  Li2FeSiO4 is an important alternative cathode for next generation Li-ion
batteries due to its high theoretical capacity (330 mA h/g). 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 P21 modification is found to be the ground
state structure. The application of pressure originates a sequence of phase
transitions according to P21 - Pmn21 - 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 Li-ion conductivity of this material is destitute.
