First principle calculations on pristine and Mn-doped iron fluorophosphates as sodium-ion battery cathode materials

Abstract

It is well known that fluorophosphate Na2FePO4F materials unveil promise in battery applications as sodium-ion cathode materials, particularly on account of their non-toxic features, economic and environmental advantages. In this paper, we mainly report on how the electronic conductivity and the voltage of the Na2FePO4F can be enhanced by Mn-doping, leading then to high energy density cathode materials. For this purpose, density functional theory (DFT) calculations are used to investigate the structural, electronic, and electrochemical properties of the pristine and the Mn-doped Na2FePO4F. The obtained results show a decrease of the band gap energy from 2.19 eV to 1.36 eV with the GGA + U approximation and from 2.80 eV to 1.93 eV with the HSE hybrid functional, by doping 50% of the Fe site in the Na2FePO4F lattice with Mn. This indicates an improvement in the electronic conductivity of the pristine material as confirmed by the calculation of effective masses. On the other hand, the Climbing Image Nudged Elastic Band (CI-NEB) calculations unveil a clear drop in the diffusion energies of the three similar paths in doped Na2FePO4F, which indicates the easy transportation of Na+ inside the lattice. The average voltage and quality factor-Q calculations reveal encouraging results for the 50% Mn-doped Na2FePO4F. These findings show the potential of Na2Fe0.5Mn0.5PO4F as a promising cathode material for Na-ion batteries.