Interaction of Carbon Coating on LiFePO4: A Local Visualization Study of the Influence of Impurity Phases

Abstract Carbon coating is a proven successful approach for improving the conductivity of LiFePO 4 used in rechargeable Li‐ion batteries. Different impurity phases can be formed during LiFePO 4 synthesis. Here, a direct visualization of the impact of impurity phases in LiFePO 4 on a carbon coating is presented; they are investigated on a model material using various surface‐characterization techniques. By using polished ingot model materials, impurity phases can be clearly observed, identified, and located on the surface of the sample by scanning electron microscopy (SEM), focused‐ion‐beam lithography (FIB), high‐resolution transmission electron microscopy (HR‐TEM), and Raman spectroscopy. During the carbon‐coating process, the phosphorus‐rich phase is found to have an inhibiting effect (or no positive catalytic effect) on carbon formation, while iron‐rich phases (mainly iron phosphides) promote carbon growth by contributing to more carbon deposition and a higher graphitic carbon content. This finding, and the methodological evaluation here, will help us to understand and reveal the influencing factors of impurity phases on the basic carbon‐deposition process to obtain high‐performance LiFePO 4 material for future energy‐storage devices.