Effects of MgO Coating on the Structural and Electrochemical Characteristics of LiCoO₂ as Cathode Materials for Lithium Ion Battery

Abstract

To understand the origin of enhanced electrochemical performances of MgO-coated LiCoO2 as cathode materials for lithium ion battery, we investigate the internal structures of the materials at the nanometer scale. The MgO-coated LiCoO2 are annealed at various temperatures of 750–810 °C so as to find the optimized heat-treatment condition. The surface morphologies and crystalline structures are characterized by SEM, TEM, EELS, and XRD. The electrochemical results show that the MgO-coated LiCoO2 delivers a high capacity with excellent retention property. In particular, the sample annealed at 810 °C, which possesses the high doping level of Mg2+ ion in the Li sites, exhibits the highest retention capacity without undergoing phase transformation in the interfaces between the MgO-coating layer and the bulk LiCoO2 during cycling. Lithium and vacancy ordering in the delithiated state is monitored by TEM measurements. By comparison with the bare LiCoO2, we reveal that the MgO-coating layer plays a role in the prevention of lithium and vacancy ordering in the near-interfaces, which is in agreement with the excellent electrochemical cycling performances of the MgO-coated sample annealed at 810 °C. As a result, depending on the thermal treatment temperature, the Mg2+ ions from MgO layer diffuse into Co and Li sites in LiCoO2, competitively, and affect the structural stability and electrochemical performances.