Polymorphism and phase transformations of Li2-xFeSiO4 (0 <= x <= 2) from first principles

Abstract

Computational study on the phase stabilities and electrochemical properties of four polymorphs of Li2-xFeSiO4 (the beta(II), gamma(II), gamma(S), and inverse beta(II) polymorph) as cathode materials for a Li-rechargeable battery is conducted using first principles calculations. While gamma(S) polymorph is found to be the most stable ground state structure of fully lithiated state Li2FeSiO4 from first principles calculation, the difference in energies among polymorphs is relatively small suggesting the possibility of a presence of different polymorphs, depending on the synthesis condition. However, upon delithiation to LiFeSiO4, the beta(II), gamma(S), and gamma(II) polymorphs of Li2-xFeSiO4 become notably more unstable than the inverse beta(II), thus they transform into the inverse beta(II) during charge. This phase transformation accompanies the voltage drop about 0.3 V, which is in agreement with recent experimental observations. The calculated redox potential for Fe2+/Fe3+ in Li2FeSiO4 varies depending on the type of polymorph, that is, 3.102, 3.078, 3.023, and 2.823 V vs Li for beta(II), gamma(S), gamma(II), and inverse beta(II) polymorph, respectively. Further delithiation from LiFeSiO4 leads to the redox potential of 4.802 V vs Li for Fe3+/Fe4+, therefore, utilization of more than one Li in Li2FeSiO4 is expected to be challenging in conventional lithium batteries.