Influence of cationic substitutions on the first charge and reversible capacities of lithium-rich layered oxide cathodes

The reversible capacity values of lithium-rich layered oxide cathodes depend on the length (capacity) of the plateau region during the first charge. With an aim to understand the factors that control the length of the plateau region and thereby enhance the reversible capacity, the effects of various cationic substitutions in Li1.2Mn0.6Ni0.2O2 have been investigated systematically. Specifically, substitutions of (i) M3+ = Al3+, Cr3+, Fe3+, Co3+, and Ga3+ for equal amounts of Mn4+ and Ni2+ in Li1.2Mn0.6−0.5xNi0.2−0.5xMxO2 with x = 0.06, 0.13, and 0.2, (ii) Ti4+ for Mn4+ in Li1.2Mn0.6−xTixNi0.2O2 with x = 0.025, 0.05, and 0.1, and (iii) Mg2+ for Ni2+ in Li1.2Mn0.6Ni0.2−xMgxO2 with x = 0.025, 0.05, and 0.1 have been investigated. The cationic substitutions affect the first charge capacity values both in the sloping region corresponding to the oxidation of the transition-metal ions to the 4+ state and in the plateau region corresponding to the oxidation of O2− ions to O2 and/or transition-metal ions beyond 4+, which control the reversible capacity values in subsequent cycles. While the changes in the sloping-region capacity could be readily understood by considering the redox activities of the transition-metal ions, the plateau-region capacity is found to depend sensitively on the metal–oxygen covalence, which is dictated by the relative positions of the metal:3d band with respect to the top of the O2−:2p band, and electron delocalization. For instance, an overlap of the Co3+/4+:t2g band with the top of the O2−:2p band along with a partially filled t2g band across the shared octahedral edges makes the oxygen loss from the lattice and/or oxidation beyond 4+ much more facile.