Selecting substituent elements for LiMnPO4 cathode materials combined with density functional theory (DFT) calculations and experiments

Abstract

LiMnPO4 cathode material has a high voltage platform and matches the existing electrolyte window, and thus researchers are constantly shifting their focus from LiFePO4 to LiMnPO4. However, LiMnPO4 has lower electron (ion) conductivity than LiFePO4, and besides, its delithiated phase MnPO4 will suffer thermal decomposition at lower temperatures more easily than FePO4. In order to effectively solve the above problems by elements substitution, DFT calculations are employed to screen for suitable dopants from a series of transition metals including Fe, Mg, Ni, V, Nb, Ti. Properties such as electronic structure, atomic Bader charge, O2 evolution Gibbs free energy, average voltages, and lithium ion diffusion energy barrier were evaluated. Based on the calculation, Fe is the most effective doping element because Fe doping is able to reduce the band gap of the material and improve the electronic conductivity, suppress the O2 evolution reaction of the delithiation phase and improve the thermal stability. The reason for such a situation is that Fe can form a stronger covalent bond with the surrounding O atoms to bind the escape of O. Fe doping reduces ion diffusion energy barrier to promote lithium ion diffusion. Electrochemical tests show that Fe doping can improve the electrochemical properties of LiMnPO4.