Preparation, Structure And Properties Of LiFePO₄ By Carbothermal Reduction Method

Among the well-known Li-insertion compounds, the olivine-type LiFePO₄ is considered as one of the most promising cathode materials for rechargeable Li-ion batteries because of its low cost, low toxicity, better thermal stability and excellent safety. However, there are three drawbacks preventing LiFePO₄ to be put into commercially used. One is its low electronic and ionic conductivity, which leads to the poor rate capability. The second problem is that the synthesis of LiFePO₄ is not easy because of the +2 oxidation state of iron in the compound, and the production cost is high as expensive divalent iron precursor compounds have to be used as the starting material to synthesize LiFePO₄. Another disadvantage is the low tap-density, which results in a low volumetric specific capacity. Tremendous efforts have so far been devoted to improve the electronic conductivity of LiFePO₄ and several effective ways have been proposed. Nevertheless, the problem concerning the high production cost and low tap-density of this material remains to be solved. In this paper, LiFePO₄/C composite cathode materials have been synthesized by solid state– carbothermal reduction method using cheap Fe³⁺ compounds as iron precursors. The micro-structures, morphologies and electrochemical performances of these composites were investigated by XRD, SEM, X-ray photoelectron spectrometry (XPS), laser diffraction and scattering measurements, Raman spectra, galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS). The tap-density of LiFePO₄/C composite materials was also tested. The main results and conclusions were listed as following:1) In order to reduce the cost of LiFePO₄, carbothermal reduction approach was employed to synthesize the LiFePO₄ by using cheap Fe³⁺ compounds as iron precursors. The thermal reaction behavior of the starting materials composed by various Fe³⁺ compounds was studied, and the effect of different Fe³⁺ sources on the structure and properties of LiFePO₄/C has been discussed in detail. TG-DSC results showed that the temperature related to the formation of LiFePO₄ crystal by citrate ferric, Fe₂O₃ and Fe3O₄ was 470℃, 505℃and 525℃, respectively. SEM and galvanostatic charge-discharge results indicated that the material synthesized with Fe₂O₃ and citrate ferric as iron precursors had small particle size and superior electrochemical properties.2) LiFePO₄/C composite was synthesized by solid state– carbothermal reduction method using Fe₂O₃ and citrate ferric as iron precursors. The effects of synthetic conditions such as sintering temperature, sintering time, and citrate ferric additive amount on the physico-electrochemical properties of LiFePO₄/C have been studied. It was found that the LiFePO₄/C composites showed multi-peaks distribution in broad particle size range, and the tap-density and electrochemical performance of LiFePO₄ could be improved by varying the synthetic processes. Increasing the sintering temperature and extending sintering time resulted in higher crystallinity and tap-density, but in a larger particle size. In the range of 600⁸00℃, 700℃is the optimum synthetic temperature for the LiFePO₄/C with small particle sizes, high tap-density and perfect crystal. Increasing citrate ferric amount from 10wt.% to 30wt.%, the tap-density and electrochemical properties of LiFePO₄/C firstly enhance then decrease. In the present case, LiFePO₄/C synthesized at 700℃for 24h with 20wt.% citrate ferric shows best electrochemical performances and high tap-density. Under this condition, the obtained LiFePO₄/C has a tap-density of 1.40g·cm^-3 and shows an initial discharge capacity of 135 mAh·g^-1, 129 mAh·g^-1, 126 mAh·g^-1 and 110 mAh·g^-1 at 0.1C, 0.2C, 0.5C and 1.0C, respectively.3) The influences of metal ion doping modification on the structure and electrochemical properties of Li-site doped Li1-xMxFePO₄/C composite were investigated. It was found that the metal ion doping method could greatly improve the tap-density and electronic conductivity, but decrease the particle size. Among the Li-site doped composites, the Li0.99W0.01FePO₄/C synthesized by phosphotungstic acid [H₃PO₄·12(WO₃)·H₂O] shows the best electrochemical performances. The average particle size and tap-density of Li_0.99W_0.01FePO₄/C and undoped LiFePO₄/C are 0.17nm and 1.50g·cm^-3, and 0.31nm and 1.40g·cm^-3, respectively. At current densities of 0.2C, 0.5C, 1.0C and 1.5C, the Li0.99W0.01FePO₄/C composite materials have initial discharge specific capacity of 146 mAh·g^-1, 133 mAh·g^-1, 130 mAh·g^-1 and 125 mAh·g^-1, respectively.4) The effects of FePO₄·xH₂O phase structure on the structures and properties of LiFePO₄/C synthesized by solid state– carbothermal reduction method have been analyzed. It was found that the material synthesized with trigonal anhydrous FePO₄ as inorganic Fe³⁺ precursor had superior electrochemical properties to that prepared with incomplete crystal hydrous FePO₄·2H₂O as inorganic Fe³⁺ precursor. The synthetic processes of LiFePO₄/C using trigonal anhydrous FePO₄ as inorganic Fe³⁺ precursor was explored. The results show that LiFePO₄/C composite prepared at 650℃for 24h with 35wt.% citrate ferric exhibits good electrochemical performances. At current densities of 0.2C, 0.5C and 1.0C, the composite materials have initial discharge specific capacity of 138 mAh·g^-1, 128 mAh·g^-1 and 116 mAh·g^-1, respectively. After 25 cycles, the composite cathode retains 99.1 % of the first cycle discharge capacity at 1.0C.