Stable Nickel‐Rich Layered Oxide Cathodes Enabled by Conformal AlF ₃ Nanoshell for High‐Voltage All‐Solid‐State Batteries

ABSTRACT Chloride‐based solid‐state electrolytes (SSEs) emerge as promising catholytes for all‐solid‐state batteries (ASSBs) because of their high ionic conductivity and good oxidative stability. However, their integration with nickel‐rich layered oxides, such as LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), remains hindered by poor interfacial stability, leading to continuous performance degradation, particularly at high operation voltages. Herein, we demonstrate that the surface modification of NCM811 particles through the construction of conformal AlF 3 nanoshells enables stable battery operation at voltages up to 4.8 V with chloride‐based SSE Li 3 InCl 6 . Notably, a solution‐based route is developed to first form a uniform (NH 4 ) 3 AlF 6 precursor nanoshell, which was subsequently converted into the AlF 3 surface layer by sintering, thereby overcoming the long‐standing synthetic challenge in building AlF 3 coatings. We found that this AlF 3 surface layer not only enhances the structural robustness of NCM811 against surface degradation during the electrochemical cycling, but also effectively suppresses oxidative decomposition of the interfacial Li 3 InCl 6 electrolyte, thereby enabling a significantly enhanced high‐voltage stability (up to 4.8 V), excellent rate capability (3 C), and prolonged cyclability (≥1000 cycles). This work elucidates the critical role of surface chemistry in governing interfacial and structural evolution in chloride‐based ASSBs and provides a generalizable pathway for designing reliable high‐energy storage devices.