Thermodynamic Analysis and Interpretative Guide to Entropic Potential Measurements of Lithium-Ion Battery Electrodes

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.