On electrochemistry of Al2O3-coated LiCoO2 composite cathode with improved cycle stability

Abstract

LiCoO2-based cathode does still have a powerful competition in high-end mobile electronics due to its relatively high true density (about 5.2 g/cm3). When the operation potential range is extended, the improvement in its cycle stability has attracted more attention. The extension of its operation potential can be realized by partial replacement of Co by Ni and Mn or by surface modification. However, Ni and Mn replacing partial Co results in decreased true density; for example, the true density of LiNi0.5Mn0.3Co0.2O2 is about 4.6 g/cm3. In this case, the increase in its practical energy density is impossible. As a result, the surface modification technology becomes very important to extend its operation potential range. In this article, an Al2O3-coated LiCoO2 cathode was synthesized. X-ray diffraction test did not show any impurity. Scanning electron spectroscopy measurements showed that the basic microstructure of pristine LiCoO2 grain is sustained after coating Al2O3. The surface characteristic of pure and Al2O3-coated LiCoO2 was also analyzed using an X-ray photoelectron spectroscopy (XPS) technique. Unusual XPS peaks of O 1s, Al 2p, and Co 2p binding energy were found and may be caused by the possible H existence in crystal structure. The electrochemical behavior was systematically investigated, and the cathode was cycled at different charge cutoff voltages (4.30∼4.60 V). The charge-discharge and cyclic voltammetry measurements showed an obviously improved cyclic performance after coating Al2O3. The electrocatalytic activity is not clearly changed before and after coating Al2O3. From our systematical investigation, it could be concluded that the Al2O3-coated LiCoO2 cathode is suitable for practical application in the potential range of 3.70∼4.50 V vs. Li/Li+.