Comparative Study on Evaluation Methods of Entropy Change of Electrochemical Reaction in Lithium-Ion Batteries

Abstract

In electrochemical system such as lithium-ion battery (LIB), the entropy change of the electrochemical reaction, Δ s, is one of important factors of heat generation or absorption during working. Without information of Δ s, we cannot estimate thermal behavior of LIB accurately, and consequently it contributes to design the LIB itself and the total energy system using LIBs as safety devices. The entropy change of the electrochemical reaction can be evaluated theoretically from the temperature dependence of the electromotive force (EMF) without calorimetric technique, and precision of voltage measurement is higher than that of calorimetry in general. However, in LIB, the compositions of both the positive and the negative electrodes gradually change during charge and discharge run because those electrode reactions are caused by insertion/extraction of lithium ions into/from the active electrode materials, and the open circuit voltage (OCV) of LIB also depends on the compositions of the electrode materials as well as the state of charge (SOC). In addition, some active materials show structural phase transition depending on the composition. Hence, SOC dependency should be considered for Δ s of LIB. In this study, we have evaluated Δ s of some LIBs in the full range of SOC by OCV measurement, using a high-precision digital voltmeter combined with temperature control system of the sample batteries. Furthermore, calorimetric approach has been also performed to obtain Δ s where it is important to remove the influence of the other heat generation factors such as the inner resistance of the batteries including the electrochemical polarization at the electrode interfaces. In the presentation, we will discuss the advantage and disadvantage of both OCV and calorimetric methods for measurement of Δ s for LIBs. Acknowledgement This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO).