Entropy changes due to structural transformation in the graphite anode and phase change of the LiCoO2 cathode

Electrochemical-calorimetric measurements, in combination with the de current interruption technique, were used to determine the change of entropy during discharge and charge of the Panasonic Li-ion cell CGR 18650H. The entropy coefficient dE(eq)/dT was found to vary from -0.6 mV K-1 in the discharged state to -0.1 mV K-1 in the fully charged state. An unexpected variation of large amplitude (from -0.1 to 0.25 and - 0.75 back to - 0.1 mV K-1) was observed in a narrow range of depth of discharge (DOD) near 0.23, between 3.95 and 4.10 V, curing charge as well as discharge. Thus, changes in the reversible heat, T Delta S, appear to be mainly responsible for the transient endothermic heat effect observed during discharge of these Li-ion cells, near DOD 0.33, between 4.035 and 3.975 V. This interpretation of the calorimetric measurements is confirmed by the fact that dE(eq)/dT measured at 4.0 V was positive (0.14 mV K-1). The unexpected strong variation of dE(eq)/dT near DOD 0.23 is due to the nearly simultaneous occurrence of a phase change in the cathode and a structural transformation (quasi-phase change) in the graphite anode. (C) 2000 The Electrochemical Society. S0013-4651(99)11-071-1. All rights reserved.