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

For the development of a full-cell battery system, typically appropriate cathodes and anodes are characterized within a half-cell setup where a metal counter electrode is installed to gather data about the employed electrodes. Ultimately, the individual capacity loadings allow for suitable balancing of the anode to cathode capacity in the full-cell. This approach seems rather unproblematic for lithium-ion batteries. For sodium-ion batteries, however, we show that the high reactivity of sodium metal strongly influences hard carbon-based electrode measurements within sodium-ion half-cells. As hard carbon is considered state-of-the-art anode material, the presented results have high impact on the development of sodium ion batteries. Specifically, we show that the type of electrolyte, as well as cell- and measurement-setup are key factors for reliable sodium half-cell measurements of hard carbon. The investigated hard carbon electrodes have a high active material loading of 7.2 mg/cm2 (with 93 % active material content) resulting in an areal capacity of 2.4 mAh/cm2, which represent application-relevant conditions. Hard Carbon is the material of choice for state of the art sodium-ion batteries. However, for its proper characterization within a half-cell configuration, it is essential to control the voltage against the current free sodium metal reference electrode within a 3-electrode setup. Furthermore, we highlight the importance of stable type of electrolyte towards elemental sodium to obtain reliable data.image