Solid-state batteries represent a significant advancement in energy storage technology, with their behavior at elevated temperatures being a critical area of study. The performance of these batteries under thermal stress is largely determined by the stability of electrode-electrolyte interfaces and the temperature-dependent evolution of ionic conductivity. These factors vary substantially across different solid electrolyte classes, namely oxide, sulfide, and polymer-based systems. Understanding these differences is essential for optimizing battery designs for high-temperature applications.

At elevated temperatures, interfacial stability becomes a major concern for solid-state batteries. The physical and chemical interactions between solid electrolytes and electrodes can degrade, leading to increased impedance and capacity fade. Oxide electrolytes, such as garnet-type Li7La3Zr2O12 (LLZO), demonstrate excellent thermal stability, maintaining structural integrity up to 600°C. However, their rigid nature often leads to poor interfacial contact with electrodes, exacerbating delamination and crack formation as temperatures rise. Sulfide electrolytes, like Li10GeP2S12 (LGPS), exhibit better interfacial adhesion at room temperature but suffer from decomposition when exposed to temperatures above 200°C. This decomposition releases volatile byproducts that further degrade battery performance. Polymer electrolytes, including PEO-based systems, soften at elevated temperatures, improving interfacial contact but also increasing the risk of short circuits due to enhanced electrode penetration.

Ionic conductivity in solid-state electrolytes is highly temperature-dependent, following Arrhenius-type behavior. Oxide electrolytes typically have low room-temperature conductivity but show a gradual increase with temperature, reaching values around 10^-3 S/cm at 100°C. Their activation energies range between 0.3-0.5 eV, indicating a moderate temperature sensitivity. Sulfide electrolytes possess higher baseline conductivity at room temperature but experience more pronounced degradation at elevated temperatures due to structural rearrangements and partial melting. Their conductivity can peak around 10^-2 S/cm near 100°C before declining sharply. Polymer electrolytes display the most dramatic temperature dependence, with conductivity increasing by several orders of magnitude between 60-100°C as the polymer matrix undergoes phase transitions. However, this comes at the cost of mechanical stability.

The thermal expansion behavior of solid electrolytes significantly impacts battery performance at high temperatures. Oxide electrolytes have low thermal expansion coefficients, typically below 10 ppm/K, which helps maintain dimensional stability but can create stress at interfaces with electrodes that expand more. Sulfide electrolytes exhibit higher expansion coefficients, around 20-30 ppm/K, leading to greater interfacial strain during thermal cycling. Polymer electrolytes show the most substantial thermal expansion, often exceeding 100 ppm/K, which can cause electrode separation or short circuits in constrained battery configurations.

Chemical stability at elevated temperatures varies markedly between electrolyte types. Oxide electrolytes are generally inert against lithium metal up to 300°C, making them suitable for high-temperature lithium-metal batteries. Sulfide electrolytes begin reacting with lithium above 150°C, forming interfacial layers that increase resistance. Polymer electrolytes can undergo oxidative degradation at the cathode interface when temperatures exceed 80°C, particularly when high-voltage cathodes are used. The decomposition products from these reactions often catalyze further degradation, creating a runaway effect that accelerates battery failure.

Mechanical properties evolve differently with temperature across electrolyte classes. Oxide electrolytes maintain their rigidity up to their melting points, providing good dendrite suppression but poor accommodation of volume changes during cycling. Sulfide electrolytes soften progressively with temperature, potentially allowing lithium penetration at stresses lower than those required at room temperature. Polymer electrolytes transition from glassy to rubbery states, losing most of their mechanical strength at temperatures above their glass transition points, typically in the 50-70°C range.

The thermal conductivity of solid electrolytes affects heat distribution within batteries. Oxide electrolytes have relatively high thermal conductivity, around 1-2 W/mK, helping to mitigate hot spots. Sulfide electrolytes exhibit lower thermal conductivity, approximately 0.5 W/mK, which can lead to uneven temperature distributions. Polymer electrolytes have the lowest thermal conductivity, often below 0.2 W/mK, exacerbating thermal management challenges in large-format cells.

Practical considerations for high-temperature operation differ among electrolyte systems. Oxide-based batteries require sophisticated engineering to compensate for poor interfacial contact, such as the use of compliant interlayers or nanostructured electrodes. Sulfide-based systems need encapsulation to prevent decomposition and gas evolution at elevated temperatures. Polymer-based batteries must balance the benefits of increased conductivity at higher temperatures against the risks of mechanical failure and accelerated degradation.

Long-term stability studies reveal distinct degradation patterns. Oxide electrolyte interfaces tend to fail through progressive crack propagation, with cycle life decreasing by approximately 50% for every 50°C increase above 100°C. Sulfide systems show rapid capacity fade above their stability threshold, often losing 80% capacity within 20 cycles at 200°C. Polymer electrolytes experience both chemical and mechanical degradation, with performance declining steadily over time at elevated temperatures rather than failing catastrophically.

The selection of solid electrolytes for high-temperature applications requires careful consideration of these factors. Oxide systems excel in extreme temperature environments where chemical stability is paramount, despite their interfacial challenges. Sulfide electrolytes offer superior performance in moderate temperature ranges but require careful thermal management. Polymer systems provide the easiest processing and best interfacial properties at slightly elevated temperatures but have the narrowest operational window. Future developments in hybrid and composite electrolytes may combine the advantages of these materials while mitigating their individual limitations for high-temperature battery applications.