Theoretical study on effect of Ge-S/F co-doping on crystal structure and properties of Li₂<i>M</i>SiO₄(<i>M</i> = Mn, Fe)

Abstract

The effects of Ge-S/F co-doping on the structural stability and electrochemical properties of Li₂<i>M</i>SiO₄ (<i>M</i> = Mn, Fe) crystal are systematically studied by the first-principle calculations based on density functional theory combined with the generalized gradient approximation (GGA) + <i>U</i> method. The calculation results show that the Ge-S/F co-doping Li₂<i>M</i>SiO₄ (<i>M</i> = Mn, Fe) system undergoes the site exchange between Li and M in the delithiation process. Compared with Li₂<i>M</i>SiO₄(<i>M</i> = Mn, Fe), the doped system has good toughness, and lithium ions migrate easily in the doped system. And the doped system with site exchange is more stable in the process of delithium, especially the volume change of Li₂Mn_0.5Ge_0.5SiO_3.5S_0.5 is very small, indicating that it has good structural cyclic stability. Moreover, the theoretical average deintercalation voltages of Li₂<i>M</i>SiO₄ (<i>M</i> = Mn, Fe) are reduced by Ge-S/F co-doping. The combination of the density of states with magnetic moment shows that the Ge-S/F co-doping can improve the conductivity of Li₂MnSiO₄ and delay the appearance of the Jahn-Teller effect in the Li₂MnSiO₄ system, which is beneficial to the improvement of the structural cycling stability of Li₂MnSiO₄. Meanwhile, the Ge-S/F co-doping can not only improve the conductivity of Li₂FeSiO₄, but also facilitate the removal of more Li⁺ from Li₂FeSiO₄ system, especially the complete delithium of Ge-F co-doping system is expected to be achieved.