Enhanced electrochemical performance of LiFePO₄/C nanocomposites due to in situ formation of Fe₂P impurities

Abstract

We have studied LiFePO₄/C nanocomposites prepared by sol-gel method using lauric acid as a surfactant and calcined at different temperatures between 600 and 900 °C. In addition to the major LiFePO₄ phase, all the samples show a varying amount of in situ Fe₂P impurity phase characterized by x-ray diffraction, magnetic measurements, and Mössbauer spectroscopy. The amount of Fe₂P impurity phase increases with increasing calcination temperature. Of all the samples studied, the LiFePO₄/C sample calcined at 700 °C which contains ∼15 wt% Fe₂P shows the least charge transfer resistance and a better electrochemical performance with a discharge capacity of 136 mA h g⁻¹ at a rate of 1 C, 121 mA h g⁻¹ at 10 C (∼70 % of the theoretical capacity of LiFePO₄), and excellent cycleability. Although further increase in the amount of Fe₂P reduces the overall capacity, frequency-dependent Warburg impedance analyses show that all samples calcined at temperatures ≥700 °C have an order of magnitude higher Li⁺ diffusion coefficient (∼1.3 × 10⁻¹³ cm² s⁻¹) compared to the one calcined at 600 °C, as well as the values reported in literature. This work suggests that controlling the reduction environment and the temperature during the synthesis process can be used to optimize the amount of conducting Fe₂P for obtaining the best capacity for the high power batteries.