Safer solid-state lithium metal batteries: Mechanisms and strategies

Abstract

Solid-state batteries that employ solid-state electrolytes (SSEs) to replace routine liquid electrolytes are considered to be one of the most promising solutions for achieving high-safety lithium metal batteries. SSEs with high mechanical modulus, thermal stability, and non-flammability can not only inhibit the growth of lithium dendrites but also enhance the safety of lithium metal batteries. However, several internal materials/electrodes-related thermal hazards demonstrated by recent works show that solid-state lithium metal batteries (SSLMBs) are not impenetrable. Therefore, understanding the potential thermal hazards of SSLMBs is critical for their more secure and widespread applications. In this contribution, we provide a comprehensive overview of the thermal failure mechanism of SSLMBs from materials to devices. Also, strategies to improve the thermal safety performance of SSLMBs are included from the view of material enhancement, battery design, and external management. Consequently, the future directions are further provided. We hope that this work can shed bright insights into the path of constructing energy storage devices with high energy density and safety.

INTRODUCTION

High-energy-density and safe energy storage devices are an urged need for the continuous development of the economy and society.^1–4 Lithium (Li) metal with the ultrahigh theoretical specific capacity (3860 mAh g⁻¹) and the lowest electrode potential (−3.04 V vs. standard hydrogen electrode) is considered an excellent candidate to replace graphite anodes in traditional Li-ion batteries.^5–8 Nevertheless, the high reactivity of Li metal and its nonuniform deposition/stripping bring great hidden danger to the application of Li metal anode.^9,10 Specifically, the Li dendrite propagation can aggravate the side reactions between Li metal and electrolytes and even lead to internal short circuit, resulting in the violent release of heat and finally the thermal runaway of batteries, which once caused a recall of Li metal batteries (LMBs) by Moli company in the 1980s.^11–13 Meanwhile, the flammability of conventional organic liquid electrolytes also increases the risk of battery fire and explosion. To be worse, when LMBs are applied in large-scale energy storage, such as grid storage, the cluster effect of batteries will exacerbate the thermal safety concerns further.^14–16

The coupling of solid-state electrolytes (SSEs) and Li metal anode has been regarded as one of the notable strategies to simultaneously improve the energy density and safety of LMBs in the last decade.^17–21 Compared with traditional organic electrolytes, inorganic SSEs are expected to avoid safety accidents due to their non-flammability, high thermal and electrochemical stability.^22–26 In general, an ideal SSE in intrinsically safe solid-state Li metal batteries (SSLMBs) should meet the following requirements besides the indispensably high Li-ion conductivity: (i) High thermal stability. The melting or decomposition of SSEs at high temperature will cause heat release and even the occurrence of internal short circuits; (ii) High chemical stability. The thermodynamical instability of SSEs with anode/cathode materials can induce chemical reactions, which easily cause heat accumulation and thereby thermal runaway of a single battery. (iii) Wide electrochemical stable window. The reduction and/or oxidation decomposition of SSEs can lead to electrode/electrolyte interface degradation and even the overcharging of batteries. At the same time, the accumulated side reaction products also increase the impedance of the battery and cause the elevation of ohmic heat; (iv) High mechanical strength. Enhanced mechanical modulus of SSEs can effectively prevent Li dendrite propagation as well as external mechanical penetration through SSEs. Despite unprecedented successes in SSEs, constructing an intrinsically safe SSLMB remains challenging. On the one hand, there is still a gap between cutting-edge SSEs and perfect SSEs, which makes current SSEs difficult to meet the requirements of high-performance SSLMBs. On the other hand, the safety of SSLMBs involves not only the stability of individual components but also the compatibility of SSEs with other electrode materials. Besides, the crosstalk reactions between the anode and cathode also induce an increase in security risks and the elusive safety failure mechanism of SSLMBs.^27–29

In general, the battery thermal runway usually begins with the heat release of side reactions between electrode materials.^30–32 The continuous exothermic reactions can lead to the accumulation of heat. When the battery temperature gradually rises, a series of chain reactions will be triggered, such as the decomposition of solid electrolyte interphase (SEI), the intensification of reactions between Li metal and electrolyte, and the release of oxygen from the cathode. Further, all these chemical reactions follow the Arrhenius equation, which will be accelerated with increasing temperature. Eventually, the heat generation rate is much higher than heat dissipation, leading to an irreversible thermal runaway. In addition, the growth of Li dendrite may give rise to internal short circuits, which will directly cause the violent release of battery energy rapidly.^33,34 In the meantime, the drastically accumulated heat in a single cell will lead to the heat spread to neighboring cells through heat conduction and radiation, resulting in a continuous thermal runaway diffusion process among packs. Consequently, the thermal runaway of batteries is continuous and chain processes originate from materials to cells and finally to devices. Only when the cognitive chain reaction processes are clearly defined can the SSLMB thermal runaways be better inhibited. Therefore, an in-depth understanding of the thermal safety failure mechanism of SSLMBs is urgently needed as a premise for the rational design of safer SSLMBs.

In this contribution, this comprehensive review focuses on the safety properties of SSLMBs. First, characterization methods on the thermal performance and thermal failure processes of SSLMB at different scales are introduced. Subsequently, thermal stabilities of independent materials in SSLMBs including Li metal anode, organic/inorganic SSEs and various cathode materials are discussed. Then, the chemical reactions between electrode materials and their thermal behaviors at electrode/SSE interfaces are analyzed (Figure 1). In addition, thermal runaway mechanisms of SSLMBs at single cell and pack levels are elucidated further. Finally, strategies for safer SSLMBs are also included. Based on these thermal failure behavior analyses, we hope to provide a bright light to guide the rational design of safer SSLMBs with high energy density.

FIGURE 1. Schematic diagram of thermal failures of SSLMBs at material and electrode levels.

CHARACTERIZATION METHODS

Sharp tools make good work. An in-depth understanding of battery thermal safety mechanisms strongly depends on advanced characterizations. Different measurement methods are summarized in this chapter.

Differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and TG-mass spectrometry (TG-MS) are the most used methods to test the thermal stability of materials and their mixtures. DSC can detect the heat flow difference between the sample and the reference at different temperatures and thus can reveal the thermal stability and the thermal failure mechanism of materials.³⁵ Moreover, DSC can also reflect the chemical compatibility between different materials and identify the reaction classifications. By observing the dependance of heat flow on temperatures, the complete internal heat release sequence diagram between different materials can be obtained. Therefore, DSC is one of the best and most convenient methods to understand the battery thermal runaway mechanism. Additionally, TG can be utilized to detect the thermogravimetric behaviors and analyze the decomposition temperatures of battery materials. The DSC-TG test can better organically unify the results and further identify the corresponding reactions. Further, the employment of the TG-MS test can strongly prove the types of reaction products and their impact on the thermal safety performance of the battery, which is convenient for the subsequent targeted design of the battery.³⁶

Although the inherent thermal stability of materials can be detected using the above thermal-sociated measurement methods, it is difficult to clearly confirm the origin of the thermal-induced instability of battery materials chemically. X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) are typical measurement technologies to examine the phases, structures, and morphologies of materials, which, therefore, can probe the material phase structure transitions before and after the heat treatment. The continuous crystal structure changes with rising temperatures can identify the source of thermal safety hazards of the battery materials from the atomic scale. The byproducts resulting from the exothermic side reactions can also be monitored by these tests. In addition, Raman and infrared spectroscopy (IR) tests can also confirm the chemical reaction by identifying the formation of new species.^37,38 Further, Cryo-TEM and time of flight secondary ion mass spectrometry (TOF-SIMS) can more professionally characterize the chemical reaction and evolution process of the metal Li-SSEs interface.^39–41 Therefore, the battery thermal failure mechanism can be deduced backwards from the material phase structure point of view, thus providing a clear understanding of the interface side reactions, especially for reactions associated with Li metal and cathode.^42,43 Moreover, the material perspective can also provide a guidance for the rational design of thermally stable battery materials in the future.

In addition, the overall thermal safety mechanism of SSBs is also related to their thermal responses in practical working batteries. Accelerating rate calorimeter (ARC) is a commonly used method to evaluate the time and heating rate of thermal reaction emergence of a single cell under heat abuse conditions. It provides a quasi-adiabatic environment constructed by thermal compensation and thus can fully reflect the internal heat release of the battery itself.^44,45 The ultrahigh detective sensitivity of ARC can catch a slight change in temperature resulting from exothermic side reactions within the battery, therefore effectively analyzing the battery safety under thermal abuse conditions. The ARC test is also able to gather the other useful parameters (the cavity pressure, gas release rate, and thermal ejection) during the thermal runaway process. The generated gas products can also be deeply analyzed when the ARC is combined with MS. In situ infrared tests can be used to detect the internal heat distribution of the SSLMBs. The further developed lock-in thermal imaging system can discover the location of heat release, even the distribution of Li dendrite punctured SSEs.⁴⁶ Moreover, the x-ray computed tomography (CT) test has demonstrated an excellent ability to study the internal structure changes of batteries under various conditions of abuse, further revealing the safety failure mechanism of SSBs and helping to guide the structure design directly.^47,48 In addition, routine internal/external short-circuit testing, such as puncture, as well as mechanical abuse testing including drops, collisions, crushing, and so forth, are essential means to measure the safety of SSLMBs.

THERMAL STABILITY OF MATERIALS

Although the thermal safety of SSLMBs has been shown to be little higher than that of traditional liquid LIBs in some systems, there are still some internal thermal safety hazards that need to be further addressed to achieve widespread application.^49–51 SSLMBs generally consist of Li metal anodes, SSE layers, cathodes, as well as residual materials such as current collectors, tabs, and battery packaging. The intrinsic thermal safety of SSLMBs is firstly determined by the thermal stability of the materials themself. Any one of those materials in the battery may become the direct fuse of the SSLMB thermal runaway. The “barrel effect” of battery thermal safety makes it possible to improve battery safety only when every material is not a weak point. To explore the hidden dangers of SSLMB thermal safety, we start with the inherent thermal safety problems of battery materials, which relate to Li metal, SSEs, and cathode active materials in this section. We hope that the discussions in this chapter will guide the improvements in the thermal stability of materials in SSLMBs. Inactive materials in SSBs will not be discussed because they are low in proportion or their thermal stability is good enough not to participate in thermal runaway.

Li metal

Li metal has attracted tremendous attention in next-generation high-energy-density batteries because of the thrilling theoretical specific capacity (3860 vs. 372 mAh g⁻¹ of graphite) and the lowest electrode potential (−3.04 V vs. standard hydrogen electrode). When it matches with the high-voltage cathodes, the battery energy density can easily achieve 400 Wh kg⁻¹ (vs. ~300 Wh kg⁻¹ of state-of-the-art LIBs), which can provide significant energy storage for electronics and electric vehicles.⁵² However, as one of the alkali metals with the highest electronegativity, Li metal can react with almost any chemical substances, including SSEs and cathode materials.⁵³ The chemical reactions are usually accompanied by great heat generation, which can probably lead to battery fire and even explosion. Therefore, the strong reactivity of Li metal and concomitant heat generation make the use of Li metal anode extremely dangerous.^54,55

Due to the low melting point (453 K), Li metal can melt and transform into a state of fluid when the battery temperature is higher than the melting point. The fluidity of molten Li can increase its reactivity with battery components, and even the infiltration of liquid Li into the cathode across the SSE layer may lead to the direct internal short circuit of a battery, causing intense heat generation subsequently. In addition, various gas-phase species in the air can react with Li and produce flames with frightening temperatures, which will lead to a disaster when SSLMBs contact with air under high temperatures (Table 1).⁵⁶ The highest flame temperatures of Li metal in three gases (N₂, CO₂, and O₂, the main components of air) are more than 1000 K, and it can even reach 2450 K when Li metal combusts in oxygen, indicating that the combustion of Li metal is a deadly potential safety hazard for SSLMBs.

TABLE 1 Exothermic reaction properties of Li metal with different gases.⁵⁶

Reaction	Chemical reaction	Reaction enthalpy (kJ mol−1) (298.15 K, 0.1 MPa)	Combustion temperature (K)	Maximum flame temperature (K)
1	2Li + 2H2O → 2LiOH + H2	−202	–	–
2	6Li + N2 → 2Li3N	−54	443	1234
3	2Li + 2CO2 → Li2CO3 + CO	−270	603	2160
4	4Li + O2 → 2Li2O	−299	903	2450

The violent heat production and high deflagrated flame temperatures are the direct reasons for catastrophic battery thermal runaway because of the high intrinsic reactivity and a low melting point of Li metal. Therefore, how to protect Li metal to limit its side reactions with battery materials will be a permanent topic for improving the safety of SSLMBs.

Cathode materials

In order to meet the demand for high energy density of energy storage devices, cathode materials with high storage capacity and high electrode potential are required.^57–59 Therefore, a class of new cathode materials with high capacity and high voltage, such as layered Li transition-metal oxide (LMO₂, M = Ni, Co, Mn) with high nickel concentration and Li-rich manganese-based oxides, has been developed.^1,60 Nonetheless, high electrode potential implies high oxidation activity. While the high reversible capacity indicates that the cathode materials usually suffer from the expansion and shrinkage in volume accompanied by repeated Li-ion intercalation and extraction. The oxygen redox reaction at a higher charging voltage induces the release of oxygen and the migration of transition metal ions. Meanwhile, the imbalance in internal stress can also result in the structural cracking and crushing of cathode materials and finally the structural degradation, therefore, inducing the release of more amount of surface oxygen and further affecting the safety of SSLMBs.

In fact, the thermal stability of cathode materials is strongly correlated with their structural stability. Recently, nickel-rich layered oxide materials (such as LiNi_0.8Co_0.1Mn_0.1O₂, LiNi_0.9Co_0.5Mn_0.5O₂, etc.) have been receiving growing attention because the increased participation of Ni in the redox reaction can contribute to higher capacity.^61,62 However, the stripping of Li-ion will lead to a transition of cathode materials into a thermodynamically unstable state. Under high temperatures, the thermal action can induce the structure transformation and even the evolution of surface oxygen.^63,64 It is generally believed that transition metal (TM) elements inside the charged cathode material will migrate from the TM cations occupying octahedral sites to the Li layer at elevated temperature, causing the cathode to undergo a phase transition, release oxygen, and generate heat (Figure 2A).⁶⁵ In situ neutron diffraction (ND) patterns showed a clear phase structure transition of Li_xNi_0.5Co_0.2Mn_0.3O₂ (NCM523) from layered phase (R$\overline{3}$m) to spinel phase (Fd$\overline{3}$m) and finally to rock-salt phase (Fm$\overline{3}$m) (Figure 2B) with the increasing of temperature, which is, at the same time, accompanying with the release of large amounts of oxygen meanwhile.⁶⁶ In addition, considering the role of cobalt and manganese elements in stabilizing the layered structure in ternary cathode materials, further increasing the Ni content, in spite of bringing the elevation of capacity, will lead to the decrease of structural stability of cathode materials and the intensified mixing of Ni and Li cations. Higher Ni content in cathode materials indicates worse thermal stability followed by the lower thermal-induced phase transition temperature and the earlier and more oxygen release (Figure 2C).⁶⁵

FIGURE 2. Potential thermal safety hazards of cathode materials. (A) Schematic illustration of TM cation migration path in the thermal decomposition of charged NMC materials. Reproduced with permission.65 Copyright 2014, Elsevier. (B) Lattice parameters of Fd3¯m spinel of Li0.5Ni0.5Co0.2Mn0.3O2 among heating processes. Reproduced with permission.66 Copyright 2021, Wiley. (C) Mass spectroscopy data for the oxygen (m/z = 32) of various cathode materials with different Ni contents during heating. Reproduced with permission.65 Copyright 2014, Elsevier. (D) The structure change, crack widening, and propagation of NCM622 material during heating. Reproduced with permission.67 Copyright 2018, Springer Nature.

The diversity in the thermal response of different grains and even the same domains but at different areas can induce significant internal stress in cathode materials and thus lead to the thermo-mechanical failure of cathode materials. The escaping of lattice oxygen also generates expansion stress and results in the structural cracking of cathode particles. Yan et al. applied in situ high-resolution transmission electron microscopy to monitor the crack evolution during the heating of LiNi_0.6Mn_0.2Co_0.2O₂ (NCM622).⁶⁷ They found that the crack continues to grow with the heating temperature from room temperature to elevated temperature due to the presence of inhomogeneous thermal stress and gas pressure effect (Figure 2D). In turn, the cracking of cathode materials further exacerbates the nonuniformity of internal stress and releases a larger amount of surface oxygen. This vicious cycle contributes to the “popcorn” intragranular cracking under heating and leads to the thermal spray of battery materials and the eventual violent battery thermal runaway.⁶⁸ In addition to NCM cathodes, other high-voltage and high-capacity cathode materials, such as LiCoO₂ (LCO), LiNi_0.8Co_0.15Al_0.05O₂, and emerging Li-rich manganese-based layered oxide (xLiTMO₂·(1 − x)Li₂MnO₃), also exhibit similar drawbacks.^69–71 Their low thermal stable temperature (~200°C) brings more disturbing security risks for batteries. In pursuit of high energy density, it is necessary to pay attention to the impact of cathode stability on SSLMBs safety.

Solid-state electrolytes

As important but inactive components in SSLMBs, SSEs play critical roles in not only conducting Li-ions but also preventing the direct contact between cathode and anode as a separator, thus offering a promising future for increasing the energy density of LMBs. Despite efforts to reduce the thickness of SSEs to pursue high energy density, the impact of SSEs on battery safety cannot be ignored. At present, SSEs are mainly classified into three categories: solid polymer electrolytes (SPEs), inorganic solid electrolytes (ISEs), and hybrid solid electrolytes (HSEs). Although SPEs have high mechanical flexibility contributing to good electrode/electrolyte interface contact, they still exhibit low ionic conductivity and poor thermal stability.^72,73 ISEs show advantages in ionic conductivity and mechanical strength, but the brittleness of ISEs limits their wide applications. The HSEs, the blending of ISEs with SPEs, combines the flexibility of SPEs and the mechanical strength of ISEs, however, the thermal stability of HSEs is still determined by the properties of both the ISEs and SPEs, so we will not discuss the thermal behaviors of HSEs separately at this section.

The thermal stability of SPEs strongly depends on the polymer structural units and the polymerization degree. Polyethylene oxide (PEO) as one of the representative SPEs, has attracted numerous attention due to its flexibility, suitable ionic conductivity, and ease of processing. However, the crystallization of polymer chains leads to inferior ion transport at room temperature, which commonly requires PEO electrolyte to be used at elevated temperatures to ensure moderate Li-ion conduction capability.⁷⁴ Once the temperature exceeds the melting limit, SPEs will distinctly shrink and melt, resulting in the internal short circuit of SSLMBs (Figure 3A).⁷⁵ The internal short circuit can even induce intense heat release and increase the possibility of final thermal runaway of SSLMBs. In addition, there is concern about the decomposition of SPEs at high temperatures.⁷⁶ For example, the thermal decomposition of PEO can generate byproducts with low boiling points such as alcohols, alkenes, non-cyclic ethers, aldehydes, water, CO, CO₂, and so forth.⁷⁷ Most of these products can react drastically with Li metal and produce hydrogen gas, increasing the risk of battery fire and even explosion. In situ polymerization of SPEs is usually applied to construct a conformal electrode/electrolyte interface. However, in situ polymerized SPEs have a wide molecular distribution and contain a large number of oligomers, and even residual monomers are retained within SPEs. These in situ polymerized SPEs generally exhibit poor thermal stability and low decomposition temperature. Li et al. demonstrated that in situ polymerized poly(1,3-dioxolane) (P-DOL) SPEs can be decomposed at around 110°C according to TG analysis (Figure 3B).⁷⁸ The decomposed products, including formaldehyde gas and other epoxides, can lead to leakage and combustion risks. Actually, the flammability of SPEs is similar to that of liquid electrolytes due to their intrinsic organic features.^79,80 The PEO-LITFSI SPE can be ignited by the flame and burn violently (Figure 3C).⁷⁵ In particular, when the SPEs are swelled with liquid electrolytes to improve the ionic conductivity, the thermal stability of SPEs will be largely dominated by volatile liquid components.^81,82

FIGURE 3. Potential thermal safety hazards of SSEs. (A) PEO/LiTFSI SSE film before/after exposure to thermal shock (150°C, 0.5 h). Reproduced with permission.75 Copyright 2020, American Chemical Society. (B) The TGA curves of different P-DOL electrolytes. Reproduced with permission.78 Copyright 2022, Wiley. (C) The flame tests of PEO/LiTFSI SSE. Reproduced with permission.75 Copyright 2020, American Chemical Society. (D) Al-doped LLZO pellets exposed to ambient air for 25 days. Reproduced with permission.86 Copyright 2018, Elsevier. (E) H2S gas generated from the xLi2O-(100 − x)(0.7Li2S-0.3P2S5) (x = 17, 20, 25) powders in air. Reproduced with permission.89 Copyright 2013, Elsevier.

ISEs are regarded as ideal electrolyte alternatives to achieve safer SSLMBs due to their intrinsic thermal stability and non-flammability. Oxide ISEs, including Li₇La₃ Zr₂O₁₂ (LLZO), Li_1+xAl_yGe_2−y(PO₄)₃ (LAGP), Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), and Li3xLa_2/3−xTiO₃ (LLTO), which possess high mechanical strength and electrochemical windows, have become a hot topic among researchers.^83–85 However, the surface alkalinity of LLZO-type ISEs can induce the reaction of LLZO with CO₂ or moisture and the formation of LiOH and Li₂CO₃. When exposed to an ambient atmosphere, the surface side reactions can cause the spontaneous cracking of LLZO electrolyte pellets and lead to the loss of structural integrity (Figure 3D).⁸⁶ Once damaged electrolyte pellets are assembled into SSLMBs, the initial defects and cracks can induce the risk of Li metal penetration and even the resultant battery short circuit. Sulfide ISEs (Li₃SP₄, Li₁₀GeP₂S₁₂, Li₆PS₅Cl, etc.), another type of representative ISEs with excellent ionic conductivity, have attracted significant attention.^87,88 Whereas, the thermal stability of sulfides is not as good in comparison to that of the oxide counterparts. Sulfide ISEs undergo obvious sulfur evolution at above 300°C.⁴² The precipitated sulfur crystals are highly volatile and can react with Li metal exothermically, which renders severe safety concerns in sulfide SSLMBs. In addition, the moisture sensibility of sulfides and the formation of toxic H₂S byproducts raise significant operational and thermal safety concerns for SSLMBs (Figure 3E).⁸⁹

Overall, SSEs exhibit higher thermal stability in comparison to conventional liquid counterparts. However, the melting and decomposition of SPEs as well as their flammability still introduce safety risks in SSLMBs safety risks. In spite of the intrinsic thermal stability and non-flammability of ISEs, their inferior environmental tolerance (CO₂, H₂O) can induce the degradation of electrolyte structures. In particular, the brittleness and low flexibility of ISEs make them difficult for large-scale applications. The cracking and breaking of ISE pellets are easy to cause internal short circuits by Li dendrites penetration and thus, resulting in heat release and severe safety hazards.

REACTIONS AND THERMAL BEHAVIORS AT ELECTRODE/SSE INTERFACE

Electrodes are essential structural components of SSLMBs and provide the main sites for electrochemical reactions. Energy storage and release processes rely on the electrode/electrolyte interfaces inside the electrode. However, the electrochemical reactions are generally accompanied by interface side reactions as well as thermal feedback. The decomposition of electrolytes and the formation of solid-state interphase (SEI) leads to the increase in Ohm resistance and the release of heat. The generated heat aggravates the interface side reactions in turn and leads to the serious accumulation of byproducts and thus the generation of more heat. In fact, the SEI is thermally unstable, which lays hidden dangers for the safe operation of the battery. Furthermore, the electrode interface evolution during cycling can lead to significant complexity in thermal safety mechanisms. Hence, this chapter will discuss the safety implications of SSLMBs at the electrode scale.

Anode/SSE interface

As noted above, the Li metal anode can spontaneously react with most SSEs and release a large amount of heat due to its high reactivity, which induces a great thermal safety risk for SSLMBs. Chen et al. investigated the chemical reactions between Li metal and LATP SSE by electrochemical impedance spectroscopy (EIS) and x-ray CT analysis.⁹⁰ The successive interface side reactions are revealed by the continuous increment of the total resistance in the Li/LATP/Li cell (Figure 4A). Furthermore, both the interfacial side reactions and bulk phase degradation are accelerated sharply at the elevated temperature of 120°C, resulting in the giant augmenting of resistance (Figure 4B). Even worse, once the temperature exceeds the melting point of Li metal, the molten Li will fill in the pores of SSEs and bring catastrophic danger. Especially for LAGP SSE containing high-valance metal elements, it can react with molten Li intensely and generate bright flame even in the Ar atmosphere (Figure 4C).⁹¹ This is attributed to the complete decomposition of LAGP with Li to form Li-based alloys like Li₂₂Ge₅, Li₄AlGe, and Li₃PO₄. Recently, Li and coworkers systematically investigated the thermal stability of four common oxide SSEs against Li metal by ARC (Figure 4E).⁵⁰ They found that all oxide SSEs can react with Li metal and generate heat and the order of thermal stability of SSEs against Li metal is ranked as LAGP < LATP < LLTO < LLZO, which further demonstrates that the interface side reactions between SSEs and Li metal may induce battery safety issues. The thermal stabilities between sulfide SSEs and Li metal are also investigated. Huang et al. showed that the mixture of Li metal and Li₆PS₅Cl SSE displays a relatively violent exothermic reaction above 225°C, but the reaction mechanism is still vague.⁵¹ Due to the excellent prospect of sulfide SSEs in practical applications, the comprehensive understanding of the thermal stability of sulfide SSEs with Li metal should be enhanced.

FIGURE 4. Thermal failures upon anode/SSE interphase. The interfacial chemical reactions between Li and SSE studied by EIS in Li/LATP/Li symmetrical cell at (A) room temperature and (B) 120°C. Reproduced with permission.90 Copyright 2021, American Chemical Society. (C) Deflagration reaction of Li with SSEs. Sequential images for contact of sintered LAGP pellet and melted Li metal at 200°C in the glovebox. Reproduced with permission.91 Copyright 2017, American Chemical Society. (D) Thermal decomposition of Li with different SPE content electrolytes. Reproduced with permission.92 Copyright 1995, Elsevier. (E) Research of thermal stability between different oxide SSEs and Li. The ARC results of LAGP/Li, LATP/Li, LLTO/Li and LLZO/Li simulation samples. Reproduced with permission.50 Copyright 2020, Elsevier.

Due to the organic nature of SPEs, the interface side reactions between Li metal and SPEs also attract concerns on the thermal safety of polymer-based solid-state batteries. Murata compared the thermal stability of Li metal with different contents of SPEs by DSC.⁹² Unlike conventional carbonate electrolytes, the exothermic reactions between SPEs and Li metal are significantly reduced and delayed, yet there is still obvious heat release (Figure 4D). Mirsakiyeva et al. applied the density functional theory (DFT) model to explore the reactions between Li metal and PEO.⁹³ PEO can decompose into C₂H₄ and H₂ on a Li(100) surface while Li is oxidized to Li₂O simultaneously, which are exothermic processes. As previously mentioned, organic SPEs can self-decompose at high temperatures and the decomposition products have the potential to react with Li metal.

In fact, the charge and discharge processes are usually accompanied by the interface microstructure evolution, which has considerable effects on battery safety. When the cell begins to work and cycle, uneven Li plating and stripping occur repeatedly on the Li metal anode surface, leading to vigorous interface fluctuation and continuous growth of Li dendrites.^94–96 The penetration of Li dendrites into the SSEs enhances contact and thus accelerates side reactions between Li and SSEs. Even negligible Li dendrites at the early stage of cycling can lead to a short circuit of the battery. This phenomenon is particularly prominent in SSLMBs. The inferior solid–solid interface contact between Li and SSEs induces excessive local current density and the continuous Li plating at the point-to-point contact regions, therefore, resulting in the sudden catastrophic Li dendrite growth.⁹⁷ The dendrites in SSEs can open additional electrical pathways and cause a drop in ohmic impedance (Figure 5A).⁹⁸ Once the Li whiskers propagate into the SSE layer and reach the cathode, a high short-circuit current causes severe joule heat, enough to melt Li in a short time (Figure 5B). The instantaneous intense release of heat will further aggravate self-heating side reactions in the SSLMBs and lead to battery thermal runaway at last.

FIGURE 5. Thermal failures upon anode/SSE interphase. (A) Short circuit of the Li/SPE/Li symmetric cell with a polarization at 0.50 mA cm−2. Reproduced with permission.98 Copyright 2002, The Electrochemical Society. (B) Cross-section SEM image after short circuit accident in Li/SPE/Li symmetric cell. Reproduced with permission.107 Copyright 2006, Elsevier. (C) Operando x-ray CT of the initiation and propagation of Li dendrite in Li/Li6PS5Cl/Li cell. Reproduced with permission.104 Copyright 2023, Springer Nature. (D) Lock-in thermal images of the top of Li/Li3PS4/Li cell when short circuit occurred. Reproduced with permission.105 Copyright 2018, The Electrochemical Society. (E) Chemical reaction triggered crack formation mechanism at the interphase between the Li metal and Li3PS4 pellet. Reproduced with permission.106 Copyright 2021, American Chemical Society.

Theoretically, the high mechanical strength of SSEs can effectively inhibit the growth of Li dendrites if the modulus of SSEs is more than twice than that of Li metal, but it often backfires and Li filament is more prone to propagate into SSEs. In fact, defects, such as grain boundaries, voids, and cracks, on the Li/SSE interfaces play critical roles in trapping electrons and inducing the reduction of Li ions surrounding the defects, which also attribute to the growth of Li dendrites. On one hand, voids and defects reduce the effective interface contact between Li and SSEs, leading to an uneven distribution of current. On the other hand, the deposition of Li on defects evokes considerable internal stress, causing cracking in SSEs. While the continuous growth of Li and the accumulation of local stress further lead to crack propagation.^99–103 Subsequently, Li dendrites grow rapidly along the crack and quickly fill the gap until the internal short circuit (Figure 5C).¹⁰⁴ Sulas et al. adopted lock-in thermography to identify the heat production areas after a critical short circuit of the Li/Li₃PS₄/Li cells (Figure 5D).¹⁰⁵ The point-like heat signatures are attributed to the drastic current flow through Li dendrites induced by punctate defects, which will create a terrifying concentration of heat in a small area fleetly and trigger a cascade of exothermic reactions.

Apart from the internal short circuit caused by Li dendrite growth, the interface side reactions between Li and SSEs are also responsible for the decrease in the thermal stability and safety of SSLMBs. The growth of Li dendrites can accelerate the side reaction process through the increase of contact area and thus the deterioration of the interface. Otoyama et al. used in situ x-ray CT to study the degradation mechanism of the interface in Li/Li₃PS₄/Li symmetry cells (Figure 5E).¹⁰⁶ They found that the short circuit of SSLMBs is attributed to the interfacial chemical reactions between Li metal and Li₃PS₄ electrolyte. The reduction and decomposition of Li₃PS₄ are accompanied by the change in volume, resulting in an uneven stress distribution and fine cracks inside the SSEs finally. After that, Li dendrites continue to penetrate along the as-formed fine cracks. As a result, with the repeated reduction-expansion-cracking-growth effect, the growth of the Li cluster and the formation of cracks further lead to the structure failure of the whole SSEs and finally the internal short circuit of SSLMBs. Actually, the generation of interface side reactions and the decomposition of SSEs usually induce the passivation of the Li/SSE interface and the increase of interface impedance, which therefore gives rise to increased ohmic heat during cycling. The accumulation of byproducts and “dead” Li exacerbate the deterioration of the anode interface, which will affect the battery capacity retention and safe operation. At the same time, fresh Li anode will be converted into a porous Li with high-specific surface after repeated Li platting and stripping, so that the interface side reactions of Li metal with the SSEs are greatly accelerated.⁹⁵ In addition, the metastable SEI is easy to further decompose, which will also produce heat and lead to the self-heating of the battery.²⁰

As one of the most important parts, Li metal anode is a double-edged sword in SSLMBs. The high specific capacity of Li metal ensures the energy density of SSLMBs, while its high reactivity brings severe safety risks and leads to the thermal runaway of batteries even without cycling. Once the electrochemical process is introduced, Li dendrite growth and interface side reactions will lead to interface degradation. The complicated coupling of interfacial electro-chemical–mechanical processes further results in the failure of interface structure and the successive propagation of Li dendrites, finally inducing the battery short circuit and safety problems. Therefore, the thermal safety of Li metal anode is one of the key factors of the SSLMB safety limit, which should need to be further systematically studied and solved.

Cathode/SSE interface

In order to pursue the high energy density of batteries, high-voltage cathode materials, such as LiMO₂, Li-rich manganese-based oxides, and so forth, are generally adopted to pair with Li metal anode to widen the working voltage of batteries. However, the high operating voltage usually means strong oxidation environments at the cathode interface especially for the batteries after charging. In general, the electrochemically stable window of most SSEs is relatively narrow and lower than the oxidation potential of high-voltage cathodes, which therefore induces the oxidation decomposition of SSEs.^20,108 Similar to the Li/SSE interface, the oxidation instability of SSEs against the high-voltage cathodes and their chemical incompatibility can also lead to the release of heat and the generation of byproducts and thus, resulting in the passivation of the cathode/SSE interface and risks of the battery thermal safety.¹⁰⁹

In addition to high oxidation potential, the delithiated cathodes are thermodynamically unstable and have poor structural stability, easy to render the phase transition and the oxygen release. The generated oxygen, as the reactive oxidizer, becomes one of the objects that must be considered and avoided during the thermal runaway process of the battery. Actually, the exothermic reaction between the cathode-released oxygen and the electrolyte is one of the important reasons for the thermal runaway of routine liquid Li-ion batteries (LIBs). This is also a concern in SSLMBs, especially in sulfide-based and polymer-based systems with weak oxidation resistance. Bartsch et al. combined isotopic labeling, titration and operando MS analysis to reveal the evolution of CO₂ and O₂ formation in indium (In)/Li₃PS₄/NCM622 cells during cycling (Figure 6A).¹¹⁰ When the cathode is charged above 4.5 V (vs. Li⁺/Li), the release of oxygen is an intrinsic characteristic of Ni-rich layered oxides, which is related to the loss of lattice oxygen. Meanwhile, due to the strong oxidability of produced oxygen, SO₂ is also detected, which is strongly correlated with the side reactions between oxygen and sulfide SSEs. As previous discussions, the oxidized cathodes are metastable and high temperatures can also induce the structure transition and oxygen release. In turn, oxygen evolution further accelerates the progress of oxidation side reactions between cathodes and SSEs and leads to more heat generation.⁴³ Kim et al. observed a direct fire between the sulfide electrolytes and cathodes in the argon glove box at high temperatures (Figure 6B).¹¹¹ More dangerous, the mechanical mixing can cause a visible flame in their tests. In addition, the highly combustible polymer SSEs exhibit obvious exothermic reactions with oxygen at elevated temperatures (Figure 6C).¹¹² Even, thermally stable oxide SSEs can also react violently with and be oxidized by the cathodes at much higher temperatures.¹¹³

FIGURE 6. Thermal failures upon cathode/SSE interphase. (A) The oxygen release during the cycling of cathode in SSLMB. Voltage profile and MS profiles of CO2, O2 and H2 within the cycling of In/β-Li3PS4/NCM622 cell at 45°C between 4.4 and 2.3 V (vs. In/InLi). Reproduced with permission.110 Copyright 2018, American Chemical Society. (B) Ignition temperatures and images of the composite cathode pellets (NCM811 and Li6PS5Cl SSEs) as a function of cutoff voltage for bare and LiNbO3-coated cathode materials. Reproduced with permission.111 Copyright 2022, American Chemical Society. (C) The research of exothermic reaction between SPEs, including PEO-LiTFSI, PEG-LiTFSI and P(EO/EM = 95/5)-LiTFSI, with LixMnO2 cathode by DSC test. Reproduced with permission.112 Copyright 2001, Elsevier. (D) Thermal expansion coefficient difference ΔTEC between each SSE and AM depending on the temperature. Reproduced with permission.117 Copyright 2021, Royal Society of Chemistry.

In particular, as a typical representative of energy-dense cathodes, LiNi_xCo_yMn_zO₂ (x + y + z = 1, NCM) ternary materials show considerably enhanced energy density with increasing Ni content, however, their structural stability decreases due to the reduction of Co and Mn content that function as the stabilizer of the lattice. Therefore, oxygen production is the major safety concern for high-energy-density batteries especially for that assembled with high-Ni content NCM cathodes and high-voltage LCO cathodes. Yang et al. compared the interface stability of LCO and LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) cathodes toward the PEO-based SSEs under high temperatures. They found that PEO cannot sufficiently passivate the polycrystalline NMC811 particles, on the contrary, released oxygen can induce the severe oxidation of PEO and thus intense heat generation.¹¹⁴ Sulfide SSEs are more likely to react exothermically with oxygen due to less resistance to oxidation. Both glassy-ceramic and crystalline sulfide SSEs exhibited significantly larger heat generation even than liquid electrolyte with the delithiated NCM811 cathode.⁴³ The violent S–O exchange between sulfide SSEs and oxygen can result in huge amounts of heat release and dangerous sulfur dioxide gas generation. Consequently, more attention should be paid to the thermal reactions between cathodes and SSEs. Moreover, more compatible SSEs with high-voltage cathodes should be exploited.

In addition to exothermic reactions, the thermal expansion of the cathode materials also contributes to cathode structure collapse and battery safety issues. The mismatching thermal expansion coefficients between cathode active materials (AM) and SSEs can lead to local stress concentration and microstructure damage at elevated temperatures. The exposure of more fresh surfaces induces severe interface side reactions and the release of surface lattice oxygen, resulting in a reduction in battery safety.^115,116 XRD can be applied to survey the difference in thermal expansion coefficients (ΔTECs) of various cathode materials and SSEs at different temperatures, beneficial for the selection of compatible materials (Figure 6D).¹¹⁷ Moreover, there presents the intrinsic change of cathode materials in volume during lithiation/delithiation processes, which gives rise to the contact loss between cathodes and SSEs. The uneven local current concentration can also result in local hot spots on the cathode/SSE interface, affecting the safe operation of the battery.⁶⁸ Therefore, the balance in structure stress between cathodes and SSEs is also important for interface stability and battery safety.

In short, the strong oxidation environment in high-voltage cathodes can induce drastic interface side reactions. cathode phase transition as well as oxygen release, which in turn intensify the side reactions and cause powerful heat release, severely affecting the battery safety. Moreover, the structure evolution at elevated temperatures also delivers thermal safety risks to batteries, because the expansion and shrinkage of cathode lattices also induce the structure degradation and even the oxygen loss of cathodes.

THERMAL RUNAWAY MECHANISM OF BATTERY DEVICES

In fact, a practical battery or a pack is a significantly complicated system, which is assembled by varied materials as well as electrodes at different dimensions. Both the properties of materials and electrode interface reactions will affect the overall electrochemical and thermal performances of the battery. The instability of materials aggravates the interface reactions and the release of heat. The increased temperature will further accelerate the exothermic processes and leads to the crosstalk of heat and gas byproducts between cathodes and anodes. Therefore, the battery will enter a self-driving and irreversible thermal runaway process. In particular, the heat can spread from one cell to neighboring cells in a pack, inducing unprecedented thermal runaway and final battery explosion. Despite the difference in the type of electrolytes, the thermal safety mechanism of SSLMBs is unified and coincident in comparison to conventional liquid LIBs. Therefore, detailed thermal runaway sequence diagrams can promote the understanding of the key triggers of battery thermal runaway.

As a basic unit of energy storage and release, the thermal behaviors of the single battery are generally affected by external working conditions including thermal abuse, mechanical abuse, and electrical abuse (Figure 7).^118,119 But, the intrinsic mechanisms of battery thermal runaway under different abuses are still attributed to the underlying thermal responses at material, electrode and cell scales. Moreover, these three kinds of abuse conditions can be mutually transformed and eventually lead to the battery thermal runaway. For instance, cell overheating (thermal abuse) will lead to the melting and cracking of electrolytes and internal short circuit occurrence (electrical abuse), the heat generated by the internal short circuit will trigger the thermal runaway further. Although the origin and evolution of thermal runaway are complicated, the battery thermal runaway under external abuse conditions is invariably attributed to the failure with the sequence from materials to electrodes, to cells, and eventually to pack (Figure 8). A comprehensive cognition of the thermal runaway mechanism under abuse conditions is required to ensure safer SSLMBs.

FIGURE 7. Schematic diagram of thermal failures triggers of SSLMBs at the cell level.

FIGURE 8. Schematic diagram of SSLMBs failure mechanisms under thermal, mechanical, and electric abuses.

Thermal abuse practically refers to an impertinent supply of external heat sources for SSLMBs, which leads to side reactions and corresponding heat accumulation inside the battery. Raised temperatures pose challenges to the thermal stability of both materials and electrode interfaces inside the battery. First, the elevated temperature is directly related to the thermal stability of materials and the reaction kinetics of interface side reactions.¹²⁰ For instance, the high reactivity of Li metal is bound to bring interface side reactions accompanied by the release of heat. While external heat input will expedite the dynamics of these side reactions and thus cause further battery temperature ascension. In particular, with regard to the cycled Li metal battery, the thermal abuse will tremendously intensify the self-heating process caused by side reactions with increasing battery service time due to the pulverization of the Li anode as well as increased contact area with SSEs. Yang et al. summarized the mechanism of thermal runaway caused by thermal abuse of practical Ah-level 100%SOC Li/Li₆PS₅Cl/NCM523 cell systematically by ARC and DSC tests.⁴⁹ First, the self-heating of the cell arouses because the elevated temperature aggravated the interfacial reaction between Li and SSE. The structural stability of cathode deteriorated under the accumulation of heat, and it gradually broke down to release oxygen which will react with SSE and lead to the thermal runaway finally. After adopting of sulfide electrolytes that are more stable to Li metal, the trigger of thermal runaway shifts to the exothermic reaction between cathode and SSE. In addition, heat abuse causes structural damage to electrode materials and oxygen release, which further induces the chemical crosstalk between the cathode and anode at the cell scale. The release of oxygen originating from the decomposition of LMO₂ cathodes can diffuse to the anode and induce violent exothermic reactions between Li and oxygen. In spite of SSEs are more thermally stable than liquid electrolytes, the diffusion of oxygen and resultant chemical crosstalk still occur and are difficult to prevent.¹²¹ As the battery temperature rises, the melting of the Li metal or SPEs causes the internal short circuit, creating more heat. Moreover, enhanced temperatures can trigger the production of flammable gas that is ascribed to the SPEs decomposition, which may lead to serious expansion of the cells.⁷⁸ If the gas concentration reaches the explosion limit or the pressure relief is too violent in a short time, serious accidents may occur.

Mechanical abuse is the simplest but prestissimo inducement type of battery thermal runaway, which is mainly caused by improper mechanical operations, such as battery extrusion, drop, rupture and puncture. Metallic sharp objects can penetrate SSLMBs in a collision, acting as an excellent electron conductor inside the battery and causing severe internal short circuits. The large current in a short period of time and rapid energy release cause a large amount of joule heat, leading to subsequent thermal abuse of the battery within a few seconds.¹²² Besides, the brittle SSEs are prone to cracking under external force and hard to self-recover. Once the battery is damaged and cracked, N₂, H₂O and CO₂ in the air will also enter the battery and react with highly active electrolytes and Li anode, which will also cause the followed-up heat release and heat abuse.

Electrical abuse is related to nonstandard working protocols, such as fast charging, fast discharging, overcharging, over-discharging, and so forth, which can lead to Li dendrite growth, cathode material damage and current collector corrosion. The Li dendrite can puncture across the SSE layers and leads to internal short circuits. If the Li dendrite cannot be effectively restrained, the violent exothermal process caused by internal short circuits will be extremely dangerous. The risks of overcharging/over-discharging are widely demonstrated in conventional LIBs, which theoretically are identical to the SSLMBs using similar cathode materials and battery configurations. Practically, battery overcharging is profoundly fatal. On one hand, immoderate Li extraction from cathode materials can lead to structural collapse and oxygen production. More importantly, overcharging can also result in Li dendrite growth. These, therefore, may directly cause thermal runaway of the battery after multiple overcharging.^123,124 At the same time, some SSE materials that have narrow electrochemical windows will also deteriorate because of high-voltage oxidation on the interface of the cathode, generating heat and combustible gas. Extremal over-discharging may also give rise to the copper stripping from the anode copper collector and the growth of copper dendrite on the cathode, which can also lead to the battery internal short circuit.¹²⁵

In order to meet practical requirements for high power and energy of electrical equipment, packs are generally fabricated by connecting with many cells in series or parallel. The increase in the number of batteries and the tight battery arrangement configuration can make the pack more dangerous. When thermal safety issues happen within one cell, the resulting high temperature or flame will quickly spread to neighboring cells, causing a series of thermal runaways in the entire pack.¹²⁶ The thermal runaway propagation is an exponential process, resulting in the whole battery pack thermal runaway at breakneck speed. Especially for the combustion of Li metal which flame temperatures can easily exceed 2000 K, hard-to-control high-temperature flames are more dangerous and incredibly induce thermal runaway propagation. In addition, the accumulation of heat and gas inside the cells creates high-temperature jets. The gas ejection also leads to the spraying of the SSEs and cathode powders inside SSLMBs, which also pose a safety hazard to nearby cells. These problems can be magnified in Li metal batteries because of the presence of liquid molten Li metal and the increased content of thermally unstable byproducts. Besides, when multiple cells are linked together to form a pack, the difference in capacity and materials quality between cells make it possible for a single cell to happens electrical abuse.^127,128

In summary, abuse conditions of every description can cause irreversible thermal runaway of SSLMBs. However, the core of various abuse cases is still the heat release, including the interface exothermic side reactions and internal short circuit, which acts as the “fuse” for battery thermal failure. The intense exothermic reactions cause the elevation of the cell temperature. Even worse, the quick battery energy release and the horrible Li combustion flame will severely accelerate the thermal runaway propagation. Therefore, inhibiting the thermal reactions and blocking the thermal propagation, that is blowing the “fuse”, will effectively enhance the battery safety and prevent the battery thermal runaway.

STRATEGIES FOR SAFER SSLMBS

Safety is a critical yet fundamental requirement for the practical application of battery systems. SSLMBs utilizing thermally stable SSEs to alternate conventional liquid electrolytes exhibit higher safety and have garnered significant attention. However, intrinsically safe SSLMBs still remain challenging. As previously discussed, the safety of SSLMBs is closely linked to the battery materials, interface reactions, and battery pack mode design. Therefore, developing stable and compatible battery materials and advanced interface protective strategies are imperative (Figure 9). In addition, the implementation of external flame-retardant materials can effectively isolate hazards and inhibit thermal spread between unit cells in a practical high-energy battery pack. An advanced battery management system also plays a critical role in ensuring the safe operation of SSLMBs by effectively regulating appropriate charging/discharging protocols. Once a battery exhibits any incidental safety risks, a timely safety detection and feedback mechanism can insulate the unit cell from the battery pack. Battery life forecasts and accurate warnings can also leave enough time for people to avoid danger. This chapter summarized the advances in improving battery thermal safety, which is expected to guide the rational design of safer SSLMBs.

FIGURE 9. Schematic diagram of thermal stable enhancements of SSLMBs at material and electrode levels.

High-stable battery materials

The safety failure of batteries is generally originated from the instability of materials, which, thus, determines the intrinsic safety of SSBs. This section will discuss the advances in thermal stability enhancement of battery materials in SSLMBs, including Li metal, SSEs, and cathode materials.

Li metal is the most active component in SSLMBs. It can react with almost every battery material and release a huge amount of heat. Moreover, these reactions can be intensified further when Li metal melts because of its low melting point. Structural design, such as applying Li alloy, has been regarded as one of the potential methods to solve the problems concerning Li metal. In order to mitigate the thermal safety issues caused by Li melting, Fu et al. exploited a Li₅B₄/Li composition anode to reserve the liquid molten Li at elevated temperatures (Figure 10A).¹²⁹ The strong capillary action and lithiophilic nature of the Li₅B₄ microstructure skeleton inhibit the leakage of molten Li and improve the thermal stability of Li metal anode. Besides, Li alloys such as Li–In, Li–Al, Li–Mg, and so forth, have an increased electrode potential as well as improved compatibility with SSEs, which, therefore, considerably inhibit interface side reactions.¹³⁰ However, the safety failure mechanism of Li alloys is still indistinct, and considerable efforts should be endowed.

FIGURE 10. Thermal stability modification of materials in SSLMBs. (A) Li5B4 framework weakens the mobility of lithium melting. Reproduced with permission.129 Copyright 2020, Wiley. (B) DBDPE fire-retardant additive brings good flame retardant for PI/LiTFSI/PEO SPE. Reproduced with permission.75 Copyright 2020, American Chemical Society. (C) Contour plots of the time-resolved XRD patterns exhibit the thermal stability of NCM811 cathode improved by the introduction of Mg/calcium (Ca) elements. Reproduced with permission.61 Copyright 2021, American Chemical Society.

The SSE decomposition and even combustion at high temperatures pose significant safety hazards for SSLMBs. Enhancing the thermal stability of SSEs can also render safer SSLMBs. As for conventional ISEs, strategies including the element doping, surface coating, structure design and sintering process are usually applied to improve their thermal and air stability.^{88,89,131–133} Besides, developing new type ISEs, such as halide SSEs (Li₃YCl6, Li₃YBr₆, etc.) and argyrodite SSE (Li₆PS₅Cl), is another effective direction, which can target to design ISEs with high-voltage tolerance and strong Li metal compatibility.¹³⁴ The organic natures of routine polyether SPEs make them decompose, melt and burn much easily. Recently, the flammability of PEO/LiTFSI-based SPEs is improved by adding functional flame-retardant groups and additives, such as halogenated and phosphate, into SPE matrix. Cui et al. introduced decabromodiphenyl ethane (DBDPE) fire-retardant additive into PI/LiTFSI/PEO SPE to mitigate the safety problem of SPEs (Figure 10B).⁷⁵ Zhang et al. constructed a risk-responding polymer separator by incorporating phosphorus-contained functional groups into a hydrocarbon-based polymer, which can release phosphoxy radical and quench the chain reaction during combustion, mitigating the battery thermal runaway.¹³⁵ Apart from the introduction of functional additives, Zhou et al. developed a thermoresponsive SPE to improve the safety property of SSLMBs.¹³⁶ This thermoresponsive SPE shows a drastic dive in ionic conductivity at 70°C by the temperature-triggered conformational changes at the molecular level, which can inhibit the ionic conduction between the electrodes and further terminate the subsequent reactions. In addition, the use of ionic liquids and polymer cross-linking framework design have also similar effects on improving the SPE thermal stability.^137,138 Organic–inorganic composite SSEs can combine the flexibility of SPEs and the high thermal stability of ISEs, which can greatly improve the thermal stability, battery safety, ion conductivity and battery cycling performance.^139,140 Although researchers have made many improvements to SSEs, there are still few researches on their thermal safety performance in practical SSLMBs, which need to be followed up with more detailed and comprehensive discussions.

In fact, the critical problem of cathode materials is the interpretative oxygen release at elevated temperatures, which can react significantly with SSEs as well as Li metal anode. Particularly, the reactions of oxygen with Li metal can lead to disastrous heat generation and even fire and explosion of SSLMBs. Therefore, stabilizing the lattice oxygen in cathode materials is imperative. Cation doping is one of the easiest and low-cost ways to suppress the phase transition and stabilize the lattice oxygen under elevated temperatures.^62,64 Wang et al. evaluated the thermal stability of NCM811 materials doped with different elements and found that 3.0 mol.% magnesium (Mg)-doped NCM811 material exhibits an increase in oxygen release temperature from 200 to 250°C (Figure 10C). Appropriate ionic size of Mg²⁺ can make it enter the lithium layer, which plays a favorable role in stabilizing the crystal structure of NCM811 and thus improve the thermal stability of NCM811. Besides, the rational design of LMO₂ particle structures with layered gradients can also improve its structural stability. Wu et al. developed an ultrathin spinel membrane coated on the surface of Li-rich manganese-based cathode materials. The ultrathin coating layers have high thermal stability, which can lock lattice oxygen and delay or reduce oxygen release. The thermal stability of modified materials increases from 197 to 225°C. In spite of the successes in improving the thermal stability and inhibiting oxygen evolution of high-voltage cathode materials, the development of new cathode materials with enhanced working voltage still confronts the battery thermal safety concerns, which should be also emphasized.⁶²

Enhanced interface stability

The chemical/electrochemical reactions at the electrode/electrolyte interfaces are the main reasons for the self-heating of SSLMBs.^141–143 Therefore, restraining the interface side reactions through interface/interphase modifications to improve interface stability has been considered one of effective strategies to enhance the thermal safety of SSLMBs. As mentioned above, the high reactivity of Li metal with SSEs is too difficult to tame. A thermodynamically stable interphase with Li metal implanted into the Li/SSE interface can avoid the direct contact between Li metal and SSEs and thus effectively suppress the Li-induced self-heating reactions. Chen et al. found that adding LiPO₂F₂ (LPF) on the LATP surface can modify LATP pellet defect sites and impede the Li/SSE interfacial side reactions and then improving the thermal stability of SSLMBs (Figure 11A).⁹⁰ In the same way, ex-suit and in-suit coating protective layers upon Li anode are also promising to suppress exothermic side reactions between Li metal and SSEs. Moreover, the protective layer can serve as a buffer layer to mitigate the mechanical stress under mechanical abuse conditions, therefore effectively inhibiting the direct contact between cathode and anode and avoiding the subsequent electrical abuse and thermal abuse. However, the mechanism of enhanced safety needed to be demonstrated in more detail.^144,145 Additionally, how to ensure that Li metal does not leak after melting is a problem that needs additional consideration.

FIGURE 11. Safety improvement strategies upon interphases of SSLMBs. (A) LPF additive improved thermal stability between Li metal and LATP. Reproduced with permission.90 Copyright 2021, American Chemical Society. (B) In situ poly-(VC) CEI coating in ASSLMB to reduce oxygen production. Reproduced with permission.146 Copyright 2020, Elsevier. (C) Schematic illustrating the interfacial stability of the LiNi0.88Co0.11Al0.01O2 electrodes in all-solid-state cells with halide or sulfide SSEs. Reproduced with permission.147 Copyright 2021, Wiley. (D) Advanced PCEE suppressed Li dendrites growth. Reproduced with permission.160 Copyright 2022, Springer Nature.

Besides, introducing thermally stable artificial cathode electrolyte interphase (CEI) on cathode particle surfaces and adding functional fillers into cathode can solve the problems of oxygen release and poor interface compatibility of cathode. Lu et al. established a thermally stable CEI by the decomposition of lithium difluoro(oxalate)borate (LiDFOB) additive. In situ heat-initiated polymerization of vinylene carbonate (VC) also contributes to enhanced stability of CEI (Figure 11B).¹⁴⁶ This artificial CEI not only blocks the direct contact between the high-voltage cathodes and PEO-base electrolytes but also constrains the lattice oxygen and thus inhibits the release of oxygen. The ARC test further testified that the Li-PEO-p(VC)-LiCoO₂ cell displays extraordinary safety performance below 350°C. Besides, the replacement of sulfide SSEs with oxidation-resistant halide SSEs also helps to improve the stability of the cathode interface at high temperature and restrain the exothermic side reaction (Figure 11C).¹⁴⁷ The halogen elements can also better construct a stable interface layer at the cathode interface and inhibit the exothermic reaction between halide SSEs and cathode materials. Introducing oxygen-evolution-free and high thermal stability fillers into the cathode can reduce the contact area and reaction rate between cathode materials and SSEs. Nanometer-sized LiMn_0.7Fe_0.3PO₄ is an excellent representative of high thermal stability active fillers, which can both maintain the energy density of the cathode and hidden oxygen release.¹⁴⁸

As we all know, uneven Li metal deposition is one of the biggest security concerns in SSLMBs, which not only induces Li dendrite growth but also intensifies the interface side reactions.^149–151 Especially, the Li dendrite penetration can cause internal short circuits, leading to severe safety risks.^152–155 In addition to the artificial interphase design, advanced SSE development is one of the effective ways to inhibit the growth of Li dendrites.^156–159 Lee et al. created an in situ formed plastic-crystal-embedded elastomer electrolyte (PCEE), which has an excellent effect on promoting uniform Li deposition (Figure 11D).¹⁶⁰ However, serious dendrite propagation still occurs after long-term cycles, which even penetrates into the SSE layer and leads to an internal short circuit of the cell.

Overall, the side reactions and the growth of Li dendrites as well as the oxygen release underneath the electrode/electrolyte interfaces are responsible for the thermal failure of SSLMBs, and all of them should be addressed urgently. More robust artificial interphase materials and advanced safety failure mechanisms should be paid considerable attention.

Advanced external thermal protection

In general, practically battery devices are assembled by the series–parallel group of numerous unit cells to achieve high energy storage. The safety of battery devices is determined by every unit cell. Heat accumulation and propagation in battery devices under external abuse conditions can lead to catastrophic thermal safety issues. Once one unit cell goes through thermal runaway, an exponential heat spread can cause the entire pack to fail quickly and catastrophically. Therefore, high-efficiency thermal dissipation and/or insulation materials are developed to enhance the safety of battery devices (Figure 12A).

FIGURE 12. External security protection and early warning for SSLMBs. (A) Advanced outer protection methods of SSLMBs. (B) Gassing detection in the SSLMBs pouch cell by ultrasonic transmission image. Reproduced with permission.169 Copyright 2022, American Chemical Society. (C) Early internal short circuit detection with a bifunctional separator in SSLMBs. Reproduced with permission.172 Copyright 2014, Springer Nature.

Thermal protective materials can alleviate the self-heating of the unit cell and heat spread into adjacent unit cells so that the temperature of the battery does not rise too high and result in thermal runaway. At present, the main way of heat dissipation is air cooling/water cooling combined with heat dissipation plates. But this system is still insufficient for battery thermal management, more powerful cooling systems and new materials are needed to be designed. Yue et al. proposed a hybrid battery thermal management system that incorporates micro heat pipe arrays, forced air, and spray water to provide efficient cooling service, which can handle the heat production of daily use and momentary heating for large-size battery packs.¹⁶¹ Besides, phase change materials (PCM), which can absorb large amounts of heat in a short period in the form of phase change, have great potential in thermal regulation and suppression of abnormal overheating of single cells.^162,163 Several studies have proved that the introduction of appropriate PCM in the heat dissipation of batteries can effectively inhibit the thermal spread caused by the thermal runaway of a single battery.^164,165 Even if a single cell exhibits a significant thermal runaway, sufficient insulation and flame-retardant materials can prevent the thermal spread process and provide sufficient time for alarming and evacuating. Although many literatures have reported the application of flame-retardant materials in traditional LIBs, the fire resistance of commonly used flame-retardant materials is relatively lower than that of Li metal combustion, which even exceeds 1500°C.^166,167 How to select appropriate flame-retardant materials for Li metal batteries is still challenging and needs to be further discussed.

Safety detection and warning

The earlier warning can effectively inhibit the occurrence of the battery thermal runaway and even provide more time for people to evacuate if inevitable. Conventional temperature detection and voltage detection methods have been quite mature, but these methods do not fully confirm the health state of the battery. The pseudo-two-dimensions model and further neural networks algorithms can quickly and efficiently calculate thermal runaway processes in systems consisting of many batteries.^162,168 With the booming development of big data, the optimization algorithms and the accumulation of databases can provide more accurate predictions of SSLMBs at operating conditions. Some advanced detection methods in LIBs can also be applied directly to SSLMBs. For example, in situ x-ray CT and ultrasonic imaging can observe the interface evolution and so determine whether the battery is functioning properly. Huo et al. employed ultrasonic imaging to nondestructively investigate the interfacial stability in SSLMBs (Figure 12B).¹⁶⁹ Moreover, the distribution of relaxation times analysis of electrochemical impedance is able to refine the interface evolution and battery operating status.^170,171 Apart from the common monitoring methods, the unique configuration of SSLMBs and the high mechanical strength of SSEs allow for more novel and targeted detection methods. Li dendrite growth in SSLMBs is generally responsible for battery failure. A functional network that can conduct both Li-ions and electrons was implanted into the SSE layer, which can detect the growth of Li dendrites in SSE earlier and simply, providing early warning for the occurrence of internal short circuit like in liquid LMBs (Figure 12C).¹⁷² In summary, the high mechanical modulus and self-supporting properties of SSEs allow researchers to develop more distinctive detection methods in SSLMBs. However, the researches on safety detection and warning in SSLMBs are still rare and much more effort should be conducted in the future.

SUMMARY AND PERSPECTIVE

SSLMBs have aroused extraordinary interest in academia and industry due to their high safety and high energy density. But a high energy density also indicates a high safety risk. Despite the continuous development and succusses in promoting safe SSLMBs, the understanding of the thermal safety mechanism of SSLMBs is still elusive, which is difficult to guide the rational design of intrinsically safe SSLMBs. Herein, a comprehensive review of the thermal safety failure mechanism is systematically summarized at material, electrode, and device scales, respectively. The thermal stability of electrode materials, interface side reactions, and battery thermal failure mechanism under different abuse conditions are also discussed in detail. Furthermore, the corresponding detection methods and solution strategies to improve the safety of SSLMBs are also included.

1. Material level: Flies in the ointment, all the major materials in SSLMBs have their own thermal safety hazards. Li metal is both the “holy Grail” to improve battery energy density and the “roadblock” to hinder SSLMBs safe applications. The use of Li metal anode not only introduces its high specific capacity and the lowest electrode potential but also brings a huge challenge to battery safety due to its high reactivity, low melting temperature and combustion. In order to improve the safety of Li metal, Li alloy skeletons are considered one of the effective measures. But the safety failure of Li alloy anodes is still unclear and needs to be researched. The O₂ released from the decomposition of cathode active materials with high operating voltage and capacity under heating/overcharging is another obstacle toward the intrinsically safe SSLMBs. Elemental doping and surface coating are considered to be one of the effective solutions to address the problems. However, the effects of these strategies on the thermal safety of practical batteries need to be further studied. Plentiful improvements have been made to the air instability and flammability of SSEs, including adding flame retardants and surface modifications, but they remain in the validation of materials and models.
2. Electrode level: New thermal hazards have been brought by the electrochemical processes and chemical reactions at both electrode interfaces. Dendrite growth upon Li metal anode and its propagation into SSEs are intractable problems despite the high mechanical strength of SSEs, which will cause severe interface side reactions, interface degradation, and even internal short circuits. The Li surface decoration, the use of Li alloy, SSE modifications as well as artificial interphases can significantly improve the interface compatibility of Li anode with SSEs and inhibit Li filament propagation. On the counter electrode, the incompatibility of high-voltage cathodes with SSEs also leads to the oxidation of SSEs and the structure degradation of cathodes after charging. Troubles are sowing by consequent exothermic reactions as well as the evolution of oxygen, which immensely affects the safe operation of SSLMBs. The surface coating, functional fillers, antioxidant and flame-retardant additives are usually used to construct a stable CEI to reduce cathode surface potential or reduce/absorb the O₂ release, thus remitting the interface side reactions. However, there is still a lack of in-depth recognition of the interface chemistry in SSLMBs. The high reactivity of Li metal and the high oxidation activity of high-voltage cathodes still pose many challenges.
3. Device level: SSLMBs are not impregnable but rather porous. Mechanical abuse, thermal abuse, and electric abuse can put SSLMBs on the verge of thermal failure. But the thermal failures of SSLMBs are always attributed to the failure of materials and subsequent electrodes. In addition to the improvements in materials and electrodes, external battery protection technologies including anti-collision materials, better thermal insulation, enhanced heat dissipation as well as advanced detections are used to ensure the safe operation of SSLMBs. Prevention is better than cure. Battery management systems and early warning are also adopted to observe the health status of every single cell, which can provide a timely response to shield the damaged cell and avoid catastrophic heat spread. In addition, artificial intelligence including machine learning and big data is regarded as one of the effective tools to predict the cycle life of SSLMBs. But better prediction models and more distinctive detection methods need to be developed.

The application of SSLMBs is a double-edged sword that increases the battery energy density while safety risk. A comprehensive understanding of thermal failure mechanisms is important not only to guide the development of SSLMBs but also to facilitate the deployment of SSLMBs under practical conditions. Therefore, we hope our review can provoke the attention to the SSLMB thermal failures among the academia and industry as well as promote the development of safer SSLMBs.

ACKNOWLEDGMENTS

This work was supported by the Beijing Municipal Natural Science Foundation (L223009), the National Key Research and Development Program of China (2021YFB2500300), the National Natural Science Foundation of China (22075029), the Key Research and Development (R&D) Projects of Shanxi Province (2021020660301013), and the Fundamental Research Funds for the Central Universities.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.