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

The search for new low cost, safe, and high capacity cathodes for lithium batteries has focused attention recently on Li2FeSiO4. 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 Li2FeSiO4 polymorphs (beta(II), gamma(s), and gamma(II)) including the newly elucidated cycled structure (termed inverse-beta(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 Li2FeSiO4 to LiFeSiO4 in which the cation-cation electrostatic repulsion competes with distortion of the tetrahedral framework.