Thermodynamic Analysis and Interpretative Guide to Entropic Potential Measurements of Lithium-Ion Battery Electrodes
Sun Woong Baek, Muna Saber, Anton Van der Ven, Laurent Pilon
Abstract
This paper reviews and clarifies the fundamental thermodynamic relationships relevant to the interpretation of potentiometric entropy measurements on lithium-ion batteries to gain insight into the physicochemical phenomena occurring during cycling. First, contributions from configurational, vibrational, and electronic excitations to the entropy of an ideal intercalation compound used as a cathode in a battery system are analyzed. The results of this analysis are used to provide an interpretative guide of open circuit voltage Uocv(x,T) and entropic potential ∂Uocv(x,T)/∂T measurements to identify different mechanisms of intercalation, including (i) lithium intercalation as a homogeneous solid solution, (ii) ion ordering reactions from a homogeneous solid solution, (iii) first-order phase transitions involving a two-phase coexistence, and/or (iv) first-order phase transitions passing through a stable intermediate phase. These interpretations illustrated with experimental data for different battery electrode materials including TiS2, LiCoO2, Li4/3Ti5/3O4, LiFePO4, and graphite electrodes with metallic lithium as the counter electrode. The systematic interpretation of Uocv(x,T) and ∂Uocv(x,T)/∂T can enhance other structural analysis techniques such as X-ray diffraction, electron energy-loss spectroscopy, and Raman spectroscopy. The thermodynamic analysis and the interpretive guide will be instrumental in the discovery of new battery materials.