Heteroatom-Engineered Carbon Coatings Enable Fluorophosphate-Rich Interphases for a Thermally Robust LiFePO₄ Cathode

The renewed interest in LiFePO₄ (LFP) as a cost-effective and intrinsically safe cathode material for electric vehicles and large-scale energy storage systems has intensified efforts to overcome its limited electronic and ionic transport properties. Although carbon coating is widely used to improve electronic conductivity in LFP cathodes, a purely carbonaceous surface can accelerate electrolyte decomposition and interfacial impedance growth, particularly under high-temperature conditions. To address this issue, this study developed a scalable heteroatom-engineered carbon coating strategy by incorporating LiPF₆ during carbon layer formation to create fluorophosphate-containing modified carbon shells on LFP particles. Structural analyses confirmed that LiPF₆ incorporation did not alter the olivine structure of LFP but introduced fluorine-containing species (mainly F^-) into the carbon matrix while modifying carbon crystallinity. Among the investigated compositions, 0.8 wt % LiPF₆ provided the optimal balance between electronic conductivity and interfacial stabilization, resulting in improved rate capability and high-temperature durability. X-ray photoelectron spectroscopy and impedance analyses revealed that the modified carbon layer promoted the formation of fluorophosphate-rich interphase species (Li_xPO_yF_z), which served as a chemically stable cathode-electrolyte interphase. This interfacial chemistry suppressed electrolyte decomposition, reduced charge-transfer resistance, and preserved structural integrity during thermally accelerated cycling. These findings demonstrate that LiPF₆-derived heteroatom-modified carbon coatings can transform the carbon layer from a passive conductor into an active interfacial stabilizer, offering a practical route toward high-rate, high-temperature LFP-based lithium-ion batteries.