Fe–Si networks and charge/discharge-induced phase transitions in Li2FeSiO4 cathode materials

Abstract

Structural phase transitions of electrode materials are responsible for poor reversibility during charge/discharge cycling in Li-ion batteries. Using previously developed structural databases, we investigate a structural landscape for Li_xFeSiO₄ systems at x = 1. Starting with low-energy Li₂FeSiO₄ crystal structures, we explore the crystal structures of the material in different states of charge. The as-prepared Li₂FeSiO₄ materials adopt low energy structures characterized by two-dimensional (2D) Fe–Si networks. After the removal of one Li per formula unit to form LiFeSiO₄, the structures with three-dimensional (3D) diamond-like Fe–Si networks become more energetically favorable without a significant impact on the charge capacity, which agrees with previous experimental and theoretical work. However, we reveal that the structure with a 3D diamond-like Fe–Si network can further transform into a new structure at x = 1. And the Li atom is hard to reinsert into these new structures. Consequently the system is prevented from returning to the Li₂FeSiO₄ state. We believe that the formation of this new structure plays an important role in the loss of reversible capacity of Li₂FeSiO₄ electrode materials.