Entropy Profiling of Hard Carbon/Na Metal Cells with Stepwise Temperature Changes: Influence of the Sodiation Kinetics on the Cell Voltage Response

Hard carbon (HC) is commonly used as negative electrode material in sodium-ion batteries. Despite its extensive use, there is still a lack of comprehensive understanding of the sodiation mechanism. To obtain thermodynamic information on the storage processes, the reaction entropy of HC/Na metal cells in NaPF6/diglyme electrolyte solution is determined by measuring the temperature dependence of the equilibrium cell voltage at different states of charge (SoCs), commonly known as entropy profiling. In contrast to former studies, we changed the temperature of the cell stepwise within less than 1 min, which reduces the influence of baseline drift of the cell voltage. The cell reaction entropy varied between −10 and 6 J mol−1K−1 for all SoCs, which indicates complete solvation/desolvation of the Na+ ions at the HC composite electrode. In addition, the fast temperature change led to characteristic features of the cell voltage response. These features were explained with a simple model including temperature gradients across the cell and different kinetics of the reactions at the HC and the Na metal electrode. From the cell voltage response, we inferred that the sodiation of HC is significantly slowed down with decreasing SoC.