Novel Non-aqueous Sol-Gel Synthesis of Carbon-Coated L1MPO 4 (M = Fe, Mn, Co) for Lithium Ion Batteries
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Novel Non-aqueous Sol-Gel Synthesis of Carbon-Coated LiMPO4 (M = Fe, Mn, Co) for Lithium Ion Batteries Jingsi Yang and Jun John Xu Materials Science and Engineering Rutgers, the State University of New Jersey Piscataway, NJ 08854, USA ABSTRACT A simple organic solvent based sol-gel method for synthesis of carbon-coated LiFePO4 is developed. Phase pure LiFePO4 with uniform size of 200-300 nm and no agglomeration is obtained, with 1.8 wt % surface carbon formed in-situ. Such a material gives specific discharge capacities of 150 mAh/g and 145 mAh/g at C/5 and 2C at room temperature, respectively, with desirable cycling stability. The dependence of particle size and morphology on precursor concentrations is opposite to conventional solution based methods. Other LiMPO4 (M = Mn, Co, Ni, etc) with favorable physical characteristics can be synthesized the same way. INTRODUCTION Lithium iron phosphate (LiFePO4) with the olivine structure has attracted much attention recently as a promising cathode material for rechargeable lithium ion batteries [1]. It operates at a flat voltage of 3.4 V vs Li+/Li, with a theoretical specific capacity of 170 mAh/g, and yields a theoretical gravimetric energy density comparable to that of LiCoO2, which is widely used as the cathode material in commercial lithium ion batteries. In contrast to the high cost and toxicity of the cobalt-based compound, LiFePO4 is inexpensive, non-toxic and environmental friendly. Furthermore, both LiFePO4 and its charged form FePO4 are naturally occurring minerals and are exceptionally stable both chemically and thermally. Consequently, LiFePO4 is promising for large scale lithium ion batteries for electrical vehicles and load leveling. However, the rate performance of the pristine LiFePO4 is very poor. Near theoretical specific capacity can not be achieved at practical current rates with the pristine material. Such a poor kinetic performance is largely related with the material’s low electronic conductivity, which is around 10-9-10-10 S/cm. One strategy to get around this problem is to synthesize a composite of LiFePO4 with small particle size and an electronic conductive phase, typically carbon [2, 3]. Surface carbon coating may effectively alleviate the problem of low electronic conductivity; small particle size would reduce the diffusion length for reaction with lithium and a uniform size distribution would be instrumental for maximizing electrode utilization. To realize such a strategy, an elaborately designed synthesis approach is required. Solution based synthesis methods are well known for yielding small particles [4-7]. Among various solution based methods, the sol gel method is of particular interest, considering the multi-element nature of the lithium iron phosphate [6, 7]. Furthermore, it should be advantageous if the solvent is an
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organic compound, since surface carbon coating may be readily formed in-situ during the heating process. We have designed a simple, organic solvent based sol gel method for the synthesis of lithium iron p
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