Nonambient Thermodynamics in Solid-State Batteries

Most solid-state electrochemical reactions, such as those in lithium-ion batteries, are described well using thermodynamics under ambient conditions. In this work, we show that the microscopic stress in the composite cathode of solid-state batteries is highly heterogeneous and can exceed several hundreds of MPa via quasi in situ confocal Raman spectroscopy, making expansion work ( p Δ V ) no longer negligible. We observed a strong correlation between the state of charge of cathode particles with their state of stress. By building a nonambient thermodynamics framework to describe cathode reactions in solid-state batteries, we identified the substantial mechanical potential (up to tens of mV) as a primary cause of capacity loss upon electrochemical cycling. Pairing a cathode and solid electrolyte of similar partial molar volumes is suggested to mitigate this issue. This work highlights the importance of considering expansion work in solid-state batteries and suggests new materials’ design rules for improved electrochemical performance.