Revealing the Underlying Role of Li₂CO₃ in Enhancing Performance of Oxyhalide-Based Solid-State Batteries

Residual lithium compounds (RLCs) in all-solid-state batteries (ASSBs) employing Ni-rich cathode materials (LiNi_xCo_yMn_zO₂, NCM) are traditionally viewed either as ionically and electronically insulating layers hindering electrochemical performance or as protective buffer layers enhancing cycling stability. In this study, a beneficial role of Li₂CO₃ in ASSBs featuring an oxyhalide-based AlOCl-2LiCl (LAOC) solid-state electrolyte (SSE) is revealed. ASSBs containing NCM with residual Li₂CO₃ demonstrate superior electrochemical performance compared to those treated with a washing pretreatment to remove Li₂CO₃. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy shows that Li₂CO₃ facilitates spontaneous Li⁺ exchange at multiple sites within the LAOC SSE. This leads to faster ion mobility and shorter relaxation times at various lithium sites, indicating enhanced ion transport and improved interface dynamics. Moreover, the beneficial effects of Li₂CO₃ are confirmed in other halide-based ASSBs. This study uncovers an unexpected role for Li₂CO₃ in halide-based ASSBs, offering insights that may inspire further exploration of RLCs with functional properties for improving ASSBs performance.