A Chemical-Potential-Driven Self-Mitigation Mechanism during Calendar Aging

Sulfide-based all-solid-state batteries (ASSBs) suffer from severe performance decay under open circuit driven by inherent thermodynamic instability, which is defined as calendar aging. It is paramount to understand the mechanism of capacity decay during storage. In this work, the calendar aging effect was systematically investigated across varying state-of-charge (SOC). It is found that Li₆PS₅Cl exhibits a lower equilibrium voltage (2.06 V vs Li⁺/LiIn) than LiNi0.94Co0.04Al0.02O₂ (>2.96 V vs Li⁺/LiIn), indicating higher lithium chemical potential of the electrolyte. This drives spontaneous Li⁺ migration during calendar aging from the electrolyte to cathode active materials (CAMs), functioning as self-mitigation to suppress the high electrochemical activity of Li-deficient CAMs. However, high-SOC aging (beyond 50%) still induces lattice oxygen release and particle cracking. In contrast, the well-ordered structure is maintained under 25% SOC, enabling a capacity of 179.5 mAh/g, 133% higher than its 100%-SOC counterpart (77 mAh/g). These findings provide critical insights for the practical storage management of ASSBs.