Electrochemical properties of Mg-added lithium nickel cobalt oxide induced by structural characteristics depending on the synthetic process

Abstract

The doping of various foreign elements into Ni-based layer-structured cathode materials has been investigated to improve their electrochemical performance. However, the dependence of structural features and the resultant electrochemical performance on the processing methods remains unclear. In this study, the effects of varied Mg addition methods on the properties of lithium nickel cobalt oxide (LNCO) are investigated by experimental and first-principles simulation approaches. Coprecipitating Mg with the transition metals causes no differences in the morphological and initial charge/discharge characteristics compared to the method of mixing the Mg-source powder with the other raw materials, but performance relating to long-term stability, such as cyclic capacity retention and swelling by gas evolution, is greatly improved. The simulation results indicate that cases in which Mg is doped within the LNCO crystal and in which Mg forms a separate MgO phase are both thermodynamically allowable, and that increasing the Mg content in the LNCO crystal structure suppresses the formation of VO- and NiLi-type point defects. This is proposed as a new mechanism of structural stabilization by Mg doping that explains the improved stability with the introduction of Mg by coprecipitation.