High Voltage Stable Polyoxalate Catholyte with Cathode Coating for All‐Solid‐State Li‐Metal/NMC622 Batteries

Abstract The solid polymer electrolyte (SPE) is one type of the most promising solid‐state electrolytes for next‐generation solid‐state batteries, due to its good compatibility with Li‐metal, high flexibility, and safety. To compete with currently used conventional Li‐ion batteries in energy density, SPEs must be integrated with high energy density cathode of LiNi x Mn y Co z O 2 ( x + y + z = 1, 0.5 < x < 1) (NMC). However, the application of SPEs with NMC is limited by the narrow redox window of single SPEs and interfacial decomposition of SPEs by NMC. To overcome these challenges, a strategy is proposed utilizing a polymeric‐catholyte/‐anolyte‐composed dual‐polymer electrolyte and a cathode coating: a low voltage stable polyether works as a separator and stabilizes the interface with Li‐metal, while a high voltage stable polyoxalate functions as the catholyte and NMC particles are precoated by TiO 2 . This cell model not only widens the voltage window of the electrolyte system, but also protects the polyoxalate in the cathode from interfacial decomposition. With this strategy, cycling stability of all‐solid‐state Li‐metal/LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) cells is significantly improved. Meanwhile, large volume expansions of deposited lithium on both the Li‐metal anode and the copper collector are observed, which deserve more attention in the investigation of all‐solid‐state cells.