Synthesis and Electrochemical Performance of Na and F Elements Co-Doped LiFePO₄/C as a Cathode Material for High-Rate Lithium-Ion Batteries and the Mechanism of Modification

LiFePO₄ has become a mainstream cathode material, owing to its good safety and low price. However, its intrinsic limitations in terms of electronic and ionic conductivities hinder its further advancements. To overcome this challenge, element doping has been explored to enhance its conductivity. Here, Na and F co-doped LiFePO₄/C was successfully synthesized via a carbothermic method. Structural characterization techniques were employed to confirm the formation of Li_0.97Na_0.03FePO_3.97F_0.03 with controllable particle sizes. The incorporation of Na and F ions into the LiFePO₄ lattice results in a reduction of the Li-O bond energy. This decrease in bond energy facilitates the extraction and insertion of lithium ions during the charge and discharge processes, subsequently improving the overall kinetics of lithium-ion diffusion. Simultaneously, lattice distortion induced by doping led to charge compensation and accelerated Li⁺ migration. Benefiting from the enhancement of intrinsic conductivity and Li⁺ diffusivity, the specific capacitance of the co-doped LiFePO₄/C cathode material was found to be 160 and 110 mAh g⁻¹ at 0.1 and 5 C, respectively. Further, density functional theory calculations showed that Na/F co-doping enhanced the structural evolution in the material and increased free carrier concentration in LiFePO₄. This work demonstrates that synergistic Na and F co-doping can optimize the rate performance of LiFePO₄ by enhancing intrinsic conductivity and Li⁺ diffusivity, suggesting promising applications of the co-doped LiFePO₄/C in high-power lithium-ion batteries for electric vehicles and grid-level energy storage systems.