First-principles study of structural stability, dynamical and mechanical properties of Li2FeSiO4 polymorphs

Li2FeSiO4 is an important alternative cathode for next generation Li-ion batteries due to its high theoretical capacity (330 mA h g(-1)). However, its development has faced significant challenges arising from structural complexity and poor ionic conductivity. In the present work, the relative stability, electronic structure, thermodynamics, and mechanical properties of potential cathode material Li2FeSiO4 and its polymorphs have been studied by state-of-the-art density-functional calculations. Among the 11 structural arrangements considered for the structural optimization calculations, the experimentally known monoclinic P2(1) modification is found to be the ground state structure. The application of pressure originates a sequence of phase transitions according to P2(1) -> Pmn2(1) -> I222, and the estimated values of the critical pressure are found to be 0.38 and 1.93 GPa. The electronic structures reveal that all the considered polymorphs have a non-metallic character, with band gap values varying between 3.0 and 3.2 eV. The energy differences between different polymorphs are small, and most of these structures are dynamically stable. On the other hand, the calculation of single crystal elastic constants reveals that only few Li2FeSiO4 polymorphs are mechanically stable. At room temperature, the diffusion coefficient calculations of Li2FeSiO4 in different polymorphs reveal that the Li-ion conductivity of this material is poor.