Explaining Performance-Limiting Mechanisms in Fluorophosphate Na-Ion Battery Cathodes through Inactive Transition-Metal Mixing and First-Principles Mobility Calculations

Fluorophosphate cathodes are currently one of the most promising polyanionic sodium-ion battery cathodes and exhibit specific energies not far below oxide cathodes. To further improve fluorophosphate cathodes, their capacity must be increased, which might be possible since some sodium (Na) remains unextracted in these cathodes during cycling. In this study we attempt to answer the question of what specific mechanism limits fluorophosphate cathode capacity, which could stem from either redox-limiting or site-limiting behavior. This paper reports the synthesis, electrochemical characterization, and computational examination of Na3GaV(PO4)2F3. This test system, which was designed explicitly for uncovering the limiting factors in these structures, exhibits reversible insertion of Na+ and redox activity for V2+ through V5+ during electrochemical cycling, indicating that fluorophosphate cathodes are not fundamentally redox-limited and must be site-limited. First-principles calculations indicate that large diffusion barriers at high sodiations impose a kinetic limit on Na+ insertion in fluorophosphate cathodes, but further investigation is needed to determine capacity limits on Na+ extraction. From our combined results we also propose possible routes to improve future fluorophosphate cathodes.