Electronic structure and transport properties of cathode material Li2FeSiO4 for lithium-ion battery

Abstract

The electronic structure and properties of silicate polyanion Li2FeSiO4 in the orthorhombic crystal structure with Pmn21 symmetry and the relevant delithiated system LiFeSiO4 are investigated by the first principles method in the framework of the density functional theory with the generalized gradient approximation. The WIEN2k software is used for the self-consistent calculation of the crystal structure to obtain the energy band, density of states, and charge density. Boltzmann transport theory is further used to obtain the values of ratio σ /τ of Li2FeSiO4 and LiFeSiO4 based on the results of the first-principles calculations. The structural stability of Li2FeSiO4 system is demonstrated by calculating and analyzing the lattice parameter and the bond length. The results indicate that Li2FeSiO4 crystal has only 2.7% volume variation in the lithiation/delithiation process and the change of the Si–O bond length is very small, which suggests that the bonding nature between silicon and oxygen atoms remains unchanged. The results of charge density analysis show that the structural stability of Li2FeSiO4 crystal during lithium deintercalation is actually a consequence of a strong covalent interaction between silicon and oxygen atoms. An analysis of density of states shows that the density in the high-energy range near the Fermi level mainly comes from Fe-3d electron states. The Fermi level moves towards the lower energy end during the deintercalation of lithium ions and the electronic conductivity decreases with the decreasing of lithium ions, indicating that the conductive properties of Li2FeSiO4 are better than those of LiFeSiO4. It suggests that Li2FeSiO4 could be modified by doping atoms to affect the electrons in orbital Fe-3d and enhance conductive properties in future research. The calculations of transport properties show that the electronic conductivity of Li2FeSiO4 is not sensitive to temperature in a range from 300 to 800 K, and Li2FeSiO4 material is a potential candidate for heat-resisting cathode material. It also indicates that Li2FeSiO4 owns a better electronic conductivity than LiFeSiO4, which is consistent with the analyses of band structure and density of states. This research reveals the microscopic mechanism such as electronic structure and electronic transport properties of Li2FeSiO4 crystal in theoretical calculations, and provides a theoretical basis for the further improvement of electrochemical properties of lithium-ion battery.