Effects of S/F doping and Li/Mn site exchange in Li2MnSiO4: A first-principles investigation of the structural, electrochemical and electronic properties

Abstract

Effects of S and F doping and Li/Mn site exchange on structural, electrochemical and electronic properties of Li2MnSiO4 cathode materials are systematically studied by the first-principles calculations based on density functional theory within the generalized gradient approximation with Hubbard corrections (GGA+U). The calculated results about Modulus ratio B/G and Poisson ratio ν show that Li2MnSiO3.5R0.5 (R= S and F) possess excellent mechanical properties. The investigation of Young’s modulus E and Debye temperature θD indicates that the doping of S and F is beneficial to the migration of Li+. In addition, the doping of S and F can reduce the theoretical average deintercalation voltage during delithiation and improve the conductivity of Li2MnSiO4. The results also show that the site exchange of Li and Mn can effectively reduce the rate of cells volume change when one Li+ is removed in per formula unit for the doped systems. Moreover, the charge compensation is completely provided by O and S ions for the first Li+ removal in the S-doped system with Li/Mn site exchange, showing that the appearance of Mn3+/Mn4+Jahn–Teller effect can be delayed, which is beneficial in improving the cycle stability of the system. Also, F doping can play the similar role in delaying the appearance of Mn3+/Mn4+Jahn–Teller effect, which is due to extra electrons generated by the substitution of O2− with F−.