Understanding the Influence of Composition and Synthesis Temperature on Oxygen Loss, Reversible Capacity, and Electrochemical Behavior of xLi2MnO3-(1 – x)LiCoO2 Cathodes in the First Cycle

With an aim to broaden the understanding of the role of Co3+ in the lithium-rich layered oxide cathodes for lithium-ion batteries, the influence of composition and synthesis temperature on the oxygen loss, reversible capacity, and electrochemical behavior of the xLi2MnO3-(1 – x)LiCoO2 series in the first cycle has been systematically investigated. The charge capacity first increases with increasing x value from 0.2 to 0.6 due to the enhanced oxygen loss, but then decreases with further increase in x from 0.6 to 0.8. Although the discharge capacity shows a trend similar to the charge capacity, the maximum value appears at x = 0.4–0.5, rather than at x = 0.6 because of its larger irreversible capacity loss. Surprisingly, raising the synthesis temperature from 800 to 1000 °C alters the electrochemical behavior of the first charge. The materials synthesized at 900 and 1000 °C show a three-step reaction process consisting of the oxidation of Co3+, simultaneous oxidation of Co3+ and O2–, and finally the oxidation of O2– only, while those synthesized at 800 °C exhibit a two-step reaction process composed of Co3+ oxidation and O2– oxidation occurring separately.