Phase stability of N substituted Li2−xFeSiO4 electrode material: DFT calculations

Abstract First principles calculations are used to focus on the phase stability of N substituted Li 2 FeSiO 4 . The ground state structures of Li 2− x FeSiO 4− y N y ( x  = 0, 1, 2; y  = 0.5, 1) are determined by including the site-exchange of Li and Fe, on the basis of the Pmn2 1 structure of Li 2 FeSiO 4 . Calculated B / G (quotient of bulk to shear modulus) values of delithiated FeSiO 4− y N y are 1.33 ( y  = 0.5) and 0.94 ( y  = 1) lower than the critical value 1.75 which separates ductile and brittle materials. The elastic–brittle FeSiO 4− y N y would induce a structural collapse and a capacity fading of Li 2 FeSiO 4− y N y during the charge/discharge cycles, under a significant volume change of more than 32.0%. The calculated results suggest that the phase stability is important for the electrochemical performance of Fe-based silicates as well as the deinsertion voltage.