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

Abstract

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 Li_1.2Mn_0.6Ni_0.2O₂ have been investigated systematically. Specifically, substitutions of (i) M³⁺ = Al³⁺, Cr³⁺, Fe³⁺, Co³⁺, and Ga³⁺ for equal amounts of Mn⁴⁺ and Ni²⁺ in Li_1.2Mn_0.6−0.5xNi_0.2−0.5xM_xO₂ with x = 0.06, 0.13, and 0.2, (ii) Ti⁴⁺ for Mn⁴⁺ in Li_1.2Mn_0.6−xTi_xNi_0.2O₂ with x = 0.025, 0.05, and 0.1, and (iii) Mg²⁺ for Ni²⁺ in Li_1.2Mn_0.6Ni_0.2−xMg_xO₂ 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 Co^3+/4+:t_2g band with the top of the O²⁻:2p band along with a partially filled t_2g band across the shared octahedral edges makes the oxygen loss from the lattice and/or oxidation beyond 4+ much more facile.