Surfactant effect on synthesis of core-shell LiFePO₄/C cathode materials for lithium-ion batteries

Abstract

Nanospherical LiFePO₄ particle with a uniform carbon coating layer (core-shell structure expressed as LiFePO₄/C) in the presence of both low-cost FeOOH as iron resource and polyoxyethylene sorbitan monopalmitate (Tween 40) as surfactant is synthesized via a solid–liquid reaction milling method consisting of high-energy milling and pyrolysis steps. X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are used for structure, morphology, and composition characterization. Both XRD and FTIR results confirm the existence of interactions between surfactant molecules and precursor, which can benefit the formation of the core-shell structure with an average particle size of about 200 nm and a high tap density of 1.7 g cm⁻³. The electrochemical properties of as-prepared LiFePO₄/C as cathode material are investigated by electrochemical impedance spectroscopy (EIS), charge–discharge, and cycling tests. The results show that the LiFePO₄/C can achieve high discharge capacities of 163.6 and 131.3 mAh g⁻¹ under 0.1 °C at room temperature (25 °C) and sub-zero temperature (−20 °C), respectively, due to its improved conductivity. In addition, the material also shows an excellent cycling performance with capacity retention of 98.8 % (0.1 °C) after 120 cycles at 25 °C and 102.9 mAh g⁻¹ at the ends of the 200th cycle corresponding to a fading of 0.01 % per cycle.