Multi-walled CNT were oxidised with nitric acid in liquid and gas-phase. By splitting the capacity and initial charge loss during lithium intercalation into different potential regions, it was possible to relate these values to the CNT surface oxygen groups as determined by XPS. Gas-phase oxidised CNT show a significantly lower amount of initial charge loss (172 mAh/g) compared to liquid-phase oxidised CNT (283 mAh/g). This decrease originates from less pronounced exfoliation likely caused by an increase of surface carbonyl groups.