A novel and precise measuring method for the entropy of lithium-ion cells: ΔS via electrothermal impedance spectroscopy

The change of entropy ΔS is a distinctive characteristic of the thermal behavior of a lithium-ion cell. Usually, the change of entropy ΔS in an individual cell type or electrode material is determined by potentiometric measurements which, although precise, are time-consuming. Measurement times could be shortened by as much as a factor of 100 by using the method presented in this paper; “electrothermal impedance spectroscopy” (ETIS). Herein, the effectiveness of ETIS is demonstrated for three different forms of commercial lithium-ion cells. Interestingly enough, ΔS of all three cell types show a hysteresis behavior for cell charging and discharging, as well-known for the open circuit voltage. Furthermore, as the ETIS method offers a high SOC-resolution of ΔS this could lead to its application in cell diagnosis.