Stable High-Areal-Capacity All-Solid-State Batteries Enabled by Ultrathin Li ₃ YCl ₆ -Based Coatings on Nickel-Rich Layered Cathodes

Interfacial side reactions between nickel-rich lithium–nickel-manganese–cobalt oxides (NMC) and sulfur-based argyrodite electrolytes degrade cathode active materials and lead to substantial interfacial resistance in all-solid-state batteries. Halide-based electrolytes can serve as protective coatings for NMC owing to their chemical compatibility with high-nickel NMC and their wide electrochemical stability window. Few-nanometer-thick Li 3 YCl 6 -based coatings on NMC811 have been successfully synthesized via a liquid deposition process followed by low-temperature annealing at 175 °C. A solid-state half-cell using the coated NMC811 and a sulfur-based catholyte shows a reversible capacity of 200 mAh/g and exhibits significantly improved cycling performance compared to bare NMC, using a dry cathode with 80 wt % active material and a high-areal loading of 4 mAh/cm 2 cycled to 4.3 V. Surface analysis combined with density functional theory calculations reveal that the improved performance originates from the stabilization of surface reactive oxygen on bare NMC through the formation of lithium yttrium oxychloride and lithium yttrium hydroxyl chloride. These processes are predominantly driven by ion-pair charge transfer and Lewis acid–base donor–acceptor interactions, respectively. This work demonstrates an example of using a small amount of halide-based electrolyte to design high-performance all-solid-state batteries.