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

Abstract Residual lithium compounds (RLCs) in all‐solid‐state batteries (ASSBs) employing Ni‐rich cathode materials (LiNi x Co y Mn z O 2 , 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 Li 2 CO 3 in ASSBs featuring an oxyhalide‐based AlOCl‐2LiCl (LAOC) solid‐state electrolyte (SSE) is revealed. ASSBs containing NCM with residual Li 2 CO 3 demonstrate superior electrochemical performance compared to those treated with a washing pretreatment to remove Li 2 CO 3 . Solid‐state nuclear magnetic resonance (ssNMR) spectroscopy shows that Li 2 CO 3 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 Li 2 CO 3 are confirmed in other halide‐based ASSBs. This study uncovers an unexpected role for Li 2 CO 3 in halide‐based ASSBs, offering insights that may inspire further exploration of RLCs with functional properties for improving ASSBs performance.