Insights into Changes in Voltage and Structure of Li₂FeSiO₄ Polymorphs for Lithium-Ion Batteries

Abstract

The search for new low cost, safe, and high capacity cathodes for lithium batteries has focused attention recently on Li 2 FeSiO 4 . The material presents a challenge because it exhibits complex polymorphism, and when it is electrochemically cycled there is a significant drop in the cell voltage related to a structural change. Systematic studies based on density functional theory techniques have been carried out to examine the change in cell voltages and structures for the full range of Li 2 FeSiO 4 polymorphs (β II, γ s, and γ II ) including the newly elucidated cycled structure (termed inverse-β II ). We find that the cycled structure has a 0.18–0.30 V lower voltage than the directly synthesized polymorphs in accord with experimental observations. The trends in cell voltage have been correlated to the change in energy upon delithiation from Li 2 FeSiO 4 to LiFeSiO 4 in which the cation–cation electrostatic repulsion competes with distortion of the tetrahedral framework.