Structural Instability of Delithiated Li[sub 1−x]Ni[sub 1−y]Co[sub y]O[sub 2] Cathodes

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Abstract
The structural stability of layered LiNi1−yCoyO2LiNi1−yCoyO2 cathodes has been assessed by investigating the crystal chemistry of chemically delithiated Li1−xNi1−yCoyO2(0⩽y⩽0.3)Li1−xNi1−yCoyO2(0⩽y⩽0.3) subjected to mild heating and comparing the data with that for Li1−xCoO2.Li1−xCoO2. The delithiated Li1−xNi1−yCoyO2Li1−xNi1−yCoyO2 samples exhibit a decrease in c/ac/a ratio at temperatures as low as 50°C due to a migration of the Ni3+/4+Ni3+/4+ ions from the nickel layer to the lithium layer. The rate of change in the c/ac/a ratio increases with increasing degree of delithiation (charging) and heating temperature and time, but decreases with increasing cobalt content. For example, the c/ac/a ratio of Li0.31Ni0.85Co0.15O2Li0.31Ni0.85Co0.15O2 decreases from 5.11 to 5.01 after heating at 70°C for 11 days, whereas that of Li0.35CoO2Li0.35CoO2 does not show a change under identical conditions. The decrease in c/ac/a ratio is also observed after mild heating of lithium cells consisting of charged Li1−xNi0.85Co0.15O2Li1−xNi0.85Co0.15O2 electrodes. © 2000 The Electrochemical Society. All rights reserved.