Structural instability of delithiated Li1-xNi1-yCoyO2 cathodes

The structural stability of layered LiNi1-yCoyO2 cathodes has been assessed by investigating the crystal chemistry of chemically delithiated Li1-xNi1-yCoyO2 (0 less than or equal to y less than or equal to 0.3) subjected to mild heating and comparing the data with that for Li1-xCoO2. The delithiated Li1-xNi1-yCoyO2 samples exhibit a decrease in cia ratio at temperatures as low as 50 degreesC due to a migration of the Ni3+/4+ ions from the nickel layer to the lithium layer The rate of change in the c/a ratio increases with increasing degree of delithiation (charging) and heating temperature and time, but decreases with increasing cobalt content. For example, the cia ratio of Li0.31Ni0.85Co0.15O2 decreases from 5.11 to 5.01 after heating at 70 degreesC for 11 days, whereas that of Li0.35CoO2 does nor show a change under identical conditions. The decrease in c/a ratio is also observed after mild heating of lithium cells consisting of charged Li1-xNi0.85Co0.15O2 electrodes. (C) 2000 The Electrochemical Society. S0013-4651(00)06-064-X. All rights reserved.