Characterization of modified NG7 graphite as an improved anode for lithium-ion batteries

Mild oxidation (burnoff) has been found to improve the performance of NG7 (natural graphite) in Li/LixC6 cells. The reversible capacity of the graphite (QR) increased (up to 405 mAh/g for the 0–2 V range at 4–11% burnoff), its irreversible capacity (QIR) decreased with burnoff and the degradation rate of the LixC6 electrode was much lower. The increase in capacity at 4–11% burnoff of NG7 was found to be associated with the formation of less than 1% void volume. On further burning beyond 11% weight loss, the density decreases faster, down to 1.92 g/cm3 at 34% burnoff. This is associated with some decrease in QR. X-ray photoelectron spectrocopy studies showed that the surface oxygen content of NG7 has a broad minimum at 4–22% burnoff. The highest oxygen peak shifted monotonically with burnoff time, rising from 531.05 eV for pristine NG7 to 534.0 eV for 34% burnoff sample. The X-ray photoemission spectra may be assigned to hydroxyl surface groups in the case of the pristine sample and to surface acid groups in the case of burnt samples. From the powder X-ray diffraction and dq/dv results, it was found that a more ordered structure appears on cycling. Performance improvement was attributed to the formation of a solid electrolyte interface chemically bonded to the surface carboxylic groups at the zigzag and armchair faces, better wetting by the electrolyte and to accommodation of extra lithium at the zigzag, armchair and other edge sites and nanovoids.