Unraveling the Mechanism of Structural Stability and Electrochemical Performance of N/F-Modified Li2FeSiO4: A First-Principles Study

The influence of N/F substitution and site-exchange of Li and Fe ions on properties of Li2FeSiO4 are investigated by first-principles calculation, including the structural parameters, mechanical, electrochemical, and magnetic properties of Li2FeSiO4. The calculated results show that site-exchange of Li and Fe ions occurs during the removal of lithium ions for Li2FeSiO3.5R0.5 (R = O, N, and F), and the cell volume changes greatly. It is found that substitution of O with F can decrease the cell volume change and improve the cyclic stability of Li2FeSiO4. Furthermore, the theoretical average deintercalation voltages can be effectively reduced by N and F doping. The densities of states and magnetic moment show that substitution of O with N and F can improve the conductivity of Li2FeSiO4, and F doping is conducive to the extraction of more Li+ from Li2FeSiO4 system.