Al-doped ZnO-coated LiCoO2 thin-film electrode: Understanding the impact of a coating layer on the degradation mechanism

Abstract

Despite the high theoretical capacity of LiCoO2 electrode, only half of it can be used in commercial cells due to chemical and structural instabilities of the electrode surface at high charge cut-off voltages. Surface modifications by coating layers are among the best approaches used to mitigate the surface degradation. Here, a systematic study of degradation mechanism of uncoated polycrystalline LiCoO2 thin-film electrodes and of the suppression mechanisms induced by an Al-doped ZnO (Al:ZnO) coating layer during electrochemical cycling was performed, using analytical transmission electron microscopy (TEM). The Al:ZnO coating layer was deposited onto the LiCoO2 electrode surface using a wet-chemical dip-coating process. The coating layer increased the first discharge capacity and improved capacity retention. In the case of uncoated LiCoO2 electrode after 40 cycles, an irreversible phase transition from a layered to a spinel phase occurred at the electrode surface due to a direct electrolyte exposure. Moreover, Li and oxygen losses as well as a reduction of the oxidation state of Co ions occurred at the electrode surface. In the case of an Al:ZnO - coated LiCoO2 electrode, the coating layer significantly mitigated the chemical and structural degradation of the electrode surface and, thereby, suppressed the capacity and voltage fading.