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

Abstract

Entropy profiles of lithium cobalt oxide (LiCoO2) electrodes were measured at various stages in their cycle life to examine performance degradation and cycling-induced changes, or lack thereof, in thermodynamics. LiCoO2 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 LixCoO2. 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 LiCoO2 structure, lithium insertion mechanism, and thermodynamics. This confirms that cycling-induced performance degradation in LiCoO2 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.