Simple Hydrothermal Preparation of High Performance LiMn0.7Fe0.3PO4/C Nano-Particle Cathode Materials

Abstract

Olivine LiFePO4 has been widely used as cathode material in lithium-ion batteries for electric vehicles (EV) due to high safety and long life. Its main drawback is low voltage plateau (3.5V). Recently, studies of LiMnPO4 have been paid more attention because it possesses the same structure and theoretical capacity as LiFePO4, but higher voltage plateau (4.1V). However, the poor conductivity limits its uses. Some modification methods such as ion-doping, carbon coating etc. have been investigated to improve its electrochemical properties, and the results in many papers indicated that LiMn1-xFexPO4/C (x=0.2~0.5) nano-particle composite cathode materials prepared by hydrothermal method can present promising electrochemical performance. For such method, how to use simple process to achieve LiMn1-xFexPO4/C nano-particles with high crystallization and dispersion is an important issue to be solved urgently at present. In this work, a simple and high effective hydrothermal preparation process for LiMn0.7Fe0.3PO4/C was proposed. In such process, the synthesis and carbon coating of LiMn0.7Fe0.3PO4 was completed in one-step. The structural and electrochemical properties of as-prepared materials were studied by using XRD, SEM, electrochemical impedance and charge-discharge tests technologies. The carbon sources and surfactant added in the process were optimized. The results show that when using CTAB as surfactant and glucose as carbon source, one-step process make LiMn0.7Fe0.3PO4/C obtain more uniform coated carbon, more controlled crystal particle growth and dispersion, more excellent electrochemical performance while compared with two-step process including hydrothermal synthesis followed by carbon-coating process. CTAB content exhibits little effect on crystal lattice parameters, structure and coated-carbon amounts of as-prepared materials. However, it displays great impact on the crystal particle size, carbon coating uniformity and particle dispersion. With gradual increase of CTAB content to 2.5g (in 40ml reaction liquid), as-prepared samples show an improved carbon coating uniformity, elevated Li+ diffusion coefficient, reduced electric charge transfer resistance, and remarkable enhanced capacity, rate capability and energy density. It presents a capacity of 143.5 mAh/g at 0.5C and 112.6 mAh/g at 5C, respectively. It is also noticeable that the sample exhibits an energy density of 499.3Wh/kg, 457.8Wh/kg and 349.7Wh/kg at 1C, 2C and 5C rate respectively. In addition, other surfactants that can also be used as carbon sources at the same time were also investigated, and it is found that some surfactant can substitute for CTAB+glucose to further simplify process and lower cost.