Kinetics and Degradation Processes of CuO as Conversion Electrode for Sodium-Ion Batteries: An Electrochemical Study Combined with Pressure Monitoring and DEMS

Copper oxide (CuO) can be used as electrode material for lithium-ion and sodium-ion batteries; however, the path toward application for rechargeable systems is still long. This is mainly related to the complex nature of the electrode reaction and to aging mechanisms that are not well understood, especially in the case of sodium. The main subject of this paper is to compare the electrode reaction of CuO in lithium (CuO/Li) and sodium (CuO/Na) half-cells and to study side reactions in: the CuO/Na system by means of differential electrochemical mass spectrometry (DEMS) and in situ pressure monitoring during galvanostatic cycling (PMGC). Electrode processes have been studied at different current densities and temperatures. In CuO/Li cells, CuO and Cu2O form during charging, their respective fraction depending on the current density. In case of CuO/Na, Cu2O is always the charging product although oxidation to CuO can be temporarily achieved by increasing the temperature to 50 degrees C. The difference between-both CuO/Na and CuO/Li is related to the electrode volume expansion/shrinkage during cycling. Differences in the temporal evolution of electrode surface films are followed by electrochemical impedance Spectroscopy (EIS). For the CuO/Na system, PMGC and DEMS studies reveal a periodic release of gaseous side products as a result of a slow but likely continuous electrolyte degradation. This degradation is due to the repeated formation of a surface film (discharge) and its partial dissolution (charge) accompanied by the release of H-2 and CO2. The degradation processes fade during cycling but remain dynamic. This means that a long cycle life of CuO electrodes in sodium-ion batteries can likely be only achieved by employing some excess electrolyte.