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 Li2FeSiO4. The structure of Li2FeSiO4 undergoes significant change on cycling, from the as-prepared gamma(s) form to an inverse beta(II) polymorph; therefore it is important to establish the structure of the cycled material. In gamma(s) half the LiO4, FeO4, and SiO4 tetrahedra point in opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the inverse beta(II) polymorph on cycling involves inversion of half the SiO4, Feat, 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.