Structure, conductive mechanism and electrochemical performances of LiFePO4/C doped with Mg2+, Cr3+ and Ti4+ by a carbothermal reduction method

Abstract

Mg²⁺, Cr³⁺ and Ti⁴⁺ with various valences at the ratio of 0.02 were used to dope, in order to improve the electrochemical performances of LiFePO₄/C. LiFe_0.98Mg_0.02PO₄/C (LFMPC), LiFe_0.97Cr_0.02PO₄/C (LFCPC) and LiFe_0.96Ti_0.02PO₄/C (LFTPC) were successfully synthesized by a carbothermal reduction method using FePO₄·2H₂O as the iron source and phenol-formaldehyde resin as the reducing agent and carbon source. The reaction mechanism is put forward. A LiFePO₄ crystallite develops very well, and the lattice constants decrease after doping. LFTPC possesses the largest conductivity of 8.01 × 10⁻⁴ S cm⁻¹, compared to LFMPC and LFCPC. The capacities of LFMPC, LFCPC and LFTPC at 0.1 C are 126.2 mA h g⁻¹, 132.3 mA h g⁻¹ and 134.7 mA h g⁻¹ respectively, which are much larger than the 122.4 mA h g⁻¹ of LiFePO₄/C. LFTPC possesses the maximum capacity of 83.1 mA h g⁻¹ at 3 C and a stable potential platform of 3.3 V. The energy gap of LFTPC is 0.61 eV, which is smaller than the 0.63 eV and 0.65 eV of Cr³⁺ and Mg²⁺ doped LiFePO₄/C, respectively. The vacancy content of LFTPC is much more than for the other samples. This improves the electronic conductivity of doped LiFePO₄/C. It is found that Ti⁴⁺ plays a significant role in improving the electronic conductivity and performances of LiFePO₄/C.