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 LiFePO4 after metal doping to Fe-sites. This is because the metal doped LiFePO4/C maintains its initial capacity at higher C-rates than undoped one. Using LiFePO4/C and doped LiFe0.97M0.03PO4/C (M=Al3+, Cr3+, Zr4+), which are synthesized by a mechanochemical process followed by one-step heat treatment, the Li content before and after chemical delithiation in the LiEePO(4)/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 LiFePO4/C after chemical delithiation is attributed to the low binding energy induced by weak Li-O interactions. The improved capacity retention of the doped LiFePO4/C at high discharge rates is, therefore, achieved by relatively low binding energy between Li and O ions, which leads to fast Li diffusivity.