Comparative Study of the Capacity and Rate Capability of LiNiyMnyCo1–2yO2 (y = 0.5, 0.45, 0.4, 0.33)

An unresolved question for the layered oxides is: what is the optimum value of y in the formula LiNiyMnyCo1–2yO2 for energy stor-age at moderate reaction rates? Here we report a systematic study of the specific capacity, rate capability and cycle life of Lix-NiyMnyCo1–2yO2 (y 0.5, 0.45, 0.4, and 0.333). The voltage of the Li/y 0.333 couple crosses over those of lower cobalt content for x< 0.55, as the Co redox begins to get involved. This early involvement of cobalt, rather than just Ni, leads to a slightly smaller specific capacity for y 0.333 than for LiNiyMnyCo1–2yO2 with y> 0.333 when charging above 4 V. Overall the y 0.4 material has the optimum properties, having the highest theoretical capacity, less of the expensive cobalt and yet rate capabilities and capacity retention comparable to the y 0.333 material.