Abstract
Pure lithium iron phosphate (LiFePO4) and carbon-coatedLiFePO4(C-LiFePO4) cathode materials were synthesized for Li-ion batteries. Structural and electrochemical properties of these materials were compared. X-ray diffraction revealed orthorhombic olivine structure. Micro-Raman scattering analysis indicates amorphous carbon, and TEM micrographs show carbon coating onLiFePO4particles. Ex situ Raman spectrum of C-LiFePO4at various stages of charging and discharging showed reversibility upon electrochemical cycling. The cyclic voltammograms ofLiFePO4and C-LiFePO4showed only a pair of peaks corresponding to the anodic and cathodic reactions. The first discharge capacities were 63, 43, and 13 mAh/g for C/5, C/3, and C/2, respectively forLiFePO4where as in case of C-LiFePO4that were 163, 144, 118, and 70 mAh/g for C/5, C/3, C/2, and 1C, respectively. The capacity retention of pureLiFePO4was 69% after 25 cycles where as that of C-LiFePO4was around 97% after 50 cycles. These results indicate that the capacity and the rate capability improved significantly upon carbon coating.
Highlights
Lithium iron phosphate (LiFePO4) is under intense investigation since its introduction in 1997 as a possible cathode material for Li-ion rechargeable batteries [1]
Card no. 40–1499) without any impurity phase compared to pure LiFePO4 synthesized without carbon mixing
Intensity ration was higher than that reported by Julien et al [35], and degree of carbon disorder is lower and conductivity may be lower due to this effect. This can be the reason for the reduced capacity at high C-rate; in the present case it may be due to the slow diffusion coefficient of Li ion in LiFePO4 cathode material and lower conductivity of the conductive carbon coating
Summary
Lithium iron phosphate (LiFePO4) is under intense investigation since its introduction in 1997 as a possible cathode material for Li-ion rechargeable batteries [1]. The above discussion on the three material related issues of LiFePO4 for cathode application clearly suggests that addition of carbon could be beneficial in addressing all three issues and yielding phase-pure olivine, enhancing its electronic conductivity and retarding the particle growth. The nature of the morphology of C-LiFePO4 composite is yet to be properly understood Earlier, it was not clear whether carbon should form a thin coating on LiFePO4 particles or point contacts between particles to have beneficial effect on the discharge capacity and the rate capability. A solid-state route under nitrogen ambient has been adopted to synthesize pure LiFePO4 and CLiFePO4 composite cathodes at a relatively low temperature and their structural and electrochemical properties have been studied and compared. The Raman spectra of CLiFePO4 at various stages of charging and discharging have been taken to study the structural reversibility
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