The Root Cause of the Rate Performance Improvement After Metal Doping: A Case Study of LiFePO4

Abstract

This study investigates a root cause of the improved rate performance of <TEX>$LiFePO_4$</TEX> after metal doping to Fesites. This is because the metal doped <TEX>$LiFePO_4$</TEX>/C maintains its initial capacity at higher C-rates than undoped one. Using <TEX>$LiFePO_4$</TEX>/C and doped <TEX>$LiFe_{0.97}M_{0.03}PO_4$</TEX>/C (M=<TEX>$Al^{3+}$</TEX>, <TEX>$Cr^{3+}$</TEX>, <TEX>$Zr^{4+}$</TEX>), which are synthesized by a mechanochemical process followed by one-step heat treatment, the Li content before and after chemical delithiation in the <TEX>$LiFePO_4$</TEX>/C and the binding energy are compared using atomic absorption spectroscopy (AAS) and X-ray photoelectron spectroscopy (XPS). The results from AAS and XPS indicate that the low Li content of the metal doped <TEX>$LiFePO_4$</TEX>/C after chemical delithiation is attributed to the low binding energy induced by weak Li-O interactions. The improved capacity retention of the doped <TEX>$LiFePO_4$</TEX>/C at high discharge rates is, therefore, achieved by relatively low binding energy between Li and O ions, which leads to fast Li diffusivity.