Recent Configurational Advances for Solid-State Lithium Batteries Featuring Conversion-Type Cathodes

Abstract

Solid-state lithium metal batteries offer superior energy density, longer lifespan, and enhanced safety compared to traditional liquid-electrolyte batteries. Their development has the potential to revolutionize battery technology, including the creation of electric vehicles with extended ranges and smaller more efficient portable devices. The employment of metallic lithium as the negative electrode allows the use of Li-free positive electrode materials, expanding the range of cathode choices and increasing the diversity of solid-state battery design options. In this review, we present recent developments in the configuration of solid-state lithium batteries with conversion-type cathodes, which cannot be paired with conventional graphite or advanced silicon anodes due to the lack of active lithium. Recent advancements in electrode and cell configuration have resulted in significant improvements in solid-state batteries with chalcogen, chalcogenide, and halide cathodes, including improved energy density, better rate capability, longer cycle life, and other notable benefits. To fully leverage the benefits of lithium metal anodes in solid-state batteries, high-capacity conversion-type cathodes are necessary. While challenges remain in optimizing the interface between solid-state electrolytes and conversion-type cathodes, this area of research presents significant opportunities for the development of improved battery systems and will require continued efforts to overcome these challenges.

Full text

Citation: Chiu, K.-C.; Chang, J.-K.;
Su, Y.-S. Recent Configurational
Advances for Solid-State Lithium
Batteries Featuring Conversion-Type
Cathodes. Molecules 2023,28, 4579.
https://doi.org/10.3390/
molecules28124579
Academic Editor: Wei Lv
Received: 12 April 2023
Revised: 25 May 2023
Accepted: 2 June 2023
Published: 6 June 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
molecules
Review
Recent Configurational Advances for Solid-State Lithium
Batteries Featuring Conversion-Type Cathodes
Kuan-Cheng Chiu 1, Jeng-Kuei Chang 2, * and Yu-Sheng Su 1,3,*
1International College of Semiconductor Technology, National Yang Ming Chiao Tung University,
1001 University Road, Hsinchu 30010, Taiwan
2Department of Materials Science and Engineering, National Yang Ming Chiao Tung University,
1001 University Road, Hsinchu 30010, Taiwan
3Industry Academia Innovation School, National Yang Ming Chiao Tung University, 1001 University Road,
Hsinchu 30010, Taiwan
*Correspondence: jkchang@nycu.edu.tw (J.-K.C.); yushengsu@nycu.edu.tw (Y.-S.S.)
Abstract:
Solid-state lithium metal batteries offer superior energy density, longer lifespan, and
enhanced safety compared to traditional liquid-electrolyte batteries. Their development has the
potential to revolutionize battery technology, including the creation of electric vehicles with extended
ranges and smaller more efficient portable devices. The employment of metallic lithium as the
negative electrode allows the use of Li-free positive electrode materials, expanding the range of
cathode choices and increasing the diversity of solid-state battery design options. In this review, we
present recent developments in the configuration of solid-state lithium batteries with conversion-type
cathodes, which cannot be paired with conventional graphite or advanced silicon anodes due to
the lack of active lithium. Recent advancements in electrode and cell configuration have resulted in
significant improvements in solid-state batteries with chalcogen, chalcogenide, and halide cathodes,
including improved energy density, better rate capability, longer cycle life, and other notable benefits.
To fully leverage the benefits of lithium metal anodes in solid-state batteries, high-capacity conversion-
type cathodes are necessary. While challenges remain in optimizing the interface between solid-state
electrolytes and conversion-type cathodes, this area of research presents significant opportunities for
the development of improved battery systems and will require continued efforts to overcome these
challenges.
Keywords:
all-solid-state battery; metallic lithium anode; chalcogen cathode; chalcogenide cathode;
halide cathode; solid-state electrolyte; Li–S battery; sulfur cathode; sulfide cathode; fluoride cathode
1. Introduction
Rechargeable lithium-ion batteries have dominated major energy storage battery
applications for the past decade, including electric vehicles, drones, consumer electronics,
and stationary and mobile energy storage systems. Traditional lithium-ion batteries consist
of graphitic anodes, polyolefin separators, organic liquid electrolytes, and intercalation-
type lithium transition metal oxides/phosphate cathodes. Among these, the cathode
material is the key component that limits the energy density of lithium-ion/lithium metal
batteries [
1
–
3
]. Therefore, conversion-type cathode materials are in the spotlight of battery
material researchers because of their high gravimetric and volumetric capacity for lithium-
ion storage [4,5].
The high-energy-density conversion-type cathode materials for lithium batteries can
be divided into three main categories: chalcogens, chalcogenides, and halides. Figure 1
displays how lithium ions react with these cathodes during the conversion-type lithiation,
and they can be reversibly transformed back to their initial states under a rational elec-
trode design. When these conversion-type cathodes are cycled in the cell with a liquid
electrolyte, dissolution of active materials can occur, inducing undesirable shuttle reactions
Molecules 2023,28, 4579. https://doi.org/10.3390/molecules28124579 https://www.mdpi.com/journal/molecules

Molecules 2023,28, 4579 2 of 17
that result in poor cycle life [
5
–
7
]. Chalcogen cathode materials form soluble intermediates
(high-order polysulfides or polyselenides) at the early stage of lithiation and then convert
into lithium sulfide/selenide as the final product upon further discharge [
8
–
10
]. The most
investigated chalcogen cathode material is sulfur, which has been optimized using various
state-of-the-art electrodes and cell architectures in Li–S cells with a liquid electrolyte [
6
,
11
].
Selenium has a much higher electrical conductivity than sulfur, leading to a potentially
better power performance [
12
], but its high material cost and low earth abundance may
hinder its commercial viability [5].
Molecules 2023, 28, x FOR PEER REVIEW 2 of 19
electrolyte, dissolution of active materials can occur, inducing undesirable shule reac-
tions that result in poor cycle life [5–7]. Chalcogen cathode materials form soluble inter-
mediates (high-order polysulfides or polyselenides) at the early stage of lithiation and
then convert into lithium sulfide/selenide as the final product upon further discharge [8–
10]. The most investigated chalcogen cathode material is sulfur, which has been optimized
using various state-of-the-art electrodes and cell architectures in Li–S cells with a liquid
electrolyte [6,11]. Selenium has a much higher electrical conductivity than sulfur, leading
to a potentially beer power performance [12], but its high material cost and low earth
abundance may hinder its commercial viability [5].
Figure 1. Schematic drawing of the lithiation reaction of conversion-type cathodes.
The second category of conversion-type cathodes are chalcogenides, mostly coupled
with transition metals (Fe, Co, Ni, Cu, Mn) [5,13]. Taking metal sulfides as an example, the
cathode compound is converted to metal and lithium sulfide (Li
2
S) after lithiation and vice
versa after delithiation [14–17]. The chalcogenide anions can be replaced by halide anions
as the third type of conversion cathode. Fluoride cathode materials are popular in this
category, forming intermediate lithiated metal fluoride compounds (Li
x
M
y
F
z
), metal, and
lithium fluoride (LiF) instead of Li
2
S after the discharge reactions [18–21]. In fact, FeF
3
has
multistep and mixed intercalation/conversion reactions during lithiation and delithiation
[18,19], which is still included in this review. All three types of conversion cathode mate-
rials must be paired with lithiated anodes, such as metallic lithium, prelithiated graphite,
and prelithiated silicon, to make a full cell, making them relatively difficult to process
compared to traditional lithium-ion baeries. Oxygen cathodes have received considera-
ble aention due to their environmentally friendly nature. This has led to a particular in-
terest in their conversion reaction chemistry, especially when paired with solid-state elec-
trolytes [22–25]. Compared to other solid-state conversion cathodes, oxygen cathodes
have fundamentally different technological barriers due to the gas–solid conversion reac-
tions and catalyst requirements. However, since several review articles have already cov-
ered Li–O
2
baeries, we will not delve into the specifics of this chemistry but will focus
only on chacolgen, chacolgenide, and halide cathodes in this review.
Figure 2 illustrates the advantages of integrating conversion-type cathodes with
solid-state electrolytes in lithium baeries. The solid-state electrolyte can enable high-en-
ergy-density metallic lithium anodes, which are considered to have extremely poor cycla-
bility in liquid electrolytes [8,26–28]. Lithium metal anodes are necessary for the non-lithi-
ated conversion cathodes to be operated in a baery. By combining both high-energy-
Figure 1. Schematic drawing of the lithiation reaction of conversion-type cathodes.
The second category of conversion-type cathodes are chalcogenides, mostly coupled
with transition metals (Fe, Co, Ni, Cu, Mn) [
5
,
13
]. Taking metal sulfides as an example, the
cathode compound is converted to metal and lithium sulfide (Li
2
S) after lithiation and vice
versa after delithiation [
14
–
17
]. The chalcogenide anions can be replaced by halide anions as
the third type of conversion cathode. Fluoride cathode materials are popular in this category,
forming intermediate lithiated metal fluoride compounds (Li
x
M
y
F
z
), metal, and lithium
fluoride (LiF) instead of Li
2
S after the discharge reactions [
18
–
21
]. In fact, FeF
3
has multistep
and mixed intercalation/conversion reactions during lithiation and delithiation [
18
,
19
],
which is still included in this review. All three types of conversion cathode materials
must be paired with lithiated anodes, such as metallic lithium, prelithiated graphite, and
prelithiated silicon, to make a full cell, making them relatively difficult to process compared
to traditional lithium-ion batteries. Oxygen cathodes have received considerable attention
due to their environmentally friendly nature. This has led to a particular interest in their
conversion reaction chemistry, especially when paired with solid-state electrolytes [
22
–
25
].
Compared to other solid-state conversion cathodes, oxygen cathodes have fundamentally
different technological barriers due to the gas–solid conversion reactions and catalyst
requirements. However, since several review articles have already covered Li–O
2
batteries,
we will not delve into the specifics of this chemistry but will focus only on chacolgen,
chacolgenide, and halide cathodes in this review.
Figure 2illustrates the advantages of integrating conversion-type cathodes with solid-
state electrolytes in lithium batteries. The solid-state electrolyte can enable high-energy-
density metallic lithium anodes, which are considered to have extremely poor cyclability
in liquid electrolytes [
8
,
26
–
28
]. Lithium metal anodes are necessary for the non-lithiated
conversion cathodes to be operated in a battery. By combining both high-energy-density
electrodes, the battery can offer ~two times higher specific energy calculated based at the cell
level [
5
]. By utilizing solid-state electrolytes instead of flammable organic liquid electrolyte,

Molecules 2023,28, 4579 3 of 17
the fire hazards of the battery resulting from the low flash point of liquid electrolytes
and the failure of low-melting-point polyolefin separators can be eliminated [
29
–
34
]. In
addition, the absence of solvents in the cathode region can significantly solve the active
material dissolution problem [
35
–
37
]. The notorious shuttle effect in Li–S batteries, caused
by the migration of soluble polysulfides, can also be excluded in a solid-state battery [
38
,
39
].
However, the dissolution of polysulfide intermediates is required to achieve high power
density and high active material utilization in Li–S batteries [
40
,
41
], which will be discussed
later in the case of the solid-state configuration.
Molecules 2023, 28, x FOR PEER REVIEW 3 of 19
density electrodes, the baery can offer ~two times higher specific energy calculated based
at the cell level [5]. By utilizing solid-state electrolytes instead of flammable organic liquid
electrolyte, the fire hazards of the baery resulting from the low flash point of liquid elec-
trolytes and the failure of low-melting-point polyolefin separators can be eliminated [29–
34]. In addition, the absence of solvents in the cathode region can significantly solve the
active material dissolution problem [35–37]. The notorious shule effect in Li–S baeries,
caused by the migration of soluble polysulfides, can also be excluded in a solid-state bat-
tery [38,39]. However, the dissolution of polysulfide intermediates is required to achieve
high power density and high active material utilization in Li–S baeries [40,41], which
will be discussed later in the case of the solid-state configuration.
Figure 2. Schematic illustration of the advantages of using conversion-type cathodes in lithium
solid-state baeries.
2. Challenges Remained in Conversion-Type Cathode Materials
Although conversion-type cathode materials have tremendous potential in terms of
high gravimetric capacity, safe operating voltage, scalable synthesis routes, etc., there are
still several obstacles to the commercialization of these cathode systems. In Li–S cells, poor
electrical and ionic conductivities of sulfur and significant cathode volume change (from
sulfur to lithium sulfide) can be addressed by intelligent 3D structural design, which is
mainly enabled by carbonaceous materials and other functional materials that can accom-
modate and trap sulfur species [6,11,42]. Nevertheless, despite efforts to mitigate the rapid
capacity degradation caused by the loss of soluble polysulfide intermediates, this problem
cannot be completely solved. Moreover, there are currently no commercially viable solu-
tions to the poor reversibility of the lithium metal anode in liquid electrolytes. In this re-
gard, the integration of solid-state electrolyte into chalcogen cathode lithium baeries
seems to be a rational strategy to mitigate the dissolution of active species in the traditional
lithium baery with a liquid electrolyte [39,43,44].
For metal sulfide and metal fluoride cathode materials, their common processing
problem is that they are both sensitive to moisture [45–47], which shares the same chal-
lenge with many solid-state electrolyte systems [48–52]. High electrical resistance and
sluggish reaction kinetics of sulfide and fluoride materials are the main causes of their
large voltage hysteresis in lithium baeries [4,5,7,18,53]. In addition, a non-uniform cath-
ode–electrolyte interface (CEI) may be generated by the catalytic reaction during cycling,
Figure 2.
Schematic illustration of the advantages of using conversion-type cathodes in lithium
solid-state batteries.
2. Challenges Remained in Conversion-Type Cathode Materials
Although conversion-type cathode materials have tremendous potential in terms
of high gravimetric capacity, safe operating voltage, scalable synthesis routes, etc., there
are still several obstacles to the commercialization of these cathode systems. In Li–S
cells, poor electrical and ionic conductivities of sulfur and significant cathode volume
change (from sulfur to lithium sulfide) can be addressed by intelligent 3D structural design,
which is mainly enabled by carbonaceous materials and other functional materials that can
accommodate and trap sulfur species [
6
,
11
,
42
]. Nevertheless, despite efforts to mitigate
the rapid capacity degradation caused by the loss of soluble polysulfide intermediates,
this problem cannot be completely solved. Moreover, there are currently no commercially
viable solutions to the poor reversibility of the lithium metal anode in liquid electrolytes. In
this regard, the integration of solid-state electrolyte into chalcogen cathode lithium batteries
seems to be a rational strategy to mitigate the dissolution of active species in the traditional
lithium battery with a liquid electrolyte [39,43,44].
For metal sulfide and metal fluoride cathode materials, their common processing
problem is that they are both sensitive to moisture [
45
–
47
], which shares the same challenge
with many solid-state electrolyte systems [
48
–
52
]. High electrical resistance and sluggish
reaction kinetics of sulfide and fluoride materials are the main causes of their large voltage
hysteresis in lithium batteries [
4
,
5
,
7
,
18
,
53
]. In addition, a non-uniform cathode–electrolyte
interface (CEI) may be generated by the catalytic reaction during cycling, which is not strong
enough to withstand the volume expansion of conversion-type cathodes [
53
,
54
]. The use of
solid-state battery design can reduce the formation of unstable solid–electrolyte interface
(SEI) at either the anode or the cathode (i.e., CEI) made from decomposed electrolyte

Molecules 2023,28, 4579 4 of 17
components, which can improve the cycling stability of the battery with a chalcogenide
or halide cathode. Other challenges, such as high overpotentials, gas generation from
decomposed electrolyte ingredients, and safety concerns, also need to be addressed before
the conversion-type cathodes can be implemented in commercial batteries [55–57].
3. Solid-State Lithium Battery with Conversion-Type Cathodes
3.1. Chalcogen Cathodes with Solid-State Electrolytes
The most popular chalcogen cathode material, sulfur, is attractive due to its high
theoretical energy density, low cost, and environmental friendliness [
6
,
11
]. However, Li–S
batteries with a liquid electrolyte have been limited by several challenges, such as short
cycle life and poor rate performance [
58
–
60
]. Figure 3a exhibits the typical charge/discharge
profiles of a Li–S cell adopting a liquid electrolyte. Two distinct discharge plateaus represent
the polysulfide dissolution reactions (upper plateau; S
8→
Li
2
S
4
; ~2.3 V) and solid-state
reactions (lower plateau; Li
2
S
4→
Li
2
S; ~2.1 V) [
10
,
40
,
61
,
62
]. In contrast, the solid-state Li–S
battery shows only a sloping curve for the reaction of sulfur converting to lithium sulfide
(Figure 3b). The same behavior can also be found in the liquid-phase carbonate electrolytes
because carbonates cannot dissolve polysulfide intermediates [
63
–
65
]. The slow kinetics
resulting from the solid-state reaction mechanism occurring in Li–S solid-state batteries
may hinder their practicality due to sluggish diffusion and limited interfacial contact area,
which can severely degrade their performance.
To reduce the charge-transfer resistance, similar to the strategy adopted in other solid-
state batteries, sulfur or lithium sulfide active materials must be blended with solid-state
electrolyte particles in the cathode to achieve better utilization [
66
–
69
].
Xu et al. demonstrated a well-mixed cathode consisting of reduced graphene oxide coated
with sulfur (rGO@S)/acetylene black (AB)/Li
10
GeP
2
S
12
(LGPS) via long-duration ball-
milling (Figure 3c), delivering an impressive reversible capacity of 830 mA h g
−1
after
750 cycles at a rate of 1 C and 60
◦
C [
66
]. The incorporation of rGO@S nanocomposite
into the LGPS-AB matrix results in a homogeneous distribution of the composite cathode,
which facilitates uniform volume changes during lithiation/delithiation. The high cathode
uniformity significantly reduces stress and strain within the solid-state cells, thereby pro-
longing their cycle life. Additionally, to address the issue of bulky solid-state electrolyte,
Wang et al. developed a cathode-supported solid-state electrolyte configuration shown
in Figure 3d, which not only reduces the ion diffusion distance between the anode and
cathode but also significantly enhances the energy density of the solid-state Li–S battery
(370.6 W h kg
−1
) [
67
]. The cathode/electrolyte/anode laminated structure was accom-
plished by using a stainless steel mesh-supported Li
2
S cathode as a starting point, followed
by adding a robust Kevlar nonwoven scaffold-reinforced Li
3
PS
4
(LPS) electrolyte as the top
layer, with a thickness of approximately 100 µm and a metallic lithium anode [67].
One alternative approach to addressing the interfacial challenges between the cath-
ode and solid-state electrolyte is to adopt hybrid electrolyte systems. Cui et al. uti-
lized Li
7
La
3
Zr
2
O
12
(LLZO) nanoparticles-filled poly(ethylene oxide) (PEO) polymer elec-
trolyte in solid-state Li–S batteries (Figure 3e) to achieve an outstanding specific capacity
of >900 mA h g
–1
at human body temperature of 37
◦
C [
68
]. The remarkable electrochemical
performance is attributed to the composite cathode and solid-state electrolyte where the
LLZO nanoparticle serves both as a filler to enhance ion conductivity and as an interfacial
stabilizer to mitigate interfacial resistance. Efficient ion transport in solid-state batteries
depends on low interfacial resistance, which can facilitate electrochemical reactions and
reduce the barrier for ions to cross the heterogeneous solid-state electrolyte/electrode
interface [
70
–
72
]. On the other hand, PEO offers reasonable mechanical stability, good
electrode compatibility, and excellent film-forming properties for the composite solid-state
electrolyte [
73
,
74
]. Polymer electrolytes can fill in the interparticle voids generated at the
cathode and solid-state electrolyte regions, effectively promoting interfacial wetting and
enabling stable cycling of lithium electrodeposition and electrostripping at relatively low
overpotentials [
75
–
77
]. Figure 3f shows another Li–S battery configuration adopting a

Molecules 2023,28, 4579 5 of 17
hybrid electrolyte system, including a sodium (Na) super ionic conductor (NaSICON)-type
solid electrolyte (Li
1+x
Y
x
Zr
2−x
(PO
4
)
3
(LYZP) (x = 0–0.15)) and a liquid electrolyte [
78
,
79
].
By integrating a solid electrolyte with a liquid electrolyte, the hybrid dual-electrolyte Li–S
battery exhibits significantly improved cyclability compared to conventional Li–S batteries
that utilize a porous polymer separator immersed with a liquid-phase electrolyte [
78
,
80
].
Here, polysulfides can still dissolve in the liquid electrolyte like a catholyte and perform
two-step electrochemical reactions to guarantee high active material utilization and rea-
sonable rate performance. In summary, the solid electrolyte membrane demonstrates
promising characteristics such as reasonable Li
+
ion conductivity, superior polysulfide
retention, chemical compatibility with cell components, and electrochemical stability under
repeated charge-discharge conditions in Li–S cells [78,80].
Molecules 2023, 28, x FOR PEER REVIEW 5 of 19
electrode compatibility, and excellent film-forming properties for the composite solid-
state electrolyte [73,74]. Polymer electrolytes can fill in the interparticle voids generated at
the cathode and solid-state electrolyte regions, effectively promoting interfacial weing
and enabling stable cycling of lithium electrodeposition and electrostripping at relatively
low overpotentials [75–77]. Figure 3f shows another Li–S baery configuration adopting
a hybrid electrolyte system, including a sodium (Na) super ionic conductor (NaSICON)-
type solid electrolyte (Li
1+x
Y
x
Zr
2−x
(PO
4
)
3
(LYZP) (x = 0–0.15)) and a liquid electrolyte [78,79].
By integrating a solid electrolyte with a liquid electrolyte, the hybrid dual-electrolyte Li–
S baery exhibits significantly improved cyclability compared to conventional Li–S bat-
teries that utilize a porous polymer separator immersed with a liquid-phase electrolyte
[78,80]. Here, polysulfides can still dissolve in the liquid electrolyte like a catholyte and
perform two-step electrochemical reactions to guarantee high active material utilization
and reasonable rate performance. In summary, the solid electrolyte membrane demon-
strates promising characteristics such as reasonable Li
+
ion conductivity, superior poly-
sulfide retention, chemical compatibility with cell components, and electrochemical sta-
bility under repeated charge-discharge conditions in Li–S cells [78,80].
Figure 3. The charge/discharge voltage curves of a Li–S baery with (a) an ether-based liquid elec-
trolyte and (b) a carbonate-based liquid electrolyte or a solid-state electrolyte. Reproduced with
permission from ref. [61]. Copyright 2020, Royal Society of Chemistry (London, UK). (c) Schematic
drawing of an all-solid-state lithium-sulfur baery with a well-mixed sulfur cathode. Reproduced
with permission from ref. [66]. Copyright 2017, Wiley-VCH (Weinheim, Germany). (d) Schematic
drawing of a cathode-supported and Kevlar fiber-reinforced all-solid-state Li−Li
2
S cell. Reproduced
with permission from ref. [67]. Copyright 2019, ACS Publications (Washington, DC, USA). (e) Sche-
matic drawing of an all-solid-state Li–S baery with a hybrid LLZO/PEO electrolyte system. Repro-
duced with permission from ref. [68]. Copyright 2017, ACS Publications (Washington, DC, USA). (f)
Schematic drawing of a hybrid Li || LY ZP || Li
2
S
6
cell with the liquid electrolyte on both sides of the
LYZP membrane. Reproduced with permission from ref. [78]. Copyright 2016, Wiley-VCH (Wein-
heim, Germany).
As a cathode active material, the insulating nature of sulfur is always problematic
during electrochemical reactions, especially in solid-state mechanisms. Much effort has
been expended in the past to improve the contact between sulfur species and carbon sub-
strates for beer cyclability [81–85]. To improve the contact between the conductive car-
bon substrate and the sulfur, a simple heating process can be used. This involves coating
the carbon surface uniformly with elemental sulfur, which promotes surface-to-surface
contact. As the sulfur melts during the heating process and then solidifies after cooling, it
forms a thin, uniform layer that adheres to the carbon surface. This layer facilitates the
Figure 3.
The charge/discharge voltage curves of a Li–S battery with (
a
) an ether-based liquid
electrolyte and (
b
) a carbonate-based liquid electrolyte or a solid-state electrolyte. Reproduced with
permission from ref. [
61
]. Copyright 2020, Royal Society of Chemistry (London, UK). (
c
) Schematic
drawing of an all-solid-state lithium-sulfur battery with a well-mixed sulfur cathode. Reproduced
with permission from ref. [
66
]. Copyright 2017, Wiley-VCH (Weinheim, Germany). (
d
) Schematic
drawing of a cathode-supported and Kevlar fiber-reinforced all-solid-state Li
−
Li
2
S cell. Repro-
duced with permission from ref. [
67
]. Copyright 2019, ACS Publications (Washington, DC, USA).
(
e
) Schematic drawing of an all-solid-state Li–S battery with a hybrid LLZO/PEO electrolyte sys-
tem. Reproduced with permission from ref. [
68
]. Copyright 2017, ACS Publications (Washington,
DC, USA). (
f
) Schematic drawing of a hybrid Li||LYZP||Li
2
S
6
cell with the liquid electrolyte on
both sides of the LYZP membrane. Reproduced with permission from ref. [
78
]. Copyright 2016,
Wiley-VCH (Weinheim, Germany).
As a cathode active material, the insulating nature of sulfur is always problematic
during electrochemical reactions, especially in solid-state mechanisms. Much effort has
been expended in the past to improve the contact between sulfur species and carbon
substrates for better cyclability [
81
–
85
]. To improve the contact between the conductive
carbon substrate and the sulfur, a simple heating process can be used. This involves coating
the carbon surface uniformly with elemental sulfur, which promotes surface-to-surface
contact. As the sulfur melts during the heating process and then solidifies after cooling,
it forms a thin, uniform layer that adheres to the carbon surface. This layer facilitates
the transfer of electrons between the carbon and sulfur, leading to more efficient active
material utilization [
86
]. Substituting selenium for sulfur is a common method used to
improve the intrinsic electrical conductivity of chalcogen cathode materials. Figure 4a
shows an all-solid-state Li–Se battery configuration with LPS as the electrolyte [
87
]. The
use of selenium in the cathode provides high electrical conductivity (1
×
10
−3
S cm
−1
),

Molecules 2023,28, 4579 6 of 17
while a high Li
+
conductivity of 1.4
×
10
−5
S cm
−1
is achieved across the Se-LPS interface.
This battery has a high reversible capacity of 652 mA h g
−1
(96% of theoretical capacity)
and exhibits favorable capacity retention during cycling. Kumar et al. also showcased the
functionality of a high-temperature molten Li–Se battery cell with a garnet-type solid-state
electrolyte (LLZO) operating at 465
◦
C (Figure 4b), which is essential for powering future
space exploration missions [
88
]. The cells demonstrated a stable open-circuit voltage for
17 h and were subjected to electrochemical cycling at various current rates. Since selenium
is a relatively high-cost material, introducing selenium into sulfur cathodes through the
formation of SeS
x
solid solutions could be an alternative means to modify the electrical
and ionic conductivities of the cathode without a significant increase in material price [
89
].
Figure 4c illustrates the solid-state cell configuration with SeS
x
cathode, sulfide-based solid
electrolyte (LPS or LGPS), and Li metal. The use of SeS
2
in high loading cells has been
found to achieve an ultrahigh areal capacity of up to 12.6 mA h cm
−2
[
61
,
89
]. The SeS
x
solid
solution cathode confirms the importance of cathode ionic and electrical conductivities in
determining electrochemical performance.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 19
Figure 4. (a) Schematic drawing of an all-solid-state Li–Se baery. Reproduced with permission
from ref. [87]. Copyright 2018, Royal Society of Chemistry (London, UK). (b) Cell configuration of
the molten Li–Se cell tested at 465 °C. Reproduced with permission from ref. [88]. Copyright 2021,
ACS Publications (Washington, DC, USA). (c) Schematic drawing of a Li–SeS
x
solid-state baery.
Reproduced with permission from ref. [61]. Copyright 2020, Royal Society of Chemistry (London,
UK). (d) Schematic drawing of an Li-In || LPSCB || LPSCB-MWCNTs cell with a monolithic structure.
(e) Schematic drawing of the LPSCB-MWCNTs composite cathode. Reproduced with permission
from ref. [90]. Copyright 2021, Wiley-VCH (Weinheim, Germany).
3.2. Chalcogenide Cathodes with Solid-State Electrolytes
Pyrite (FeS
2
) shows promise as an electrode material for lithium-ion baeries due to
its natural abundance, low cost (commercialized by Energizer, St. Louis, MO, USA), non-
toxicity, and ultrahigh theoretical energy density of 1313 W h kg
−1
[93–95]. Moreover, re-
cent studies have reported improved electrochemical properties in all-solid-state second-
ary Li/FeS
2
baeries [96–99]. Yang et al. investigated the usage of FeS
2
as a dopant for
Li
7
P
3
S
11
-type glass-ceramic electrolytes, which has been shown to enhance ionic conduc-
tivity while reducing interfacial resistance between the FeS
2
cathode and electrolyte (Fig-
ure 5a,b) [100]. This design was different from previous reports where a conventional FeS
2
cathode was doped into the sulfide electrolyte by a suitable proportion (99.5(70Li
2
S–
30P
2
S
5
)–0.5FeS
2
) and the largest crystallinity was obtained, boosting the ionic conductivity
of the electrolyte [100]. The FeS
2
composite cathode and sulfide electrolyte had similar
chemical potential, resulting in lower interfacial resistance and superior cycling stability.
Although all-solid-state baeries have the capability to support reversible four-lithium-
ion storage for FeS
2
(FeS
2
+ 4 Li = Fe + 2 Li
2
S), issues such as strain/stress concentration
resulting in electrode pulverization and sluggish electrochemical reactions between lith-
ium sulfide and sulfur can impact the long-term cycling stability of the baery [101]. Fig-
ure 5c represents an approach to utilize the loose-structured Co
0.1
Fe
0.9
S
2
-based all-solid-
state lithium baeries that have been optimized via nanoengineering to achieve impres-
sive electrochemical performance. After initial charging to 3.0 V, it was observed that the
element cobalt had an extremely homogeneous distribution with iron and sulfur, and no
Fe/S aggregation was detected [101]. This indicates that cobalt has a catalytic effect on the
electrochemical reaction, which improves the reaction between Li
2
S and Fe. Moreover,
previous studies have reported that transition metals and their metal sulfides can effec-
tively enhance the redox reaction kinetics and reduce the shule effect in lithium−sulfur
Figure 4.
(
a
) Schematic drawing of an all-solid-state Li–Se battery. Reproduced with permission
from ref. [
87
]. Copyright 2018, Royal Society of Chemistry (London, UK). (
b
) Cell configuration of
the molten Li–Se cell tested at 465
◦
C. Reproduced with permission from ref. [
88
]. Copyright 2021,
ACS Publications (Washington, DC, USA). (
c
) Schematic drawing of a Li–SeS
x
solid-state battery.
Reproduced with permission from ref. [
61
]. Copyright 2020, Royal Society of Chemistry (London,
UK). (
d
) Schematic drawing of an Li-In||LPSCB||LPSCB-MWCNTs cell with a monolithic structure.
(
e
) Schematic drawing of the LPSCB-MWCNTs composite cathode. Reproduced with permission
from ref. [90]. Copyright 2021, Wiley-VCH (Weinheim, Germany).
In addition to the high ionic conductivity of sulfide-based solid-state electrolyte,
lithium argyrodite sulfide (Li
6
PS
5
Cl
0.5
Br
0.5
, LPSCB) offers the advantage of serving a
bifunctional role as both a solid electrolyte and a precursor material for the chalcogen
cathode [
90
,
91
]. In Figure 4d, a monolithic cell configuration that uses the LPSCB ma-
terial all over the electrolyte and cathode region was designed, and the LPSCB in the
cathode region, along with multiwall carbon nanotubes (MWCNTs), forms a multiphase
conversion-type cathode by partial decomposition during the first discharge. Meanwhile,
the remaining LPSCB electrolyte remains unchanged and provides low-impedance ionic
transport pathways, which enhances the cathode performance. The discharge capacity of
the all-solid-state cell exhibits an initial sharp incline, followed by a gradual increase as

Molecules 2023,28, 4579 7 of 17
the MWCNT content is increased [
90
]. This observation suggests that the active material
formation occurs exclusively at the interface between the MWCNTs and the electrolyte par-
ticles, as illustrated in Figure 4e [
90
,
92
]. This finding paves the way for the development of
high-performance all-solid-state batteries using thiophosphate solid electrolytes where the
high cycling stability can be attributed to the intimate contact between the electrochemically
reduced cathode and electrolyte interface.
3.2. Chalcogenide Cathodes with Solid-State Electrolytes
Pyrite (FeS
2
) shows promise as an electrode material for lithium-ion batteries due
to its natural abundance, low cost (commercialized by Energizer, St. Louis, MO, USA),
non-toxicity, and ultrahigh theoretical energy density of 1313 W h kg
−1
[
93
–
95
]. More-
over, recent studies have reported improved electrochemical properties in all-solid-state
secondary Li/FeS
2
batteries [
96
–
99
]. Yang et al. investigated the usage of FeS
2
as a
dopant for Li
7
P
3
S
11
-type glass-ceramic electrolytes, which has been shown to enhance
ionic conductivity while reducing interfacial resistance between the FeS
2
cathode and
electrolyte (Figure 5a,b) [
100
]. This design was different from previous reports where a
conventional FeS2cathode was doped into the sulfide electrolyte by a suitable proportion
(99.5(70Li
2
S–30P
2
S
5
)–0.5FeS
2
) and the largest crystallinity was obtained, boosting the
ionic conductivity of the electrolyte [
100
]. The FeS
2
composite cathode and sulfide elec-
trolyte had similar chemical potential, resulting in lower interfacial resistance and su-
perior cycling stability. Although all-solid-state batteries have the capability to support
reversible four-lithium-ion storage for FeS
2
(FeS
2
+ 4 Li = Fe + 2 Li
2
S), issues such as
strain/stress concentration resulting in electrode pulverization and sluggish electrochemi-
cal reactions between lithium sulfide and sulfur can impact the long-term cycling stability
of the battery [
101
]. Figure 5c represents an approach to utilize the loose-structured
Co
0.1
Fe
0.9
S
2
-based all-solid-state lithium batteries that have been optimized via nanoengi-
neering to achieve impressive electrochemical performance. After initial charging to 3.0 V,
it was observed that the element cobalt had an extremely homogeneous distribution with
iron and sulfur, and no Fe/S aggregation was detected [
101
]. This indicates that cobalt has
a catalytic effect on the electrochemical reaction, which improves the reaction between Li
2
S
and Fe. Moreover, previous studies have reported that transition metals and their metal
sulfides can effectively enhance the redox reaction kinetics and reduce the shuttle effect in
lithium
−
sulfur batteries. This is attributed to the catalytic properties of these materials and
their strong ability to absorb polysulfides [102,103].
Certain chalcogenide materials, including vanadium sulfide (VS
2
) and titanium sulfide
(TiS
2
), exhibit exceptional electrical conductivity, making them suitable for use as efficient
electrode materials [
104
–
107
]. Figure 5d demonstrates a high-performance solid-state
battery with a VS
2
/S nanocomposite cathode, which combines intercalation-type vanadium
sulfide with conversion-type sulfur chemistry [
108
]. A facile, low-cost, and low-energy
mechanical blending process was implemented to prepare the S/VS
2
/LPS composite
cathode [
108
]. The VS
2
nanomaterial has a layered structure with fast Li-ion transport
channels, metallic conductivity, and extra capacity contribution, providing an ideal platform
for the solid-state S/Li
2
S redox couple to achieve its high gravimetric capacity. A similar
idea was adopted by Jung et al., demonstrating that controlled ball-milling of a sulfide-
based active material (TiS
2
) and a solid-state electrolyte (LPS) for the electrode led to a
significant increase in capacity in all-solid-state lithium batteries without compromising the
ionic and electronic conduction pathways [
109
]. The increased Li
+
ion storage is believed to
be associated with the formation of an amorphous Li–Ti–P–S phase during the controlled
ball-milling process. The aforementioned structural designs indicate that utilizing a mixed
ion/electron conductive transition metal sulfide and an active cathode material in an all-
solid-state cell configuration is a promising strategy for the development of next-generation
solid-state batteries.

Molecules 2023,28, 4579 8 of 17
Molecules 2023, 28, x FOR PEER REVIEW 9 of 19
Figure 5. Schematic drawing of an all-solid-state lithium baery (a) without cathode-doped electro-
lyte and (b) with cathode-doped electrolyte. Reproduced with permission from ref. [100]. Copyright
2020, Elsevier (Amsterdam, The Netherlands). (c) Schematic drawing of the redox reactions of FeS
2
and Co
0.1
Fe
0.9
S
2
cathodes. Reproduced with permission from ref. [101]. Copyright 2019, ACS Publi-
cations (Washington, DC, USA). (d) Schematic drawing of the solid-state hybrid Li–S/VS
2
/LPS bat-
tery. Reproduced with permission from ref. [108]. Copyright 2021, Wiley-VCH (Weinheim, Ger-
many). (e) Schematic drawing of the solid-state hybrid Li–TiS
2
/LPS baery. Reproduced with per-
mission from ref. [109]. Copyright 2014, Springer (Berlin/Heidelberg, Germany).
3.3. Halide Cathodes with Solid-State Electrolytes
Although intercalation-type oxide cathodes and conversion-type chalcogen/chalco-
genide cathodes have high capacity, their voltage output is relatively low, limiting their
potential to achieve high energy density as positive electrodes. However, early studies
have shown that metal fluorides can be used in the conversion process, enabling higher
voltage materials through the use of nanomaterials and composites (as shown in Figure
6a) [110–112]. The voltage of these materials is about 1 V higher than that of chalco-
gens/chalcogenides. Metal fluorides thus offer a promising means towards achieving both
specific and volumetric energy densities that greatly surpass the theoretical limits of cur-
rent positive electrode materials (lithium cobalt oxides, lithium nickel manganese cobalt
oxides, lithium nickel cobalt aluminum oxides, and lithium iron phosphates) [113–115].
Halides have an intrinsic advantage of exhibiting extraordinarily specific and volumetric
Figure 5.
Schematic drawing of an all-solid-state lithium battery (
a
) without cathode-doped elec-
trolyte and (
b
) with cathode-doped electrolyte. Reproduced with permission from ref. [
100
].
Copyright 2020, Elsevier (Amsterdam, The Netherlands). (
c
) Schematic drawing of the redox reac-
tions of FeS
2
and Co
0.1
Fe
0.9
S
2
cathodes. Reproduced with permission from ref. [
101
]. Copyright
2019, ACS Publications (Washington, DC, USA). (
d
) Schematic drawing of the solid-state hybrid
Li–S/VS
2
/LPS battery. Reproduced with permission from ref. [
108
]. Copyright 2021, Wiley-VCH
(Weinheim, Germany). (
e
) Schematic drawing of the solid-state hybrid Li–TiS
2
/LPS battery. Repro-
duced with permission from ref. [109]. Copyright 2014, Springer (Berlin/Heidelberg, Germany).
3.3. Halide Cathodes with Solid-State Electrolytes
Although intercalation-type oxide cathodes and conversion-type chalcogen/chalcogenide
cathodes have high capacity, their voltage output is relatively low, limiting their potential to
achieve high energy density as positive electrodes. However, early studies have shown that
metal fluorides can be used in the conversion process, enabling higher voltage materials
through the use of nanomaterials and composites (as shown in Figure 6a) [
110
–
112
]. The
voltage of these materials is about 1 V higher than that of chalcogens/chalcogenides. Metal
fluorides thus offer a promising means towards achieving both specific and volumetric
energy densities that greatly surpass the theoretical limits of current positive electrode ma-
terials (lithium cobalt oxides, lithium nickel manganese cobalt oxides, lithium nickel cobalt

Molecules 2023,28, 4579 9 of 17
aluminum oxides, and lithium iron phosphates) [
113
–
115
]. Halides have an intrinsic advan-
tage of exhibiting extraordinarily specific and volumetric energy density while operating
at moderate voltages compared to those of commercial intercalation-type cathodes. This
allows for the use of various solid and liquid electrolytes that are not stable at the higher
voltages (over 4 V) used with traditional positive electrodes. In addition, previous reports
suggested that the lack of active oxygen in halides leads to improved safety compared to
conventional layered oxides [116].
Halide cathodes, like other conversion-type cathode materials, face agglomeration chal-
lenges that can lead to increased electrical/ionic resistance, ultimately deteriorating battery
performance [
117
–
119
]. Numerous efforts have been dedicated to developing carbon/metal
fluoride nanocomposites in order to mitigate active material aggregation [
120
–
124
]. Another
promising approach to prevent agglomeration is through the use of surfactants [
125
,
126
].
The aforementioned drawback of increased impedance in halide cathodes can be exacer-
bated when they are combined with a solid-state electrolyte, as this introduces additional
solid–solid interfacial resistance and potential solid-state reactions. When two materials
come into contact, the difference in their standard chemical potentials drives the flow of
free ions across their interface, which could potentially lead to redox reactions [
127
]. To the
best of our knowledge, no prior research has been conducted on the interfacial reactions
between the solid-state electrolyte and halide cathode surface up until now. In the case of
the interface between Li
1.3
Al
0.3
Ti
1.7
(PO
4
)
3
(LATP) and Li metal, this flow is influenced by
the fact that the Li chemical potential of LATP (
−
4.3 eV) is lower than that of Li metal (0 eV).
As a result, Li
+
ions may transfer from Li metal to LATP upon contact, while electrons are
injected from Li metal into the Ti 3d unoccupied orbital in LATP, resulting in the reduction
of Ti
4+
to Ti
3+
, followed by structural cracks (Figure 6b) [
128
,
129
]. Li et al. developed a
sericin protein (SP) buffer layer with the confined ionic liquid, which can assist in minimiz-
ing the contact between ionic liquid (IL) electrolyte and Li metal, improve the solidification
of ionic liquid, and ensure a homogenous dispersion of Li-ion flux at the anode/solid-state
electrolyte interface. Consequently, it promotes smooth Li deposition and mitigates side
reactions that may occur. Additionally, the intermolecular interaction between TFSI
-
anions
and sericin-chain can also alleviate the reduction of free TFSI
−
by Li metal, which prevents
the fast accumulation of SEI and evident passivation of symmetric cells [
128
]. The use of
the sericin protein film protected LATP solid-state electrolyte has been shown to facilitate
the successful reversible operation of Li/FeF3conversion solid-state batteries.
Table 1summarizes the new cell configurations discussed in this review. Operating
temperatures play a critical role in the practical applications of solid-state batteries. Most of
the reported battery systems can be operated at room temperature or near body temperature.
However, there are a few exceptions. The amorphous sulfur-coated reduced graphene
cathode paired with a sulfide-based solid-state electrolyte requires heating to 60
◦
C. Despite
the elevated temperature, this cell shows excellent performance in terms of capacity and
cycle life at a high rate of 1 C [
66
]. On the other hand, a Li–Se battery designed for
space applications must be operated at a much higher temperature of 465
◦
C [
88
]. As for
chalcogenide and halide cathode systems, their cycle life in solid-state batteries is still under
development, and their rate performance is not yet satisfactory. In conclusion, while most
solid-state batteries can operate effectively at moderate temperatures, certain configurations
require higher operating temperatures. However, chalcogenide and halide cathode systems
require further development to improve their cycle life and rate performance.

Molecules 2023,28, 4579 10 of 17
Molecules 2023, 28, x FOR PEER REVIEW 11 of 19
Figure 6. (a) Schematic drawing of the crystallographic reaction mechanism that occurs during the
discharge of metal fluoride electrodes. Reproduced with permission from ref. [116]. Copyright 2007,
Elsevier (Amsterdam, The Netherlands). (b) Schematic drawing of interface environment of Li/IL-
LATP (left) and Li/IL@SP-LATP (right) during cycling process. Reproduced with permission from
ref. [128]. Copyright 2022, Elsevier (Amsterdam, The Netherlands).
Table 1 summarizes the new cell configurations discussed in this review. Operating
temperatures play a critical role in the practical applications of solid-state baeries. Most
of the reported baery systems can be operated at room temperature or near body tem-
perature. However, there are a few exceptions. The amorphous sulfur-coated reduced gra-
phene cathode paired with a sulfide-based solid-state electrolyte requires heating to 60
°C. Despite the elevated temperature, this cell shows excellent performance in terms of
capacity and cycle life at a high rate of 1 C [66]. On the other hand, a Li–Se baery designed
for space applications must be operated at a much higher temperature of 465 °C [88]. As
for chalcogenide and halide cathode systems, their cycle life in solid-state baeries is still
under development, and their rate performance is not yet satisfactory. In conclusion,
while most solid-state baeries can operate effectively at moderate temperatures, certain
configurations require higher operating temperatures. However, chalcogenide and halide
cathode systems require further development to improve their cycle life and rate perfor-
mance.
Figure 6.
(
a
) Schematic drawing of the crystallographic reaction mechanism that occurs during
the discharge of metal fluoride electrodes. Reproduced with permission from ref. [
116
]. Copyright
2007, Elsevier (Amsterdam, The Netherlands). (
b
) Schematic drawing of interface environment of
Li/IL-LATP (
left
) and Li/IL@SP-LATP (
right
) during cycling process. Reproduced with permission
from ref. [128]. Copyright 2022, Elsevier (Amsterdam, The Netherlands).
Table 1. Summary of the solid-state battery configurations with various conversion-type cathodes.
Cell Configuration Temp. Capacity Rate Cycle
Life Feature Ref.
Chalcogen Cathode
rGO@SkLi10GeP2S12 k75Li2S/24P2S5/1P2O5kLi 60 ◦C830 mAh g−11 C 750 The conformal S coating minimizing
interface resistance & stress/strain [66]
80Li2S/20LiI+LPSkLPS+KevlarkLi 25 ◦C537.8 mAh g−10.2 C 100 Thick cathode-supported all-solid-state
lithium batteries [67]
S@LLZO@CkPEO-LiClO4kLi 37 ◦C>900 mAh g–1 N/A 90 A LLZO nanoparticle-decorated porous
carbon foam for high S utilization [68]
Li2S6kLYZPkLi 25 ◦C≈1000 mAh g−10.2 C 150 A NaSICON solid-electrolyte/separator
suppressing polysulfide crossover [78]
Se+Li3PS4kLi3PS4kLi or LiSn alloy 25 ◦C652 mAh g−150 mA g−1100 The Se cathode improving charge transfer
in solid-state batteries [87]
SekLi7La3Zr2O12kLi 465 ◦C824 mAh g−130 mA g−1N/A A high-temperature molten Li-Se battery
for stable OCV and cyclability [88]
SeS2kLi10GeP2S12 +Li3PS4kLi 25 ◦C1100 mAh g−150 mA g−1100 SeSxsolid solutions introduced into S
cathode for enhanced utilization [89]
LPSCB-MWCNTskLi6PS5Cl0.5Br0.5kLi-In 25 ◦C12.56 mAh cm−2≈0.7 C 1030 The electrochemically decomposed LPSCB
forming a multiphase cathode [90]

Molecules 2023,28, 4579 11 of 17
Table 1. Cont.
Cell Configuration Temp. Capacity Rate Cycle
Life Feature Ref.
Chalcogenide Cathode
FeS2k99.5(70Li2S/30P2S5)/0.5FeS2kLi–In 25 ◦C543 mAh g−10.03 mA cm−220 A FeS2-doped solid electrolyte lowering
interfacial resistance [100]
Co0.1Fe0.9 S2kLi10GeP2S12 /75Li2S/24P2S5/1P2O5kLi N/A 685.8 mAh g−1500 mA g–1 100 The catalytic cobalt in FeS2cathode
enhancing electrochemical activity [101]
S+VS2+Li3PS4kLi3PS4kLi-In 25 ◦C7.8 mAh cm−20.12 mA cm−2200 The hybrid S/VS2cathode achieving high
sulfur utilization [108]
TiS2+75Li2S/25P2S5k75Li2S/25P2S5kLi0.5In 30 ◦C837 mAh g−150 mA g−160 An amorphous Li-Ti-P-S phase offering
increased capacity [109]
Halide Cathode
FeF3kIL@SPF+LATPkLi 25 ◦C524.3 mAh g−10.1 C 100 A conformal sericin protein film stabilizing
the Li-LATP interface [128]
4. Summary and Outlook
In a traditional lithium battery configuration with a conversion-type cathode and a
liquid electrolyte, there are several scenarios that can lead to battery failure, as shown
in Figure 7. On the anode side, during repeated cycling, dendritic lithium can form in
the liquid electrolyte, potentially penetrating the separator and causing a short circuit
(Figure 7a). On the cathode side, some conversion-type cathode materials can dissolve in
the electrolyte during redox reactions, resulting in capacity loss and cathode structural
instability (Figure 7b). The soluble active material species may further shuttle to the lithium
metal anode, leading to rapid degradation. Brittle and fragile CEI could also form in
conversion-type cathode systems, which would fracture and thicken further due to cathode
swelling and shrinkage (Figure 7c). A thick and damaged CEI will significantly increase
the impedance and thereby deactivate the cathode. The above challenges can be solved
by introducing a solid electrolyte to replace the liquid electrolyte. Undoubtedly, lithium
dendrite formation can be greatly suppressed, and the solid-state electrolyte can also act as
a rigid separator to prevent short-circuiting. In addition, both the dissolution of the cathode
active material and the formation of the CEI layer can be suppressed in the solid-state
battery, so a stable cathode structure can be achieved.
Molecules 2023, 28, x FOR PEER REVIEW 13 of 19
electron contact resistance can be high (green arrow in Figure 7d). Second, the movement
of lithium ions occurs through the channels established by the solid electrolyte, and the
presence of any gaps between the electrolyte and cathode material can potentially render
the cathode inactive or impair its efficiency (white gap in Figure 7d). Moreover, the tran-
sition metals present in chalcogenide and halide cathode materials can act as catalysts that
degrade the solid-state electrolyte at the interface, resulting in unstable redox reactions
(blue interface in Figure 7d). As a result, the challenges of interfacial instability, volume
change, and chemical incompatibility should be effectively addressed through smart cell
configuration and meticulous electrode preparation.
Figure 7. The failure mechanisms of conversion-type cathodes in a liquid electrolyte: (a) lithium
dendrite formation on the anode, (b) cathode active material dissolution, and (c) fractured cathode
electrolyte interface. (d) Schematic drawing of possible interfacial conditions in the cathode region
of a solid-state lithium baery with a conversion-type cathode.
The development of commercial solid-state baeries is still a long way off, and solid-
state baeries with conversion-type cathodes will require extensive efforts to become
practical. However, solid-state Li–S cells have emerged as a promising advanced baery
system due to the elimination of polysulfide dissolution and have aracted tremendous
aention in recent years. Although there is not much work on solid-state baery design
focusing on sulfide or fluoride cathodes, both of these cathode systems integrated into
solid-state baeries have the potential to achieve high energy density if carefully de-
signed. A rationally designed cell configuration can effectively improve the utilization of
active materials, rate performance, and cycle life, which requires continuous research and
development efforts in the future.
Author Contributions: K.-C.C., J.-K.C. and Y.-S.S. drafted this review paper together and partici-
pated in the discussion and editing of the draft. All authors have read and agreed to the published
version of the manuscript.
Funding: This research was funded by the National Science and Technology Council (NSTC) of Tai-
wan (110-2113-M-A49-027-MY2) and by the Ministry of Education (MOE) of Taiwan under the
Yushan Young Fellow Program.
Data Availability Statement: All collected data are presented in the manuscript.
Figure 7.
The failure mechanisms of conversion-type cathodes in a liquid electrolyte: (
a
) lithium
dendrite formation on the anode, (
b
) cathode active material dissolution, and (
c
) fractured cathode
electrolyte interface. (
d
) Schematic drawing of possible interfacial conditions in the cathode region of
a solid-state lithium battery with a conversion-type cathode.

Molecules 2023,28, 4579 12 of 17
While the solid-state electrolyte system appears to resolve several critical problems
associated with the conversion-type cathodes in lithium batteries, there may be several
concomitant problems that come with its implementation. First of all, in case the attachment
between the carbon black and the cathode material surface is weak, the resulting electron
contact resistance can be high (green arrow in Figure 7d). Second, the movement of lithium
ions occurs through the channels established by the solid electrolyte, and the presence of
any gaps between the electrolyte and cathode material can potentially render the cathode
inactive or impair its efficiency (white gap in Figure 7d). Moreover, the transition metals
present in chalcogenide and halide cathode materials can act as catalysts that degrade the
solid-state electrolyte at the interface, resulting in unstable redox reactions (blue interface
in Figure 7d). As a result, the challenges of interfacial instability, volume change, and
chemical incompatibility should be effectively addressed through smart cell configuration
and meticulous electrode preparation.
The development of commercial solid-state batteries is still a long way off, and solid-
state batteries with conversion-type cathodes will require extensive efforts to become
practical. However, solid-state Li–S cells have emerged as a promising advanced battery
system due to the elimination of polysulfide dissolution and have attracted tremendous
attention in recent years. Although there is not much work on solid-state battery design
focusing on sulfide or fluoride cathodes, both of these cathode systems integrated into solid-
state batteries have the potential to achieve high energy density if carefully designed. A
rationally designed cell configuration can effectively improve the utilization of active mate-
rials, rate performance, and cycle life, which requires continuous research and development
efforts in the future.
Author Contributions:
K.-C.C., J.-K.C. and Y.-S.S. drafted this review paper together and participated
in the discussion and editing of the draft. All authors have read and agreed to the published version
of the manuscript.
Funding:
This research was funded by the National Science and Technology Council (NSTC) of
Taiwan (110-2113-M-A49-027-MY2) and by the Ministry of Education (MOE) of Taiwan under the
Yushan Young Fellow Program.
Data Availability Statement: All collected data are presented in the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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