Chemical and structural instabilities of lithium ion battery cathodes

The chemical and structural stabilities of various layered Li1−xNi1−y−zMnyCozO2 cathodes are compared by characterizing the samples obtained by chemically extracting lithium from the parent Li1−xNi1−y−zMnyCozO2 with NO2BF4 in an acetonitrile medium. The nickel- and manganese-rich compositions such as Li1−xNi1/3Mn1/3Co1/3O2 and Li1−xNi0.5Mn0.5O2 exhibit better chemical stability than the LiCoO2 cathode. While the chemically delithiated Li1−xCoO2 tends to form a P3 type phase for (1 − x) < 0.5, Li1−xNi0.5Mn0.5O2 maintains the original O3 type phase for the entire 0 ≤ (1 − x) ≤ 1 and Li1−xNi1/3Mn1/3Co1/3O2 forms an O1 type phase for (1 − x) < 0.23. The variations in the type of phases formed are explained on the basis of the differences in the chemical lithium extraction rate caused by the differences in the degree of cation disorder and electrostatic repulsions. Additionally, the observed rate capability of the Li1−xNi1−y−zMnyCozO2 cathodes bears a clear relationship to cation disorder and lithium extraction rate.