Solid-state electrolytes (SSEs) have emerged as a promising alternative to conventional liquid electrolytes in lithium-ion batteries due to their potential for improved thermal stability and safety. Unlike liquid electrolytes, which are flammable and prone to decomposition under high temperatures, SSEs offer inherent resistance to thermal runaway. However, their behavior under abuse conditions must be thoroughly evaluated to ensure reliability in real-world applications. This analysis focuses on the thermal stability, decomposition mechanisms, and safety profiles of SSEs, with comparisons to liquid electrolytes and reference to relevant ASTM and UL testing standards.

Thermal stability is a critical parameter for SSEs, as it determines their performance under high-temperature conditions. Most SSEs exhibit higher decomposition temperatures compared to liquid electrolytes. For example, oxide-based SSEs such as LLZO (Li7La3Zr2O12) demonstrate stability up to 1000°C, while sulfide-based SSEs like Li10GeP2S12 (LGPS) decompose at around 500°C. In contrast, conventional liquid electrolytes containing LiPF6 in organic carbonates begin decomposing at approximately 200°C, releasing flammable gases such as CO2 and HF. The higher decomposition thresholds of SSEs reduce the risk of thermal runaway, a major safety concern in lithium-ion batteries.

Under abuse conditions such as overheating, short-circuiting, or mechanical damage, SSEs exhibit distinct exothermic reactions. While liquid electrolytes undergo rapid vaporization and combustion, SSEs typically experience slower decomposition processes. For instance, polymer-based SSEs like PEO (polyethylene oxide) melt at around 60°C but do not ignite, whereas sulfide-based SSEs may release H2S gas at elevated temperatures, posing toxicity risks. Oxide-based SSEs, though thermally robust, may fracture under mechanical stress, leading to internal short circuits. These behaviors highlight the need for material-specific safety assessments.

ASTM and UL standards provide frameworks for evaluating the thermal and safety characteristics of SSEs. ASTM E537-20 measures the thermal stability of materials via differential scanning calorimetry (DSC), identifying onset temperatures for exothermic or endothermic events. UL 1973 outlines safety requirements for stationary battery systems, including thermal abuse tests such as oven exposure at 130°C for 30 minutes. SSEs must also comply with UL 9540A, which assesses fire propagation risks in energy storage systems. These standards ensure that SSEs meet minimum safety benchmarks before deployment.

Comparative studies between SSEs and liquid electrolytes reveal trade-offs in safety and performance. Liquid electrolytes excel in ionic conductivity but suffer from low flash points and high volatility. SSEs, while less conductive, eliminate leakage risks and reduce flammability. For example, garnet-type LLZO retains structural integrity up to 1000°C, whereas liquid electrolytes decompose explosively above 200°C. However, sulfide-based SSEs require careful handling due to their moisture sensitivity and potential H2S emissions, complicating large-scale manufacturing.

Decomposition pathways further differentiate SSEs from liquid electrolytes. Liquid electrolytes decompose via chain reactions involving solvent oxidation and salt degradation, often catalyzed by transition metals in cathodes. In contrast, SSEs decompose through solid-state reactions, such as phase transitions or interfacial degradation. For instance, LGPS undergoes partial decomposition to Li2S and GeS2 above 500°C, while polymer SSEs depolymerize without gas emission. These pathways influence failure modes and safety protocols.

Mechanical abuse testing reveals another dimension of SSE safety. Under crush or penetration tests, SSEs exhibit brittle fracture (oxides) or plastic deformation (polymers), whereas liquid electrolytes leak and ignite. ASTM D5485-16 evaluates material response to mechanical stress, simulating vehicle collisions or impact scenarios. SSEs with high fracture toughness, such as composite polymer-ceramic electrolytes, outperform liquid systems in these tests but may still face challenges in maintaining interfacial contact with electrodes.

Long-term thermal cycling also affects SSE stability. Repeated heating and cooling can induce microcracks in ceramic SSEs or phase separation in polymer SSEs, degrading ionic conductivity. ASTM D8336-21 provides guidelines for thermal cycling tests, exposing materials to temperature fluctuations between -40°C and 85°C. SSEs with low coefficients of thermal expansion, such as LLZO, show better resilience than sulfide or polymer alternatives.

In summary, solid-state electrolytes offer superior thermal stability and safety compared to liquid electrolytes, but their performance under abuse conditions varies by material class. Oxide-based SSEs excel in high-temperature resistance, while sulfide and polymer SSEs face challenges related to gas emission or mechanical integrity. Standardized testing under ASTM and UL protocols ensures consistent evaluation of these materials, guiding their adoption in next-generation batteries. Future research should focus on optimizing SSE compositions to balance conductivity, stability, and manufacturability without compromising safety.