Hexagonal to Cubic Spinel Transformation in Lithiated Cobalt Oxide

A transmission electron microscopy (TEM) investigation was performed on before and after it had been subjected to charge/discharge cycling in electrochemical cells, as well as on delithiated before and after thermal aging. Turbostratic disorder involving small rotations of the O-Co-O slabs was found in as-received material, and in material subjected to a few cycles. In subjected to extensive charge/discharge cycling, it was found that increasing amounts of the trigonal O3 phase had transformed to H1-3 phase and to the cubic spinel phase. The transformation appears to initiate on the surfaces of trigonal crystals. The orientation relationship between the trigonal and spinel phases was determined from diffraction patterns to be parallel and parallel The difference in unit cell dimensions leads to transformation stresses when spinel crystals are formed, and spallation of surface layers was observed. The formation of a spinel phase could suppress electrochemical performance of cathodes in heavily cycled cells. Aging in the charged state also can alter particle surfaces and therefore the performance. © 2004 The Electrochemical Society. All rights reserved.