Surface Structure Characterization and Electrochemical Characteristics of Carbon-Coated Lithium Iron Phosphate (C-LiFePO
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Surface Structure Characterization and Electrochemical Characteristics of Carbon-Coated Lithium Iron Phosphate (C-LiFePO4) Particles Xiangcheng Sun1, Kai Sun2 and Bo Cui1 1
Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Canada 2 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 US
ABSTRACT Carbon-coated lithium iron phosphate (C-LiFePO4) particles have been synthesized by a solid-state reaction process. Particles surface morphology, olivine-type phase structures and the carbon shell-core structures are investigated in details by transmission electron microscopy (TEM, HRTEM) imaging and electron diffraction (SAED) patterns. Homogenous features of carbon coating of the LiFePO4 particles surface are obviously revealed. HR-TEM imaging and X-ray photoelectron spectroscopy (XPS) confirmed an amorphous sp2 type conducting coating layer on the surface of LiFePO4 particles. Particles shape and size showed the clear single-crystal nature of the phospho-olivine type structures with the rough spherical features of 50-250 nm size range. The characteristics of sp2 type carbon-coating on the LiFePO4 particles surfaces allows improving the electrical conductivity and reducing the diffusion path of the lithium ions, as directly evidenced from electrochemical tests of charge-discharge cycling. INTRODUCTION Considerable attention has been contributed to the phospho-olivine type lithium iron phosphate (LiFePO4) for use as an attractive cathode candidate, which was first reported as a cathode electrode for the rechargeable lithium-ion batteries in 1997 by John Goodenough [1]. Olivine-type LiFePO4 exhibits various unique advantages such as low toxicity, low cost, high thermal and chemical stability, and good electrochemical performance in the fully charged state. It has a higher theoretical specific capacity (170 mAh g−1) and a flat charge–discharge profile at intermediate voltage (3.45 V vs Li/Li+), and reasonable cycle life [2-3]. However, LiFePO4 has inherently low electrical conductivity, which results in its poor rate capability due to the poor kinetics of lithium intercalation/deintercalation process [2-3]. This raises a bigger challenge for power-demanding applications such as hybrid electric vehicles and electric vehicles [4-6]. Many approaches have been successfully employed to improve the performance, and to overcome its poor conductivity, such as metal cation doping and carbon-coating [7-12], minimizing the particle size and using smaller particle sizes [13-18]. The reason is that a small particle size of LiFePO4 could shorten the diffusion length of Li-ion while the carbon-coating would increase the surface electrical conductivity. It is apparent that carbon coating is very effective in the enhancement of capacity and rate capability [6-20]. In this study, large scale synthesis, structural morphology and electrochemical characterization of C-LiFePO4 particles has been presented. EXPERIMENT Firstly, in order to prepare homogenous carbon-co
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