Two-Phase Composition (LiFePO₄/FePO₄) and Phase Transformation Dependence on Charging Current: <i>In Situ</i> and <i>Ex Situ</i> Studies

Abstract

The study on the two-phase (LiFePO4/FePO4) transformation mechanism of LiFePO4 during charge–discharge is reported. The study is conducted using ex situ and in situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The Rietveld refinements are performed on ex situ XRD data of electrode materials collected at various stages of the first charge–discharge cycle. The results reveal the existence of solid solution at room temperature in the narrow nanophase region (0 < x < μ and 1 – η < x < 1). The biphase LixFePO4 is a mixture of Li-deficient phase LiμFePO4 and Li-rich phase Li1−ηFePO4 in μ < x < 1 – η, where μ = 0.056 and η = 0.043. The present study is conducted on samples prepared by electrochemical reaction, which provides a more realistic situation than the previous study reported on samples prepared by the chemical process. Moreover, a similar study is also repeated by in situ XRD and XPS techniques, which also confirm the transformation of LiFePO4 into the FePO4 phase during charging and vice versa during discharging. Furthermore, the study also conducted on the fully charged state (at different current rates from 0.1 to 5C) of various electrodes by ex situ XRD found that phase transformation (LiFePO4 to FePO4) depends on the charging current rate. At the lower current, complete transformation of the LiFePO4 phase into FePO4 is observed, whereas at the higher current rate, a trace amount of residual phase LiFePO4 along with FePO4 is found in the fully charged state. Therefore, the findings of the study reveal the time-dependent lithium-ion diffusion phenomenon in LiFePO4 causing lower capacity at the higher current rate.