Interaction of carbon coating on LiFePO₄: A local visualization study of the influence of impurity phases

Carbon coating is a proven successful approach for improving the conductivity of LiFePO₄ used in rechargeable Li-ion batteries. Different impurity phases can be formed during LiFePO₄ 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 LiFePO₄ material for future energy-storage devices.