Determination of a sodium-ion cell entropy-variation

The entropy-variation of a battery is responsible for heat generation or consumption during operation. It is an important part of a thermal model, but it can also provide information on the cell behavior. This study is focused on the determination of the entropy-variation curve of a sodium-ion cell whose cathode is based on Na3V2(PO4)2F3 and anode on hard carbon (NVPF/HC). The measurement method is a calorimetric approach based on the inversion of a one-node thermal model. It requires a few tests in a climatic chamber, making it accessible for a lot of laboratories. Results obtained at three different current-rates (0.5 C, 1 C and 2 C) are consistent, leading to an entropy-variation curve that varies a lot and is negative over most of the state of charge range (minimum value around −0.5 mV/K). As showed in this article, the associated entropic heat is strongly negative during charge and positive during discharge. This study brings an accurate entropy-variation curve for NVPF/HC cells that can be used for thermal models or for other physical analysis. It also shows that entropy-variation plays an important role in the thermal behavior of NVPF/HC cells and that it should be taken into account when modeling their thermal behavior.