Intercalation voltages and ion diffusion in Mn-based transition metal fluorophosphates as cathode materials for Na-ion batteries: a synergistic experimental and theoretical approach

Transition metal fluorophosphates of general formula Na2MPO4F (M = transition metal) are interesting cathode materials for Na-ion batteries (NIBs), thanks to predicted high intercalation voltages and high theoretical capacities. However, the practical applications of several compositions in this family of compounds is limited by effective capacities lower than the theoretical ones and by high capacity fading, particularly at high charging rates. Thanks to the synergy of a broad spectrum of experimental and theoretical techniques, this work presents an extensive characterization of the electrochemical behavior of the prospect cathode material Na2-xMnPO4F (x = 0, 0.5, 1 and 2). Ab initio calculations, performed using Hubbard-corrected Kohn-Sham density functional theory (DFT) according to the DFT + U + V scheme, confirmed that the calculated intercalation voltages for the NaMnPO4F/MnPO4F couple are outside the stability window of conventional liquid electrolytes, thus limiting the material's performances to the extraction of 1 Na-ion per formula unit. To investigate the role of Na-ion diffusion on the electrochemical properties of this system, bond valence site energy (BVSE) analyses and molecular dynamics were used to identify the main diffusion pathways, and to correlate their energetics to the distortion of the Mn-O/F octahedra at various Na concentrations. The resulting 3D diffusion pathway is characterized by a relatively high diffusion coefficient for Na-ion in the perfect crystal, suggesting that the experimentally observed performances, lower than expected particularly at high charging rates, can only marginally be attributed to limited Na-ion diffusion in the system.