Dynamic study of sub-micro sized LiFePO₄ cathodes by in-situ tender X-ray absorption near edge structure

Olivine-type phosphates (LiMPO₄, M = Fe, Mn, Co) are promising cathode materials for lithium-ion batteries that are generally accepted to follow first order equilibrium phase transformations. Herein, the phase transformation dynamics of sub-micro sized LiFePO₄ particles with limited rate capability at a low current density of 0.14 C was investigated. An in-situ X-ray Absorption Near Edge Structure (XANES) measurement was conducted at the Fe and P K-edge for the dynamic studies upon lithiation and delithiation. Fe K-edge XANES spectra demonstrate that not only lithium-rich intermediate phase Li_xFePO₄ (x = 0.6-0.75), but also lithium-poor intermediate phase Li_yFePO₄ (y = 0.1-0.25) exist during the charge and discharge, respectively. Furthermore, during charge and discharge, a fluctuation of the FePO₄ and LiFePO₄ fractions obtained by liner combination fitting around the imaginary phase fractions followed Faraday's law and the equilibrium first-order two-phase transformation versus reaction time is present, respectively. The charging and discharging process has a reversible phase transformation dynamics with symmetric structural evolution routes. P K-edge XANES spectra reveal an enrichment of PF₆^-1 anions at the surface of the electrode during charging.