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. Lithium-ion batteries (LIB) are the most common power sources for portable electronic devices and emerging electric vehicles. Fundamental understanding of the electrode reactions and degradation mechanisms of such batteries will help lead to improvements in shelf life and operational life (also known as cycle life), which are necessary for the widespread commercialization of LIB-powered electric vehicles.1 To this end, many in situ characterization methods 2 have been developed to characterize LIB electrode reactions during lithiation or delithiation, including X-ray diffraction, 3 synchrotron X-ray techniques, 4 atomic force microscopy, 5 Raman spectroscopy, 6 and transmission electron microscopy.7 Most of these methods require electrochemical cells that are specially designed and electrode materials that are nanostructured or immobilized in a particular fashion. A simple and often overlooked method of battery characterization is the electrochemical measurement of thermodynamic quantities for individual electrode reactions or full electrochemical cells.8 Thermodynamic studies provide fundamental insight into electrode reactions, which can lead to performance improvements, and quantitative information for thermo-electrochemical models of cells and batteries, which are essential for predicting heat generation and preventing thermal runaway. The electrochemical measurement of thermodynamic quantities can be performed as an in situ diagnostic tool for cells or batteries of any form factor, including commercial cells, or as an ex situ evaluation of composite electrodes before or after cycling.The three major thermodynamic quantities for an electrochemical cell are the Gibbs free e...