Factors Influencing the Irreversible Oxygen Loss and Reversible Capacity in Layered LiLi1/3Mn2/3O2−LiMO2 (M = Mn0.5-yNi0.5-yCo2y and Ni1-yCoy) Solid Solutions

The electrochemical charge−discharge properties of the layered (1 − z)LiLi1/3Mn2/3O2·(z)LiMn0.5-yNi0.5-yCo2yO2 (y = 1/12, 1/6, and 1/3 and 0.25 ≤ z ≤ 0.75) and (1 − z)LiLi1/3Mn2/3O2·(z)LiNi1-yCoyO2 (0 ≤ y ≤ 1 and 0.2 ≤ z ≤ 0.7) solid solution series have been investigated with an aim to identify the factors that control the amount of oxygen loss from the lattice during the first charge and the reversible capacity values. The first charge profiles exhibit an initial sloping region A followed by a plateau region B around 4.5 V. The sloping region A is found to be determined by the initial average oxidation state of the transition metal ions and their oxidation to Ni4+ and Co3.6+ with Mn remaining as Mn4+. The plateau region B, which corresponds to an irreversible loss of oxygen from the lattice, is found to be determined by the amount of lithium η in the transition metal layer of the solid solution LiLiηM1-ηO2. However, high Mn4+ content causes a decrease in oxygen mobility and loss. Moreover, the tendency of Ni3+ to get reduced to Ni2+ and the consequent volatilization of lithium during synthesis alter the lithium content in the transition metal layer and thereby influence the degree of oxygen loss and reversible capacity values.