Structure and lithium transport pathways in Li₂FeSiO₄ cathodes for lithium batteries

The importance of exploring new low-cost and safe cathodes for large-scale lithium batteries has led to increasing interest in Li₂FeSiO ₄. The structure of Li₂FeSiO₄ undergoes significant change on cycling, from the as-prepared γ_s form to an inverse β_II polymorph; therefore it is important to establish the structure of the cycled material. In γ_s half the LiO ₄, FeO₄, and SiO₄ tetrahedra point in opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the inverse β_II polymorph on cycling involves inversion of half the SiO₄, FeO₄, and LiO₄ tetrahedra, such that they all now point in the same direction, eliminating edge sharing between cation sites and flattening the oxygen layers. As a result of the structural changes, Li⁺ transport paths and corresponding Li-Li separations in the cycled structure are quite different from the as-prepared material, as revealed here by computer modeling, and involve distinct zigzag paths between both Li sites and through intervening unoccupied octahedral sites that share faces with the LiO₄ tetrahedra.