Understanding Calendar Aging of Thiophosphate-Based Solid-State Batteries

While significant progress has been made to improve the energy density, power density, and cycle life of solid-state batteries (SSBs), their calendar life, which dictates the capacity retention during storage, has been seldom studied. One key difference between liquid electrolytes and solid electrolytes is the residual electronic conductivity in solids. Despite being small, the electronic conductivity in solid electrolytes can lead to perceptible self-discharge especially when the battery is not under constant use. However, its contribution to the calendar decay of SSBs has not been considered with existing studies solely focusing on the irreversible capacity loss caused by the side reactions between electrodes and solid electrolytes. Here, we present our study on calendar aging of Li 6 PS 5 Cl-based SSBs throughout 8 months of measurement at 25 and 60 °C. We report that apparent capacity decay, 4.1% at 25 °C and 7.0% at 60 °C, occurs over the entire period of the test, highlighting the critical challenge of calendar aging of SSBs. More importantly, by quantifying the irreversible and reversible capacity losses, we demonstrate the predominant role of reversible self-discharge during calendar aging of SSBs and provide new insights into improving the calendar life of SSBs.