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 LiNi{sub y}Mn{sub y}Co{sub 1-2y}O{sub 2} for energy storage at moderate reaction rates? Here we report a systematic study of the specific capacity, rate capability and cycle life of Li{sub x}Ni{sub y}Mn{sub y}Co{sub 1-2y}O{sub 2}(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 LiNi{sub y}Mn{sub y}Co{sub 1-2y}O{sub 2} 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.