Entropymetry of Active Materials for Li-Ion Batteries

Batteries' performance and safety depend on the thermodynamic properties of active materials. Electrochemical measurements provide an effective way to determine the thermodynamic properties of batteries. For example, the open-circuit voltage (OCV) of an electrochemical cell is a direct measure of the Gibbs free energy (ΔG) as a function of the state of charge (SoC). Moreover, electrochemical entropymetry determines the reaction entropy (ΔS) of an electrochemical cell from the temperature dependency of the OCV. Considering that the reaction enthalpy (ΔH) is derived from those ΔG and ΔS values, entropymetry is a nondestructive tool to study the complete thermodynamic properties of lithium-ion batteries (LIBs) in depth. Herein, we conduct the entropymetry analysis of the following representative active materials for LIBs: LiCoO2, LiNi0.8Co0.1Mn0.1O2, LiMn2O4, and graphite. Among various methods of measuring the reaction entropy, temperature scanning at 1 °C min–1 enables reliable measurements on an efficient time scale. Also, we discuss the method to accurately determine the reaction entropy of active materials using the coin-type half-cell by compensating the reaction entropy of the lithium metal deposition reaction. The reaction entropy is directly related to the reversible heat, which must be considered for the heat management of LIBs.