The effect of non-polar solvent on the structural properties and electrochemical performance of LiFePO4/C cathode materials synthesized by solid-state reaction

Abstract

LiFePO₄/C has been successfully synthesized using surfactant-assisted solid-state reaction method to investigate the effects of non-polar solvents on structural properties and electrochemical performance. Petroleum jelly, oleic acid, and sucrose were used as non-polar solvents, surfactants and carbon sources. The ratio of petroleum jelly and oleic acid were 0.5:1 (LFP A), 1:1 (LFP B), and 2:1 (LFP C). The XRD, FE-SEM, and HR-TEM results show that adding petroleum jelly in LFP C enhances crystallinity and improves the morphology of nanoplates in LiFePO₄ material. The EDS and Raman Spectroscopy tests show that the higher addition of petroleum jelly increases carbon percentage and carbon layer defects. The highest Li-ion diffusion coefficient was calculated by LFP C of 4.21 \(\times\) 10^–15 cm².s⁻¹. Furthermore, the highest discharge test results at 0.1 C of LFP A, LFP B, and LFP C were 125 mAh.g⁻¹, 103 mAh.g⁻¹, and 144 mAh.g⁻¹, respectively. However, C-rate performance shows that the specific capacity of LFP A, LFP B, and LFP C at 5 C were 74 mAh.g⁻¹, 35 mAh.g⁻¹, and 59 mAh.g⁻¹, respectively. The cyclability test results showed that LFP A capacity retention after testing for 100 cycles was better than LFP C, and the lowest stability was obtained by LFP B. The addition of petroleum jelly improved the performance of LiFePO₄/C but resulted in excess carbon in active material which decreased battery stability and specific capacity at high C-rate. Our results suggest that non-polar solvents can be added to LiFePO₄/C synthesis to improve electrochemical performance but less carbon chains must be chosen.