The structural and chemical stabilities of layered Li1-xCoO2-δ, Li1-xNi0.85Co0.15O2-δ and Li1-xNi0.5Mn0.5O2-δ (0 ≤ (1-x) ≤ 1) cathodes have been investigated by chemically extracting lithium from the corresponding LiMO2 with the oxidizer NO2BF4 in acetonitrile medium. While Li1-xCoO2-δ and Li1-xNi0.85Co0.15O2-δ begin to form a P3-type and a new O3-type (designated as O3’) phases, respectively, for (1-x) < 0.5 and (1-x) < 0.3, Li1-xNi0.5Mn0.5O2-δ maintains the initial O3-type structure without forming any second phase. Chemical analysis with a redox titration indicates that the Li1-xCoO2-δ, Li1-xNi0.85Co0.15O2-δ, and Li1-xNi0.5Mn0.5O2-δ systems begin to lose oxygen from the lattice, respectively, for (1-x) < 0.5, < 0.3 and < 0.4, which is accompanied by an onset of a decrease in the c parameter. The oxygen loss signals chemical instability and the trend in instability correlates with the charging voltage profiles of the cathodes.
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Manthiram, A., Venkatraman, S. Crystal Chemistry of Chemically Delithiated Layered Oxide Cathodes of Lithium Ion Batteries. MRS Online Proceedings Library 756, 57 (2002). https://doi.org/10.1557/PROC-756-EE5.7