Computational design of phosphate fluoride cathode materials for Na-based batteries

Na-ion batteries are sustainable, low-cost alternatives to Li-ion batteries. However, their limited energy density has hindered a widespread adoption. Among positive electrode materials, polyanionic compounds approaching the performances of LiFePO₄ are being investigated. The Na₃V₂(PO₄)₂F₃ family of phosphate fluorides in particular has demonstrated sufficient specific capacity at high operating voltage. Combined with remarkable capacity retention and power capabilities, it entered applications in power tools. However significant concerns exist about the availability of vanadium. To find alternatives, we explored the substitution of V with other transition metals. We considered Ti, Cr, Mn, Fe, Co, Ni, Mo, Zr and Nb using first-principles calculations based on density functional theory with the r²SCAN functional. For all compounds, we investigated in detail the expected operational voltage, as well as the structural characteristics and Na⁺ mobility <i>via</i> nudged-elastic band calculations (NEB). Most metals yield too high voltages for operation within the stability window of common electrolytes, with the notable exceptions of Mn and Mo that show promising voltages over the reversible (de)intercalation of 3 Na/f.u. In all cases, the electrochemical operation is found to occur with small volume change (maximum 6% for Mn) and the computed migration barriers remain similar to vanadium's ones. Finally, we propose potential synthesis reactions for all compounds and calculate their Gibbs free energy. The never-before reported Co-, Mn- and Mo-based compounds are predicted to be synthesizable. Our work suggests the existence of novel promising positive electrode materials for Na-ion batteries, and it suggests potential synthetic routes to experimentally achieve them.