Reduction of the irreversible capacity in hard-carbon anode materials prepared from sucrose for Li-ion batteries

Chemical vapor deposition (CVD) of a carbonaceous substance from ethylene gas at temperatures >700 degrees C on hard-carbon anode materials causes a dramatic reduction in irreversible capacity from >150 to <70 mAh/g. The reversible capacity (approximately 540 mAh/g) and the voltage profile were not changed, suggesting that the hydrogen content and lithium insertion mechanisms are not significantly affected in the new materials. The authors believe that the ethylene treatment modifies the surface and reduces the samples' ability to adsorb/chemisorb/react with species from air which have previously been correlated to an increase in irreversible capacity. This treatment can be performed either during or after pyrolysis, with similar results, and three processes that control the amount of carbonaceous material deposited are outlined in this paper. Wide-angle X-ray scattering measurements confirm the growth of a soft-carbon phase resulting from the CVD process, presumably on the surface. The reduction in irreversible capacity occurs without significant changes in the underlying structure of the hard carbon as measured by small-angle X-ray scattering, X-ray photoelectron spectroscopy, and nitrogen adsorption Brunauer-Emmett-Teller surface area measurements.