Influences on Reliable Capacity Measurements of Hard Carbon in Highly Loaded Electrodes

Lithium-ion batteries (LIBs) are the current state of the art energy storage technology. However, to tackle the steady increasing demand for clean but also sustainable energy, other complementary cell chemistries are gaining significant importance. Sodium-ion batteries (NIBs) are considered to be a drop-in technology that can benefit from the knowledge about processing of LIBs and its existing manufacturing structures. For the development of a sodium-ion full-cell, commercially available hard carbon (HC) as an anode material was characterized within a half-cell setup. On the one hand, those measurements are essential to characterize individual electrodes in order to obtain electrochemical information that is needed to set up the full cell, e.g. balancing. On the other hand, the exact determination of the HC capacity is prone to errors, mainly due to the cut-off potential being close to 0 V against metallic sodium. This behavior and also electrolyte degradation in presence of elemental sodium were studied. It has been found that the potential of the current carrying sodium counter electrode in half-cells is not stable, resulting in measurement inaccuracies, particularly at elevated C-rates when charging is controlled by a cut-off potential. On the contrary, by using a three-electrode setup, the current-free reference electrode provides stable cut-off conditions. An improved setup for half-cell measurements is suggested to obtain reliable and reproducible capacity readings for HC anodes with an areal capacity of up to 2.4 mAh/cm². Figure : Schematic overview about the investigated control voltages and their effect on the electrochemical performance. [1] The initial coulombic efficiency (ICE) of HC is a current challenge because it is relatively low at about 80% and it can vary for different HC materials. Therefore, precise measurements are a necessity to develop HC materials with higher ICEs. Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC 2154. [1] Müller, C., Wang, Z., Hofmann, A., Stüble, P., Liu-Théato, X., Klemens, J., Smith, A., “ Influences on Reliable Capacity Measurements of Hard Carbon in Highly Loaded Electrodes ” Batteries & Supercaps 2023, e202300322.