Synergistic Effects in Mixed-Metal Fluoride Conversion Cathodes in Thin-Film Solid-State Batteries

Abstract Transition-metal fluoride (TMF) conversion cathodes offer high theoretical capacities but are limited by sluggish kinetics, large voltage hysteresis, and poor reversibility. We explore if TMF performance can be improved by leveraging synergies among metal mixtures in TMF cathodes. We systematically evaluate mixed-metal TMF cathodes (TM = Cr, Mn, Fe, Co, Ni, Cu) in a thin-film solid-state battery configuration. Across 30 compositions, most mixtures exhibit synergistic effects through improved cycling and energy efficiency. Cu/Fe mixtures stand out, delivering higher average discharge voltage and capacity with reduced hysteresis compared to pure Cu and Fe cathodes. An optimum of 20–30% Cu is identified, reaching up to 1.05 Wh/g, 530 mAh/g, 2.10 V, and 70% voltaic efficiency. Long-term cycling shows the best compositions retain ∼300 mAh/g at 1C after 1000 cycles. This study shows that metal mixing is a broadly effective strategy to enhance TMF conversion cathodes.