Understanding and unveiling the electro‐chemo‐mechanical behavior in solid‐state batteries

Solid‐state batteries (SSBs) are attracting growing interest as long‐lasting, thermally resilient, and high‐safe energy storage systems. As an emerging area of battery chemistry, there are many issues with SSBs, including strongly reductive lithium anodes, oxidized cathodes (state of charge), the thermodynamic stability limits of solid‐state electrolytes (SSEs), and the ubiquitous and critical interfaces. In this Review, we provided an overview of the main obstacles in the development of SSBs, such as the lithium anode|SSEs interface, the cathode|SSEs interface, lithium‐ion transport in the SSEs, and the root origin of lithium intrusions, as well as the safety issues caused by the dendrites. Understanding and overcoming these obstacles are crucial but also extremely challenging as the localized and buried nature of the intimate contact between electrode and SSEs makes direct detection difficult. We reviewed advanced characterization techniques and discussed the complex ion/electron‐transport mechanism that have been plaguing electrochemists. Finally, we focused on studying and revealing the coupled electro‐chemo‐mechanical behavior occurring in the lithium anode, cathode, SSEs, and beyond.


INTRODUCTION
Due to the incorporation of desirable solid-state electrolyte (SSE) with mechanical resistance and its ability to resist potential dendrite growth, the standard graphite anode of conventional lithium-ion batteries (LIBs) is expected to be replaced by a lithium metal anode, which could increase the theoretical specific capacity by more than 10 times (lithium 3860 mAh g −1 vs. graphite 372 mAh g −1 ). 1 Taking the advantages of high energy density for metallic Li, solidstate cells configuration containing Li metal anode and SSEs (either inorganic, organic, or composite electrolyte) have strong potential to deliver improved safety, accompanying longer cycle lives, and wider range of operating temperature compared with liquid-based LIBs.The increasing pursuit of high-safety electrochemical storage technologies will compel an unescapable shift from conventional lithium batteries to solid-state batteries (SSBs). 2 SSBs function by a similar mechanism to commercial liquid-based (rechargeable) batteries, which are charged and discharged by transporting lithium ions between the anode and cathode.However, as the liquid electrolyte is replaced by SSE, the solid-solid contact between the diverse components in the SSB presents more complex interfacial issues, [3][4][5][6][7][8][9][10] and therefore the development of SSBs is greatly hindered.In order to unveil the underlying science and engineering challenges, it is critical to study and track how lithium ions are transported and distributed in SSBs under working conditions.Nevertheless, as the lightest metal element, methods and techniques for quantifying lithium during battery operation are limited to the extent that deep insights into lithium transport has not been gained.
Next-generation lithium-metal SSBs with theoretically high-energy densities are highly competitive, but its commercial application is currently hampered by safety issues arising from dendrite growth at the anode sides on the one hand, 1,11,12 and the inevitable chemical/electrochemical reactions 13 and mechanical failure 14 at the cathode sides on the other.At the anode/electrolyte sides, lithium dendrite penetration is one of the main failure modes in SSBs and limits the electroplating rate during battery charging. 15,16Studying and understanding the interface between SSE and lithium metal are considered critical, which influences dendrite growth and determines SSBs performance.The critical current density (CCD), which F I G U R E 1 The electro-chemo-mechanical behaviors occurring in the solid-state electrolytes, and composite cathodes, and the corresponding interfacial modification strategies.
is the current density at which the dendrite penetrates an SSE, is an important guideline in the study of the nature of the lithium metal, the electrolyte, and their interface.Furthermore, stack pressure of SSBs plays a vital role on the CCD, thus detailed experimental conditions (e.g., lithium metal thickness and stacking pressure) need to be reported in order to accurately interpret the data. 179][20] In addition, severe mechanical failure and volume changes are triggered by the repeated insertion and de-insertion of lithium ions.The expansion and contraction of the active particles could cause microstructural changes, which inevitably leads to internal stresses on the SSE due to the intimate contact characteristic of the components.
In this Review, we present a comprehensive overview of the interface issues and challenges that dramatically affect SSB performance.We propose that the study of Li-ion transport behavior can aid in revealing the root causes of dendrite nucleation at the anode|SSEs interface and side reactions at the cathode|SSEs interface (Figure 1).In addition, we summarize the research progress of advanced characterization techniques in revealing the failure behavior of SSBs.In particular, we focus on how these characterization techniques can shed light on complex electro-chemo-mechanical behavior of the anode|SSEs interface and cathode|SSEs interface, as well as the solid electrolytes.By digging out the complicated ion transport mechanisms closely associated with interfaces, it is expected to provide a reasonable solution for the construction of functional interfaces to suppress lithium dendrites and side reactions and achieve performing SSB.

DENDRITES AND INTERFACES CHALLENGES
On the anode side, it is generally considered preferable to use lithium metal anodes for SSBs in order to achieve higher energy densities than commercial LIBs.Despite its many advantages of good workability, nonflammability, zero leakage, wide operation temperatures, and mechanical resistance, 1,21 the SSE cannot completely overcome the safety issues arising from lithium dendrites growth.The investigation on Li dendrites triggered by the interfacial behavior remain insufficient compared to the extensive research work on SSEs development.Inhomogeneous electric field distribution resulting from poor interfacial contact can further promote dendritic deposition.Uneven distribution of the electric field due to poor interface con-tact can drive dendrite deposition and further lead to rapid short-circuiting of SSBs.
The interaction of lithium dendrites with mechanical deformation of SSE has attracted growing interest since the pioneering work of Monroe and Newman. 22,23They predicted that SSEs even with a small shear modulus of 8 GPa could mechanically inhibit lithium dendrites, because the electrochemical potential of lithium is stress dependent.Nevertheless, one of the most reported oxide-type SSE, Li 7 La 3 Zr 2 O 12 , although having a high shear modulus as high as approximately 60 GPa, 24,25 still fails to avoid dendrites at a current density of less than 1 mA cm −2 , 26 which is even lower than the case when using liquid electrolytes.This unexpected result not only contradicts the general understanding that SSBs are safer than liquidelectrolyte batteries, but also puts the initial development of inorganic SSEs in jeopardy. 1Theoretical models, which typically assume bulk and homogeneous material properties, fail to capture the technical challenges in the complex electrochemical behavior of SSEs, leading to some results that deviate from reality.These technical challenges urgently require advanced characterization techniques to be explored and unlocked.The growth of lithium dendrites is closely associated with three main mechanisms, pores/voids formation, [27][28][29][30] fracture mechanics, 17,[31][32][33] and electronic conductivity. 17,34he inherent morphological instability of the SSE and the metal electrode is a universal phenomenon that can be observed in SSB system, regardless of the specific chemistry.The difference is in the origin of this instability and its effect on charge transport.Krauskopf et al. reported that the contact loss is caused by vacancy accumulation and the resulting pore formation at the interface (Figure 2A), 27 which further increases the interfacial resistance and impedes ionic transport.Direct observation of hidden and buried solid-solid interfaces presents significant challenges.Lu et al. constructed a solid-state void nucleation and growth model, revealing the interfacial Li void evolution behaviors (Figure 2B). 28Under the high current density, the "void bubbles" accumulated and formed rapidly.They also observed this morphological feature and evolution processes using plasma-focused ion beam-equipped scanning electron microscope (PFIB-SEM).Vishnugopi et al. studied the correlation between contact evolution and the electrochemical-mechanical behaviors. 29They argued that the inhomogeneity of the external pressure and temperature fields results in reaction heterogeneity and further leads to contact loss.Identifying the CCD for pore formation is extremely important, which can cause uneven current distribution and dendrite generation.Kasemchainan et al. found that external pressure applied to the cell governs the CCD and pores formation, as well as dendrite growth. 30 I G U R E 2 (A) The schematic of dissolution process and contact loss.Copyright, 2019 American Chemical Society. 27(B) The site energy evolution after void injection and the phase diagram of the void evolution.Copyright, 2022 American Association for the Advancement of Science. 28pressively, Wang et al. designed a mechanical testing platform (Figure 3A) and innovatively utilized "critical stack pressure" to correlate electrochemical and mechanical behavior of lithium and SSE. 35It is well known that during the lithium-stripping process, pores form, with a consequent increase in resistance and potential polarization, which greatly limits high-power-density SSB.The formation of pores in lithium anode during electrochemical stripping and inhomogeneous Li-ion transport at the Li|SSEs interface may be responsible for dendrite nucleation. 22,36,37Wang et al. provided a practical framework for how to avoid pore formation and decrease internal cell resistance, projecting a pathway to understanding the Li|SSEs interface and designing a highly performing SSB. 35Impressively, a nominal external pressure is expected to mitigate the formation of pores during lithium stripping, further affecting the CCD. 31 Furthermore, the formation and evolution of lithium dendrites, which are closely related to the mechanical fracture of the SSEs, remain challenging to study in-depth due to the closed system nature of SSBs and the close contact of each component.Many reports have indicated that dendrites were more likely to occur at the defects of the solid-state electrodes and electrolytes (e.g., cracks, impu-rities, grain boundaries, and porosity). 31,38,39CCD closely correlated with dendrites formation is mainly influenced by the chemical, electrochemical, and mechanical stability at the interface.Thus, a detailed dynamic process related to Li-transport and lithium dendrites growth at the Li|SSEs interface needs to be unveiled.
The root cause of dendrite formation has been the subject of debate and is thought to be more complicated because of its localized and buried nature.In SSB chemical systems, interface presents a huge influence on dendrite nucleation and growth (Figure 3B), and therefore characterization techniques need to be extended to different length scales to track the dynamic behavior at the Li|SSEs interface.Using advanced neutron depth profiling, Han et al. used advanced neutron depth profiling to monitor the dynamic evolution of lithium concentration profiles in three representative SSEs (LiPON, LLZO, and Li 3 PS 4 ) during charging, 34 revealing the root cause of dendrite formation.Their findings showed that the high electronic conductivity of LLZO and Li 3 PS 4 is the main reason for the nucleation and growth of dendrite, further leading to a short circuit.However, McConohy et al. argued that the combination of current focusing and the presence of nanoscale cracks is responsible for the lithium intrusion F I G U R E 3 (A) Mechanics of lithium/SSEs interface and the role of the stacking pressure in SSBs design.Copyright, 2019 Elsevier Inc. 35 (B) The schematic of the widespread interface and the formation of dendrites.The bulk LGPS in Li/LGPS/Li cell after cycling (C1) and in Li/LGPS-LiMg 22 /Li cell before cycling (C2) and after cycling (C3).Copyright, 2021 American Chemical Society. 31to the solid electrolytes, rather than the well-known electronic leakage or electrochemical reduction. 40By employing operando SEM microprobe, they observed that a high mechanical loading significantly increased the probability of intrusion events, inducing more lithium intrusions and whiskers, and that the propagation behavior of lithium intrusions is expected to be mechanically controlled.In addition, the authors emphasize that the nanoscale defects triggering dendrite nucleation are difficult to observe with conventional microscopy, and thus more advanced characterization techniques are desperately needed to elucidate the complex coupled electro-chemo-mechanical behavior at the Li|SSEs interface.
Dendritic problem that hinders the commercialization of SSBs essentially stems from inhomogeneous lithium ion transport at the interface.On this basis, a potential solution to inhibit dendrite growth by improving dynamic transport using a functional interface is proposed.Specifically, sulfide-type Li 10 GeP 2 S 12 (LGPS) electrolyte with a high ionic conductivity of up to 12 mS cm −1 is thermodynamically unstable with the lithium anode, forming a mixed conductive species (Li 2 S-Li 3 P-Li x Ge), further leading to continuous dendrite growth and the SSE's fracture (Figure 3C1). 31It should be noted that interfacial engineering strategies are expected to make the application of Li-metal anodes possible.The LiTFSI-Mg(TFSI) 2 -DME liquid electrolyte is added between the lithium anode and the LGPS electrolyte (TFSI = bis(trifluoromethanesulfonyl)imide; DME = dimethoxyethane) to form a composite layer rich in Li x Mg alloy and LiF through the reduction of salts and solvent, where the lithium can be guided to preferentially deposit at the interface between Li x Mg and LiF during charging.As a result, the treated LGPS electrolyte showed no cracks before and after cycling (Figure 3C2,3), and the CCD was increased to 1.3 mA cm −2 , which contrasts with Figure 3C1.Furthermore, Huo et al. proposed a flexible electron-blocking interfacial shield (EBS) to achieve uniform interfacial contact and prevent dendritic deposits.The authors prepared Li-inserted PAA (LiPPA) by reacting polyacrylic acid (PAA) polymer with molten Li at 250 • C at the interface between LLZTO electrolyte and lithium anode.Such LiPPA composite is both flexible and electronically shielded, exhibiting a CCD up to 1.2 mA cm −2 at 25 • C, and effectively accommodating volumetric dynamic changes and dendrite growth of lithium anode during electrochemical cycling (Figure 4A1). 41wever, it has been arduous to realize acceptable long cycle lives for practical integration owing to the limitations of available interlayers.3][44][45] Therefore, a mechanically robust interlayer capable of undergoing repeated lithium plating/stripping is desperately needed.Strikingly, Kim et al. innovatively proposed crystalline-direction-controlled carbon material to achieve isotropic Li-ion transport and to guide the direction of lithium nucleation to the current collector (Figure 4B). 46These electrically conductive interlayers (optimized amorphous carbon) allow for preferential lithium deposition between the Cu collector inter-layer and the interlayer without destructing the already existing interlayer/LLZO interface, presenting significant physical and chemical stability (Figure 4C).The authors also revealed that the crystal orientation of the carbon domains is crucial for the homogenization of the Li-ion flux through the interlayers.The lithium-metal SSB with this carbon interlayer exhibits excellent room-temperature electrochemical performance, providing 4.0 mAh cm −2 at 2.5 mA cm −2 with no significant reduction in capacity. 46his compelling discovery promises to open up new frontiers in the development of SSB by providing key strategies to ensure a highly stable interface between the lithium metal and the oxide-based electrolyte.Spencer-Jolly et al. constructed Ag-carbon composite interlayers to hinder the growth of lithium dendrites. 47On charging, lithium electrochemically intercalates into graphite, and then F I G U R E 5 (A) x-Ray tomography of the LLZO/Au/Li interface after cycling.Copyright, 2023 American Chemical Society. 48(B) Schematics of the three-diemnsional (3D) PZL/Li interfaces formation.Copyright, 2021 Elsevier Inc. 49 chemically reacting with Ag to form Li-Ag alloys.These alloys lead to more homogeneous Li distribution and deposition.Similarly, Thenuwara et al. designed Li-Au alloys interlayers to increase the wettability of Li anode with LLZO electrolytes, mitigating void formation and realizing stable long-term cycling. 48The authors used advanced xray imaging to show the morphology of the gold interlayer without destroying any electrode structure after cycling (Figure 5A).In addition, three-dimensional (3D) alloy skeleton can be used as interlayers to improve uniform Li-ion transport and impede volume expansion.A porous Zn layer is constructed on LLZTO surface, and lithium metal in situ reacts with it to form a 3D Li-Zn alloy interface (Figure 5B), which can not only suppress the volume changes, but also reduce the interfacial impedance. 49

INTERFACES WITHIN COMPOSITE CATHODES
Chemo-mechanical effects play a critical role at the interface between the active material and the SSEs within composite electrodes.Cathodes are generally composed of active particles surrounded by solid-ion-conducting phase, providing an abundant lithium ion transport pathway for the active materials.Due to the insertion and removal of Li + , active particles may undergo mechanical deformation during the electrochemical reaction.1][52] Additionally, these interfaces could be electrochemically/chemically unstable, giving rise to interfacial phases with increased electrochemical impedance.Understanding and controlling the chemo-mechanical evolution of interfaces within the composite cathode with the aid of advanced characterization technique is a key driver for rechargeable SSBs with long cycling life.
The solid-phase reaction between electrolyte and cathode active materials (CAMs) would unavoidably occur at the interface, especially for the sulfide-based electrolyte.Mutual diffusion of chemical element (Co and P) rooted from LiCoO 2 (LCO) cathode and Li 2 S-P 2 S 5 (LPS) electrolyte is visualized, and thereby form an interfacial layer with the thickness of up to 100 nm (Figure 6A). 20his unexpected interfacial layer leads to a large interfacial resistance, which greatly hinders the diffusion of Li + across the interface.Based on the first-principles calculations, using a density functional theory (DFT+U) treatment, Co (LCO) and P (LPS) diffuse into each other due to highly favorable thermodynamics. 53The coatings, such as LiNbO 3 (LNO) and Li 2 SiO 3 , are critical to inhibit the interdiffusion of Co and P cations, as the strong bond between the Si (or Nb) cation and the O anion prevents the interdiffusion of the Nb and P elements.This desirable result was later experimentally verified by transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS), in which both Co and P distributions were effectively tracked at the interface with the presence of protective coatings. 20In addition, Neumann et al. performed x-ray tomography to study interfacial effects between Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 and β-Li 3 PS 4 , showing that tortuosity and structural inhomogeneities lead to capacity losses (Figure 6B). 54

F I G U R E 7 (A)
The schematic image of an ultrasound imaging method to investigate the SSB interfaces.Copyright, 2022 American Chemical Society. 57(B) Electro-chemo-mechanical degradation in LLZTO-based ASSBs.Copyright, 2023 American Chemical Society. 50ectro-chemo-mechanical degradation at the cath-ode|SSEs interface has been experimentally demonstrated in composite cathodes.During the charging process, various SSEs are exposed to relatively high-voltage CAM, and undesired oxidation occurs, 55,56 leading to the interlayer formation and high impedance.It is therefore highly likely that the performance degradation originates from these chemically unstable interfaces and impedances.Koerver et al. investigated the interface behavior using EIS and xray photoemission spectroscopy (XPS) and confirmed that the irreversible initial cycling capacity loss was caused by the decomposition of the sulfide-based electrolyte (β-Li 3 PS 4 ). 19Beyond the SSE decomposition, the contraction of the active material LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM 811) upon delithiation can isolate CAMs from SSEs (Figure 6C), further blocking ion diffusion pathways and capacity loss.This work sheds light on the important impact of (electro-)chemo-mechanical effects on SSBs.It should be noted that mechanical disassemble for postern analysis may trigger the contact loss between SSEs and CAMs.
Therefore, non-destructive and spatial inspection of buried interfaces is essential.Huo and coworkers performed operando ultrasonic imaging to track gas generation and interfacial degradation in LiCoO 2 /SSE/Li solidstate pouch cell (Figure 7A). 57The SSE used here consists of the polymer and Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) particles.They proposed that the increase in overpotential and impedance during long-term cycling is mainly due to the growth of the interlayers rather than the interfacial pores caused by contact loss.Han et al. constructed SSBs based on full oxide garnet-type LLZTO using LiCoO 2 and LiNi 1/3 Co 1/3 Mn 1/3 O 2 CAMs.They revealed that the volume changes in these CAMs is constrained by the rigid LLZTO framework, eventually resulting in mechanical failure.This mechanical failure is closely ralated to strain accumulation of the CAMs.Therefore, to construct well-functioning SSBs, a rational design between com-monly used CAMs and SSEs that combines their chemical and mechanical compatibility is desperately needed (Figure 7B). 50dvanced x-ray imaging and its associated applications show the distinct advantage of visualizing the chemomechanical degradation of composite cathodes in a way that has been described as "what you see is what you get". 1 However, it cannot directly map Li-ion diffusion pathways.Kobayashi et al. reported that Li-ion transport in SSB could be quantitatively probed and tracked during battery function with thermal neutron source and lithium-6 as tracer. 58They proposed that lithium ions in the Li 3 PO 4 SSE are not transported uniformly throughout the electrolyte (Figure 8A), but follow a vacancy migration mechanism (Figure 8B).However, these experiments were performed under specific conditions using thin film samples, thus it is not clear whether there is a generalized mechanism of vacancy migration for Li-ion transport in thick SSEs, or whether there are competing processes.
Considering the human rights issues associated with its mining, the use of cobalt must be completely eliminated in order to realize socially responsible batteries. 59Cronk et al. first applied LiFePO 4 (LFP) to the field of chlorideand sulfide-based SSB and studied the compatibility of LFP particles with two typical types of SSEs, Li 6 PS 5 Cl (LPSCl) and Li 2 ZrCl 6 (LZC). 18Structural and electrochemical tests revealed that LPSCl decomposes into S, LiCl, and Li-P 2 S x at 2.3 V and 2.7 V, corresponding to two oxidation peaks (Figure 9A1).In contrast to LPSCl electrolyte, LZC begins to oxidize and decompose at 4 V (Figure 9A2), forming Cl 2 (g) and ZrCl 4 .The electrolyte voltage window shows the low oxidative stability of LPSCl electrolyte, while LZC electrolyte is highly stable at 4 V, beyond the operation voltage of LFP, making it an ideal SSE material for this solid-state system.In addition to electrochemical stability, good contact between LFP particles and SSE powders is necessary to confirm Li-ion transport and reduce interfacial resistance.
F I G U R E 8 (A) Illustration of the depth profile of each cell in each layer before cycling, and the depth distribution of each element in each layer after 1-2 h of charging. 58(B) The migration path model in the Li 3 PO 4 solid electrolyte includes the original distribution of Li-6 and Li-7, the final state distribution after uniform migration, and the equivalent distribution for the migration model in selected regions, where the change in the number of Li-6 along the position is represented.Copyright, 2022 Wiley. 58od electrolyte deformability is regarded as a key factor, which requires high densification under manufacturing pressure to produce minimal voids and better connectivity of solid-ion-conducting phase throughout the cathode composite (Figure 9B).Cronk et al. studied the densification properties and the porosity of LPSCl and LZC electrolyte under a typical cathode stack pressure of 375 MPa, using cross-section images (Figure 9C).Compared to LPSCl showing clear voids (Figure 9C1), LZC electrolyte with high density surfaces is observed (Figure 9C2).Under the pressure of 375 MPa, the relative density of the LZC electrolyte is high at 94.7%, higher than the 85.5% of LPSCl (Figure 9C3).High electrochemical stability and mechanical deformability ensure a physically and chemically stable interface and good interfacial contact, achieving excellent cycling performance of 1000 cycles (80% retention) at 1C. 18 The application of the amorphous oxides coatings is expected to inhibit parasitic reactions while reducing the interfacial resistance by optimizing the contact between the cathode particles and the SSEs powders. 20Whatever the material is, any coatings employed on CAMs should be thin enough to avoid negative influences on battery impedance.Engineering a functional interface may be a powerful solution to enable SSBs.In addition, the construction of 3D cathodes [60][61][62] can greatly optimize cathode/electrolyte interface and shorten Li + transport distance, thus improving the rate performance of SSBs.

F I G U R E 9
The stability window of (A1) LPSCl and (A2) LZC electrolyte. 18(B) Schematic representation of the applied pressure to the SSB.Cross-sectional FIB-SEM of cold-pressed pellets, with SEM insets at 10 μm scale, reveal (C1) LPSCl and (C2) LZC electrolytes, and (C3) the relationship between fabrication pressure and density for both electrolytes.Copyright, 2023 American Chemical Society. 18

THERMAL SAFETY OF THE SSBS
Although the high safety of SSBs is generally attributed to the non-flammability of the SSE, 63,64 66 Impressively, Li 4 SnS 4 had good air stability, but was less thermally stable with lithium metal, Li-Sn alloys are thought to be the thermodynamic driving force, leading to thermal runaway.In addition to the thermal runaway caused by the anode side (SSE/lithium), the possible thermal runaway caused by the cathode side needs to be widely explored. 67,68Kim et al. reported that the composite cathode composed of NCM 811 and LPSCl caught fire when charged to ≥4.4 V even in an Ar-filled glovebox at 150 • C.This is mainly due to the violent reaction between oxygen from the NCM cathodes and LPSCl SSEs, forming phosphorus sulfide, lithium sulfide, sulfur dioxide gas, and so forth. 68

CONCLUSION AND OUTLOOK
Understanding and controlling localized solid-sate interface are thought to be a major scientific challenges in the field of SSB chemistry.Fundamentally, both structural/chemical evolution of the electrode materials and mechanical characteristics of the SSE play a crucial role in the stability of such ubiquitous interface.Defects in the electrode and SSE, such as cracks, impurities, grain boundaries, and porosity, can cause inhomogeneous distribution of the electric field and further drive dendrite deposition.These defects may be related to or originate from the mechanical damage.Furthermore, some of the SSEs become chemically/electrochemically reduced in contact with highly reductive lithium metal, forming an unwanted interphase and leading to an increase in impedance.Dendrite formation can be attributed to the effects of solid-state lithium and SSE mechanics on lithium ion transport at the surface/interface during cell cycling.Similar to the lithium anode, the CAMs may also react electrochemically and chemically with SSE, forming insulating products that further enhance the electrochemical impedance and severely degrade SSB performance.Additionally, dramatic volume changes due to the expansion and contraction of the CAMs could cause mechanical microcracks in composite electrode or SSE, which inevitably leads to contact loss and increased impedance.In order to gain fundamental insights into the electro-chemo-mechanical behaviors, powerful in situ/operando characterization techniques and the associated applications are highly desirable.The characterization of electrodes, SSEs, and their interfaces is fundamental to our understanding on SSBs.Advanced characterization techniques can aid in answering various questions regarding dendrite growth, such as how lithium plates upon electrochemical cycling, what is the exact relationship between the deformation of SSE and the plate of lithium, and how electrodeposited lithium evolves during cell cycling. 1,691][72] In addition, the combination of artificial intelligence and x-ray imaging can be leveraged to develop predictive models to analyze the effect of electrode microstructure on battery performance, further advancing the design, discovery, and optimization of battery materials. 71,73nly a few materials can unite all the advantages: good ionic-electronic conductivity, high redox potential, structural/chemical stability, and environmental sustainability, 59,74 all of which are critical to practical integration.Achieving high-performance cathodes therefore cannot be limited to the purely chemical design of new phases, but needs to involve chemical composition tuning and particle structure/morphology (shape and size) optimization, as well as surface functionalization.In summary, time-spatial-resolved advanced characterization techniques, materials science, and promising interfacial engineering strategies are expected to be the ultimate solution to all the issues and challenges confronted by SSB.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

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I G U R E 4 (A1) SEM cross-sectional image of the Li/LLZTO@EBS interface.(A2) Schematic demonstration of Li dendrite growth at the pristine Li/LLZTO, LLZTO@Au/Li, and LLZTO@EBS/Li interface.Copyright, 2021 Huo et al.This is an open access article distributed under the terms of the Creative Commons Attribution License. 41(B) Computation of interfacial energy based on the position of lithium plating and interlayer material.(C) SEM and EDS images of the LLZO/anode interfaces after Li deposition through the interlayer of amorphous carbon with pre-existing Li metal at 25 • C. Copyright, 2022 American Chemical Society.

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I G U R E 6 (A1) The EDX mapping of Co element in the vicinity of the LiCoO 2 electrode/Li 2 S-P 2 S 5 interface and (A2) cross-sectional HAADF-STEM image of the LiCoO 2 /Li 2 S-P 2 S 5 interface after first charging and corresponding EDX line profiles of Co, P, and S. Copyright, 2010 American Chemical Society. 20(B) 3D microstructure-resolved visualization of Li + concentration and current density distribution at Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 and β-Li 3 PS 4 interface.Copyright, 2020 American Chemical Society. 54(C) SEM images of the NCM 811and β-Li 3 PS 4 interfaces with uncycled, after first charge to 4.3 V and 50 full cell cycles.Copyright, 2017 American Chemical Society.
This work was supported by the Talent Scientific Research Project of Qilu University of Technology (grant number 2023RCKY181), Natural Science Foundation of Shandong Province Youth Project (File nos.ZR2022QB178, ZR2020QB197), National Natural Science Foundation of China (Nos.52272136, 22108135), Natural Science Foundation of Jiangsu province (BK20221402), Special Support of China Postdoctoral Science Founudation (2023T160471), and Basic Research Project of Science, Education and Production Integration Pilot Project (2022PY053).
65 3 PS 4 , Li 6 PS 5 Cl, and Li 10 SnP 2 S 12 ) SSBs system, illustrating that the pristine Li 10 SnP 2 S 12 /Li interface exhibited thermal runaway even in the case of uncycling, which severely intensified with cycling.65Wuet al. studied a variety of sulfide-based SSEs, and their interfacial thermal stability was in the order Li 6 PS 5 Cl > Li 3 PS 4 > Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 > Li 4 SnS 4 > Li 7 P 3 S 11 .
the underlying mechanisms regarding thermal behavior at high temperatures and thermal stability of SSE systems need to be further explored.Vishnugop et al. used accelerating rate calorimetry (ARC) to determine the correlation between the thermal stability and interfacial characteristics at the SSE/Li side in sulfide-based (