In the process of Li+ intercalation-deintercalation, electron removal is accompanied simultaneously. Oxygen was found to compensate electron removal both in theoretical calculations and practical experiments. Chlorine addition to LiNi0.7CO0.3O2 was expected to exchange electrons in that Cl- was easier to lose electrons than O2-. LiNi0.7CO0.3O2-xClx was identified as a pure hexagonal lattice of alpha-NaFeO2 type by X-ray diffraction. X-ray photoelectron spectroscopy was used to analyze the influence of chlorine substitution on the oxidation state of transition-metal ions. Charge-discharge experiments and cyclic voltarnmetry confirmed that chlorine addition was an effective way to improve reversible capacity and structural stability in cycles. (c) 2007 Elsevier Ltd. All rights reserved.