Enhanced Performance of Different Carbon Coated Nano-LiFePO4/C Composites for Lithium Battery By Vacuum Thermal Decomposition

Abstract

Nano-LiFePO4/C composite cathode materials were synthesized by vacuum thermal decomposition method, using sucrose, polyvinyl alcohol (PVA) and citric acid (CA) as different carbon sources. The samples had systematically been investigated with X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Raman spectroscopy, element analysis, electrochemical impedance spectroscopy and charge/discharge measurements. Under vacuum sintering condition, the thermal decomposition rate of organic carbon source was significant accelerated and active carbon were formed to coat LiFePO4 particle. The obtained LiFePO4/C sample showed highly crystalized olivine-type structure and spherical-like particles of 100-300 nm, delivering an enhancement of rates capacity than traditional argon sintered LiFePO4/C material. Besides, the effect of different carbon sources (sucrose, PVA and CA) on the properties of LiFePO4/C composite was paid special attention. SEM and TEM analysis showed that all the LiFePO4/C samples had nano size particle but differ in carbon morphology and layer thickness. After vacuum thermal decomposition, the residual carbon content decomposed by sucrose, PVA and CA was about 2.5%, 3.1% and 1.8%, respectively. EIS spectra indicated that the sucrose coated LiFePO4/C electrode had the lowest charge transfer resistance (Rct=50 Ω) and highest lithium diffusion coefficient. As a consequence, the sucrose coated LiFePO4/C synthesized under vacuum decomposition exhibited the best discharge capacity of 155.1, 153.7, 148.6, 140.3, 123.9 and 80.1 mA·h·g-1 at 0.2 C, 0.5 C, 1 C, 2 C, 5 C and 10 C, respectively. The results indicated that organic carbon sources can be prior carbonized and the performance of LiFePO4/C composite can be further improved by vacuum thermal decomposition.