Structural properties and energetics of Li2FeSiO4 polymorphs and their delithiated products from first-principles

Structural properties, thermodynamic stability and delithiation process for Li2FeSiO4 polymorphs are investigated by using density functional theory (DFT) within the DFT + U framework. Three Li2FeSiO4 polymorphs crystallizing in space group Pmn2(1), P2(1)/n, and Pmnb have been considered. The investigations demonstrate that the strong Si-O bonds remain almost unchanged during the lithiation-delithiation process for all the polymorphs, which contribute significantly to the structural stability. On the other hand, the differences in local environment around FeO4 tetrahedra will be translated into varying degrees of distortion, which shows a significant influence on the structural stability and average voltages. The average voltages obtained here are in good agreement with the experimental values. Furthermore, the possibility of extracting more than one lithium ions per formula unit from Li2FeSiO4 of P2(1)/n is also discussed.