High-capacity potato peel-shaped graphite for lithium-ion batteries
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High-capacity potato peel-shaped graphite for lithium-ion batteries T. Sri Devi Kumari, Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630006, Tamil Nadu, India R. Surya and A. Manuel Stephan, Electrochemical Power Systems Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630006, Tamil Nadu, India D. Jeyakumar, Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630006, Tamil Nadu, India T. Prem Kumar, Electrochemical Power Systems Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630006, Tamil Nadu, India Address all correspondence to T. Prem Kumar at [email protected] (Received 17 June 2011; accepted 22 August 2011)
Abstract Highly graphitic carbons are obtained by precipitating carbon from molten steel inoculated with bismuth. Scanning electron microscopy images show that the products have a potato peel morphology. The inoculant leads to a breaking of the local symmetry of the graphitic structure as evidenced by Raman spectroscopic studies. The products exhibit flat charge–discharge profiles below about 200 mV versus Li+/Li, reversible capacities even exceeding the theoretical limit of 372 mAh/g for perfectly graphitic structures, low first-cycle irreversible capacities, and sustained hundreds of cycles.
Introduction Carbonaceous materials investigated as anodes for lithium-ion batteries can broadly be classified into graphitic and disordered carbons. Disordered carbons lack long-range crystalline ordering and exhibit lithium insertion capacities much higher than the 372 mAh/g theoretically possible with perfectly graphitic structures. However, their sloping discharge profiles, which translate to decreasing cell voltage as the discharge proceeds,[1,2] and their large irreversible capacities[1] limit their applicability in practical cells. Graphitic carbons, on the other hand, deliver moderately high lithium storage capacities, often around 300 mAh/g. They also cycle well and present flat potential profiles. With new applications demanding highcapacity materials, the desirability of developing an anode material with even an incremental jump in capacities is felt more than ever before. If that material is based on a cheap, abundant, and environmentally benign material as carbon, it would be a welcome development. An oft-cited problem with graphites is the anisotropic orientation of graphite grains, with the basal planes preferentially oriented parallel to the substrate surface, which restricts the kinetics of lithium intercalation across the edges. Graphites with less anisotropic morphologies have, therefore, attracted attention. For example, a spherical carbon produced by thermal processing of mesocarbon microbeads at 2800 °C[3] was considered. However, it gave a maximum reversible capacity of only 280 mAh/g. Natarajan et al.[4] demonstrated capacities of 368 mAh/g with disordered carbons produced by mechanical milling of graphite. In other studies, Deng and Lee[5] obtained a firs
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