Understanding the Effect of Co3+ Substitution on the Electrochemical Properties of Lithium-Rich Layered Oxide Cathodes for Lithium-Ion Batteries

The effect of the substitution of Co 3+ for equal amounts of Mn 4+ and Ni 2+ in the lithium-rich layered oxide cathode Li[Li 0.2 Ni 0.2– x /2 Mn 0.6– x /2 Co x ]O 2 (0 ≤ x ≤ 0.24) has been investigated systematically. Electrochemical charge/discharge measurements in lithium-ion cells indicate that the oxygen loss from the lattice during the first charge is a kinetically slow process, but it can be improved by the substitution of Co 3+ due to an overlap of the Co 3+/4+:t 2g band with the top of the O 2–:2p band and an increase in metal–oxygen covalence. Although the increased oxygen loss with increasing Co 3+ substitution leads to a higher reversible capacity, the first cycle discharge voltage decreases slightly due to the higher concentration of Mn 3+ ions created during the first discharge. However, both the capacity fade and the voltage decay increase during extended cycling with increasing Co 3+ substitution due to an increasing migration of the transition-metal ions to the lithium layer, which is promoted by the higher number of oxide-ion vacancies in the lattice, to form a spinel-like phase.