Entropy Changes Due to Structural Transformation in the Graphite Anode and Phase Change of the LiCoO[sub 2] Cathode

Abstract

Electrochemical‐calorimetric measurements, in combination with the dc 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 was found to vary from in the discharged state to in the fully charged state. An unexpected variation of large amplitude (from −0.1 to 0.25 and −0.75 back to ) 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,TΔS, appear to be mainly responsible for the transient endothermic heat effect observed during discharge of these Li‐ion cells, near DOD 0.23, between 4.035 and 3.975 V. This interpretation of the calorimetric measurements is confirmed by the fact that measured at 4.0 V was positive . The unexpected strong variation of 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. © 2000 The Electrochemical Society. All rights reserved.