The structural and chemical stabilities of Li1-xCoO2-delta and Li1-xNi0.85Co0.15O2-delta [0 less than or equal to (1 - x) less than or equal to 1] cathodes have been investigated by chemically extracting lithium with an acetonitrile solution of NO2BF4. The Li1-xCoO2-delta and Li1-xNi0.85Co0.15O2-delta systems maintain the initial O3-type structure, respectively, for 0.5 less than or equal to (1 - x) less than or equal to 1 and 0.3 less than or equal to (1 - x) less than or equal to 1. While Li1-xCoO2-delta begins to form a P3-type phase for (1 - x), 0.5, Li1-xNi0.85Co0.15O2-delta begins to form a new O3-type phase, designated as O3' phase, for (1 - x) < 0.3. The P3-type and the O3' phases have smaller c parameters than do the O3-type phase and oxygen contents of <2, resulting in a loss of oxygen from the lattice for (1 - x) < 0.5 and (1 - x) < 0.3, respectively, for Li1-xCoO2-delta and Li1-xNi0.85Co0.15O2-delta. The loss of oxygen is also confirmed for electrochemically charged samples. The formation of new phases with lower oxygen contents at different levels of charge, (1 - x) < 0.5 vs. (1 - x) < 0.3, for the two systems may play a role in limiting the practical capacities of the two systems (140 vs. 180 mAh/g). (C) 2002 The Electrochemical Society.