
Lithium plating is a Faradaic side reaction in which Li
þ
ions in
the electrolyte form lithium metal on the surface of the negative
electrode instead of intercalating into it [66]. This reaction mainly
occurs during fast charging when the electrostatic potential of the
negative electrode approaches or falls below that of a Li/Li
þ
refer-
ence electrode. It is aggravated by factors that slow down the
competing intercalation reaction, including low temperatures and
insufcient negative electrode material into which to insert
lithium.
3.2.1. Structure of lithium deposits
Initially, lithium metal forms in droplets so as to minimise the
surface energy [83]. The surface of the droplets quickly reacts with
the electrolyte to form SEI. As additional plating occurs on the
droplets beneath the SEI, the droplets are subject to increasing
Fig. 5. Temperature distribution of a prismatic LFP cell during a) 1C, b) 2C and c) 5C discharge process. Discharge time is: a) 1020 s, b) 990 s, and c) 360 s [56].
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 7
mechanical stress until the SEI is broken and the lithium escapes, in
what is variously termed mossygrowth, root growth[84]or
base-controlled growth[85]. In their X-ray imaging study of
lithium deposition in symmetrical lithium metal cells with a 1 M
LPF
6
in 1:1 EC/DMC electrolyte, Eastwood et al. [86] showed that
the mossy structure may also contain higher atomic number ele-
ments in addition to metallic lithium, which were interpreted as
lithium salts. New SEI forms on the mossy lithium, preventing the
formation of uniform lms as observed with less reactive metals
such as copper. Continuous mossy growth reduces the electrolyte
concentration to values far below equilibrium, decreasing the rate
of mossy growth and instead promoting growth perpendicular to
the surface, which is referred to as dendritic or tip growth[84]. As
the dendrite grows, mechanical stress in the dendrite will act to
either break it, or in the worst-case scenario, compress it causing
exponential growth. Jana and García [85] use the analogy of
squeezing toothpaste out of a tube; in order to do so, there must be
sufcient toothpaste in the tube to create the necessary pressure
gradients.
3.2.2. Effects on battery performance
Dendritic growth is referred to as a worst-case scenario for good
reason; if a dendrite goes all the way through the separator to reach
the positive electrode, an internal short-circuit is formed, resulting
in rapid heating of the cell [87]. In most cases, the resulting heat
generation melts the dendrite immediately and breaks the short
[88], but the temperature rise can trigger other degradation
mechanisms such as SEI formation, electrolyte drying and disso-
lution of the positive electrode. While the risk of short-circuit alone
is enough to render fast charging at low temperatures unsafe, even
mossy growth causes capacity fade through loss of lithium in-
ventory and power fade by reducing the porosity of the negative
electrode and/or the amount of electrolyte in the cell. Often plated
lithium forms at the electrode-separator interface and thus has the
potential to form a blocking layer to the rest of the active material.
Metallic lithium is also much more reactive than graphite, pro-
moting further side reactions that result in SEI growth, gas gener-
ation and electrolyte decomposition [66].
3.2.3. Model based investigation of lithium plating
The rst extension of the standard pseudo-two-dimensional
(P2D) model of Fuller, Doyle and Newman [89] to include lithium
plating and the inverse process, lithium stripping, was proposed by
Arora, Doyle and White [18]. A second Butler-Volmer equation was
proposed for the side reaction:
j
n;sr
¼i
0;sr
Fexp
a
a;sr
F
h
sr
RT exp
a
c;sr
F
h
sr
RT ;(4)
where j
n;sr
is the side reaction ux (positive for stripping, negative
for plating), i
0;sr
is the side reaction exchange current density, Fis
Faraday's constant, Ris the universal gas constant, Tis absolute
temperature, the side reaction overpotential
h
sr
is given by
h
sr
¼f
s
f
l
U
sr
Fj
n;sr
d
film
k
film
(5)
and f
s
and f
l
are the local electrostatic potentials in the solid and
electrolyte phases respectively. If the potentials are dened with
respect to a Li/Li
þ
reference electrode, as in the standard Newman
model [89], U
sr
¼0bydenition. Arora, Doyle and White assumed
the plating to be partially reversible, choosing values of
a
a;sr
¼0:3
and
a
c;sr
¼0:7 for the transfer coefcients accordingly. Their model
assumed a uniform lm, where the lm thickness
d
film
is an addi-
tional variable to be solved for:
v
d
film
vt¼j
n;sr
M
film
r
film
:(6)
The electrical conductivity
k
film
, molar mass M
film
and density
r
film
of the lm can be either set to those of lithium or modied in
order to account for fast side reactions: Arora, Doyle and White
consider a lm consisting of both lithium metal and lithium car-
bonate Li
2
CO
3
. The effect of local electrolyte concentration con
mossy lm growth is accounted for by assuming i
0;sr
to take the
form
i
0;sr
¼Fk
sr
c
a
c;sr
;(7)
where k
sr
is a reaction rate constant. Inspection of (4) and (5) shows
that plating occurs when the electrostatic potential of the electro-
lyte exceeds that of the electrode, explaining the potential depen-
dence discussed previously. The above model is hitherto referred to
as the Arora model for the sake of brevity.
Perkins et al. devised a control-oriented reduced-order model
[90] based on the Arora model. The relative error on j
n;sr
is less then
10% compared to the full partial differential equation (PDE)-based
model for charging rates up to 3C. More important for control
purposes is the charge rate required to obtain plating at any given
state of charge, for which the relative error is less than 5%. Hein and
Latz [91] devised a three-dimensional microstructure-resolved
model based on the Arora model. Notably, Hein and Latz also
simulated chemical intercalation, a non-Faradaic process where
plated lithium intercalates into graphite, assuming a constant ux
for said chemical intercalation. Ge et al. [92] used the Arora model
to include the effect of low temperatures, by assuming the solid
state diffusion coefcients, electrolyte conductivity and reaction
rate constants to have Arrhenius temperature dependence. They
found that at 25
+
C, plated lithium accounts for 2% of total charge
by the end of a 1.5C charge. The model was also able to reproduce
the experimental result that the plated lithium accounted for 1:55%
of total charge on reaching 80% SOC, the rst direct experimental
comparison with a lithium plating model.
Yang et al. [93] added the porosity variation equation of Sikha,
Popov and White [94] to the model:
vε
vt¼av
d
film
vt;(8)
where εis the porosity or electrolyte volume fraction and ais the
surface area to volume ratio. Integrating (8) with respect to time
and assuming the initial lm thickness to be zero yields
ε¼ε
0
a
d
film
;(9)
where ε
0
is the initial value of the porosity. Like Arora, Doyle and
White, Yang et al. assumed the lm to contain SEI as well as lithium
metal, but used a separate cathodic Tafel equation for SEI forma-
tion. A lumped thermal model was also included. They found that,
after around 3000 cycles at room temperature, SEI growth causes
enough pore clogging to raise the electrolyte potential above that of
the graphite and thus cause plating, at cycling rates as low as C/3.
This plating reduces the porosity further, causing positive feedback
and nonlinear reduction in useable capacity. However, they did not
conrm the presence of plating with a cell teardown.
The same team [95] later devised a modied version of (4)
where the stripping rate depends on the concentration c
Li
of plated
lithium:
A. Tomaszewska et al. / eTransportation 1 (2019) 1000118
j
n;sr
¼i
0;sr
F"c
Li
c
Li
exp
a
a;sr
F
h
sr
RT c
c
exp
a
c;sr
F
h
sr
RT #;(10)
where i
0;sr
is redened in terms the reference concentrations c
Li
and c
:
i
0;sr
¼Fk
sr
c
Li
a
a;sr
ðc
Þ
a
c;sr
:(11)
In that paper [95], the lm is assumed to consist of only plated
lithium with no SEI, resulting in a simple relationship between c
Li
and
d
film
in terms of the density
r
Li
and molar mass M
Li
of pure
lithium:
c
Li
¼a
d
film
r
Li
M
Li
:(12)
In a rst attempt to model irreversible capacity loss due to
plating, Ren et al. [96] use a modied form of (4) that considers
both the reversible lithium, which can be stripped, and irreversible
or deadlithium that is electrically isolated and therefore cannot be
stripped.
3.2.4. Detection
Non-destructive diagnosis of lithium plating is of great signi-
cance for onboard application as well as periodic inspection.
Characterisation techniques that can be used to observe the
morphology and distribution of lithium plating include optical
microscopy [97e100], scanning electron microscopy (SEM)
[101e103], transmission electron microscopy (TEM) [104 e106],
NMR spectroscopy [107e109 ] and X-ray diffraction (XRD) [20,110].
However, these methods generally require the cell to be dis-
assembled or manufactured with a special structure. The
commonly used non-destructive methods to detect lithium plating
take advantage of observable external characteristics, including
aging rates, voltage plateaus exhibited during lithium stripping,
model-based prediction and others.
Lithium deposits gradually accumulate at the microscale during
cycling. The side reaction between the plated lithium and the
electrolyte forms new SEI, resulting in capacity fade and the rise of
the impedance. Trends in these aging behaviours can be used to
identify lithium plating. Available methods include the Arrhenius
plot [20,111], resistance-capacity plot [111 ], impedance spectros-
copy [112 ] and coulombic efciency analysis [113 ].
The relationship between the aging rate rand the temperature T
can be described by the Arrhenius equation, the parameters of
which are acquired from cycling tests under different temperatures
[20]. An example is shown in Fig. 6a. The aging rates increase at low
temperatures (<0
+
C) due to the increased likelihood of lithium
plating, whereas aging due to SEI growth accelerates with
increasing temperatures. The resistance-capacity plot [111] reveals
that lithium plating primarily inuences the rate of capacity fade
and has a lesser effect on the rate of impedance rise due to the
higher conductivity of lithium metal compared to SEI [111 ]. Two
aging modes can therefore be distinguished: one dominated by
lithium plating, with a lower rate of impedance rise but high rate of
capacity fade (Aging mode 2
00
in Fig. 6b), and the other dominated
by SEI growth, with a higher rate of impedance rise and less
prominent capacity fade (Aging mode 1).
Impedance spectroscopy can also reect lithium plating.
Nonlinear frequency response analysis (NFRA) provides a new po-
tential detection criterion in the form of the correlation between
two harmonics as described in Ref. [112 ]. The authors showed that
the analysis of higher harmonic responses by NFRA captured
changes in the impedance response with aging that could not be
observed by only analysing the rst harmonic as measured with
electrochemical impedance spectroscopy (EIS). The evolution of the
second and third harmonics (Y
2
and Y
3
respectively in Fig. 6c) was
observed for cells aged at -10
+
C and 25
+
C. The ratio between their
values was proposed as an indicator of lithium plating, with the
third harmonic increasing signicantly more sharply than the
second with cycling at -10
+
C but not at 25
+
C. This observation was
attributed to the high sensitivity of the third harmonic to the
increasing charge transfer resistance between the anode and the
electrolyte caused by the lithium plated on the surface of the anode.
High precision coulometry can be used to detect lithium plating
based on variations in coulombic efciency. A slight drop of
coulombic efciency as shown in Fig. 6d indicates the consumption
of active lithium as a result of the plating reaction. Burns et al. [113]
measured the coulombic efciency with a high precision charger,
revealing that lithium deposited slightly at a charging rate of C/2 at
12
+
C.
Parts of the plated lithium can re-intercalate into the anode or
become stripped during a discharge. These processes can give rise
to voltage plateaus during the relaxation or discharging periods
immediately after charging as shown in Fig. 7. The occurrence of
these plateaus can be explained by the changes in total over-
potential during the rest or discharge period after charge due to the
re-intercalation and stripping of lithium as shown in Fig. 7a. The
depressions in the differential voltage and incremental capacity
curves in Fig. 7b and c indicate lithium plating had occurred.
However, care must be taken to distinguish depressions resulting
from lithium stripping from those resulting from phase changes in
the electrode materials [114 ]. Furthermore, Campbell et al. [114]
have shown that the absence of a plateau does not prove that
lithium plating has not occurred. Self-heating and concentration
equilibration phenomena immediately after a fast charge may also
inuence the shape of the voltage curve [114]. Differential voltage
analysis (DVA) [96,114e116] and incremental capacity analysis
(ICA) [114 ,117e119] are useful techniques to identify the occurrence
of voltage plateaus. However, these methods require using slow
discharge rates to capture the necessary detail; high currents lead
to higher overpotentials which become dominant over the features
of interest in the voltage curve [114 ]. Abnormal exothermic peaks
may also be observed during the lithium plating and stripping
process [110], acting as another indicator of deposition.
Increases in the cell thickness may provide an alternative indi-
cator of lithium plating, but the mechanisms require further study
[120,121]. Finally, electrochemical models can be used to predict
the occurrence of plating depending on the charging conditions as
discussed earlier. However, these complex and computationally
intensive models would require simplication to be used for online
detection.
Few methods for in-situ diagnosis and quantication of lithium
plating after abnormal charging have been proposed to date. We
believe the detection approaches that rely on the abnormal voltage
plateaus are the most promising to be used onboard. However,
there is still a signicant knowledge and technological gap before
these methods can be reliably applied in devices.
3.3. Mechanical effects
Mechanical pulverisation is another important aspect of fast
charging induced degradation, and widely observed for various
electrode materials including graphite [122e124], lithium nickel
manganese cobalt oxide (NMC) [125e127], LCO [12 8e130], LMO
[131], NCA [13 2], Si [133,134] and others.
Here we summarised mechanical degradation based on scale:
cracks within the electrode particles [125e127,129,130,135], isola-
tion of electrode particles, conductive material and binders [136],
separation between active material and current collector,
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 9
delamination between electrode sheets. The main reason for these
phenomena is the strain mismatch resulted from the lithium
concentration gradient [125,128,136], which is aggravated by fast
charging. During fast charging, Li
þ
ions intercalate into the anode
Fig. 6. Lithium plating detection methods based on aging behaviours. a) Arrhenius plot [20], b) Resistance-capacity plot [111], c) Nonlinear frequency response analysis (NFRA)
[112 ], d) Coulombic efciency [113 ].
Fig. 7. Detection of lithium plating based on the lithium stripping process. a) Simulated evolution of overpotentials during a CC-CV charge and subsequent rest period, explaining
the occurrence of the lithium stripping voltage plateau. In Stage I, no lithium is plated on the particle; during Stage II, lithium plating occurs; during Stage III (the start of the rest
period), the reversible part of lithium reintercalates into the anode or is stripped, while the remaining lithium becomes electrically isolated from the anode (dead lithium); in Stage
IV, steady state is achieved, with the dead lithium no longer being available for cycling [96]. b) Differential Voltage Analysis (DVA). The curves show the differential voltage of Li-ion
cells during a rest period after charging at 1/6C, 1C, and 2C at -5
+
C. The peaks seen in the 1C and 2C curves correspond to voltage plateaus and may indicate lithium stripping [96]. c)
Incremental Capacity Analysis (ICA) showing the incremental capacity of cells during a C/20 discharge after being charged at 1C to 0.7, 0.8, 0.9 and 1.0 SOC at -20
+
C. Again, peaks
correspond to voltage plateaus [118 ].
A. Tomaszewska et al. / eTransportation 1 (2019) 10001110
and deintercalate from the cathode rapidly, leading to a severe
lithium concentration gradient, strain mismatch between different
parts of the electrode particle and stress development. Cracks
propagate in particles when the energy release rate or stress in-
tensity factor exceeds a certain value [128,129]. This is always
accompanied by SEI/CEI cracks. Furthermore, the lattice strain
along different directions varies for almost all kinds of electrode
materials [131]. Primary particles are randomly oriented [125], in
direct contact with adjacent primary particles. As a result, fast
charging induced strain between nearby primary particles cannot
be concordantly accommodated, which may cause isolation be-
tween electrode particles or conductive material and binders. The
strain mismatch between electrode materials and current collector
can also lead to cracks [137 ] or detachment [138]. Since higher C-
rates induce more severe current inhomogeneity across electrode
sheets, delamination may occur for cells without tight external
constraints [139].
The effects of mechanical disintegration on cell performance can
be divided into loss of active material (LAM), loss of lithium in-
ventory (LLI) and impedance growth. Firstly, cracks lead to poor
electrical conductivity and even total detachment [125]. Secondly,
more fresh surface is uncovered by the cracks and reacts with the
electrolyte [126,130,140]. Such side reactions are facilitated by the
high temperature induced by fast charging [131]. The process re-
sults in further growth of the SEI layer which increases the resis-
tance and causes both LAM and LLI. Finally, consumption of the
electrolyte may also reduce the wettability of electrodes and hinder
ion transport [126]. The following positive feedback mechanism
has been proposed [126]: higher rates result in increased micro-
crack formation; cracks then aggravate the difference between
electron and ion transport as the ions are able to diffuse through
the electrolyte inltrated into the cracks while electrons cannot,
leading to severe SOC inhomogeneity and further cracking.
Many studies on the mechanical effects of fast charging have
discussed size dependence. Smaller particles are more resilient to
cracks because of lower lithium gradients [140 ] and energy release
rates [141]. However, small particles have higher specic surface
areas, leading to more SEI and impedance growth. Moreover, small
particles result in a lower energy density due to their lower packing
density [126,135 ,140 ]. Thus, primary particles are sometimes
packed into secondary particles. However, the mechanical strength
of secondary particles is much lower than the intrinsic material
strength of primary particles due to the weak Van der Waals in-
teractions [127]. Another way to increase energy density is to
fabricate larger primary particles [135,140]. Researchers have
attempted to explore the critical size. Model-based shock maps for
LCO [129] and LMO [128,131] have been proposed, showing that for
relatively low C-rates and small particle size zones fracture is not
expected. Similar results have also been obtained experimentally
for Si nano-pillars [142 ] and Si nano-particles [141]. For a very small
diameter of Si nano-pillars [142] (140 nm), cracks are avoided at
any C-rate; while for large diameters (290 nm, 360 nm), fracture
occurs at most C-rates. Models based on fracture mechanics in
terms of stress intensity factor [128] or energy release rate [129]
give some guidance for particle design. However, most of such
models are not validated and their implementation into real elec-
trodes may be difcult since many parameters, including the frac-
ture strength factor or initial crack length, are difcult to obtain.
The real boundary conditions are also more complicated compared
to the free-standing conditions in these models.
Whether higher C-rates certainly cause more cracks for sec-
ondary particles is still subject to debate. For NMC333 in Ref. [125],
operation at 1C did not lead to more cracks than 0.1C. This is
consistent with the results for NMC532 under 0.5C and 2C [127].
However, for NMC622 operated at 1e10C cracks increased with C-
rates, which is similar for NMC532 in Ref. [135] and NCA in
Ref. [143]. This may be due to the so-called skin effect [127]. The
hoop stress at rst increases with current rate, then drops after a
maximum according to simulations. Most Li
þ
ions may aggregate at
the particle surface while the lithium in other regions remains
dispersed even when too high C-rates are applied. Therefore, hoop
stress drops quickly within a thin region near the surface, and the
stress intensity factor will not be large.
Mechanical constraints can be used to design fast charging
protocols [144,145]. Lu et al. designed an exponential charge cur-
rent protocol using the hoop stress on the particle surface as a
constraint [144 ]. This protocol can theoretically shorten the charge
time, with little loss of cycle life. However, no other degradation
effects are considered in the model and no experimental validation
of capacity retention has been carried out. Spingler et al. proposed a
stepwise charging protocol based on constraining the maximum
local irreversible swelling of the cell and temperature rise [145 ].
Experiments proved that the method could shorten charge time
and prolong cycle life compared to a 1C constant current - constant
voltage (CC-CV) protocol.
Overall, much remains to be studied regarding mechanical
degradation in Li-ion batteries under fast charging conditions.
Experimental studies on the subject have reached different con-
clusions, and there is still disagreement regarding important issues
such as the dependence of the rate of crack formation on the
charging C-rate. Mechanical degradation is often difcult to
decouple from other aging mechanisms, although a recent study
has proposed a method to identify the effects caused by cracking
using ICA, nding it to be the dominant degradation mechanism in
single layer NMC532/gaphite cells charged at 9C [146]. Compared
to degradation mechanisms such as SEI layer growth or lithium
plating, relatively few modelling studies on mechanical effects
under high currents have been published, and even fewer have
been validated. The difculty of obtaining the required parameters
and formulating realistic boundary conditions remains a major
hindrance to developing reliable models.
4. Multi-scale design for fast charging
The extent and mode of fast charging induced degradation can
be affected by the battery material components (inherent proper-
ties of the electrodes and electrolyte), operational conditions (high
rate of charge/discharge, extreme voltages and temperatures),
battery manufacturing processes and pack design [147]. Multi-scale
design and hybrid approaches present signicant opportunities
towards developing high performance fast charging batteries.
4.1. Electrode materials
The selection of suitable electrolyte and electrode chemistries
with high capacities that can also be operated at a high rate is one of
the most challenging issues in cell design. Extensive research has
been carried out to develop fast charging anode materials which
can prevent the formation of lithium dendrites. Some well-known
anode materials, such as the carbon-based alternatives (graphite,
carbon nanotubes, graphene or graphene oxides), metal based
composites (TiO
2
, lithium titanium oxide (LTO), MnO, Co
3
O
4
,Fe
3
O
4
,
NiO) and alloy composites (such as Si and Sn based compounds),
have been investigated with some success.
Conventional graphite anodes are characterised by potentials
very close to the potential of Li/Li
þ
formation, making them desir-
able for maximising cell energy density but also particularly sus-
ceptible to lithium plating. Modifying anode materials has
therefore been one of the most fruitful approaches toward
improving the fast charging capability of Li-ion cells. For example,
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 11
surface engineered graphite with only 1 wt% Al
2
O
3
coating exhibits
a reversible capacity of about 337.1 mAh g
-1
at a high rate of
4000 mA g
1
[148 ]. In another instance, LTO materials are desirable
for batteries capable of extreme fast charging with long lifetimes
due to the fact that they do not exhibit lithium plating or SEI layer
formation, but they are seriously limited by their high operating
potentials, leading to decreased full cell voltage and limited energy
density [149]. The modication of LTO mainly aims to increase its
electronic conductivity, lithium-diffusion coefcient or to maxi-
mise the active powder content per unit volume by in situ intro-
ducing carbon sources [150 ], carbon coating [151], and metal/
nonmetal ion doping [152 ,153 ].
Alternatively, some metal oxide and alloy based materials have
been proposed for high power Li-ion battery applications. However,
these materials are often limited by their fast charge useable ca-
pacity and cycling stability because of their severe volume change,
pulverisation and agglomeration of primary particles, and their
poor electronic conductivities. 2D-structured graphenes have
shown great advantages [154] such as a high intrinsic carrier
mobility, excellent thermal and electrical conductivity, high theo-
retical specic surface area, and superior mechanical strength. They
can potentially be applied as anode materials by N-doping
[155 ,156] (achieving high reversible capacity of over 1040 mAh g
1
at 50 mA g
1
) or integrating with large volume change anode ma-
terials such as Si and Sn. For example, Agyeman et al. [157]. re-
ported a carbon coated Si/rGO composite anode with a sandwich
structure, which simultaneously achieved a high capacity and a
stable cycle life (about 93% capacity retention over 400 cycles).
Other graphene-like 2D materials have been extensively
explored as promising anode materials due to their high surface-to-
mass ratio and unique physical and chemical properties, leading to
shortened Li
þ
diffusion pathways, fast electron transport and
increased number of sites for ion storage. Such 2D anode materials
mainly include transition metal oxides (TMOs, such as Co
3
O
4
, NiO,
Fe
3
O
4
, and MnO
2
), transition metal dichalcogenides (TMDs, such as
MoS
2
,WS
2
, MoSe
2
, and WSe
2
, FeS, FeS
2
, and CoS
2
) and the transi-
tion metal carbides/nitrides (mainly highly conductive MXenes,
such as Ti
3
C
2
T
x
,Ti
2
CT
x
,Mo
2
Ti
2
C
2
T
x
,Cr
2
TiC
x
T
x
,Nb
4
C
3
T
x
and V
2
CT
x
).
Among the TMOs, titanium and niobium based oxides are proposed
as promising anode candidates with benecial redox potentials
ranging from 1.0 to 1.6 V, in good match with commercial electro-
lytes [158e160]. Recently, Goodenough's group proposed the high
rate TiNb
2
O
7
anode with a theoretical capacity comparable to
graphite which can achieve rapid Li
þ
intercalation/deintercalation
and longer cycle life, demonstrating potential to replace LTO [161].
Electrode structure designs in the nano-scale have also been
demonstrated to achieve increased energy and power densities. 2D
structures with hollow, core-shell, and yolk-core designs, e.g. 2D
holey ZnMn
2
O
4
nanosheets [162] showed a capacity of 770 mAh g
-1
at 200 mA g
-1
and 430 mAh g
-1
at 1200 mA g
-1
, while the control
spinel ZnMn
2
O
4
only achieved 32% and 6% of these values respec-
tively. Integration of 2D materials into 3D macroscopic structures,
such as porous lms, scaffolds, and networks, can enhance both
ionic and electronic transport in electrode materials. The 3D hier-
archical structures, e.g. MoS
2
with column-like structures [163 ]
have been shown to achieve a high reversible specic capacity of
840 mAh g
1
at 200 mA g
1
, cyclic stability up to 500 cycles, and
outstanding rate performance. The electrically conductive and
electro-active MXenes have been attracting increasing attention
since their discovery in 2011 [164]. Strategies were successfully
proposed to achieve 3D hierarchical structures, including fabri-
cating 2D MXene akes into hollow spheres and 3D architectures
[165 ] to prepare free-standing, exible, and highly conductive 3D
macroporous MXene based lms and thick vertically aligned
MXenes with high rate charging/discharging capabilities [166].
Metallic lithium is one of the most favoured anode material
choices for increasing energy density [167] but suffers from low
power density due to the low surface area of a pure lithium foil.
However, incorporating Li metal into 3D structural matrices can be
benecial to rate capability by enabling faster Li-ion diffusion. For
example, a composite material prepared by lithium melt infusion
into an electrospun 3D porous carbon matrix showed stable cycling
with a small Li plating/stripping overpotential (below 90 mV) [168].
Approaches to enhance the fast charging performance are not
limited to anode material selection, modication and nanoscale
structure design. Electrolyte and interfaces are also critical ele-
ments that affect the performance of anodes [169 ]. Available ap-
proaches towards optimising the anode/electrolyte interface
include amorphous carbon coatings for graphite [170 ,171] that
result in more homogeneous SEI layer formation with suppression
of unwanted active points on graphite where lithium deposition is
more likely [171 ], metal coating and doping, especially with Cu and
Sn on the anode [172 ], and smart choice of Li salt and co-solvents
[38]. Qian et al. [173 ] reported a high rate and non-dendritic
lithium metal anode by using highly concentrated electrolytes.
Their Cu/Li cell was cycled at 4 mA cm
-2
for 41,000 cycles with a
high Coulombic efciency of 98.4% in the 4 M LiFSI-DME electro-
lyte. Also beyond nano structured materials, Grifth et al. reported
Nb
16
W
5
O
55
and Nb
18
W
16
O
93
anode materials which achieved
excellent high-rate performance without nanoscaling [174], deliv-
ering a potential low cost way to produce fast charging electrode
materials. Research effort has also been dedicated to electrode
microstructure optimisation to enable faster diffusion, improving
the rate capability. Billaud et al. [175 ] reported architectured
graphite electrodes composed of graphite akes aligned perpen-
dicularly to the current collector using a low magnetic eld that
achieved three times higher specic charge compared to similarly
loaded conventional graphite anodes when cycled at up to 2C. Low
magnetic eld was also used by Sander et al. [176] to fabricate LCO
electrodes with directional pore arrays that were capable of
maintaining signicantly higher areal capacities compared to reg-
ular LCO electrodes at high C-rates. Also in conventional carbon/
graphite anodes, crystal structure has a strong inuence on the
diffusion of lithium. Fang et al. [177] investigated the electro-
chemical performance of mesophase soft carbon (MSC), mesophase
graphite (SMG) and hard carbon (HC) anodes, nding that MSC
exhibited the highest specic capacity at high C-rates, likely due to
more extensive interlayer spacing. The more crystalline SMG was
characterised by markedly lower capacity than the other two ma-
terials at high C-rates, which was contributed to narrower inter-
layer spacing that obstructed diffusion.
Material selection and modication is undoubtedly a promising
direction for future research. Many materials or material architec-
tures have shown improved fast charging characteristics compared
to the materials commonly used today. However, it is important to
stress that early material development and large scale commerci-
alisation are usually separated by a relatively long period of time. In
many cases, new manufacturing techniques and facilities need to
be developed to enable the production of a novel material at the
required scale. Furthermore, any such material needs to be cost-
competitive compared to the ones currently in use, which may be
a signicant hindrance particularly for alternative anode materials.
The performance of many materials or structuring approaches
discussed here has only been assessed on a laboratory scale, which
may not always translate to similar improvements when integrated
into commercial cells and packs. While material science will play an
important role in the development of future batteries, other engi-
neering efforts are certainly needed to address the issue of fast
charging capability in the near term.
A. Tomaszewska et al. / eTransportation 1 (2019) 10001112
4.2. Cell and pack design
Apart from the choice of materials and their microstructure, the
choice of electrode geometrical parameters is also an important
design aspect. Increasing the porosity [178] and the width of the
anode [179] can help suppress lithium plating but may also cause
capacity loss. The negative-to-positive electrode (N/P) capacity
ratio can signicantly inuence lithium deposition [18], with values
greater than 1 used in commercial cells for that reason. Addition-
ally, while graphite cracking is not expected during lithiation [26],
the higher N/P ratio may also help reduce mechanical stresses on
the anode, which would reduce additional SEI formation and
associated LLI [180 ]. The N/P ratio has been shown to decrease in
NMC811/graphite cells charged at high C-rates due to the areal
capacity of graphite reducing more sharply with the charging rate
compared to the areal capacity of the NMC811 electrode [22]. The
effect was found to be signicant, with the N/P ratio falling from
1.15 at 0.1C to 1.0 at 3C and reaching 0.5 at 4C. Kim et al. [181]
investigated 0.85C charging of 1.40 Ah pouch cells with different N/
P capacity ratios at 25
+
C and found that having 20% more capacity
in the negative electrode than in the positive electrode was the best
compromise between maximising capacity and preventing plating.
For faster charging rates and lower temperatures, this may have to
be increased. The optimal ratio may also depend on other factors
such as the electrode materials or cell form factor. Historically, the
anode has often been designed to be slightly larger than the cath-
ode in order to prevent lithium plating [182 ]. The additional area is
known as the anode overhang and was long assumed to be a pas-
sive part, essentially not participating in the lithiation or delithia-
tion of the anode [183]. However, the tendency of lithium plating to
develop in the anode areas adjacent to the overhang has recently
prompted more thorough research into the processes taking place.
It was found that during rest times, the lithium from the main
anode area tends to diffuse into the overhang due to the concen-
tration gradient [184]. During the following discharge, the edge
area of the cathode receives more lithium as an effect. Then, during
the subsequent charge, the extra lithium is transferred to the area
of the anode directly opposite the cathode edge and adjacent to the
overhang. This causes a higher local concentration of lithium with
lower local anode potential, increasing the probability of lithium
plating [184 ]. The anode overhang should therefore be minimised
in order to avoid deposition.
Cell geometrical design is another factor that can signicantly
affect the fast charging capability of Li-ion batteries. Cell shape and
form factor inuence the distribution of temperature and current,
with large form factor cells being particularly prone to developing
inhomogeneities [185 ]. Tab position within the cell, tab materials,
structures, and welding methods are very important to homogenise
the current distribution and limit localised heating and degradation
as described in Section 3.1 [185,186 ]. Erhard et al. [187 ] simulated
and measured current density distributions in large form factor
single layer NMC/graphite pouch cells by placing multiple tabs at
different locations along the electrodes. Current was always applied
to the same pair of reference tabs, located at one side of the cell.
Although the temperature differences along the cell length were
negligible due to the single layer design, signicant differences in
local current densities were reported, highlighting the importance
of tab placement. Locating the tabs at opposite sides of the cell and
increasing the number of tabs [188] can reduce current and tem-
perature gradients.
The links between pack-level performance and cell-level char-
acteristics are still not well understood [34]. While modelling of
degradation induced by fast charging at cell level is relatively
developed, few studies have so far attempted to extend similar
approaches to pack design, largely due to the added complexity and
multitude of design and operating parameters that need to be
considered. Tanim et al. [34] demonstrated that Nissan LEAF battery
packs charged at 2C experienced signicantly higher rates of ca-
pacity fade compared to individual cells charged at the same C-rate.
Liu et al. [189 ] recently investigated the effects of cell-to-cell vari-
ations on pack performance using a thermally coupled single par-
ticle (SP) model, showing that signicant current imbalances
existed between cells. These imbalances had the effect of reducing
the overall accessible energy as well as increasing the degradation
rate. Interestingly, placing higher resistance cells near the load
points reduced the imbalance. Meintz et al. [190 ] reviewed the
implications of fast (over 400 kW) charging on vehicle system
design, concluding that there was a number of problems with EV
electrical architecture and vehicle components that would need to
be addressed before higher charging powers can be safely imple-
mented. Many challenges remain for successful design of fast
charging battery packs, including: 1) fast charging packs require
high performance cells with low cell-to-cell variability, new ma-
terials and advanced manufacturing; 2) more advanced BMSs with
additional sensing and control circuits are required for monitoring
and balancing purposes; 3) advanced thermal management sys-
tems are needed to maintain safe temperatures and reduce thermal
gradients within the cells and packs.
5. Fast charging strategies
While many of the material level solutions discussed in Section
4.1 do indeed show promising results, most are not expected to
reach the market on a wider scale in the near future. Many re-
searchers have therefore turned towards cell and pack level ap-
proaches which can often be implemented in real world systems in
a signicantly shorter time. Charging strategies, which determine
how the current density is varied during the charging process, are
an important category of such solutions.
5.1. Types of charging protocols
Standard protocols. Numerous charging protocols have been
proposed for Li-ion batteries. CC-CV is by far the most common one.
It consists of a constant current charging phase where the battery
voltage increases up to a cut-off value (CC phase), followed by a
constant voltage hold until the current falls to near-zero (CV phase).
The CV phase allows for the concentration gradients within the
electrode particles to disperse and is usually necessary to obtain
high capacity utilisation without exceeding the maximum voltage.
However, since the current gradually decreases during the CV
phase, the charging time is signicantly increased compared to CC-
only charging. The simplicity and ease of implementation of CC-CV
has made it the standard charging protocol in most applications.
However, many other protocols have been shown to achieve
reduced charging times, increased efciencies and/or improved
capacity or power retention. Fig. 8 illustrates some of the alterna-
tive protocols proposed for fast charging.
Zhang et al. [192 ] studied the effects of the CC-CV charging
protocol on the reversible capacity and anode potential evolution in
experimental LCO/graphite cells with embedded reference elec-
trodes as they were charged at ambient temperatures from -20
+
Cto
10
+
C with C-rates between 0.16C and 1.2C. The results showed a
positive correlation between increasing charging current in the CC
phase and increasing time taken by the CV phase to complete. The
authors observed that increasing the current beyond a certain value
did not bring any further reduction in overall charging time due to
this correlation. Both increasing the charging current and
decreasing the ambient temperature was shown to result in a
reduction in anode potential, with all test schemes causing the
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 13
anode potential to become negative versus Li/Li
þ
at some point
during charging. Negative anode potential was assumed to inevi-
tably cause lithium plating, which was suggested as the main
reason for the reduction in charged capacity at high currents and
low temperatures. However, the occurrence of plating was not
conrmed through other methods. Ouyang et al. [193 ] demon-
strated that the shape of the voltage prole of large format LFP cells
changed signicantly with the number of cycles at 0.5C CC-CV at
-10
+
C, resulting in an elevated average voltage in the CC stage and
longer duration of the CV stage. In comparison, the voltage prole
of cells charged at 0.2 and 0.3C at the same temperature retained a
similar shape. The increased average voltage as a result of repeated
cycling at higher C-rates may contribute to increased degradation
rate.
Multistage constant current (MCC) protocols. Many researchers
have proposed that adjusting the current levels during the charging
process may limit cell degradation while reducing the charging
time. Such approaches are often motivated by reducing heat gen-
eration, avoiding conditions that enable lithium plating or reducing
mechanical stresses when the diffusion of Li
þ
ions is constrained.
MCC protocols are one of the earliest types designed specically for
fast charging. Such protocols consist of two or more constant cur-
rent stages, often followed by a CV stage. Higher current levels are
usually chosen for the earlier CC stages since the anode potential is
less likely to become negative in the beginning of the charging
process. Nevertheless, some authors have used the opposite
approach with current levels increasing in later CC stages due to the
lower cell resistance. Zhang [194] observed faster capacity loss in
cells charged with the latter MCC-CV approach compared to CC-CV
and constant power - constant voltage CP-CV protocols with the
same average C-rate of 1C. The study concluded that CP-CV resulted
in best capacity retention when the cell was fast charged (1C), while
CC-CV was less damaging for cells charged at 0.5C. Another work
[195 ] reported an 83% capacity retention after 4200 cycles in an LFP
cell charged with a 25-min MCC-CV protocol with two CC stages of
decreasing current. Waldmann et al. [19] studied the anode
potential evolution in reconstructed commercial NCA/graphite
cells. Three MCC charging protocols were then tested in fresh cells
of the same type. Current level was decreased just before reaching a
voltage at which negative anode potential was observed in the
reconstructed cells. All three protocols were faster than 0.25C CC-
CV protocol and led to a comparable rate of capacity fade. How-
ever, this protocol design method is both time consuming and
expensive as it requires reconstructing the cell with a reference
electrode and measuring the anode potential in the range of tem-
peratures and currents of interest in the practical application.
Spingler et al. [145 ] postulated that lithium deposition leads to local
volume changes which can be directly measured in pouch cells in
operando using laser triangulation, removing the need to measure
the anode potential. Based on this assumption, they designed an
MCC protocol with multiple stages in which current level was
decreased every time the maximum local expansion reached a
predetermined value. Cells cycled with this protocol experienced
dramatically improved capacity retention compared to a CC-CV
protocol with the same average current. However, the method
still requires extensive experimentation and is limited in applica-
tion to pouch cells.
Pulse charging protocols. Pulse charging protocols, where the
charging current is periodically interrupted by short rest periods or
discharge pulses, are also common in literature. The strategy aims
to reduce concentration polarisation, reducing the risk of local
anode potential becoming negative or reducing mechanical
stresses due to uneven insertion and extraction of lithium in the
solid particles. Aryanfar et al. [196] used a Monte Carlo simulation
of lithium dendrite growth to show that pulse charging can also
potentially inhibit dendrite propagation. One of the early studies
[197] reported that a 1C pulse charging protocol could reduce the
charging time from 3.5 h for a 1C CC-CV protocol to approximately
1 h due to the absence of the CV stage that accounted for most of
the total charging time in CC-CV. Higher discharge capacities were
observed for the pulse protocol relative to CC-CV which was
attributed to better active material utilisation. However, since the
Fig. 8. Schematic representation of common types of charging protocols proposed for fast charging. a) Constant Current - Constant Voltage (CC-CV), b) Constant Power - Constant
Voltage (CP-CV), c) Multistage Constant Current - Constant Voltage (MCC-CV), d) Pulse charging, e) Boostcharging with a CC-CV-CC-CV scheme, f) Variable Current Prole (VCP,
based on Ref. [191]).
A. Tomaszewska et al. / eTransportation 1 (2019) 10001114
capacities of the cells were not determined in a standard charac-
terisation test at the beginning of life and the rate of capacity fade
did not vary substantially, the discrepancy in capacities could be
caused by manufacturing differences. The same study [197 ]
compared SEM images of both electrodes after 300 cycles, nding
cracks in the cathode particles of all cells but signicantly less SEI
formation on the anodes of the pulse charged cells. Abdel-Monem
et al. [198] monitored capacity fade and impedance changes in LFP
cells cycled with eight different fast charging protocols, all
belonging to CC-CV, MCC or pulse charging categories. They
observed comparable rates of capacity fade until the 700th cycle,
when the capacity of the cell charged by CC-CV started deterio-
rating considerably faster. The rate of capacity fade between pulse
charging and MCC remained similar, however a higher rate of
impedance rise was observed for the MCC protocol than for pulse
charging. The discrepancy in the rates of capacity fade and
impedance rise was interpreted as a manifestation of different
aging mechanisms occurring for each protocol. Chen et al. [19 9]
proposed a sinusoidal-ripple-current (SRC) charging strategy using
the minimum impedance frequency signal to minimise heat gen-
eration. Experiments revealed that the SRC strategy could offer
signcant improvements in charging time, efciency, temperature
rise and lifetime compared to 1C CC-CV, and slight improvements
compared to a square pulse charge with the same parameters. The
method was only validated with one type of cell at a relatively high
starting temperature of 28.5
+
C, and it is unclear if the ambient
temperature was controlled in any way during the tests. Amanor-
Boadu et al. [200] used the Taguchi orthogonal arrays method to
optimise the pulse charging parameters to maximise charging ef-
ciency, again nding the minimum AC impedance frequency to be
optimal for charging. Signicantly shorter charging times and
higher efciencies were reported for the pulse charging protocol
compared to CC-CV with the same average current in the CC phase
at 23
+
C, however the temperature rise was higher and the effects
on cycle life were not studied. Yin et al. [201] designed a module
charger that performed pulse parameter optimisation on-line to
limit battery polarisation. The authors implemented the charger on
four cells connected in series, showing that the charge time could
be reduced compared to 2C CC-CV, although the resulting tem-
perature rise was slightly higher. No cycle life experiments were
performed, while the complexity of implementation was
acknowledged by the authors. As mentioned in Section 3.1, pulse
loads may lead to increased and uneven heat generation in packs
connected in parallel due to the rebalancing events between pulses
[62]. The implications of these effects have not been extensively
studied in the context of charging protocol optimisation.
Boostcharging. Boostcharging is characterised by high average
current in the beginning of charge, followed by a CC-CV part with
more moderate currents. The rst, boostcharge stage could simply
comprise a CC prole (making the protocol identical to MCC-CV), a
CV prole where the cell is immediately brought to a set maximum
voltage by means of high initial current (CV-CC-CV), or an entire
CC-CV prole (CC-CV-CC-CV) [202]. The boostcharge stage should,
in any case, allow higher currents or higher maximum voltage
compared to the following CC-CV part in order to reduce the overall
charging time. Notten et al. [202] tested a few variations of the
boostcharge protocol on both cylindrical and prismatic LCO cells
with a 5 min boostcharge period. Compared to a 1C CC-CV protocol,
the charging time was reduced by about 30e40% with no notice-
able acceleration in capacity fade for cylindrical cells. For prismatic
cells, a smaller reduction in charging time was reported along with
slightly higher rates of capacity fade. Interestingly, some of the
boostcharging protocols seemed to achieve marginally better ca-
pacity utilisation at the beginning of life compared to the slower 1C
CC-CV. It is, however, unclear whether this could be due to
manufacturing differences as cell capacities were not measured by
a standardised test at the beginning of life. Keil and Jossen [203]
arrived at contradicting conclusions after experimentally
comparing cycling data from three types of 18650 cells of different
chemistries charged by CC-CV, boostcharging and pulse charging
protocols. An increased rate of capacity fade was reported for
boostcharging compared to CC-CV with a similar charging time,
while no signicant differences in capacity fade rates were
observed for pulse charging. The authors concluded CC-CV was
suitable for fast charging high power cells, however mentioning
that MCC could be useful in conditions when lithium plating is
likely to occur.
Variable current proles. A number of more complex variable
current proles have also been proposed for fast charging. Sikha
et al. [204] investigated a Varying Current Decay VCD) protocol,
designed to charge faster than a conventional CC-CV protocol while
being less damaging than pure CV charging. The proposed protocol
achieved improved capacity utilisation in the early cycles, but still
resulted in a signicantly higher rate of capacity fade compared to
CC-CV with a similar average current. Possible overcharge of the
cathode was suggested as the reason for accelerated degradation.
Building on a similar idea, the Universal Voltage Protocol (UVP)
[191] was designed to reduce both charging time and energy losses
due to heating, thus improving charging efciency. The UVP is
derived from a set of CC-CV charging curves using an optimisation
algorithm for a specied charging time and target terminal voltage.
A variable current prole is then calculated from the UVP and cell
resistance. As the cell ages, the current prole needs to be recal-
culated due to changes in resistance while the voltage prole re-
mains the same as for a fresh cell. Irrespective of the cell age, the
current is always very low and rapidly increasing in the beginning
of charge due to the cell resistance being highest at 0% SOC and
then rapidly dropping. Maximum current is reached at relatively
low SOCs, and subsequently it is gradually reduced due to the mass
transport of Li becoming more constrained in increasingly lithiated
graphite particles. The study demonstrated that a very high
charging efciency could be maintained even for highly aged cells.
The capacity of the tested LCO/NMC cell dropped to 80% after 370
cycles with the UVP protocol compared to only 100 cycles achieved
by the 2C CC-CV protocol with a similar charging time. While the
voltage prole was shown to be universal with respect to cell age,
the effects of the charging conditions, most notably temperature,
on the cycling performance were not investigated. Ye et al. [205]
developed a multistage constant heating rate charging protocol
aiming to minimise charging time and temperature rise while
maintaining the same charge capacity as in CC-CV. The heating rate
was decreased in stages as the battery SOC increased, producing a
variable current prole. Reduced charging time and temperature
rise compared to CC-CV were reported at ambient temperatures
ranging from 10
+
Cto40
+
C, although the effects on cycle life were
not studied. Another charging strategy concept, Constant Temper-
ature - Constant Voltage (CT-CV) was proposed by Patnaik et al.
[206]. The temperature was set to a pre-dened maximum value
during the CT stage and then decreased during CV, producing a
current prole similar to boostcharging. The method was demon-
strated to achieve about 20% shorter charge time compared to CC-
CV with the same temperature rise, however the cycling perfor-
mance was again not investigated. It should be stressed that while
temperature plays a key role in determining degradation rates, the
assumption that higher temperature rise is always detrimental may
not apply in all cases. Temperature gradients within packs and cells
are crucial but often not considered in charging protocol studies
that use surface temperature as the main indicator of degradation
rate.
Schindler et al. [207] took the novel approach of combining
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 15
previously published physically-motivated fast charging strategies.
Four current-neutral current proles were superimposed, sepa-
rately and in combination, on a 1C CC-CV protocol. The proles are
shown in Fig. 9 and included: a) AC pulse, motivated by a decrease
in cell resistance due to additional irreversible heating; b) cold
derating, a prole in which a very low current is used initially due to
high resistance at 0% SOC and the current is brought to a higher
constant level as soon as the resistance drops; c) overpotential
reserve, a prole in which the overpotential is always maintained
below a certain safety limit to prevent negative anode potential,
producing a gradually decreasing current prole; and d) pulse
charging. Capacity fade was monitored in eighteen cells charged by
different protocols, and compared to four cells charged by standard
CC-CV. The best performing cell, charged by a combination of all
proles, retained 80% capacity after 800 cycles. In comparison, cells
charged by CC-CV alone experienced the same capacity fade after
only about 400 cycles, while the worst performing cell, charged by
CC-CV combined with cold derating, completed about 330 cycles. In
general, protocols that included the overpotential reserve prole
resulted in best capacity retention. All tests were conducted at a
temperature of 25
+
C and the results should not be generalised to
other conditions.
While many fast charging protocols have been proposed based
on an array of physical motivations, most of them have only been
validated at standard temperatures and for certain cell chemistries
or form factors. Since high currents induce higher mechanical
stresses in electrode particles along with more severe current and
temperature distributions, care must be taken when generalising
the results of experimental studies to different cell types. At the
moment, the applicability of many charging protocols to different
conditions is difcult to establish without further experiments,
which are often time consuming and expensive to conduct. There is
a clear need for accurate cell and pack models that would enable
the design of charging protocols without the need for extensive and
complicated laboratory testing. As the EV industry grows in colder
climates, more research on charging protocols suitable for low
temperature fast charging will also be necessary. More consider-
ation should also be given to the cell temperature proles induced
by different charging protocols, as it is the cell temperature rather
than ambient that determines performance. Finally, studies on the
effects of the fast charging protocols discussed in this section at the
pack level are still lacking.
5.2. Model-based protocol optimisation
5.2.1. Optimisation based on ECM type models
Some researchers have attempted to design optimal charging
protocols by using equivalent circuit model (ECM)-based models,
which are reformulated and embedded into single-objective or
multi-objective constrained optimisation problems.
In these problems, rst-order [191,208e212] or high-order
ECMs [213,214] are used to describe cell behaviour, and various
cost functions are set to achieve maximum charging efciency or
minimum charging loss while fast charging. The charging loss is
determined by the internal resistance, current rate, and charging
time.
The thermal effect or battery aging caused by charging can be
embedded into the basic ECM with an electro-thermal-aging
coupling model to diminish temperature rise or battery degrada-
tion during charging. Except for the commonly used lumped ther-
mal model in which the cell is taken as a whole, some enhanced
models that can distinguish the core and surface temperature or
have improved delity at higher rates have been proposed as well
[212,213]. The aging effect is modelled in terms of an extended
Arrhenius formula, which is related to the current rate, activation
energy, total discharged capacity, temperature, etc. [212].
Once the framework of the optimisation problem is established,
an appropriate optimisation algorithm should be used according to
the cost functions and constraints. The conventional algorithms
include: dynamic programming [211], Pontiac minimum principle
Fig. 9. Current trajectories of superimposed proles from Ref. [207]. a) AC pulse, b) Cold derating, c) Overpotential reserve, d) Pulse charging.
A. Tomaszewska et al. / eTransportation 1 (2019) 10001116
[210], genetic algorithm [191,213], Legendre-Gauss-Radau (LGR)
pseudo-spectral method [212], and minimum-maximum strategy
[209].
ECMs can capture the external characteristics of the battery but
fail to provide internal state information, especially for side re-
actions caused by charging such as SEI growth, lithium deposition,
etc. Therefore, electrochemical models have drawn increasing
attention.
5.2.2. Optimisation based on electrochemical models
The electrochemical-based models can be used to predict side
reactions during charging due to their abilities to estimate internal
states such as solid and electrolyte potential, ion concentration, and
reaction ux. The most commonly used electrochemical model is
the P2D model proposed by Doyle, Fuller, and Newman [215], into
which key degradation mechanisms such as lithium plating can be
incorporated as discussed in Section 3.2.3. However, it is compu-
tationally intensive to solve the PDEs in the full order model (FOM).
Therefore, efforts were made to reduce the order of the FOM to
achieve a faster computational speed with similar delity, allowing
for realistic application. Purushothaman et al. [216] derived an
analytical solution of the solid phase concentration through vari-
able separation method. Fast charge methods of variable current
pulse and nonlinear decay current were developed aiming at
limiting the solid surface concentration within the boundary.
Essentially, the model used in this case was a simplied form of the
single particle (SP) model with a number of simplifying assump-
tions: only diffusion was modelled without considering reaction
kinetics, a constant diffusion coefcient was assumed and all
lithium-ion reduction was assumed to take place at the anode/
separator interface. The model was not validated. The SP model can
also predict the temperature, ion concentration distribution or
overpotential by extension. The extended models are embedded
into optimisation problems to solve for fast charging strategies with
temperature or ion concentration restrictions [217,218]. In order to
improve the delity of the model at various temperatures, the SP-
thermal coupling model is established by adding parameter-
temperature correlation. By using the coupling model, the perfor-
mance of fast charging strategies at wider temperature ranges can
be enhanced [219,220]. It is important to mention that while some
of these models are validated against experimental results
[218,219], others are only validated against other models [220]or
not validated at all [216,217]. SP models generally lose accuracy at
high current densities [221] and thus the results of studies should
be interpretted with caution unless parameterised and validated
against independent experimental data sets.
Some modelling approaches have been developed that consider
side reactions. Rahimian et al. [222] used an SP-based model that
included SEI layer growth to optimise the battery cycle life through
adjusting the charging C-rate as the cell aged. The simulation re-
sults suggested that cycle life could be improved if charging C-rate
was increased after a certain number of cycles. The model was not
validated. SEI growth models were also used to develop MCC [223]
and variable current protocol [224], but neither of these studies
included full experimental validation. Lithium deposition preven-
tion has drawn intensive attention in recent years [193,225]. The
models that aim to limit lithium plating normally do so by main-
taining positive anode potential, which can be estimated by a
reduced order model (ROM). Klein et al. built a time-optimal
charging problem based on a reduced-order electrochemical-
thermal coupling model to predict anode potential and tempera-
ture during fast charging [226,227]. The model was validated
against the FOM and experimental data for a fresh cell. Han et al.
proposed a novel approach to simplify the FOM to form a simplied
P2D model (SP2D) while maintaining the FOMs delity at high rates
[228,229]. Experimental validation was not conducted. The SP2D
model was then used to develop a non-destructive fast charging
strategy with a variable current prole by maintaining the anode
potential above a safety threshold [230]. Two cells were inspected
post-mortem after ve cycles using the proposed protocol and a
CC-CV protocol with a comparable charging time (1.15C), revealing
visible lithium deposition on the anode of the cell charged with CC-
CV but no visible deposition on the cell charged with the proposed
protocol.
Recently, some physics-based ECM models have also been
developed that capture key electrochemical processes and internal
states of the battery while being easier to parameterise than P2D
models [231] or including additional effects such as double layer
capacitance [232]. Such hybrid models vary in terms of computa-
tional demand and accuracy of prediction. While the more complex
physics-informed ECMs could be useful for assessing effects in the
frequency domain that standard P2D models do not capture, the
simpler ones could have the advantage of being less computa-
tionally expensive or easier to parameterise than P2D with an
improved delity and physical insight compared to standard ECMs.
To the best of the authorsknowledge, these models have not yet
been applied to charging protocol optimisation.
Overall, model-based charging optimisation studies often use
ECM, SP or ROM rather than FOM approaches due to their lower
computational demand, which makes them potentially suitable for
real-time implementation. However, this advantage often comes at
the expense of accuracy, prompting the need for careful validation
particularly when abuse conditions such as fast charging are being
modelled. While many model-based approaches to fast charging
optimisation have been proposed, relatively few of them have been
fully validated against experimental results. Furthermore, valida-
tion is often conducted only against fresh cell data, which may not
be sufcient for modelling studies where the main purpose is to
limit long-term degradation.
6. Implications on thermal management
Section 3.1 outlined the effects of fast charging on the temper-
ature uniformity within cells and packs along with potential local
degradation effects that may evolve. Fast charging is normally
accompanied by high heat generation rates and signicant in-
homogeneities. At the same time, high charging currents applied at
low temperatures may be detrimental to battery lifetime and
safety. As such, effective and exible thermal management strate-
gies are critical to enabling fast charging in all conditions. The re-
quirements on the Battery Thermal Management Systems (BTMSs)
can vary greatly depending on temperature. While high thermal
conductivity is crucial when cooling a hot battery pack, low tem-
perature operation prompts the desire for good insulation to retain
the heat produced by the battery. Passive thermal regulators that
adjust their thermal conductance depending on temperature could
be one way to address this problem [233].
6.1. Cooling
The cooling media commonly used for EV battery packs can be
divided into air, liquid and phase change materials (PCMs). Air
cooling systems are low cost and relatively simple, but fail to ach-
ieve sufcient cooling rates or good temperature uniformity due to
the low heat capacity and thermal conductivity of air [234e236]
and are therefore unsuitable for fast charging. Liquid cooling can be
3500 times more efcient than air [235] but its drawbacks include
high cost, complexity and potential of leakage. Liquid immersion
cooling, whereby batteries are immersed directly in the cooling
medium, has been receiving attention from researchers due to its
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 17
effectiveness and ability to achieve good temperature homogeni-
zation [237]. To prevent short circuits, it is vital that the cooling
liquid is dielectric. Examples of such liquids include deionised
water and mineral oils. While liquid immersion cooling has not yet
been widely adopted in EV battery packs [235], some researchers
have shown promising results. For example, the two-phase thermal
management liquid Novec700 0 (3 M, USA) has been shown to
achieve very good surface temperature uniformity in an
immersion-cooled cylindrical Li-ion battery, particularly when the
battery temperature was high enough to induce boiling [237]. The
same liquid has also been reported to effectively maintain the
temperature of a battery module around 35
+
C even at a 20C
discharge rate [238]. In PCM cooling, the latent heat of phase
change of the cooling medium is used to absorb heat produced by
the battery. However, the method has signicant drawbacks: at
high ambient temperatures, the PCM could melt completely
without any heat being produced by the battery, and the low
thermal conductivity of the liquid PCM would then act as a barrier
to heat transfer [235]. On the other hand, Hmery et al. [239]
coupled PCM with a liquid cooling system to enable solidication of
the melted PCM. The coupled system was shown to effectively cool
a battery module during a 2C fast charge.
Since fast charging inevitably leads to higher temperature gra-
dients, both within a pack and within a cell, effective and uniform
cooling becomes even more critical than under standard charging
conditions. Heat generated during charging is more difcult to
dissipate from the centre of a cell compared to the regions closer to
the outer surfaces due to poor thermal conductivity across the
electrodes [235]. At the same time, it is the outer surface of cells
that is typically in contact with the cooling medium, further
amplifying the temperature gradients. Similar observations apply
to battery modules and packs. The dense packing of cells in EV
battery packs to minimise the volume aggrevates the issue [240],
making the cooling system design critical to pack safety and
longevity. Lu et al. modelled the performance of a forced aircooling
system with either 15 or 59 air ow channels, showing the latter
option to achieve more uniform temperature distribution [240].
The diameters of the channels were adjusted so that both systems
required the same volume within the pack and the same heat ux
was imposed. Xia et al. [235] reviewed a number of cooling system
congurations, concluding that parallel and mixed series-parallel
congurations were more effective in minimising temperature
gradients between cells compared to parallel congurations in uid
(air or liquid) cooled packs. However, cooling efciency and uni-
formity are sometimes in contradiction. Hunt et al. [241] reported
substantially lower rates of capacity fade for tab cooled pouch cells
compared to surface cooled ones, which was attributed to lower
temperature gradients between cell layers. Zhao et al. [242]
modelled the effects of surface and tab cooling on the temperature
distribution in a prismatic Li-ion cell. The study demonstrated that
tab cooling resulted in a substantially more uniform temperature
distribution across the cell thickness compared to surface cooling.
However, surface cooling was able to reduce the average cell tem-
perature more effectively than tab cooling. The model assumed a
set coolant temperature for both methods, which may not be
representative of the conditions in operation.
Finally, recent developments in increasing EV charging powers
have been accompanied by some interest in external cooling
technologies that could be provided by the charging stations. Such
an approach, if successful, could help reduce the cost and weight of
onboard cooling systems. Ford Global Technologies LLC have
applied to patent a charging station that communicates with the
connected EV and supplies cooled air to the vehicle radiator during
fast charging [243]. Lightning Energy patented a charging station
with coolant pipes integrated into the charging connector [244].
The connector locks in place to prevent coolant spillage if removed,
and coolant is cycled through by the controller in the charger in
response to battery information from the car. Tesla patented a
similar idea, with an automated charging connector placed beneath
the vehicle and being capable of supplying both cold and hot liquid
to optimise the battery temperature during charging [245]. None of
these technologies have been commercialised at the time of writing
and their ability to provide sufciently uniform cooling for fast
charging applications has not been proven. Leakage of the thermal
uid is a potential issue.
6.2. Preheating in cold conditions
As explained in earlier sections, low temperature fast charging
of Li-ion cells proves particularly difcult. While relatively few
studies have attempted to address the issue through designing
appropriate charging protocols, signicant research effort has been
dedicated to exploring different preheating strategies. In this sec-
tion, only the methods that can result in rapid heating of the entire
cell are discussed. This is because high speed is an imperative
requirement for any preheating strategy that could be integrated
with fast charging.
Internal preheating methods are favourable due to higher ef-
ciency (as less heat is lost to the surroundings) and better unifor-
mity [246,247]. Ji and Wang [248] used a coupled electrochemical-
thermal model to compare the performance of four heating
methods: self-heating by discharging the battery, convective
heating using a fan and a resistance heater powered by the battery,
mutual pulse heating, and AC heating. Mutual pulse heating is a
strategy whereby a battery pack is divided into two groups of equal
capacity and charge is exchanged between the two groups in pul-
ses, taking advantage of the resistance to generate heat. Self-
heating by discharging was found inefcient, as can be expected
considering the cell generates both heat and power during
discharge. Convective heating resulted in relatively fast but inef-
cient and non-uniform heating. The efciency of the mutual pulse
heating strategy was, on the other hand, shown to be much higher
and mainly limited by the efciency of the DC/DC converter used.
Only 120 s were needed to raise the temperature of the simulated
2.2 Ah 18650 cell from -20
+
Cto20
+
C using this method. AC heating
could reach even higher heating speeds, achieving the same tem-
perature rise in 80 s with a 10 mV sinusoidal voltage wave at a
frequency of 1000 Hz. However, the impacts on degradation and
cyclability were not studied.
For AC heating, the time to reach the desired cell temperature
can theoretically be minimised by using high current amplitudes,
yet exceeding a safe current limit can lead to non-uniformity (as the
resistance may vary within the cell), exceeding maximum cell
voltage, and to the anode potential becoming negative [246]. Ge
et al. [247] used an ECM-type model to estimate the maximum
alternating current that could be applied depending on the fre-
quency and temperature while maintaining a positive anode po-
tential and therefore preventing lithium deposition. The study
found that the maximum permissible current amplitude increased
with increasing temperature and signal frequency. Based on these
observations, a method for designing a stepwise AC heating strat-
egy with the current amplitudes increasing with the average cell
temperature was proposed. The frequency of the signal was kept
constant at 100 Hz as it was demonstrated that gradually increasing
the current had a stronger effect on the heating rate than lowering
the frequency. The 1 Ah pouch cell used in the study could be
heated from -20
+
Cto5
+
C in 800 s according to both the model and
experimental data. However, the effects on the cyclability were not
reported. The method for designing the AC prole required placing
a reference electrode within the cell in order to obtain EIS spectra of
A. Tomaszewska et al. / eTransportation 1 (2019) 10001118
both electrodes needed to t the model. The authors argued that
preparing and testing one cell per cell type would not be a
considerable cost to manufacturers. However, even assuming all
cells of a certain type can be reproduced with the same equivalent
circuit with the same parameters, it is crucial to note that these
parameters would change as the cells age, and may change differ-
ently for different cells. Another study [246] proposed a similar AC
heating prole with gradually increasing current amplitudes, again
conrming that adjusting the amplitude was more benecial than
adjusting the frequency as the cell temperature increased. Based on
an electro-thermal coupled model, the optimal frequency resulting
in highest heat generation for the study case was calculated at
1377Hz, slightly lower than the minimum impedance frequency.
The frequency optimisation process used a simple model requiring
only overall cell data without the need for a reference electrode.
However, it was assumed that only cell voltage limits needed to be
adhered to; the effect of the AC signal on the anode potential was
not considered. The commercial 18650 cell of 2.75 Ah capacity was
heated up from -15.4
+
C to 5.6
+
C in 338 s. No signs of aging were
observed in incremental capacity (IC) curves after 300 heating tests.
Capacity retention in preheated and subsequently cycled cells was
not explored. Zhu et al. [249] experimented with different fre-
quencies and amplitudes for a preheating AC prole on a 30 Ah LFP
pouch cell, although the maximum amplitudes were low relative to
the cell capacity (60 A, or 2C). With amplitudes limited to this value,
a low frequency signal was more successful in heating the cell
rapidly compared to higher frequencies. The most effective AC
prole tested was able to heat the cell from -25
+
Cto5
+
Cin1800s
using a frequency of 0.5 Hz and an amplitude of 60 A. SEM images of
the anode after 240 heating tests did not feature signs of lithium
deposition. While these were compared to images of an anode
subjected to 36 DC charge/discharge cycles at 0.5C and -25
+
C which
showed a clearly altered morphology, no attempt was made to
study a cell subjected to the AC preheating procedure with subse-
quent cycling. A thermocouple inserted in one of the cells enabled
the authors to quantify the variation between the internal and
surface temperature of the AC heated cell at about 5
+
C, providing a
useful insight into the homogeneity of the AC heating method.
Designing Li-ion cells specically to enable rapid preheating is
another approach to address low temperature fast charging. Elec-
trically insulated thin nickel foils inserted between two single-
sided anode layers of a cell provide a means to raise the tempera-
ture when necessary using standard direct current [16,250,251].
Current through the foils can be controlled using an activation
switch. Turning the switch on directs the current to the nickel foils,
causing rapid heat generation. When the switch is in the offpo-
sition, the current ows through the electrodes only, and the nickel
foils are electrically isolated. Yang et al. [251] modelled this cell
design, using 10 Ah and 40 Ah prismatic cells as examples. The
study demonstrated that heating uniformity and speed improved
substantially with the number of nickel foils inserted, even when
the total thickness of the foils was unchanged. The total thickness of
nickel foils was 200
m
m which reduced the volumetric energy
density by only about 0.58%. The same group [16 ] followed up with
an experimental study on a 9.5 Ah NMC622 pouch cell with two
nickel foils. The results showed that the cell could be preheated and
charged to 80% SOC in 15 min even from -50
+
C. Cycling tests at 0
+
C
ambient revealed a cycle life of 4500 cycles before reaching 20%
capacity fade for a cell preheated and subsequently fast charged at
3.5C, compared to only 50 cycles for a cell that was not preheated.
Although it is generally accepted that internal heating methods
offer both better efciency and more homogeneous temperature
distribution, still little research has been done to evaluate the
effects of internal preheating in conjunction with fast charging on
cycle life. Since current ows preferentially through paths of lower
resistance and therefore higher temperature, it is possible that even
small temperature gradients produced by internal preheating
methods would be exacerbated if immediately followed by a fast
charge. Since internal temperature is difcult to measure experi-
mentally, cycle life testing or reliable models will be needed to fully
assess the performance of AC preheating. Nickel foil preheating,
while potentially promising, would require a non-standard cell
design with additional weight, instrumentation, and possible reli-
ability issues. As the technology was proposed only recently, more
research on its performance and cost will be needed before po-
tential commercialisation.
7. Safety
7.1. Effects of fast charging on thermal runaway characteristics
Lithium plating [30,96,252] and the temperature rise due to the
heat accumulated in the cells or bus-bars at the end of fast charging
[253,254] pose potential safety risks in addition to causing accel-
erated degradation.
Research has shown that the thermal runaway behaviour of
batteries changes after experiencing fast charging [255]. By con-
ducting ARC tests on a fast-charged high energy pouch battery, it
was found that the self-heating temperature and the thermal
runaway triggering temperature drastically reduced for cells sub-
jected to fast charging compared to fresh cells. These effects do,
however, seem to be reversible if sufcient rest time is allowed.
Waldmann et al. [256,257] conducted ARC tests on batteries rested
for different periods of time after fast charging. While a signicant
change in thermal runaway characteristics was observed for cells
after only a short rest time, the thermal runaway behaviour of cells
subjected to long rest times was similar to that of a fresh cell. This
phenomenon can be explained by the re-intercalation of lithium
into the anode during the rest time, as well the reaction of the
plated lithium with electrolyte to form new SEI [96,115 ]. With the
rest time extending, the active plated-lithium content participating
in the thermal runaway process is reduced, and the thermal
runaway characteristics of the battery recover to their normal state.
Thermal runaway is known as being composed of a series of
chain reactions [258]. In a fresh battery it is generally triggered by
an internal short circuit, and the maximum temperature is quickly
reached with the participation of the electrolyte [259e266]. To
clearly describe the chain reactions of fast-charged batteries, the
evolution process of thermal runaway can be divided into three
stages, as shown in Fig. 10. For the fast charged battery which ex-
hibits abnormal thermal runaway behaviour, the reaction between
lithium and electrolyte is dominant in the thermal runaway pro-
cess, as opposed to that of fresh batteries. In the rst stage
(60
+
C<T<110
+
C), the plated lithium reacts with the electrolyte
and heats the battery. The SEI lm on the plated lithium surface is
continuously decomposed and regenerated, while the temperature
rise rate remains relatively low. In the second stage (thermal
runaway triggering process), the plated lithium is consumed in a
large amount in the violent reaction with the electrolyte, causing a
sharp increase in temperature. The separator collapses, and the
cathode and anode connect with each other. In the third stage
(thermal runaway developing to the highest temperature), other
reactions are involved. The reaction of the anode and electrolyte,
the reaction of the anode and the cathode, and the reaction of the
cathode and electrolyte are triggered due to the sudden rise of
temperature. As a result, the battery reaches the maximum
A. Tomaszewska et al. / eTransportation 1 (2019) 100011 19
temperature of thermal runaway.
7.2. Overcharge-induced thermal runaway
Some cells in a fast-charged battery pack may become over-
charged due to the inconsistency among the cells [34,267,268],
potentially leading to thermal runaway [269,270]. This process
consists of four stages [259].
Stage 1 (100% <SOC <120%): The battery voltage exceeds the
cut-off voltage and increases slowly. The excess anode material,
normally incorporated in Li-ion cells for safety, can still withstand
excessive embedded lithium in the beginning of overcharging
[271e273]. Some side reactions of the battery material may be
triggered and the temperature and internal resistance of the bat-
tery slightly increase.
Stage 2 (120% <SOC <140%): A dissolution reaction of transition
metal ions such as Mn
2þ
may be triggered in positive electrodes of
certain chemistries, due to the excessive deintercalation of lithium
[274,275]. At the same time, electrolyte oxidation may also begin
since the battery potential exceeds the stable operating window.
The negative electrode of the battery gradually becomes unable to
withstand further transfer of lithium atoms, leading to signicant
amounts of Li depositing on its surface [275,276]. The deposited
lithium reacts with the electrolyte to form new SEI lm, increasing
the internal resistance of the cell [276,277]. The joule heating effect
of the overcharge current causes a signicant increase in the bat-
tery temperature.
Stage 3 (140% <SOC <160%): The heat produced by the
exothermic reactions of the battery material begins to be compa-
rable to and subsequently dominant over the joule heat due to the
further removal of lithium from the positive electrode, the large
deposition of lithium in the negative electrode, and the increase of
the electrode potential. Oxidative decomposition of the electrolyte
produces a greater amount of heat [278,279], accompanied by the
generation of gas, causing the battery to expand. The lithium-
electrolyte reaction also becomes more pronounced as the
amount of lithium deposition increases. When the SOC is close to
160%, a large amount of Mn
2þ
is dissolved if present in the cathode
material; the SEI lm gradually collapses [280,281]; the structure of
the cathode material changes, causing the battery voltage to reach a
maximum value and then begin to decrease [282e284].
Stage 4 (140% <SOC <160%): The oxidative decomposition of
the electrolyte generates a large amount of gas, resulting in the
battery rupture instantaneously. The battery separator is displaced
by the shock, and a large internal short circuit occurs in the battery.
The contact between the positive and negative electrodes of the
battery produces a severe redox reaction [260]. Thermal runaway
occurs eventually.
High-precision thermo-electrochemical coupling models have
been created to investigate overcharged batteries, based on the
amount of internal materials and reaction kinetic parameters of the
battery, and proposed two design methods to help protect the
battery from overcharging [259]:
1) Raising the electrolyte oxidation reaction potential from 4.4 V
to 4.7 V. This results in a more stable electrolyte and the SOC of
thermal runaway increased to 183%. It can be achieved by adding
functional additives or redox shuttle additives [285,286], i.e.
chemical species that can be reversibly reduced and oxidised at
potentials slightly higher than the maximum safe cathode poten-
tial, thus preventing it from increasing further [287]. Due to the
range of requirements for such compounds, including stable elec-
trochemistry over many cycles, appropriate redox potential and
non-reactivity in both oxidised and reduced forms with any of the
components of a Li-ion cell, still more research is needed to nd
suitable redox shuttle species [287].
2) Increasing the temperature at which the battery thermal
runaway occurs to 300
+
C can delay the occurrence of large-scale
internal short circuit inside the battery, and increase the SOC of
Fig. 10. Chain reactions of the thermal runaway process in fast charged batteries [255].
A. Tomaszewska et al. / eTransportation 1 (2019) 10001120
thermal runaway to 180%. This measure could potentially be real-
ised by optimising the battery pressure relief design or using a
diaphragm with a higher heat exchange stability [80,260] to post-
pone the rupture of the battery [259].
8. Conclusion
Electrication of transport is undisputably one of the key stra-
tegies to address climate change. The need to reduce range anxiety
and meet customer expectations has driven many manufacturers to
target fast charging capability as a critical design feature for EV
battery packs. While signicant research efforts have been dedi-
cated to various aspects of fast charging in the recent years, many
knowledge gaps still exist.
To date, no reliable onboard methods exist to detect the occur-
rence of crucial degradation phenomena such as lithium plating
or mechanical cracking.Techniques for detecting lithium plating
based on the characteristic voltage platueaus are promising for
online application, but fully reliable methods to distinguish
lithium stripping from other plateau-inducing phenomena, or to
detect plating where no plateau is observed, have not yet been
reported.
Many alternative electrode materials have been proposed that
could potentially improve fast charging capabilities of Li-ion
cells, however much remains to be studied regarding their sta-
bility, possible degradation mechanisms, ease of manufacture,
and cost. Despite the fact that graphite anodes are particularly
susceptible to lithium plating, they will likely continue to
dominate the Li-ion battery market in the foreseeable future due
to their low cost, wide availability, and technology maturity.
Existing modelling approaches have signicant limitations:
ECM-based models, with the exclusion of some physics-
informed ones, do not capture information about the internal
states of the battery and are only reliable in a limited range of
conditions which cannot normally be extended to abuse con-
ditions. On the other hand, the high delity of full order elec-
trochemical models renders them inappropriate for real-time
implementation. There is therefore a clear need for accurate
reduced order models capable of reproducing key internal states
that could be implemented in future fast charging enabled
BMSs. Physics-based ECMs could also offer advantages in terms
of computational demand or ability to capture phenomena not
reected by standard P2D models.
Many studies on fast charging protocols have been of empirical
or experimental nature, and therefore their performance has
only been assessed for a limited range of cell chemistries, form
factors, and operating conditions. Such results cannot be easily
extended to other cell types or ambient temperatures, as sup-
ported by the often conicting ndings reported by different
authors. On the other hand, many of the model-based charging
optimisation studies are based on SP or ECM type models that
may not be accurate at high currents, while their results are
often validated only against other models or not at all. Accurate
and validated cell and pack models and improved understand-
ing of limiting phenomena are needed to enable the design of
charging protocols without the need for extensive laboratory
testing.
Few charging optimisation studies to date have addressed the
special case of low temperature fast charging, which will be
increasingly important as deployment of EVs accelerates in
colder climates.
To further optimise the charging process for individual cells in a
pack and avoid local degradation or overcharge, advanced BMSs
with cell balancing capabilities will be required.
While much attention has been afforded to the design of ther-
mal management systems already, further research is still
needed to assess the efciency and homogeneity achieved by
various preheating and cooling technologies. For instance, few
studies have attempted to assess the implications of AC pre-
heating with subsequent fast charging on cell lifetime, or to
quantify the temperature gradients induced by this method.
Optimisation of tab design and placement and of the geomet-
rical design of cooling systems may be another signicant route
to improving temperature and current homogeneity. External
cooling technologies integrated with EV chargers could be
helpful in reducing the weight and cost of on-board cooling
systems, but whether such solutions could achieve the required
cooling rates or temperature uniformity remains to be studied.
Finally, the links between cell level and pack level degradation
rates are still not well understood. Multiple charging and pre-
heating strategies have been demonstrated for single cells, but
the effects, feasibility, and cost of their implementation in bat-
tery packs have not been studied. It is conceivable that some
charging protocols that result in improved performance for
single cells may lead to current or temperature heterogeneities
when performed on a pack. Research on this topic will be
required before any non-conventional protocols are applied in
real-world systems. Furthermore, few modelling works have
addressed the impacts of cell-to-cell variation on pack level
performance. Since fast charging can be expected to amplify
heterogeneities, such multiscale studies are urgently needed.
Advancements in multiscale modelling will be critical to inte-
grating cell and pack level design and control, playing an
important role in linking research on different scales to im-
provements in the performance of commercial systems.
Acknowledgements
The authors gratefully acknowledge the helpful discussions held
with the Advanced Energy Storage team at Shell. This work was
kindly supported by: Shell, the EPSRC Faraday Institution Multi-
Scale Modelling Project (EP/S003053/1, grant number FIRG003),
energy storage for low carbon grids project (EP/K002252/1), the
EPSRC Joint UK-India Clean Energy centre (JUICE) (EP/P003605/1),
Integrated Development of Low-Carbon Energy Systems (IDLES)
(EP/R045518/1), the Innovate UK Advanced Battery Lifetime
Extension (ABLE), the Innovate UK Battery Advanced for Future
Transport Applications (BAFTA) project and the National Natural
Science Foundation of China (grant number U1564205, 51706117).
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