First Principle Study Of Cathode Material Li₂FeSiO₄ For Lithium-ion Battery

Orthosilicate polyanion cathode material Li₂FeSiO₄ for lithium ion battery is considered a potential candidate cathode material for application due to its good electrochemical performance such as low cost, high safety, and environmentally friendly. However, Li₂FeSiO₄ suffers from the low electronic conductivity after the battery discharged, which have been the bottleneck to be used commercially. How to solve the problem of low electronic conductivity is the key of the research on Li₂FeSiO₄ material. Although the researches on using transition metal ion doped to improve electronic conductivity of Li₂FeSiO₄ have made some progress in experiments, but the relevant theoretical research of Li₂FeSiO₄ were not investigated deeply.The electronic structure and conductive properties of Li₂FeSiO₄ system in the orthorhombic crystal structure with Pmn21 symmetry and the transition metal V-doped Li₂FeSiO₄ system have been investigated by first-principle calculations based on the density functional theory（DFT） and Boltzmann theory, the average voltage of Li2Fe1-xVxSiO4 also have been calculated at the same time. The WIEN2 k software was used to the self-consistent calculation of the crystal structure, obtained the charge density, energy bands, and density of states. Then the BoltzTraP code based on the Boltzmann theory was used to calculate the conductive properties, and the curves of the ratio σ/τ varies with temperature is presented. The main research contents and results are as follows:（1）. The results of electronic structure and conductive properties of Li₂FeSiO₄ system show that the structural stability of Li₂FeSiO₄ crystal is actually a consequence of a strong covalent Si–O bond, and Li₂FeSiO₄ crystal have only 2.8% volume variation during the lithiated/delithiated cycle. It is also found that the density of states near the Fermi level significantly contribute from Fe-3d orbital electrons. The band gap of Li₂FeSiO₄ get wider and the electronic conductivity get lower after lithium extractions. The ratio σ/τ calculation also suggested that Li₂FeSiO₄ own a better electronic conductivity than that of Li FeSiO4. In addition, the computational results reveal that Li₂FeSiO₄ material can be used as potential candidate for heat-resisting cathode for advanced lithium ion batteries.（2）. The structural stability of doped materials is demonstrated by calculating and analyzing the lattice structure. Compared with Li₂FeSiO₄ crystal, it can be found that the structure of Li2Fe1-xVxSi O4 alter slightly. The density of states near the Fermi level is mainly consists of Fe-3d and V-3d orbital electrons. The bad gap of doped materials is narrower than Li₂FeSiO₄, and the electrons can easily transition into the conduction band, indicating that conductive properties of Li2Fe1-xVxSiO4 are better than that of Li₂FeSiO₄.（3）. It is also found that the change of the doped materials structure is quite inconspicuously during the lithium ions extraction, suggesting that the V-doped materials has good structural stability during the lithiated/delithiated cycle. The density of states near the Fermi level is significantly contributes from Fe-3d and V-3d orbital electrons. The band gap of delithiated phases of Li2Fe1-xVxSiO4 still narrower than Li2 FeSi O4, indicating that LiyFe1-xVxSiO4（y=1.5, 1） possesses good conductive properties.（4）. Compared with the Li₂FeSiO₄, the average voltage of Li2Fe1-xVxSiO4 change marginally, indicating that the voltage platform of Li2Fe1-xVxSiO4 is very stable, which is useful for Li2Fe1-xVxSiO4 materials to be used in commercially.