Antiperovskite Ionic Conductor Layer for Stabilizing the Interface of NASICON Solid Electrolyte Against Li Metal in All‐Solid‐State Batteries**

Abstract The instability of the interface between solid‐state electrolytes (SSEs) and Li metal anodes is a critical factor restricting the application of NASICON‐type ionic conductors in all‐solid‐state lithium batteries (ASSLBs). Here, the lithium‐rich antiperovskite (LiRAP) Li 2 OHBr ionic conductor is introduced as a protective layer for the NASICON‐type solid electrolyte Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) by a simple way of rapid solidification of molten Li 2 OHBr, to prevent the adverse reaction caused by direct contact between LAGP and Li metal. The Li/Li symmetric cells with Li 2 OHBr protective layer exhibit long‐term cycle stability compared with the bare LAGP, and the mechanisms are analyzed by XPS measurements for the interface between SSEs and Li metal. Also, the LiFePO 4 /Li ASSLBs have been assembled to further demonstrate this advantage. This work shows that LiRAP Li 2 OHBr can improve the cycling stability of ASSLBs from the perspective of Li metal anode protection.