The all-solid-state lithium battery (ASSLB) with lithium-rich manganese oxide (LRMO) cathode material is one of the strongest competitors for the next generation energy-storage device with high energy density. However, due to the serious interfacial reaction between LRMO and halide solid-state electrolyte (SSE), its electrochemical performance cannot meet the practical demand. In this work, in-situ electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) tests and high resolution transmission electron microscope (HR-TEM) are carried out to clarify the interfacial reaction mechanism between LRMO and Li₃InCl₆ (LIC). The active oxygen released from LRMO and phase transition of LRMO are confirmed to seriously deteriorate the LRMO/LIC interface. To solve the above problems, a uniform LiNbO₃ coating is introduced to alleviate the interfacial side reaction and phase transition of LRMO. With moderate coating amount (1.5 wt%, ∼10 nm thick), the LNO@LRMO cathode shows an initial discharge capacity of 185 mA h g ^{− 1} with 67% capacity retention after 100 cycles in halide-based ASSLB, which greatly exceeds that of bare LRMO cathode under voltage range of 2.3–4.8 V. This work provides a new perspective for improving the performance of LRMO in halide-based ASSLBs to achieve high energy density.