Cycling-Induced Changes in the Entropy Profiles of Lithium Cobalt Oxide Electrodes

Entropy profiles of lithium cobalt oxide (LiCoO 2 ) electrodes were measured at various stages in their cycle life to examine performance degradation and cycling-induced changes, or lack thereof, in thermodynamics.LiCoO 2 electrodes were cycled at C/2 rate in half-cells (vs.lithium anodes) up to 20 cycles or C/5 rate in full cells (vs.MCMB anodes) up to 500 cycles.The electrodes were then subjected to entropy measurements (∂E/∂T, where E is open-circuit potential and T is temperature) in half-cells at regular intervals over the approximate range 0.5 ≤ x ≤ 1 in Li x CoO 2 .Despite significant losses in capacity, the cycling did not result in any change to the overall shape of the entropy profile, indicating retention of the LiCoO 2 structure, lithium insertion mechanism, and thermodynamics.This confirms that cycling-induced performance degradation in LiCoO 2 electrodes is primarily caused by kinetic barriers that increase with cycling.Electrodes cycled at C/5 exhibited a subtle, quantitative, and gradual change in the entropy profile in the narrow potential range of the hexagonal-to-monoclinic phase transition.The observed change is indicative of a decrease in the intralayer lithium ordering that occurs at these potentials, and it demonstrates that a cycling-induced structural disorder accompanies the kinetic degradation mechanisms.