Optimization of the electrochemical behavior of vapor grown carbon nanofibers for lithium-ion batteries by impregnation, and thermal and hydrothermal treatments

Several methods to improve the electrochemical behavior of vapor grown carbon nanofibers (VGCnF) are discussed. Experiments concerning X-ray diffraction, Li-7 magic angle spinning nuclear magnetic resonance (MAS NMR), electrochemical impedance spectroscopy, and galvanostatic discharge/charge cycles are carried out to study the origin of the observed changes. NMR results show that lithium is stored in the carbon surface as pure ionic lithium, and inserted as lithium with less ionic character. The relative extension of the two NMR-peaks corresponding to each type of lithium depends on the diameter of the fibers and the treatment. Carbon nanofibers have been impregnated with lithium oxalate, and an enhancement of the capacity retention upon cycling in lithium batteries has been observed. The impregnation with Li2C2O4 facilitates the formation of a passivating surface layer on the carbon fibers. Removing impurities by thermal and hydrothermal treatments increases the reversible capacity. Hydrothermal treatment of VGCnF in the presence of tin oxalate yields VGCnF-SnO2 composites electrodes, and the tin dioxide crystallite size is controlled by the duration of the hydrothermal treatment. After activating by opening the carbon nanofibers with refluxing acid, the reversible capacity increases over 400 mAh g(-1). According to Li-7 MAS NMR spectra, this extra capacity is ascribed to lithium that exhibits purely ionic character. (c) 2005 The Electrochemical Society.