Microcalorimetry electrothermal impedance spectroscopy (ETIS) informs entropy evolution at individual electrodes of PNb9O25 or TiNb2O7 battery cells

This study proposes a novel and fast microcalorimetry electrothermal impedance spectroscopy (ETIS) method based on heat generation rate measurements at each electrode of a lithium-ion battery cell. This new method is capable of retrieving the open-circuit voltage, the entropic potential, and the partial entropy changes at each electrode from measurements at a single temperature. It also shortens the measurement duration to a few hours compared to several days using the galvanostatic intermittent titration technique (GITT). The method consists of imposing a sinusoidal current on the cell assembled in an operando isothermal calorimeter. The induced sinusoidal potential response is used to calculate the open-circuit voltage of the cell as a function of the state of charge. The measured heat generation rates are analyzed by fast Fourier transform to determine not only the entropic potential of the cell but also the partial entropy changes at each electrode. This novel microcalorimetry ETIS method was first validated with numerical simulations. Then, it was experimentally demonstrated on battery cells consisting of PNb9O25 or TiNb2O7 working electrodes and metallic lithium counter electrodes in 1 M LiPF6 in EC:DMC 1:1 v/v electrolyte. The results of the open-circuit voltage and the normalized entropic potential matched those previously determined by potentiometric entropy measurements based on GITT measurements at different temperatures. Furthermore, while the normalized partial entropy changes exhibited notable features at the PNb9O25 or TiNb2O7 working electrodes, they varied little at the metallic lithium counter electrodes.