In situ Fe K-edge X-ray absorption spectroscopy study during cycling of Li2FeSiO4 and Li2.2Fe0.9SiO4 Li ion battery materials

Abstract

In situ X-ray Absorption Spectroscopy (XAS) results are presented for Li₂FeSiO₄ and Li_2.2Fe_0.9SiO₄, promising cathode materials for lithium-ion batteries. The aims are to establish the valence and local structure of Fe during charge and discharge to understand if the Fe³⁺/Fe⁴⁺ redox pair can be reached in the current battery design. It is found that the valence state changes between Fe²⁺ and Fe³⁺, with no evidence of Fe⁴⁺ before the onset of electrolyte degradation. There is a reversible contraction and extension of the Fe–O bond lengths during cycling while the Fe–Si distance remains constant, which underlines the stability of the Li₂FeSiO₄ material. The same observations apply to Li_2.2Fe_0.9SiO₄ cathode material indicating that changing the stoichiometry does not provide any additional structural stability.