Synthesis of F-doped LiFePO4/C cathode materials for high performance lithium-ion batteries using co-precipitation method with hydrofluoric acid source

F-doped LiFePO4/C materials were first synthesized using a co-precipitation method followed by high-temperature treatment with hydrofluoric acid source. The structure, morphology, valence state and electrochemical performance of F-doped LiFePO4/C materials are investigated systematically. The structure analysis shows that the introduction of F alters the lattice parameters slightly, increases the lattice volume, and changes the interatomic distances. The morphology analysis indicates that the particle size of F-doped LiFePO4/C samples are slightly increased compared with LiFePO4/C sample, F doping promotes the growth of the primary particles. An interesting red shift in FTIR analysis shows that F doping induces the rearrangement of the electron cloud in the PO43−, thus impacts the intrinsic conductivity and enhances the electrochemical performance. Raman analysis reveals that the LiFePO4/C and F-doped LiFePO4/C composites almost have the same amount of sp2-coordinated carbon in the residual carbon, thus F doping is more critical to the electrochemical performance compared with the carbon coating. XPS analysis shows that F is successfully incorporated into the product, and F doping does not change the valance of elements. Therefore, F doping impacts the above intrinsic and extrinsic properties of LiFePO4/C, and those changes will significantly influence the electrochemical performance. The electrochemical analyses show that the F-doped LiFePO4/C samples perform a better high rate performance and cycling life compared with the undoped LiFePO4/C composites. Especially, the LiFePO4−xFx/C (x = 0.15) sample performs the most remarkable high rate performance and an excellent cycling life and capacity retention, the discharge capacities are 165.7, 161.1, 155.3, 150.8, 140.3, 129.8 and 115.7 mAh·g−1 at 0.1, 1, 3, 5, 10, 20 and 30 C rates, respectively. F doping can improve the inherent demerits of LiFePO4 materials, enhance the electronic conductivity, accelerate the Li+ ions diffusion coefficient, and improve the structure stability.