Molten Salt Synthesis of a Single-Crystal LiNi0.5Mn1.5O4 Cathode with an In Situ Constructed Stable Interface for 4.8 V-Class All-Solid-State Batteries
High-voltage, Co-free spinel LiNi0.5Mn1.5O4 (LNMO) is a cost-effective cathode for all-solid-state lithium batteries (ASSLBs), but its interface with solid electrolytes (SEs) suffers from severe side reactions and poor contact, particularly at voltages exceeding 4.5 V. Here, commercially available polycrystalline LNMO powders are converted into submicron single-crystal LNMO via a molten salt synthesis route. The molten salt acts as a flux, fusing adjacent primary particles and promoting grain growth. Concurrently, a thin, uniform Li2MoO4 layer forms in situ on the single-crystal surface, which suppresses interfacial side reactions at high voltages. When paired with the halide SE Li3InCl6 (LIC), the single-crystal LNMO delivers a higher specific capacity than pristine polycrystalline LNMO, owing to improved contact and kinetics in the composite cathode. The in situ Li2MoO4 coating enables stable long-term cycling with a 4.8 V upper cutoff voltage. This work demonstrates that molten salt synthesis simultaneously addresses microstructural and interfacial limitations, providing a viable path for high-energy-density ASSLBs using commercially scalable LNMO precursors.