Study On Synthesis And Modification Of LiFePO₄ Used As Cathode Materials For Lithium-Ion Batteries

Lithium ion battery olivine-type cathode material LiFePO₄ is gaining particular interest as a potential candidate cathode material for rechargeable Li-ion batteries because of low cost , environmental benigh, excellent cyclic performance and stand-out safety etc. In this paper, LiFePO₄ dopping with metallic ion and LiFePO₄/C composite cathode materials have been synthesized through solid-state route. Adopting TG-DTA measurement to research on the reaction mechanism of the precursor mixtures, the micro-structures and morphologies of these compounds were investigated by XRD, SEM and XPS, meanwhile. The electrochemical performances have been evaluated by galvanostatic charge-discharge and cyclic voltammetry. The effects of the synthesis parameters on the physico-electrochemical properties of LiFePO₄/C composite cathode materials have been discussed in detail. The materials were synthesized using Fe₂O₃ or Fe₃O₄ as iron source and using asphaltum as both conductive and reductive agent precursor, respectively.Metallic ion dopped LiFePO₄ were synthesized by solid-state reaction using ferrous compound as iron precursor, include dopping in Fe site (Mg²⁺) and Li site (Cr³⁺ and Zr⁴⁺). The effects of some synthesis factors including dopant content and sintering temperature on the electrochemical properties were investigated. The results showed that the electrochemical properties of metallic ion dopped LiFePO₄ were rather bad.The cathode material was synthesized using Fe₂O₃ as iron source and asphaltum as both reductive agent and carbon source. The influences of the asphaltum's addition on the electrochemical properties of materials were investigated. It was found that 17 mass% would be the best addition of asphaltum in our experimental conditions. We also did some research on the influences of pre-treatment technics, ball-milling speed and the opportunity adding carbon, accordingly confirm the 200 r/min's ball-milling speed, thoroughly grinding of the material during the synthesis, and the results showed that the best time for adding asphaltum is after ball-milling before pre-sintering. The effects of synthesis conditions such as pyrolyzing temperature, sintering atmosphere and sintering time on the physico-electrochemical properties of the materials prepared were investigated. It was found that increasing the sintering temperature leads to higher crystallinity, but to a larger particle size, and the formation of electrochemically inert Fe₂P in the product. 700℃was the optimum synthetic temperature for LiFePO₄ with perfect crystal and uniform small particle sizes. The sintering atmosphere had great impact on the purity and electrochemical properties of the production, the result showed that LiFePO₄ with high purity, high crystallinity and pefect electrochemical performance would be gained when the sintering atmosphere was all argon. When sintering in argon at 700℃, the effect of sintering time on the electrochemical performance was great, it was found that 6 h was the optimum sintering time under our experiment condition. The results of XPS measurement showed that the valence of element of Fe and P on the surface of materials is +2 and +5, respectively.High crystallinity, pure LiFePO₄/C composite was synthesized at 700℃in Ar/H2 using Fe₃O₄ as iron source and asphaltum as both reductive agent and carbon source. Its initial discharge capacity is 112 mAh/g at 0.1 C rate, and the capacity gradually decreased with cycling, this behavior fitted the trency of conventional LiFePO₄, and the material showed good cyclic stability at 0.2 C and 1.0 C. The XPS results showed that there are no Fe³⁺ and Fe₂P on the surface of the product.The effects of sintering atmosphere and the addition of asphaltum on the electrochemical properties of the materials prepared were researched. It was found that LiFePO₄/C with pefect electrochemical performance would be gained when the sintering atmosphere was all argon, and confirmed that the better content of asphaltum added was 17 mass%. Also, through experiment we could deny that the more carbon we added, the better performance we would get.