Constructing Uniform Ionic Conductor Coatings on LiCoO₂ Cathode to Realize 4.6 V High‐Voltage All‐Solid‐State Lithium Batteries

All solid‐state lithium batteries (ASSLBs) are identified as the next‐generation energy storage technology due to their prospects of nonflammability and improved energy density. Elevating the charging cutoff voltage of cathode materials is an effective strategy to improve the energy density of ASSLBs. However, the limited oxidative stability of solid‐state electrolytes (SEs) and structural and chemically irreversible changes in the cathode active material result in inferior electrochemical performance. Here, we synthesized nano‐Li 1.2 Al 0.1 Ta 1.9 PO 8 (LATPO) coatings on the surface of lithium cobalt oxide (LCO) by a facile ball‐milling method combined with heat treatments. This artificial intermediate phase effectively enhances the structural stability and interfacial transport kinetics of the cathode and mitigates continuous side reactions at the cathode/solid electrolyte interface. As a result, the ASSLBs with modified LCO cathode exhibit a reversible capacity of 203.5 mAh g −1 at 0.1 C and 4.0 V (corresponding to the potential of 4.6 V vs. Li + /Li), superior cycling stability (85.4% capacity retention after 500 cycles), a high areal capacity (4.6 mAh cm −2 ), and a good rate capability (62 mAh g −1 at 3 C). This study emphasizes the importance of cathode surface modification in achieving stable cycling of halide‐based ASSLBs at high voltages.