Hierarchical LiFe_0.98M^II_0.02PO₄/C (M^II = Ni, Mn, Mg) composites via co-doping for lithium ion batteries

Hierarchical LiFe_0.98M^II_0.02PO₄/C (M^II = Ni, Mn, Mg) composites via co-doping with a variety of cations synthesized through carbothermal reduction assisted with spray pyrolysis are first studied. The physicochemical properties are characterized by X-ray diffraction (XRD), Elemental analyzer, Raman spectroscopy measurements, Field-emission scanning electron microscope (FE-SEM) and High-resolution transmission electron microscope (HR-TEM). The electrochemical performance is characterized via galvanostatic charge/discharge tests, Cyclic voltammetry (CV) test and Electrochemical impedance spectroscopy (EIS) measurement. The results show that successful co-doping with different substitution portfolios can stabilize the crystal structure. What's more, trace ions incorporated into LiFePO₄ can also serve as catalyst for graphitization and crystal growth. Especially, among all the co-doped LiFe_0.98M^II_0.02PO₄/C (M^II = Ni, Mn, Mg) composites, LiFe_0.98Ni_0.01Mn_0.01PO₄/C exhibits the most outstanding electrochemical activities. The LiFe_0.98Ni_0.01Mn_0.01PO₄/C composite displays a high discharge capacity of ~135 mAh/g at 1 C with the capacity retention close to 95% after 100 cycles. The much more excellent ratability and superior cyclability of LiFe_0.98Ni_0.01Mn_0.01PO₄/C might be ascribed to the synergistic effect of vicariant lithium phosphates, that is, LiFePO₄, LiNiPO₄ and LiMnPO₄.