Role of Desolvation upon the Sodiation of Hard Carbon in Sodium-Ion Batteries: A Microcalorimetric Study of the Sodiation Entropy

Hard carbon (HC) is one of the most promising anode materials for sodium-ion batteries, but the different sodiation processes contributing to its reversible capacity are still under debate. In order to obtain thermodynamic information, we measured the heat exchanged at a single HC composite electrode during the (de)sodiation of HC in NaClO4/propylene carbonate and NaPF6/diglyme to determine the reaction entropy of the sodiation process of HC at different states of charge (SoCs). We found that it is dominated by the positive entropy contribution of the concomitant desolvation, leading to reaction entropies up to 85 J mol-1 K-1 for NaClO4/propylene carbonate and 250 J mol-1 K-1 for NaPF6/diglyme. Together with information on the electrochemical analysis of millisecond charging/discharging pulses, we postulate a capacitive adsorption process with incomplete desolvation at low SoC, while at medium to high SoC, a Faradaic insertion process with complete desolvation takes place.