Two-Phase Composition(LiFePO_4/FePO_4) and Phase Transformation Dependence on Charging Current: In Situ and Ex Situ Studies

摘要

The study on the two-phase (LiFePO_(4)/FePO_(4)) transformation mechanism of LiFePO_(4) during charge–discharge is reported. The study is conducted using iex situ and iin situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The Rietveld refinements are performed on iex 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  ix   μ and 1 – η ix 1). The biphase Lii_(x) FePO_(4) is a mixture of Li-deficient phase Li_(μ)FePO_(4) and Li-rich phase Li_(1−η)FePO_(4) in μ  ix   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 iin situ XRD and XPS techniques, which also confirm the transformation of LiFePO_(4) into the FePO_(4) 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 iex situ XRD found that phase transformation (LiFePO_(4) to FePO_(4)) depends on the charging current rate. At the lower current, complete transformation of the LiFePO_(4) phase into FePO_(4) is observed, whereas at the higher current rate, a trace amount of residual phase LiFePO_(4) along with FePO_(4) is found in the fully charged state. Therefore, the findings of the study reveal the time-dependent lithium-ion diffusion phenomenon in LiFePO_(4) causing lower capacity at the higher current rate.