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Electrochemo‐Mechanical Stresses and Their Measurements in Sulfide‐Based All‐Solid‐State Batteries: A Review

Advanced Energy Materials

November 2022
13(2):2203153

DOI:10.1002/aenm.202203153

Authors:

Jiabao Gu

Xiamen University

Ziteng Liang

Xiamen University

Sulfide‐based all‐solid‐state batteries (ASSBs) are one of the most promising energy storage devices due to their high energy density and good safety. However, due to the volume (stress) changes of the solid active materials during the charging and discharging process, the generation and evolution of electrochemomechanical stresses are becoming serious and unavoidable problems during the operation of all‐solid‐state batteries due to the lack of a liquid electrolyte to partially buffer the stress generated in the electrodes. To understand these electrochemo‐mechanical effects, including the origins and evolution of mechanical or internal stresses, it is necessary to develop some highly sensitive probing techniques to measure them precisely and bridge the relationship between the electrochemical reaction process and internal stress evolution. Herein, recent progress on uncovering the origins of the internal stresses, the working principle and experimental devices for stress measurement, and the application of those stress‐measuring techniques in the study of electrochemical reactions in sulfide‐based ASSBs are briefly summarized and overviewed. The investigation of precise and operando monitoring techniques and strategies for suppressing or relaxing these electrochemomechanical stresses will be an important direction in future solid‐state batteries.

Comprehensive schematic diagram of electrochemo‐mechanical stress origins, stress measuring devices, and applications in sulfide‐based ASSBs.

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a) Schematic diagram of the assembly and pressure measurement of an anode/LPSC/cathode cell. Reproduced with permission.[14b] Copyright 2021, Elsevier. b) Schematic of in situ pressure‐monitoring in an ASSB. Reproduced with permission.[14a] Copyright 2022, Elsevier. c) Image of a fully assembled lithium‐ion battery coin cell with FBG sensor integrated. Reproduced according to the terms of the CC‐BY license.[¹⁰⁹] Copyright 2017, The Authors, published by MDPI AG. d) Photographs of the optical fiber embedded within the InLix anode of an InLi|Li3PS4|LTO ASSB. Reproduced with permission.[¹⁶] Copyright 2022, The Authors, published by Springer Nature. e) The working principle of an FBG optical sensor. Reproduced with permission.[¹⁶] Copyright 2022, The Authors, published by Springer Nature. f) The scheme of the birefringence phenomena in an FBG sensor when a relatively high external pressure is applied. Reproduced according to the terms of the CC‐BY license.[¹⁶] Copyright 2022, The Authors, published by Springer Nature.

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a–c) Change of the net stress σ11 for ASSBs with different electrode materials. Reproduced with permission.[14k] Copyright 2018, The Royal Society of Chemistry. d) Operando electrochemical pressiometry profiles for FCG75 and NCA80 during first charge‐discharge at 0.1 C and 30 °C. Reproduced with permission.[⁴⁷] Copyright 2019, Wiley‐VCH. e) Cycling behavior and pressure change of a Li0.3In0.7/Li7P3S11/S:Li7P3S11:Ketjen black (50:30:20 wt%) ASSB cycled at room temperature. Reproduced with permission.[¹³] Copyright 2016, American Chemical Society. f) Discharge/charge profiles combined with the in‐situ cell pressure evolution of an all‐solid‐state Li–Se battery. Reproduced with permission.[14h] Copyright 2022, Wiley‐VCH. g) Exemplary potential profile combined with the in situ measured pressure change of an In/InLi|LPSCB|LPSCB‐MWCNTs cell during cycling (top) and a magnification of the data for one cycle (bottom). Reproduced according to the terms of the CC‐BY license.[¹⁴ⁱ] Copyright 2021, The Authors, published by Wiley‐VCH.

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a) Stack pressure evolution after pressing to an initial stack pressure of 30 MPa and holding at open circuit for two different symmetric cells based on LSPS (red) and LPSC (blue). Reproduced with permission.[14m] Copyright 2021, American Chemical Society. b) Voltage curves (red) and stack pressure profiles (blue) of two LSPS symmetric cells (operated at 0.5 mA cm⁻² (solid lines), and held at OCV (dotted lines). Reproduced with permission.[14m] Copyright 2021, American Chemical Society. c,d) Charge‐discharge voltage profiles NCM/Li ASSB and corresponding pressure changes for c) uncoated and d) In/LixIn‐coated thin LMAs. Reproduced with permission.[⁹³] Copyright 2022, Elsevier. e) Capacity‐normalized pressure change difference (Δ(ΔP)Q) as a function of cycle number. Reproduced with permission.[⁹³] Copyright 2022, Elsevier. f) Time‐resolved voltage (top), external cycling pressure (middle), and an internal optical signal (bottom) for an LTO‐LiIn ASSB cycled at C/30 (5.83 mA g⁻¹) and 25 °C in an operando mode. Reproduced with permission.[¹⁶] Copyright 2022, The Authors, published by Springer Nature. g) Stress evolution obtained from the FBG sensor placed inside Li–In anode by demodulating the optical signal (blue curve). Reproduced according to the terms of the CC‐BY license.[¹⁶] Copyright 2022, The Authors, published by Springer Nature. h) 2D stack view of the operando collected spectra by the FBG sensor placed between LTO/Li–In interface, with the corresponding galvanostatic charge/discharge cycle. A typical birefringence phenomenon could be found. Reproduced according to the terms of the CC‐BY license.[¹⁶] Copyright 2022, The Authors, published by Springer Nature. i) Galvanostatic cycle (top), λx and λy evolution (middle), and operando stress evolution obtained internally by the FBG sensor and with the experimental calibration of the sensor (bottom). Reproduced according to the terms of the CC‐BY license.[¹⁶] Copyright 2022, The Authors, published by Springer Nature. j–l) Galvanostatic cycling of j) Sb, k) Sn, and l) Si composite anodes in ASSBs with LPSC solid electrolyte and NMC‐111 cathodes, along with the measured uniaxial stress (stack pressure) changes in each cell. Reproduced with permission.[14b] Copyright 2021, Elsevier.

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a) Cyclic voltammetry tests voltage steps (top), current response (middle), and corresponding pressure response (bottom) upon cycling of ASSBs using 7525‐glass/C65 composite (red) and β‐Li3PS4/C65 composite (blue) as working electrodes. Reproduced according to the terms of the CC‐BY license.[14g] Copyright 2021, The Authors, published by Wiley‐VCH. b) The SEM images of the corresponding composites, 7525‐glass/C65 (top) and β‐Li3PS4/C65 (bottom), after the CV tests. Reproduced with permission.[14g] Copyright 2021, The Authors, published by Wiley‐VCH. c) CV profiles, current response, and pressure response of Super C65 electrodes glassy SE (1.5Li2S–0.5P2S5–LiI) (left) and crystalline SE (Li6PS5Cl) (right). Reproduced according to the terms of the CC‐BY license.[⁹⁴] Copyright 2022, The Authors, published by IOP Publishing Ltd. d) First‐cycle charge voltage profiles and corresponding pressure change (and DEP profiles) for poly‐NCA electrodes employing LYC or LPSX. Reproduced with permission.[14f] Copyright 2021, Wiley‐VCH. e) First cycle charge‐discharge voltage profiles and corresponding pressure changes for NCM electrodes using pristine and vulcanized BR binder. Reproduced with permission.[14a] Copyright 2022, Elsevier.

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Review

Electrochemo-Mechanical Stresses and Their

Measurements in Sulﬁde-Based All-Solid-State Batteries:

A Review

Jiabao Gu, Ziteng Liang, Jingwen Shi, and Yong Yang*

DOI: 10.1002/aenm.202203153

to further push the energy density of

LIBs to the limit, lithium metal is urged

as a “holy grail” anode material owing

to its high theoretical speciﬁc capacity

(3860 mAh g−1) and the lowest redox

potential (−3.040 V vs standard hydrogen

electrode (SHE)).[3] However, the applica-

tion of lithium metal anodes in traditional

LIBs will inevitably lead to lithium den-

drite issues, posing serious safety risks.[4]

ASSBs oer intrinsic safety advantages by

replacing liquid electrolytes with nonﬂam-

mable, high mechanical moduli solid-state

electrolytes (SSEs). [5] In addition, bipolar

stacking can be achieved in ASSBs, thus

minimizing inactive components in the

battery module pack and achieving higher

energy density.[6] Among various SSE

candidates (such as oxides, sulﬁdes, poly-

mers, etc.), the sulﬁde solid-state electro-

lyte, with its high room temperature ionic

conductivity (10−3–10−2 S cm−1) and good

formability, oers the potential for prac-

tical applications of all-solid batteries. [7]

Despite their promise, electrochemomechanical problems

in sulﬁde-based ASSBs remain poorly understood. These elec-

trochemomechanical eects (mainly resulting from volume

changes of active materials during cycling) in LIBs with liquid

electrolytes have already been studied. [8] However, they will

become more complex and severe when it comes to solid-state

batteries.[8a,9] Unlike the liquid battery, the internal (mechan-

ical) stress generated in the particles or electrodes in ASSBs

cannot be transferred quickly and uniformly with rigid SSEs,

thus resulting in the concentrated accumulation and release

of the internal stress of the battery.[10] Therefore, the internal

stress generated in an all-solid-state battery will lead to severe

electrochemomechanical issues such as physical contact losses

between active materials and solid electrolytes, breakdown

of the active materials, short circuits of Li metal anode, etc.[11]

These electrochemo-mechanical problems will block ion/elec-

tron transport and increase battery impedance, drastically dete-

riorating the electrochemical performance and hindering the

application of ASSBs.[10b,f,12]

Measuring the corresponding stress evolution precisely

during battery cycling is crucial for understanding the electro-

chemo-mechanical problems. To the best of our knowledge,

the earliest internal stress measurement work of sulﬁde-based

ASSBs was in a Li–S battery system, reported by Janek’s group

Sulﬁde-based all-solid-state batteries (ASSBs) are one of the most promising

energy storage devices due to their high energy density and good safety.

However, due to the volume (stress) changes of the solid active materials

during the charging and discharging process, the generation and evolution

of electrochemomechanical stresses are becoming serious and unavoidable

problems during the operation of all-solid-state batteries due to the lack of a

liquid electrolyte to partially buer the stress generated in the electrodes. To

understand these electrochemo-mechanical eects, including the origins and

evolution of mechanical or internal stresses, it is necessary to develop some

highly sensitive probing techniques to measure them precisely and bridge the

relationship between the electrochemical reaction process and internal stress

evolution. Herein, recent progress on uncovering the origins of the internal

stresses, the working principle and experimental devices for stress measure-

ment, and the application of those stress-measuring techniques in the study

of electrochemical reactions in sulﬁde-based ASSBs are brieﬂy summarized

and overviewed. The investigation of precise and operando monitoring

techniques and strategies for suppressing or relaxing these electrochemome-

chanical stresses will be an important direction in future solid-state batteries.

J. Gu, Z. Liang, J. Shi, Y. Yang

State Key Laboratory for Physical Chemistry of Solid Surfaces

Collaborative Innovation Center of Chemistry for Energy Materials

and Department of Chemistry

College of Chemistry and Chemical Engineering

Xiamen University

Xiamen 361005, P. R. China

E-mail: yyang@xmu.edu.cn

Y. Yang

School of Energy Research

Xiamen University

Xiamen 361005, P. R. China

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/aenm.202203153.

1. Introduction

Lithium-ion batteries (LIBs) have been widely used in consumer

electronics, electric vehicles (EVs), and grid energy storage sys-

tems since commercialized by Sony in 1991.[1] Despite great

progress in lithium-ion battery research over the past decades,

thermal runaway risks under abusive conditions due to ﬂam-

mable organic electrolytes remain challenging.[2] Furthermore,

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in 2016.[13] In their work, an external pressure sensor was used

to monitor stress changes during cycling. Subsequently, internal

stress measurements with external pressure sensors have been

reported in various battery materials and systems, including

NCM materials, alloy anode materials, and solid-state electro-

lyte materials. [14] Recently, the ﬁber Bragg grating (FBG) sen-

sors, which are widely used in liquid LIBs, [15] have been imple-

mented into the solid-state battery system. [16] These in situ or

operando measurements provide a powerful tool for examining

the link between electromechanical eects and battery electro-

chemical performance. Recently, Song et al. performed a com-

prehensive overview of stress sources and the resulting elec-

trochemomechanical eects on composite electrodes and cell

stacks in ASSBs. [17] However, few review articles are available

in the literature to provide a comprehensive summary of the

working principles and experimental devices for stress measure-

ment (such as external pressure sensors and FBG sensors) and

the application of these stress-measuring techniques.

This review examines the recent research progress on

internal stress measurement in sulﬁde-based ASSBs. First,

volume changes of active materials and solid-state electrolytes

during battery cycling and corresponding stress changes in

electrode level are categorized. Then the basic principles and

measurement devices (including external pressure sensors

and built-in FBG sensors) for internal stress measurement in

sulﬁde-based ASSBs are introduced. We also summarize and

discuss the relationship between stress changes and electro-

chemical reaction processes in ASSBs. Finally, we present our

views on the future development of internal stress measure-

ment in sulﬁde-based ASSBs. The overall content of the review

is summarized in a comprehensive schematic diagram in

Figure 1.

2. The Origin of Internal Stress

The electrochemo-mechanical problems caused by internal

stress are frequently ignored in the research of sulﬁde-based

ASSBs. The internal stress is mainly induced by the coupling of

the volume changes of the active materials during the electro-

chemical process. The volume changes of SSEs caused by Li+

migration are negligible due to their intrinsic ceramic nature.[18]

Therefore, the variation of the active materials will be con-

strained by rigid SSEs, leading to internal stress development.

Figure 1. Comprehensive schematic diagram of electrochemo-mechanical stress origins, stress measuring devices, and applications in sulﬁde-based

ASSBs.

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When active materials with huge volume changes are applied

and the battery capacity is high, the internal stress can even

reach the MPa level.

The volume change of active materials can be described

by the partial molar volume of lithium

()

VLi in the corre-

sponding compound.[19] The partial molar volume is deﬁned as

the dierential of the considered homogeneous volume caused

by the addition of lithium (maintaining all other state variables

constant, i.e., pressure p, temperature T, and amount of phase

in the system ni).[14k]

() ()

≡∂

∂









≠

Li Li

,·

nTpnn

(1)

The absolute volume change ΔV of the active materials

during lithium insertion/extraction can be obtained by

integrating.

∫

()

∆=

()

VV n

(2)

The volume changes derived from crystallographic data for

each active material during electrochemical cycling are summa-

rized in Table 1.

2.1. Cathode Materials

2.1.1. Layered Oxide Materials

LiMO2 (M for transition metals) layered oxides possess a hexag-

onal α-NaFeO2 layered structure with the R-3m space group.[20]

It corresponds to the cubic close-packed (CCP) oxygen arrays

with metal ions occupying octahedral sites, forming consecu-

tive LiO2 and MO2 layers.[21] These layered oxides mainly include

LiCoO2 (LCO), LiNiO2(LNO), LiNixMnyCozO2 (x+ y+ z= 1)

(NMC), and LiNixCoyAlzO2 (x+ y+ z= 1) (NCA), etc. The struc-

tural changes and the corresponding volume changes of these

materials during cycling are discussed in the following section.

LiCoO2 (LCO) is widely used in consumer electronics, and

its theoretical speciﬁc capacity is 270 mAh g−1. LCO under-

goes complex structural changes during the electrochemical

process.[22] During the Li1-xCoO2 delithiation process, two

hexagonal phases (H1 and H2) and one monoclinic phase

(M1) can be observed. H1 appears in the initial charging stage

(x< 0.04) and is partially converted to H2 when x reaches

0.04, and H2 coexists with H1 until x increases to 0.25.

When x exceeds 0.25, H1 is completely converted to H2, and

when x> 0.47, Li+ is further removed from Li1-xCoO2, which

makes H2 convert to M1. When x exceeds 0.56, M1 trans-

forms into H2 again. When Li-ions are completely extracted

from Li1-xCoO2, CoO2 with a hexagonal monolayer structure

is formed.[23] Among them, the hexagonal ↔ hexagonal

transition or the hexagonal ↔ monoclinic transition causes

severe volume change. Considering the structural irrevers-

ibility (hexagonal to monoclinic) at the amount of lithium

changes > 0.5mol, its practical capacity is limited to about

145 mAh g−1. When charged to Li0.5CoO2, the volume of the

LiCoO2 cell increases by +2%.

LiNiO2 (LNO), an oxide material isomorphic to layered LCO,

is considered a promising cathode material for power LIBs with

high energy density due to its large theoretical speciﬁc capacity

(275 mAh g−1) and lower cost (avoiding Co).[24] During delithi-

ation/lithiation, LNO undergoes complex structural changes

(hexagonal H1-monoclinic M-hexagonal H2-hexagonal H3

phases) and corresponding huge volume changes (maximum

volume change ≈9%).[25] Speciﬁcally, during the delithiation

process, three two-phase regions: H1-M, M-H2, and H2-H3, are

located at 0.80 ≥ x (Li) ≥ 0.75, 0.40 ≥ x (Li) ≥ 0.36 and 0.26 ≥ x

(Li) ≥ 0.16, respectively. Interestingly, the LNO unit cell exhibits

a nonlinear volume change. Taking the charging process as an

example, the H1-M-H2 phase transition contributes only ≈3%

of the total ≈9% volume change. Most of the volume change

(≈6%) occurs when x≤ 0.26, where the H2–H3 phase transition

results in a ≈3.8% drop in unit cell volume.[25,26]

By introducing both dopants, Co (to enhance the layered

ordering) and Mn (to stabilize the local structure), into LNO,

Table 1. Volume changes of typical electrode materials during battery

cycling.

Electrode

materials

Reactions Volume change

(%)

Cathode side LiCoO2= 0.5Li + Li0.5CoO21.9[36]

LiNiO2= xLi + Li1−xNiO29[25]

Li[Ni1/3Co1/3Mn1/3]O2= xLi + Li1−x[Ni1/3Co1/3Mn1/3]O2−3.4[60]

Li[Ni0.6Co0.2Mn0.2]O2= xLi + Li1−x[Ni0.6Co0.2Mn0.2]O2−5.2[61]

Li[Ni0.8Co0.1Mn0.1]O2= xLi + Li1−x[Ni0.8Co0.1Mn0.1]O2−7.8[61]

Li[Ni0.8Co0.15Al0.05]O2= xLi + Li1−x[Ni0.8Co0.15Al0.05]O2−5.9[62]

LiFePO4= Li + FePO4−6.5[36]

LiMn2O4= Li + 2MnO2−7.3[36]

S+2Li = Li2S79[39a]

Se+2Li = Li2Se 98[39a]

Te+2Li = Li2Te 104.7[39a]

CuS+2Li = Cu+Li2S75[39e]

Anode side C6+ Li = LiC613.1[63]

Li4Ti5O12+ Li = Li7Ti5O12 ≈0.2[46]

Si + 4.4Li = Li4.4Si 310[64]

In + Li = InLi 105.6[14j]

Sn +4.4Li = Li4.4Sn 260[64]

Al + Li = LiAl 100[65]

Li foil + xLi = Li1+x4.85 ×

10−4 cm3 mAh−1a)

a)The theoretical volume change of Li foil is calculated as follows.[66]

∆=

VQM (3)

where Q is the lithium deposition capacity (mAh), MLi is the molar mass of

lithium (6.94g mol−1), F is the Faraday’s constant, and ρLi is the density of lithium

(0.534g cm−3). Therefore, the thickness change of Li foil will reach 48.5 µm at an

areal capacity of 10 mAh cm−2.

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the developed ternary NMC material has become one of the

most essential layered oxide materials in LIBs technology.[27]

Interestingly, there is an approximately linear relationship

between the unit cell volume change and the Co/(Ni + Co) mole

ratio in NCM materials.[28] The volume change increases as the

Co/(Ni + Co) molar ratio increases (charged to 4.4 V vs Li+/Li

NCN111≈-1%, NCM811≈-6%). During delithiation/lithiation,

nickel-rich NCMs (LiNixMnyCozO2: x+ y + z= 1, x≥ 0.8)

undergo similar structural changes to LNO materials.[29] Taking

NCM811 as an example, complex structural changes occur

during the delithiation process: hexagonal to monoclinic

(H1 → M), monoclinic to hexagonal (M → H2), and hexagon

to hexagon (H2 → H3, >4.15 V vs Li+/Li). Among them, the

H2 ↔ H3 phase transition leads to dramatic volume change,

resulting in signiﬁcant volume change/internal stress.[30]

Another ternary layered oxide, NCA, has shown promising

advantages in high-power applications such as electric vehicles

(EVs).[31] Al substitution is considered to improve the thermal

stability of the material due to the stability of Al3+ at the tetra-

hedral sites. During the delithiation process, the NCA cathode

undergoes a series of phase transitions similar to LNO mate-

rials: from the original H1 phase to the monoclinic (M) phase,

followed by the H2 and H3 phases in turn. In particular, for

the Li[Ni0.8Co0.16Al0.04]O2 (NCA80) material, the maximum

volume change during delithiation is about 5.63%, and a dra-

matic volume change is observed at about 4.2V (corresponding

to the H2–H3 phase transition).[32] Similar to NCM materials,

the volume change of NCA materials increases as Ni content

increases (7.10% for NCA88 and 8.37% for NCA95).[32]

2.1.2. Spinel Oxide Materials

Spinel LiMn2O4 has the advantages of low cost and excellent

safety. Its unique 3D lithium-ion transport channel results

in superior rate performance, making it a promising cathode

material for lithium-ion batteries.[33] Severe structural changes

also occur in LiMn2O4 during the Li-ion intercalation and dein-

tercalation process.[34] Three phases (cubic, orthorhombic, and

tetragonal) are observed in the phase transition of LiMn2O4 by

in situ TEM analysis.[35] During the discharging process, the

cubic phase is transformed into the orthorhombic phase in

the ﬁrst stage and ﬁnally into the tetragonal phase. The calcu-

lated volume change from pristine LiMn2O4 to fully delithiated

Mn2O4 is about −7.3%.[36]

2.1.3. Olivine Oxide Materials

Olivine LiFePO4, with a theoretical speciﬁc capacity of

170 mAh g−1, is widely used in hybrid and electric vehicles due

to its low cost, good cycling performance, and high safety.[37]

LiFePO4 undergoes a two-phase reaction mechanism (the ﬁrst-

order phase transition of LiFePO4↔ FePO4 occurs with the

progress of charge/discharge) and a two-phase solid-solution

reaction mechanism (only one phase occurs during the charge

and discharge process) across the ﬂat voltage plateau.[38] Due

to the unique solid solution reaction mechanism, the partial

molar volume of LiFePO4 is constant regardless of the change

in lithium content. The calculated volume change from pristine

LiFePO4 to fully delithiated FePO4 is about −6.5%.[36]

2.1.4. Conversion-Type Materials (Sulfur Based)

Conversion-type materials (sulfur-based materials, such

as sulfur, lithium sulﬁde, metal sulﬁdes, etc.) are another

common cathode material in sulﬁde-based ASSBs.[39] These

materials have the advantage of high theoretical speciﬁc

capacity, but their operating voltage is lower than that of lay-

ered oxide cathode materials. The “shuttle eect” in liquid

batteries can be eliminated due to the use of solid-state elec-

trolytes, thus the cycling performance of the battery can be

improved. [40] Conversion-type materials exhibit relatively

large volume changes. Taking S as an example, Li2S will gen-

erate during the discharging process. Considering that the

densities of S and Li2S are 2.03 and 1.66g cm−3, respectively,

the complete conversion of S to Li2S will result in about 80%

volume expansion.[41]

2.2. Anode Materials

2.2.1. Lithium Intercalation Materials

Graphite materials have the advantages of low cost and low

potential to lithium (0.01–0.2 V vs Li+/Li) and are one of the

most successful anode materials for lithium-ion batteries.[42]

The theoretical speciﬁc capacity of graphite is 372 mAh g−1.

Multi-step two-phase reactions through various stages (i.e., 1L,

4L, 3L, 2L, 2, and 1) can be observed in the Li-ion intercalation

and deintercalation process, leading to a nonlinear volume

change. The total volume expansion of fully lithiated graphite

(LiC6) is 13.2%.[43]

Li4Ti5O12 has good rate performance and high safety and is

often used in electric buses and other ﬁelds.[44] The theoretical

capacity of LTO is 175 mAh g−1, and a two-phase reaction of

Li4Ti(IV)5O12→Li7Ti2(IV)Ti3(III)O12 occurs during cycling.[45] The

volume change of LTO during cycling is only 0.2%, and it is

called a zero-strain material.[46] The zero-strain nature of LTO

allows it to be used as a counter electrode for pressure meas-

urement analysis of working electrodes.[14b,f,47]

2.2.2. Lithium-Alloying Materials

Si has an extremely high theoretical capacity of 4200 mAh g−1,

corresponding to Li22Si5. However, the fully lithiated Si phase at

room temperature is Li15Si4, corresponding to 3579 mAh g−1.[48]

Crystalline silicon is amorphized during the ﬁrst lithiation pro-

cess and forms amorphous silicon (LixSi) through a two-phase

reaction that appears around 170 mV versus Li+/Li. Further

lithiation leads to the crystallization of amorphous silicon into

Li15S4 below about 60mV versus Li+/Li.[49] The volume expan-

sion of Si during the ﬁrst alloying process is nearly 300%.[50]

Si undergoes irreversible structural changes during the ﬁrst

charging-discharging process, e.g., during the ﬁrst charging

process, Si undergoes a crystalline to amorphous transition at

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a lower electrode potential than in subsequent cycles, during

which the material remains amorphous.[51]

Indium anode is also a widely used alloyed material in

sulﬁde-based ASSBs. Indium can form a lithium-indium alloy

during cycling and has a stable potential (0.62V vs Li+/Li).[52]

Besides, the lithium-indium alloy also has good mechanical

properties and chemical stability with sulﬁde SSEs.[53] Indium

is lithiated during the charging process into a slightly nonstoi-

chiometric InLi1-x phase, accompanied by a transition from the

pure indium tetragonal phase to the cubic InLi1−x phase. The

volume of the tetragonal unit cell of InLi is 113.23 Å3, which

is approximately two times larger than that of the pure In

tetragonal phase (55.069 Å3). Therefore, the change from In

= 15.71 cm3 mol−1) to InLi ( m

= 23.597 cm3 mol−1 of the

stoichiometric phase) is accompanied by a volume expansion

of 105.6%.[14j]

Lithium metal is considered an ideal anode material because

of its high theoretical speciﬁc capacity (3860 mAh g−1) and the

lowest redox potential (−3.04V vs SHE).[54] However, Li metal

anodes suer from large volume changes (related to the amount

of deposition) during cycling. The inﬁnite volume change

of pure lithium metal stems from its “hostless” nature.[55] In

a constrained sulﬁde-based all-solid-state lithium metal bat-

tery, the stress evolution caused by this large volume change

of the Li metal anode during cycling is considered to be the

main cause of the nonuniform distribution of current density

of the electrodes and results in the formation of lithium den-

drites (micro-short circuits) as well as the mechanical failure of

the battery. However, the complex stress evolution of Li metal

anodes, including Li-alloy anodes and composite Li electrodes

with ﬂexible Li hosts during electrochemical cycling, is not yet

well understood and needs careful examination in the future.

2.3. Solid-State Electrolyte Materials

Due to the narrow electrochemical stability window, the sulﬁde

SSEs without any modiﬁcation, will inevitably undergo decom-

position while mixing with the conductive carbon materials in

the layered oxide composite cathode, resulting in some irrevers-

ible stress changes.[7b,c,56] For example, during charging, sulﬁde

SSEs will undergo oxidation reactions accompanied by volume

reduction.[14f ] In layered oxide composite cathode, the volume

change caused by the oxidation of the sulﬁde SSEs is mar-

ginal for the total internal stress change compared with that

of the active materials.[14f ] However, the dierences in volume

changes and mechanical parameters (such as Young’s modulus

and bulk modulus, etc.) between the SSEs and the layered oxide

will cause uneven local stress distribution inside the composite

cathode, resulting in contact loss at the three-phase interface.[17]

It is worth noting that the operating voltage range of ASSBs

is relatively narrow when sulfur-based conversion-type active

materials are used in the composite cathodes (such as 1.5–3V

for sulﬁde-based all-solid-state Li–S batteries).[57] In this case,

reversible redox reactions of sulﬁde SSEs may occur, providing

a considerable reversible capacity.[58] Therefore, the contribu-

tion of the stress change caused by the reversible redox of the

sulﬁde SSEs to the total electrode stress change is no longer

negligible.

On the anode side, the electrochemical reduction will occur

when the voltage is as low as the electrochemical decomposi-

tion voltage of sulﬁde SSEs. For example, in a composite anode

consisting of LGPS, when voltage is below 0.6V versus Li/Li+,

LGPS will be electrochemically reduced to Li3P, LixGe, and

other products, resulting in an increase in internal stress.[59] In

particular, a study reported by Jung et al. showed that in the

LGPS/Gr composite anode, the internal stress change caused

by the electrochemical reduction of LGPS was comparable to

that caused by Li+ intercalation into graphite. [14c] The measure-

ment of the stress change caused by the volume changes of the

SSEs and the corresponding electrochemo-mechanical issues

will be discussed in detail in Section4.3.

3. Principles and Devices of Internal Stress

Measurement

3.1. Principles of Internal Stress Measurement

As mentioned above, when a volume-constrained all-solid-

state battery is cycling, the internal stress of the battery system

changes due to the volume change of the cathode and anode

active materials. The volume changes of the active materials

and the pressure change of the battery system can be bridged

by the mechanical properties of battery components: Young’s

modulus (E), shear modulus (G), bulk modulus (K), and Pois-

son’s ratio (υ).[67] The mechanical parameters of major elec-

trode materials and sulﬁde SSEs are listed in Table 2. It is

worth noting that the elastic properties of electrode materials

are greatly aected by the degree of lithiation, which makes it

challenging to evaluate real-time stress changes in ASSBs.[63,68]

Janek etal. proposed a simple mechanical model to estimate

the relationship between material volume change and pressure

change in ASSBs.[14k] They assumed that the all-solid-state bat-

tery is made of one single material (β-Li3PS4 glass-ceramic),

and the solid-state electrolyte is elastic enough to behave like

a ﬂuid in a system, which is constrained on all sides. The pres-

sure change inside the battery can then be deduced by using

the isothermal compressibility factor κT consisting of Poisson’s

ratio υ and Young’s modulus E.

()

== −1312

(4)

By integrating the isothermal compressibility, Hencky’s loga-

rithmic strain can be obtained. Where V0 is the initial volume,

and εvol =ΔV/V0 is the volumetric strain.

∫∫

=−

(5)

κεκ

()

∆=

−+

∆









=− +ln 11ln 11

vol

V (6)

The approximate relationship between the internal stress

change, the bulk strain εvol, and the bulk modulus K of SE can

be obtained by linearizing the logarithmic strain.

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κεκε

∆≈−

∆

=− ∆=−

vol

(7)

In an actual experiment, the internal stress change of the

system can be measured by external pressure sensors or built-

in FBG sensors.

3.2. Stress Measuring Devices

Until now, the stress measuring devices reported in sulﬁde-based

ASSBs mainly fall into two categories: the external pressure

sensor type and the built-in optical FBG sensor type. The former

detects the stress change during cycling by placing an external

pressure sensor outside the battery system, while the latter uses

a built-in optical FBG sensor to measure the battery system pres-

sure indirectly through the conversion between the Bragg wave-

length and the physical parameters of the FBG sensor.

3.2.1. External Pressure Sensor

Some pressure-measuring devices in sulﬁde-based ASSBs

reported so far are shown in Figure 2a,b.[14a,b] A typical pressure-

measuring device reported by McDowell et al. consists mainly

of three parts: a volume-constrained mechanical framework, a

battery cell, and a pressure sensor (Figure2a).[14b] Among them,

in order to ensure the reliability of the stress measurement,

a reasonably designed mechanical framework is reasonably

designed. This ﬁxture can apply uniform pressure to the battery

by tightening the four bolts with a precision torque wrench.

Therefore, the inﬂuence caused by the relaxation of the battery

components can be minimized.

At present, commercially available pressure sensors mainly

include capacitive sensors,[82] resistive sensors,[83] piezoelectric

sensors,[84] and piezoresistive sensors.[85] These sensors can

convert the measured pressure into a corresponding electrical

signal output (voltage or current).

Take the most widely used force-sensing resistor (FSR) pres-

sure sensor as an example. The metal strain gauge will deform or

stretch under external pressure, changing its resistance value. The

relationship between the deformation of the metal strain gauge

and the change in resistance can be expressed by Equation8:[86]

∆

∆R

(8)

where k represents the sensitivity coecient; R represents the

resistance of the metal strain gauge; ΔR represents the change

in resistance; L represents the length of the strain gauge; ΔL

represents the change in the length of the strain gauge.

Table 2. Mechanical parameters of major electrode materials and sulﬁde SSEs.

Materials Young’s modulus

E [GPa]

Poisson’s ratio

[ν]

Shear modulus

G [GPa]

Bulk modulus

K [GPa]

Cathode materials LiCoO2191.0 0.24 80 122.4[69]

LiNi0.33Co0.33Mn0.33O2199.0 0.25 78 132.6[70]

LiFePO4117.8 0.30 45.5 98.2[71]

LiMn2O4194.0 0.26 77.0 134.7[34c]

α-S812.8 0.23 5.2 8[72]

Li2S 76.62 0.18 32.35 39.9[73]

TiS2 (1T)a) 228 0.11 102 97.4[74]

MoS2 (2H)a) 230 0.3 88 191.7[74]

Sulﬁde SSEs β-Li3PS429.5 0.29 11.4 23.3[75]

75Li2S–25P2S523 0.32 8.7 21[76]

Li7P3S11 21.9 0.35 8.1 23.9[75]

Li10GeP2S12 21.7 0.37 7.9 27.3[75]

Li6PS5Cl 22.1 0.37 8.1 28.7[75]

Li6PS5Br 25.3 0.35 9.3 29.0[75]

Li6PS5I 30.3 0.33 11.3 29.9[75]

Anode materials Li 4.9 0.42 4.2 11.0[77]

In 12.6 0.45 4.4 42.2[78]

Si 96.0 0.29 62.0 90.0[79]

Sn 50.5 0.34 21.1 54.4[80]

Li13Sn548.7 0.21 24.4 33.3[80]

Li2.25Al 62.4 0.21 – –[34c]

LTO 181.0 0.25 73.1 125.1[81]

Graphite 32.0 0.31 12.0 28.1[63]

a)2H and 1T represent trigonal prismatic coordination and octahedral coordination conﬁguration, respectively.

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The key speciﬁcations of pressure sensor performance

include linearity, hysteresis, temperature sensitivity, creep and

repeatability,[87] etc.

1. Linearity: Linearity is deﬁned as the parameter of the accu-

racy of the corresponding relationship between the load and

the electrical signal output of the sensor.

2. Hysteresis: Hysteresis can be determined by observing the

output signal of the sensor when it is loaded and unloaded.

Hysteresis will signiﬁcantly aect the symmetry of the meas-

ured pressure.

3. Temperature sensitivity: Temperature sensitivity is deﬁned

as the stability of output sensitivity when the ambient

temperature changes. It is generally measured in the unit

Figure 2. a) Schematic diagram of the assembly and pressure measurement of an anode/LPSC/cathode cell. Reproduced with permission.[14b] Copyright

Authors, published by MDPI AG. d) Photographs of the optical ﬁber embedded within the InLix anode of an InLi|Li3PS4|LTO ASSB. Reproduced with

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of drift generated within the range of every 10 °C, such

as 0.05% F.S./10 °C.Temperature sensitivity drift can be

reduced by temperature compensation.

4. Creep and repeatability: Creep is related to the time-depend-

ent permanent deformation of a material when subjected to

constant pressure. Repeatability refers to producing reliable

results even over a long measuring time.

Therefore, selecting an appropriate pressure sensor

according to the dierent solid-state battery systems is cru-

cial. Nevertheless, the performance parameters of pressure

sensors are rarely introduced in the reported literature. In

addition, another factor aecting pressure measurement

accuracy is pressure baseline changes caused by mechanical

relaxation.[14m] Some work compensated for baseline changes

in the measured pressure curve,[14j,k] while others rested the

battery system for a long time enough to make the baseline

changes negligible. [14b]

3.2.2. Built-in Fiber Optic Sensor

As shown in Figure2c,d, the built-in optical FBG sensor stress

measurement device needs to be skillfully implanted into the

battery system. As illustrated in Figure2e, when light travels

through the optical ﬁber, the FBG sensor acts as a reﬂector for

Bragg wavelength (λB), which is deﬁned as λB = 2neΛ, where

ne is the eective refractive index and Λ is the Bragg grating

period. Any changes in temperature (T), hydraulic pressure (P),

or strain (ε) that occur around the FBG sensor will modify ne

and/or Λ, which will be translated into a change in reﬂected

wavelength, visualized as a peak shift (ΔλB).[15a,c,d,f,g,88]

FBG sensors can obtain rich thermal/mechanical informa-

tion.[89] However, when we only want to focus on certain system

information, the richness of information will be a double-edged

sword. Therefore, when we want to detect stress changes in solid-

state batteries with FBG sensors, some settings must be made to

ignore the contributions coming from T and hydraulic P varia-

tions. [16] First, the battery tests should be performed in a temper-

ature-controlled climate chamber to eliminate any ﬂuctuations

in ambient temperature and ensure that the temperature change

of the battery during cycling is negligible. Besides, low-loading

active materials should be used to minimize inherent heat

release and gas generation to ensure limited hydraulic pressure.

When the FBG sensor is strained under these conditions,

the basic principle of the FBG sensor can be expressed by

Equation9:[16]

λρε υε

()

∆=− =− −+

















11 2

B,0

eff

12 11 12

np pp (9)

where λB,0 is the Bragg wavelength at the initial moment, ρe is the

eective photo-elastic coecient, p11 and p12 are the strain optical

coecients of the ﬁber, and ν is the Poisson’s ratio. The above

parameters are known for silica ﬁbers. The measured strain ε can

be directly converted to stress by using Hooke’s law: σ= εE, where

E is Young’s modulus of the silica ﬁber, equal to 69.9GPa. In addi-

tion, the stress-wavelength shift curve of FBG sensors can be cali-

brated ﬁrst to obtain the real-time stress value in the linear region.

Interestingly, peak splitting occurs in FBG sensors when a

relatively high external pressure is applied. This phenomenon,

known as birefringence, is caused by the elliptical deformation

of the ﬁber driven by an external force ﬁeld (Figure 2f). This

symmetry break causes the initial eective refractive index to be

anisotropic, resulting in two distinct refractive index values (nx

and ny), where the x polarization (nx) varies much more than the

y polarization (ny). Thus, when a suciently high lateral load is

applied to the FBG sensor, the single resonant peak of the FBG

(λB, given by λB = 2neΛ) splits into two peaks (λx and λy).

The birefringence (B) is given by Equation10: [16]



=−=+

∆−∆

⊥

eff,0

n (10)

where n|| and n⊥ are the refractive index in parallel and per-

pendicular directions to the applied load, Δnx and Δny are the

refractive index changes in x and y light polarization due to the

externally applied load, and ne,0 is the initial eective refrac-

tive index. B0 is the birefringence caused by grating fabrication

and is negligible for low birefringence FBG sensors. By decou-

pling λx and λy and combining them with a calibrated pressure-

wavelength working curve, the local stress information of the

electrode can be obtained.

Table 3 compares the advantages and disadvantages of the

external pressure sensors and built-in FBG sensors. In gen-

eral, the main advantage of the external pressure sensor is that

it is simple to operate and can directly and nondestructively

track and measure the stress change in the battery system.

However, the stress change measured by the external pres-

sure sensor is only in the axial direction of the cell scale. Since

the built-in FBG sensor needs to introduce the optical ﬁber

into the battery system, the operation is complicated, and the

battery structure will inevitably be damaged. The biggest fea-

ture of the FBG sensor is that the local stress in the axial and

longitudinal directions can be obtained at the electrode scale,

which is currently impossible for external pressure sensors.

4. Application

Accurately measuring internal stress changes during battery

cycling is crucial to understanding the electrochemo-mechan-

Table 3. Advantages and disadvantages of the external pressure sensors and built-in FBG sensors.

Pressure-measuring devices Destructive Operational complexity Stress scale Local stress

External pressure sensors × × Device level ×

Build-in FBG sensors √ √ Material level √ (Axial + longitudinal)

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ical eects. Depending on the degree of volume change caused

by the reaction mechanism (intercalation/conversion/alloying

reactions) and the variation in mechanical property parame-

ters, the internal stress changes from dierent active materials

into devices tend to vary widely. The magnitude of the change

in internal stress will aect the degree of electrode structure/

interface evolution, resulting in mechanical degradation of

battery capacity. In addition, the mechanical information

provided by the pressure measurement is also related to the

electrochemical behaviors, such as the reaction mechanism of

the active materials, the state of charge (SoC), and the state of

health (SoH) of the battery. Therefore, the pressure measure-

ment can bridge the gap between mechanics and electrochem-

istry. In the following sections, the application of pressure

measurement to sulﬁde-based ASSB materials and devices is

reviewed.

4.1. Cathode Active Materials: Electrochemistry and Mechanics

Layered oxide cathode materials are widely used in sulfide-

based ASSBs. Janek’s group systematically measured the

pressure changes of different materials (NCM, NCA, and

LCO) during cycling by using a zero-strain LTO anode as the

counter electrode.[14k] As shown in Figure 3a–c, the experi-

mentally measured pressure changes for these three mate-

rials are comparable (10−2 MPa level, depending on areal

capacity). In addition, the changing trend of measured pres-

sure of the three materials is consistent with the volume

change obtained by crystallography; that is, the measured

pressure of NCM and NCA materials decreases during the

charging process and increases during the discharging

process, while the LCO material shows the opposite trend.

When matched with the same anode, the opposite pressure

changes of NCM and LCO materials will lead to different

electrochemo-mechanical effects, resulting in different

degrees of mechanical damage to the battery structure.

Interestingly, McDowell’s group recently reported that when

lithium metal is used as an anode, the ASSB with NCM/LCO

as the active material shows completely different micro-

short-circuit behavior.[90] Under identical operating condi-

tions (current density of 2 mA cm−2 and areal capacity of

6 mAh cm−2), the Li/LPSC/NMC811 battery exhibited

micro-short-circuit behavior in the first charging process

(indicated by a sudden voltage drop and an overcharged

capacity far exceeding the theoretical areal capacity). In

contrast, the Li/LPSC/LCO battery was charged normally

in the first cycle. This phenomenon is very interesting as

it may incorporate information on the micro-short-circuit

failure mechanism caused by the “internal stress” (stress

coupling between cathode and anode) generated in Li metal

full cells, which cannot be obtained in the Li/Li symmetric

cell.[16] The structure of the symmetrical cell is destined to

be incapable of considering the influence of internal stress

(one side plating and the other side plating, the system is

in an internal stress balance). Recently, Budiman et al.

performed electrochemo-mechanical analysis on NCM/

LGPS/LiIn and LCO/LGPS/LiIn configuration batteries by

finite element analysis (FEA).[91] In this study, the authors

simulated the stress distribution within the SSE caused

by electrode volume changes and further mechanical

damage behavior, including plastic deformation, interfacial

debonding, and crack propagation. The simulation showed

that the most severe mechanical damage to SSE occurred

when cathode and anode active materials exhibited oppo-

site volume changes (such as the NCM cathode shrinkage

and the LiIn anode expansion during the charging pro-

cess). While different electrochemo-mechanical effects in

NCM/LCO-Li full cells make intuitive sense when LiIn is

replaced by a Li metal anode, their influence on the lithium

plating/stripping behavior or SSE mechanical damage and

the resulting micro-short-circuit of the Li metal anode still

needs to be further studied. Although challenging, tracking

the pressure changes in the battery during cycling may

shed light on understanding these complex electrochemo-

mechanical effects. The crystal structure design of Ni-rich

layered cathode materials greatly influences their elec-

trochemo-mechanical effects. Jung et al. investigated the

structure evolution of two different Ni-rich materials, a

commercial-grade Li[Ni0.80Co0.16Al0.04]O2 (NCA80) having

randomly oriented grains and a full-concentration gradient

Li[Ni0.75Co0.10Mn0.15]O2 (FCG75) having radially oriented

rod-shaped grains by in-situ pressure measurement.[47] As

shown in Figure3d, NCA80 exhibited a lower pressure drop

(-0.15MPa) than FCG75 (-0.30MPa) after the first charging

process. This result directly indicates the formation of voids

or cracks in NCA80. The pressure of NCA80 is higher than

that of FCG75 after one cycle, although fewer Li+ ions are

intercalated in the lattice of NCA80 compared to FCG75

(corresponding to a lower discharge capacity), suggesting

severe mechanical degradation of the NCA80 electrode.

Compared with layered oxide materials, the volume

changes of conversion cathode materials are much larger.

Janek’s group monitored the pressure changes during

cycling in a Li0.3In0.7/Li7P3S11/S: Li7P3S11: Ketjen black

50:30:20 wt% cell (Figure 3e).[13] In their experiment, a

pressure decrease during discharging was observed. Inter-

estingly, the authors seem perplexed by this result, as

they expected the S8→ Li2S reaction to being a volume-

increasing process. In fact, since the pressure sensor moni-

tors the resultant force in the direction of σ11, the decrease

in net pressure during the discharging process means that

the net volume change of the battery is dominated by the

Li–In anode. When a conversion-type material with a larger

volume change (e.g., Se, which exhibits a 98% volume

change when fully lithiated) is used on the cathode side,

the net pressure change is likely to be controlled by the

cathode. Recently, Sun’s group confirmed this possibility

in an all-solid-state Li–Se battery (Figure 3f).[14h] In their

experiments, the net pressure change of the battery was

determined by the volume change of the Se cathode. How-

ever, since most of the conversion active materials do not

contain lithium, zero-strain LTO cannot be directly used as

the counter electrode, and it is still a challenge to measure

the pressure change during the cycling of a single conver-

sion active material electrode.

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Some sulﬁde SSEs can undergo reversible redox reactions

within a speciﬁc voltage range and have also been reported as

conversion-type cathode active materials. However, the complex

redox reaction mechanism remains unclear. Based on the high

sensitivity and fast response between electrochemical reactions

and speciﬁc volume changes (pressure changes), tracking elec-

trochemo-mechanical changes via in situ pressure measure-

ment can be used to reveal possible reaction pathways. Wang

et al. reported an ASSB (In/InLi|LPSCB|LPSCB-MWCNTs)

using Li6PS5Cl0.5Br0.5 as the cathode active material.[14i] In

this work, pressure measurement is ﬁrst used to reveal the

reaction mechanism. As shown in Figure 3g (bottom), the

measured pressure curves show nonlinear behavior with

three distinct regions during the charging process. Consid-

ering that the partial molar volume change of the Li–In anode

∆

(In/InLi

)

V=+7.89 cm3 mol−1 is constant, it is suggested

of Chemistry. d) Operando electrochemical pressiometry proﬁles for FCG75 and NCA80 during ﬁrst charge-discharge at 0.1 C and 30 °C. Reproduced

potential proﬁle combined with the in situ measured pressure change of an In/InLi|LPSCB|LPSCB-MWCNTs cell during cycling (top) and a magniﬁcation

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that at least three reactions with dierent volume changes are

involved on the cathode side. The potential reaction mechanism

can be further given by comparing the partial molar volume

change of the cathode with the partial molar volume change

of the Li–In anode. If we suppose that the PS43− phase formed

by a reaction of Li2S and P0 (representing as (Li2S)0.75(P2S5)0.25)

should ﬁrst be converted to (Li2S)0.67(P2S5)0.33 (representing as

Li4P2S7, P2S74−), and ﬁnally to (Li2S)0.50(P2S5)0.50 (representing as

Li2P2S6, P2S62−) during charging. Therefore, the corresponding

reaction volume changes can be calculated as follows:

Reaction (1)-Li3PS4 (LPS) phase formation:

()

∆+→+







=−

+−

−

4LiS 2LiS PS 5Li5

5.60 cm mol

rm 22

0.75 25

0.25

VP e (11)

Reaction (2)-LPS oxidation I:

() ()

∆→

++ +







=−

+−

−

4LiS PS 3LiS PS S2Li 2

8.50 cm mol

rm 20.75 25

0.25 20.67 25

0.33

(12)

Reaction (3)-LPS oxidation II:

() ()

∆→

++ +





=

−

+−

−

3LiS PS 2LiS PS S2Li 2

5.76 cm mol

rm 20.67 25

0.33 20.50 25

0.50

(13)

Only reaction (2) can lead to a decrease in Δp upon charging.

Therefore, here, we assumed that the oxidation of the LPS glass

phase starts in region 2. Consequently, the formation of the

LPS glassy phase occurs in region 1. Although the complete

electrochemical reaction pathway is more complex, the above-

simpliﬁed reaction volume can explain the observed Δp trend,

providing a new perspective for revealing electrochemical reac-

tion mechanisms and pathways.

4.2. Anode Active Materials: Electrochemistry and Mechanics

Lithium metal is considered the “Holy Grail” among anode

materials. Due to the thermodynamic instability between the

sulﬁde SSEs and Li metal, however, interphases will be gener-

ated at the interface.[92] Due to the partial molar volume dier-

ence between the interphase and the Li metal, the formation

and evolution of the interphase at the interface will aect the

battery pressure. Therefore, pressure measurement can pro-

vide a new electrochemo-mechanical perspective on interface

evolution. Lee etal. systematically investigated two types of rep-

resentative SSEs (LSPS and LPSCl)/Li interfaces evolution by

in situ monitoring the pressure changes in SSE/Li symmetric

cells (Figure 4a).[14m] Compared with LPSCl symmetric cells,

LSPS symmetric cells exhibit a more obvious pressure drop

at open circuit voltage (OCV). This is owing to the formation

of electrically conductive Li–Sn alloys upon contact between

LSPS and lithium metal, which allows the interfacial reactions

to continue. The authors further investigated the relationship

between pressure changes and the electrochemical processes

in LSPS symmetric cells. Due to the formation of the electro-

chemical interface, as shown in Figure4b, the symmetric cell

pressure drops more rapidly. Interestingly, when the short cir-

cuit caused by the penetration of lithium dendrites occurs in

the symmetric cell, the pressure change tends to plateau (dP/dt

approaches zero). This electrochemomechanical response fea-

ture suggests that pressure measurement can serve as a pow-

erful tool to detect short-circuit failures in Li-metal ASSBs.

A recent work reported by Lim et al. conﬁrms this possi-

bility. As shown in Figure 4c,d, the authors monitored the in

situ pressure changes during cycling in both NCM/uncoated

Li and NCM/LixIn-coated Li batteries.[93] Since the volume

change during Li plating/stripping is much larger than that of

the NCM materials, the periodic linear pressure changes are

mainly dominated by the Li metal anode when the battery is

cycled normally. In NCM/uncoated Li battery, the pressure no

longer changes linearly when a short circuit behavior occurs.

This is caused by the penetration and growth of Li dendrites

in the mechanical stress-caused defects within the SSE. The

authors further normalized the pressure change dierences

(Δ(ΔP)) with respect to the discharge capacity (Q) at each cycle

(denoted as Δ(ΔP)Q) (Figure4e). The capacity-normalized pres-

sure change dierence Δ(ΔPQ) was calculated by the following

equation: Δ(ΔPQ) =Δ(ΔP)/Qdischarge where Qdischarge and Δ(ΔP) is

corresponding discharge capacities and pressure change dier-

ence at a corresponding cycle. The decrease in Δ(ΔP)Q is associ-

ated with the penetrating growth of Li dendrites within SSE.

Therefore, the growth mechanism of Li dendrites can be eec-

tively predicted by monitoring the Δ(ΔP)Q value.

Due to the severe interfacial reactions and microshort cir-

cuit issues, lithium metal is dicult to be practically applied in

the short term. As an alternative, Li-alloy anodes are expected

to achieve high energy density ASSBs due to their high spe-

ciﬁc capacity and low potential. Among them, Li–In alloys are

the most widely used alloy anode materials in the laboratory.

Although Li–In anode and sulﬁde SSE are thermodynami-

cally and kinetically stable, Li–In anode also exhibits huge

volume changes during cycling, causing serious electrochemo-

mechanical problems in batteries. Recently, Tarascon’s group

introduced the FBG sensors into ASSBs and systematically

measured the stress evolution inside the Li–In anode and at the

anode/SSE interface (with LTO as the counter electrode).[16] As

a comparison, the author also used an external pressure sensor

to measure the pressure change. In the pressure measure-

ment inside the Li–In anode, although the pressure changes

measured by the external pressure sensor and the FBG sensor

are symmetrical and periodic, the stress change monitored

in the FBG sensor is an order of magnitude larger than that

of the external pressure sensor (Figure4f,g). It is particularly

worth pointing out that in this work, for the ﬁrst time, the

local stress evolution between the Li–In anode/SSE interface

was monitored through the “birefringence eect” of the FBG

Sensor (Figure 4h,i). Measuring such local stress evolution

at the interface is essential for a deeper understanding of the

electrochemo-mechanical eects in ASSBs. In addition to Li–In

anode, Han etal. systematically investigated the pressure evolu-

tion in ASSBs with various composite alloy anodes (Sb, Sn, and

Si) (Figure4j–l).[14b] Since the volume change of NCM111 on the

cathode side is smaller than that of the alloy anode, the net pres-

sure change is controlled by the alloy anode. A typical feature of

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alloy anodes is the structural change that occurs during the ﬁrst

cycle, which is reﬂected in the mechanical information as the

most severe stress hysteresis and irreversibility during the ﬁrst

cycle. This result indicates the sensitivity of the pressure meas-

urement to the response of electrode structure evolution. It is

worth noting that the particle size for the stress evolution of

the Si anode was studied intensively in this work. The authors

found that the Si nanoparticles showed less stress change than

the Si microparticles by normalizing the stress change and the

areal capacity. Smaller normalized stress variation may lead

to less electrochemo-mechanical degradation, thus improving

cycling stability.

Figure 4. a) Stack pressure evolution after pressing to an initial stack pressure of 30MPa and holding at open circuit for two dierent symmetric cells

sure proﬁles (blue) of two LSPS symmetric cells (operated at 0.5mA cm−2 (solid lines), and held at OCV (dotted lines). Reproduced with permission.[14m]

an internal optical signal (bottom) for an LTO-LiIn ASSB cycled at C/30 (5.83mA g−1) and 25 °C in an operando mode. Reproduced with permission.[16]

h) 2D stack view of the operando collected spectra by the FBG sensor placed between LTO/Li–In interface, with the corresponding galvanostatic charge/

Authors, published by Springer Nature. i) Galvanostatic cycle (top), λx and λy evolution (middle), and operando stress evolution obtained internally by

The Authors, published by Springer Nature. j–l) Galvanostatic cycling of j) Sb, k) Sn, and l) Si composite anodes in ASSBs with LPSC solid electrolyte and

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4.3. Inactive Components: Electrochemistry and Mechanics

Besides the active materials, the mechanical properties of

the inactive components (SSE and binder) in the composite

electrode are equally critical to the electrode structure stability.

Wang etal. investigated the eect of thiophosphate SSE with

dierent crystallinity (7525-glass and crystalline β-Li3PS4) on

the electrochemical performance of ASSBs.[14g] As illustrated in

Figure 5a, the redox processes of two SSEs can be correlated

with pressure changes by cyclic voltammetry (CV) test and in-

situ pressure monitoring. β-Li3PS4 has a more severe redox than

7525-glass, which will lead to a more severe volume change

(pressure change). Due to the mismatch in the volume change

between the SSE and the cathode active material, more severe

physical contact loss and cracks will occur (Figure5b), deterio-

rating the electrochemical performance of the battery. Similar

conclusions were obtained in work reported by Teo et al. The

authors investigated the electrochemo-mechanical behavior of

glassy (1.5Li2S–0.5P2S5–LiI) and crystalline (Li6PS5Cl) thiophos-

phate SSEs by in situ pressure measurement[94] (Figure 5c).

Likewise, crystalline Li6PS5Cl has more pronounced redox

reactions and pressure changes. Compared with sulﬁde SSEs,

halide SSEs have recently attracted great attention due to their

wider electrochemical stability window and better chemical

stability. However, the dierence in mechanical properties

between sulﬁde and halide SSEs remains unclear. Han et al.

investigated the electrochemo-mechanical eects of sulﬁde

(Li6PS5Cl0.5Br0.5, LPSX) and halide (Li3YCl6, LYC) SSEs on bat-

tery performance by pressure measurement.[14f ] As shown in

Figure5d, the poly-NCA electrode with LPSX showed an abrupt

drop in pressure at the beginning of the ﬁrst charging process.

This feature is more pronounced in the capacity-derivative dif-

ferential pressure (or dierential electrochemical pressiom-

etry (DEP)) curve. In addition, the minimum point of the DEP

Figure 5. a) Cyclic voltammetry tests voltage steps (top), current response (middle), and corresponding pressure response (bottom) upon cycling of

ASSBs using 7525-glass/C65 composite (red) and β-Li3PS4/C65 composite (blue) as working electrodes. Reproduced according to the terms of the

current response, and pressure response of Super C65 electrodes glassy SE (1.5Li2S–0.5P2S5–LiI) (left) and crystalline SE (Li6PS5Cl) (right). Reproduced

Wiley-VCH. e) First cycle charge-discharge voltage proﬁles and corresponding pressure changes for NCM electrodes using pristine and vulcanized BR

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curve of P/LPSX (which can be regarded as an indicator of SOC

as discussed in Section4.2) shifts in the positive direction com-

pared with P/LYC, reﬂecting the delayed charging due to the

more severe decomposition reaction of LPSX (or Li+ deinterca-

lation). It is necessary to consider the electrochemo-mechanical

eect of SSEs as a design factor for the mechanical stability of

the electrode structure. A suitable SSE candidate should con-

sider its mechanical properties and the electrochemical stability

window.

In addition to considering the mechanical properties of SSEs

in the composite electrodes, the capability of the SSE sepa-

rator to resist the stress generated during the manufacturing

process and the volume changes of the electrode active mate-

rials during cycling are also closely related to their mechanical

properties.[95] An ideal SSE separator requires low porosity (to

achieve high ionic conductivity) as well as moderate elastic

modulus (including Young’s modulus (E), shear modulus (G),

bulk modulus (K), etc.) and fracture toughness, KIC. In gen-

eral, sulﬁde SSEs have favorable mechanical properties (lower

Pugh’s ratio, e.g., LPSCl with a G/B of 0.28, where G and B

are shear and bulk modulus, respectively) so that densiﬁcation

of the electrolytes can be achieved at lower external pressures

(e.g., 360 MPa) and room temperature.[96] In comparison to

sulﬁde SSEs, another typical class of inorganic solid-state elec-

trolytes, oxide SSEs, due to distinctive dierences in mechan-

ical properties (e.g., Pugh’s ratio of 0.54 for LLZO), require a

much higher external pressure (up to hundreds of GPa) and

high temperature (up to 1000 °C) to achieve densiﬁcation.[97] In

addition, the higher hardness (H) and elastic modulus (hard-

ness H= 9.2, 9.1, 7.1GPa, and Young’s modulus E= 200, 150,

115 GPa for LLTO, LLZO, and LATP, respectively) of oxide

SSEs than sulﬁde SSEs (detailed in Table 2) make them less

prone to elastic deformation.[11g] Therefore, it can be expected

that greater stress will be generated at the electrolyte/electrode

interface in pure oxide-based ASSBs, leading to more severe

electrochemo-mechanical issues.

Particularly, when Li metal is used as an anode in ASSBs,

the mechanical properties of the oxide and sulﬁde SSE will

exhibit dierent eects on the working conditions of the Li

metal anode. Monroe and Newman proposed a classical model

for elaborating the suppression of Li dendrite growth and SSE

mechanical parameters.[77] According to their model, dendrite

growth would be suppressed when the shear modulus of SSE is

greater than twice that of Li metal (≈4GPa). Therefore, the high

mechanical modulus of oxide SSE should suppress dendrite

growth, but the actual situation is complicated. However, the

growth of lithium dendrites has been reported in many oxide

SSEs (including LLZO, LLZTO, LATP, etc.), where the shear

modulus of LLZO (60GPa) is more than tenfold higher exceeds

that of Li metal, contrary to Monroe and Newman’s theory.[98]

This result suggests a more complex micro-short-circuit mecha-

nism in ASSBs. For sulﬁde SSEs, their Young’s modulus (e.g.,

E (LPSCl) = 22 GPa) is not much dierent from that of Li

metal (7.8 GPa), and the fracture toughness KIC is lower (e.g.,

KIC LPSCl = 0.23MPa m1/2), so the internal stress during bat-

tery cycling is likely to lead to a heterogeneous distribution of

lithium protrusions, which increases the local KIC of the SSE/

Li interface.[96] The increased local KIC will cause the cracks

to occur within the SSE layer, and then the lithium dendrites

will grow along the cracks, resulting in the short-circuit phe-

nomenon. For oxide SSEs, due to their relatively high Young’s

modulus (e.g., E (LLZO) = 175GPa) and KIC (e.g., KIC LLZO =

1.25 MPa m1/2), the internal stress of the battery may lead to

plastic ﬂow of lithium rather than crack growth within the

SSE.[96] In this case, micro-short-circuit behavior in oxide SSEs

may be dominated by internal lithium nucleation (lithium den-

drites propagate through voids and grain boundaries with pref-

erential ion pathways) rather than the electrochemo-mechanical

damage/dendrite penetration behavior in sulﬁde SSEs.[99]

Scalable sheet-type electrodes are necessary for the prac-

tical application of sulﬁde-based ASSBs. Due to their key role

in maintaining microstructural integrity, binders are indis-

pensable in the preparation of ﬂexible electrodes. Kwon etal.

reported a novel vulcanized butadiene rubber (BR) binder

with excellent mechanical properties, which can partially sup-

press the electromechanical eects during cycling, thereby

improving the electrochemical performance of batteries.[14a] The

authors monitored the pressure changes during the cycling of

NCM/LTO batteries with pristine and vulcanized BR binders.

As depicted in Figure5e, the overall pressure change trend is

consistent with the lattice volume change of Ni-rich layered

oxides upon the (de)lithiation process. However, with a lower

discharge depth, the cells using pristine BR binder showed

higher stress values than those using vulcanized BR binder.

The authors assumed that higher pressure change reﬂects

the formation of cracks and/or voids caused by electrochemo-

mechanical degradation. In addition, they quantiﬁed the crack

area fractions in the bulk and interfacial regions using cross-

section ﬁeld-emission scanning electron microscopy backscat-

tered electron mode (FESEM-BSE) to examine the microstruc-

tural evolution of NCM electrodes with pristine and vulcanized

BR binders after the ﬁrst charging and discharging process.

After the ﬁrst charging, the crack area fractions in both bulk

and interfacial regions increased signiﬁcantly in both batteries.

However, the area fraction of cracks in the interfacial region of

the battery using a pristine BR binder is higher than the vul-

canized BR binder. After the ﬁrst discharging, the average area

fraction of cracks in the interfacial region of the battery using

a pristine BR binder is also slightly higher than that of the vul-

canized BR binder. In addition, the area fraction of cracks in

the bulk region showed similar values after the ﬁrst charging

and discharging. These results suggest that the debonding of

the electrode/electrolyte interface has a more severe eect on

electrochemo-mechanical degradation than the contact loss

inside the electrode. This work indicates the possibility of com-

plementary analysis using FESEM-BSE and operando pressure

measurements to reveal the evolution of electro-mechanical

degradation.

4.4. Correlation of State of Charge (SOC) of the Batteries and

Changes of Stresses of the Batteries

Accurately estimating the state of charge (SOC) of the batteries

is crucial but also challenging. Many electrochemical techniques

based on measurable parameters (e.g., voltage, current, imped-

ances, and their dierentiates) have been developed to estimate

the SOC of batteries.[100] However, due to the fact that these tech-

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niques generally require measurements at low current densities

or at open circuit potential in order to exclude the eects of

polarizations, as well as complex computations, the SOC’s esti-

mation accuracy will decrease and become more complicated

as the battery ages. The correspondence between the battery

pressure and the volume change (related to the electrochemical

process) makes it possible to track SOC changes roughly but

also quickly. The sensitivity and simplicity of estimating bat-

tery SOC by pressure measurements have been demonstrated

in liquid batteries.[101] Due to the intrinsic rigidity, the pres-

sure changes in ASSBs will be more pronounced. Therefore,

pressure measurement can provide a new tool for estimating

the SOC of ASSBs. Jung et al. tracked the SOC of NCM/

graphite full cells by the operando dierential electrochemical

Figure 6. a–c) Charge discharge voltage proﬁles of each electrode, corresponding pressure change curves (ΔP), and DEP proﬁles (dP/dt) of Gr elec-

Wiley-VCH. d–f) Schematic of stress compensation by using 55:45 wt% NCM-811:LCO cathode composite. Reproduced with permission.[14k] Copyright

2018, The Royal Society of Chemistry.

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pressiometry (DEP) method.[14c] Figure 6a–c shows the voltage

curves and pressure changes (ΔPGr) of NCM/graphite cells

with dierent n/p ratios. ΔPGr was estimated from the pressure

change in NCM/Gr battery minus that in NCM/LTO battery.

The results show that, as the n/p ratio increases, the minimum

DEP value (red diamonds in the ﬁgure) shifts in a positive direc-

tion during the charging process. The maximum point value

moves in the negative direction in the discharging process.

Because the total Gr electrode volume change is caused by Li+

intercalation (or deintercalation), this observation implies an

association between DEP proﬁles and speciﬁc SOCs. Compared

to traditional voltage-based estimation methods, pressure meas-

urements are more sensitive to SOC (especially for materials

with voltage plateaus during cycling, such as LFP), making it

more convenient to estimate SOC. In addition, since the pres-

sure can respond instantaneously to the lithium content in the

electrode, it can eectively detect the SOC dierence caused

by self-discharge compared to the traditional ampere-hour

counting method.

5. Summary and Outlook

Through the external pressure sensor or the built-in optical

FBG sensor, the internal stress change during the cycling

process of the solid-state battery can be obtained. This review

overviews the recent work on stress-measuring techniques and

their applications in sulﬁde-based ASSBs. It is expected that

stress(pressure)-measuring techniques in ASSBs, including

sulﬁde-based ASSBs, will have rapid development in the next

few years, and some future perspectives of the techniques are

outlined below.

1) The accuracy and resolution of pressure (or stress)-measur-

ing techniques need further improvement. For example, an

FSR pressure sensor with force resolution better than 0.01%

full scale is expected when studying active electrode mate-

rials with small volume changes (such as NCM materials).

In the pressure measuring experiment using the external

pressure sensor, the pressure change interferences caused by

the relaxation of mechanical components are not negligible.

Therefore, rational designs are required to suppress the me-

chanical noise and keep the pressure baseline as stable as

possible. In many pressure measurement experiments using

external pressure sensors, the actual measured pressure is

the addition of the total resultant force of the cathode and

anode stress changes, while the contributions of the stress

changes of these two electrodes are often inconsistent. There-

fore, it is necessary to decouple the cathode and anode stress

contributions separately. The stress decoupling methods for

electrodes composed of lithium-containing and lithium-free

active materials are dierent. For lithium-containing cathode

or anode active materials, such as NCM, LCO, Li2S, Li metal,

Li–In alloy, etc., the zero-strain LTO counter electrode can

be directly used to decouple the electrode stress. [14k,16,55] For

lithium-free active materials, such as S, graphite, Si, etc., it is

more dicult to decouple the stress changes of their corre-

sponding electrodes. Using the LTO electrode as a bridge is a

feasible method to deduct the stress contribution of the other

electrode from the stress change of the full cell (as reported

in an NCM-Gr battery[14c]). In addition, it has been proved

that decoupling of the pressure/volume change of a single

cathode electrode in liquid battery systems by using (elec-

tro)chemically prelithiated LTO as a counter electrode.[43,102]

However, its application in sulﬁde-based ASSBs remains to

be explored and veriﬁed. Comparing the dierential pressure

(dP/dV) and dierential capacity (dQ/dV) data is also needed

to correlate the electrochemical-stress relationship,[103] as

stress changes may not be apparent in the raw measurement

data.

2) Systematic local stress measurement of electrodes should be

emphasized, and it is still lacking in the literature. During

battery cycling, the uneven electrochemical reaction within

the electrode will lead to uneven local stress distribution and

result in serious electrochemo-mechanical degradation of the

batteries. Apparently, the local stress cannot be measured by

only a single external pressure sensor. Using an optical FBG

sensor is considered to be able to obtain local stress informa-

tion to a certain extent. In the follow-up experiments, more

sophisticated stress measuring methods, such as pressure

sensor arrays or the combination of multiple groups of op-

tical FBG sensors, may be able to obtain more detailed in-

formation on local stress distribution at the electrode/cell/

module level.

Due to their non-destructiveness and cost-friendliness,

external pressure sensor arrays have been applied to monitor

the local stress distribution of electrodes and the mechanical

distribution of battery modules in conventional liquid LIBs.[104]

As shown in Figure 7a, local stress mapping of anode lithium

deposition can be obtained using an external pressure sensor

array in a single cell.[105] The analysis of nonuniform stress dis-

tribution can provide some useful information for improving

local lithium deposition and detecting lithium dendrite growth.

In addition, in a battery module/pack composed of parallel-

series connected cells, the heterogeneity caused by the manu-

facturing process and the non-uniform working conditions will

lead to inconsistency between individual cells.[97] Inconsistency

between individual cells will lead to an uneven distribution of

internal stress in the module/pack, posing challenges to the

safety design and cyclic performance of the batteries. There-

fore, accurate monitoring of the stress distribution at the cell

level and mechanical inhomogeneity at the module/pack level

through the pressure sensor array is also essential for designing

the optimal mechanical structure and achieving superior elec-

trochemical performance (Figure7b).[106]

Implanted inside the cell, the optical FBG sensors can

provide local stress distribution within the electrodes and

between cell components. Local stress spatial resolution can

be achieved through quasi-distributed and fully distributed

FBG sensor designs. The design of the quasi-distributed FBG

sensor includes multiplexing of multiple FBG sensor groups

(Figure 7c) and integrating more than one grating with dif-

ferent Bragg wavelength (λB) in a single ﬁber by tuning the

Bragg grating period (Λ) (Figure 7d).[88b,104] Fully distrib-

uted FBG sensors based on Rayleigh, anti-Stokes Raman, or

Brillouin scattering can realize stress/temperature distribu-

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tion mapping in a single FBG sensor (Figure7e).[88b] Recently,

Huang et al. successfully obtained temperature distribution

mapping with a high spatial resolution of µm by a fully dis-

tributed FBG sensor based on Rayleigh scattering in a sodium-

ion battery.[88b] Although there have been no reports on stress

distribution mapping in batteries by fully distributed FBG sen-

sors, we believe that it is promising to achieve high-resolution

stress mapping by rationally designing experimental conditions

to eliminate the eects of temperature (T) and strain (ε) on λB

shift.[16]

For local stress information measurements of sulﬁde-based

ASSBs, the above schemes, which have been applied in liquid

batteries, can provide some guidance and implications. Trans-

planting external pressure sensor arrays from liquid to solid-

state batteries may not be so dicult. However, for built-in FBG

sensors, many new challenges, such as minimizing the impact

of FBG sensor implantation on the performance of solid-state

batteries and investigating chemical stability between FBG sen-

sors and sulﬁde SSEs, remain to be addressed.

3) The complexity of internal stress sources should be con-

sidered and analyzed more carefully. Taking layered oxide

materials as an example, NCM materials in liquid batteries

can cause gas evolution, which originates from electrolyte/

surface impurity decomposition and the release of lattice

oxygen at high voltage, mainly composed of CO2 and/or

O2. Interestingly, recent studies have shown that gas evolu-

tion due to side reactions can also be observed in sulﬁde-

based ASSBs.[107] Strauss et al. investigated the gas evolu-

tion of NCM622 and solid electrolyte (β-Li3PS4 or Li6PS5Cl)

by isotopic labeling and dierential electrochemical mass

spectrometry.[108] Results showed that CO2 and O2 could be

detected during the cycling process. Both species originate

from the electrochemical decomposition of Li2CO3 surface

contaminants, and O2 originates from oxygen evolution

from the NCM lattice at high SOC. This irreversible struc-

tural change of the active material is also one of the sources

of irreversible stress in the battery. Janek etal. ﬁrst proposed

the concept of “stress balance” in a composite cathode for

sulﬁde-based all-solid-state battery. [14k] They proposed to

use NCM/LCO two active materials mixed in a 55:45 mass

ratio in the composite cathode. Due to the fact that the vol-

ume changes of NCM and LCO during the charging and

discharging process are opposite, the stress generated in the

composite cathode will cancel each other out (Figure6d–f).

However, considering that the internal stress changes in the

battery are derived from the stress coupling caused by the

cathode and anode, it is not enough to optimize the stress

balance of one electrode only. One possible approach is to

make sulﬁde-based ASSBs work in a designed stress-com-

pensating device. By externally keeping the pressure con-

stant, the electrochemo-mechanical issues will be alleviated.

In conclusion, although the stress (pressure)-measuring

technique is a powerful tool for detecting internal stress

changes directly, in order to correlate the relationship between

the electrochemical reactions and internal stress evolutions

in the batteries, a combination of several characterization

tools is still needed to understand and resolve these complex

electrochemo-mechanical issues in sulﬁde-based ASSBs. For

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example, combining pressure-probe with in-situ X-ray dirac-

tion (XRD), X-ray absorption spectroscopy (XAS), or solid-

state NMR can obtain information on stress changes and local

structural/valence changes of active materials, and imaging

methods such as SEM or CT can more intuitively observe the

relationship between stress change and battery level structure

change. In addition, computational mechanical models of the

solid-state batteries constructed by phase-ﬁeld methods will

be helpful in analyzing experimental data on the stresses and

predicting more complex electrochemo-mechanical pheno-

mena during the charging/discharging processes of the

batteries.

Acknowledgements

This work was ﬁnancially supported by National Natural Science

Foundation of China (Grant No. 21935009) and the National Key R&D

Program of China (Grant No. 2021YFB2401800).

Conﬂict of Interest

The authors declare no conﬂict of interest.

Keywords

electrochemo-mechanical eects, electroche-mechanical stresses, stress

measurement, sulﬁde-based ASSBs

Received: September 16, 2022

Revised: November 2, 2022

Published online:

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Adv. Energy Mater. 2022, 2203153

www.advenergymat.de

2203153 (21 of 21)

www.advancedsciencenews.com

Jiabao Gu is a Ph.D. candidate at Xiamen University under the supervision of Prof. Yong Yang. His

current research topic focuses on the electrochemo-mechanical issues in all-solid-state batteries.

Ziteng Liang received his Bachelor’s degree in chemistry from Xiamen University in 2018. He

is now pursuing his Ph.D. degree in Collaborative Innovation Center of Chemistry for Energy

Materials in Xiamen University, under the supervision of Prof. Yong Yang. His research is dedi-

cated to investigating the failure mechanisms of sulﬁde-based solid-state batteries and presenting

corresponding solutions.

Yong Yang is now working as a distinguished professor at Xiamen University. He also serves

as Editor for J. Power Sources and The First Vice-President of Board Member of International

Battery Materials Association (IBA) and International Meeting of Lithium Battery (IMLB). His

main research interests are electrode/electrolyte materials for Li/Na-ion batteries, Solid State

Battery, and Battery Degradation Analysis of Li-ion Battery etc. He has published 400+ papers in

referred journals and got 20000+ citations (Goggle Scholar, H = 75). He is the recipient of 2020

Technology Award and Battery Division, ECS and Technology Award presented by International

Battery Materials Association (2014).

Adv. Energy Mater. 2022, 2203153

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Article

Aug 2022

Mechanical damages in solid electrolytes of solid-state battery (SSB) during the charging-discharging process remain a challenging issue for battery implementation. This paper demonstrates a numerical simulation of the damages in Li10GeP2S12 (LGPS) solid electrolyte of SSB due to compressive loading generated by electrode volume changes. Three models of anode/electrolyte/cathode arrangements were examined numerically with different expansion-shrinkage behavior. Crack formation inside the electrolyte models was realized by inserting cohesive elements, following traction-separation law. The result shows that when the cathode shrunk and the anode expanded, as occurs in NCM/LGPS/In configuration, the mechanical damages inside the LGPS solid electrolyte are more severe. Due to high-stress generation, there is a plastic deformation in the electrolyte and debonding at the electrode-electrolyte interface. The cracks also appear in both center and edge of the electrolyte because of high-stress concentration. These cracks do not occur when Li4Ti5O12 (LTO) anode with a very low expansion rate is used. This finding confirms that SSB was prone to mechanical damages due to expansion-shrinkage behavior in the electrodes, meaning that the mechanical strength of SSB material constituents must be considered in designing long-lasting SSB.

Influence of external pressure on silicon electrodes in lithium-ion cells

Article

Apr 2022
ELECTROCHIM ACTA

The influence of external pressure on lithium-ion cells containing a silicon anode is investigated. The performance of pouch-type full cells under rigid compression is compared to unconstrained cells, where the electrode stack is allowed to swell during cycling. The negative electrode contains only silicon as active material, while prelithiated lithium titanium oxide (LTO) is used as the positive electrode. The results show that the main failure mechanism in such cells is a continuous irreversible consumption of lithium ions, likely due to repeated solid electrolyte interphase breakage and reformation. At high pressures, the lithium depletion has a larger influence than at lower pressures. This effect is examined by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) as well as dilation measurements of flexibly-constrained cells and can be traced back to an increase of the ionic pore resistance being more pronounced under high pressure. A new approach is used to compensate the lithium loss, i.e. internal relithiation of the LTO electrode via a lithium reservoir. This not only proves the theory of irreversible lithium consumption being the main challenge in these cells, but also enables cycling for 1000 cycles at 1200mAhgSi−1 without capacity fading.

Three-Dimensional Networking Binders Prepared in situ during Wet-Slurry Process for All-Solid-State Batteries Operating under Low External Pressure

Article

Apr 2022

For all-solid-state Li batteries (ASLBs), the external operating pressure offsets the detrimental electrochemo-mechanical effects. In this work, a new scalable in situ protocol to reinforce binders for sulfide-electrolyte-based ASLBs operating under low or no external pressures is reported. The vulcanization of butadiene rubber (BR) using elemental sulfur proceeds in situ during the wet-slurry fabrication process for electrodes, forming a mechanically resilient crosslinked structure. The electrochemical performance of LiNi0.70Co0.15Mn0.15O2 electrodes fabricated using pristine or vulcanized BR diverge significantly as the operating pressure is lowered from 70 MPa to a practically acceptable value of 2 MPa. Complementary analysis using cross-sectional scanning electron microscopy and operando electrochemical pressiometry measurements confirms that the vulcanization of BR suppresses the electrochemo-mechanical degradation of electrodes, which suggests that the scaffolding structure of the vulcanized BR helps maintain the microstructural integrity of the electrodes upon charge and discharge. The significantly enhanced performance of the vulcanized BR is also demonstrated for pouch-type LiNi0.70Co0.15Mn0.15O2/Li4Ti5O12 full cells operated under no external pressure (reversible capacity of 121 vs. 150 mA h g⁻¹ at 0.2C for electrodes with pristine vs. vulcanized BR, respectively).

Electrochemo‐Mechanical Stresses and Their Measurements in Sulfide‐Based All‐Solid‐State Batteries: A Review

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