To fill the current research gap in practical lifetime prediction for sodium-ion battery cells (SIB), we utilized float current analysis (FCA) which was already successfully applied to many lithium-ion cells. FCA evaluates the recharge current that is required to maintain the cell voltage at a defined value after all reversible effects have decayed. This recharge current can be assigned to electrode specific aging mechanisms giving a deeper understanding of calendar aging process. Additionally, we examined established methods to assess the aging state based on independent measurements.

This poster focuses on large-format SIBs with a commonly used Manganese-Iron-Nickel (MFN) cathode and hard carbon (HC) anode, which is currently the preferred cell configuration due to its high energy density and stability. Different cell formats and sizes for industrial and automotive applications are investigated, ranging from four cylindrical cells with a nominal capacity of 18 Ah to eight prismatic cells with up to 220 Ah. To cover the degradation mechanisms at realistic states of charge and temperatures, this study analyses the calendar aging in the range of 25–80 % state of charge and 15–50 °C.

For an initial assessment of aging, we analyzed characteristic changes in the voltage curve and impedances at discrete time points throughout the test period. In addition to the loss of active sodium ions, likely due to SEI growth, the results also indicate changes in the cathode, potentially triggered by side reactions at high voltages. Combined with the characterization of the cells and the calculation of the corresponding scaling factors, all necessary information for a detailed analysis were available. During the FCA, the measured float currents and capacity loss rates were grouped according to their corresponding voltage and temperature. This allowed the prediction of separate aging currents at the two electrodes, as well as an estimation of capacity loss under various operating modes.

Our findings showed significant differences in manufacturing quality among the different SIBs. All prismatic SIBs failed early due to excessive gas generation and could not be further evaluated. The measurements of the cylindrical SIBs were evaluable and showed competitive capacity loss rates to modern lithium-ion cells. The dominant aging mechanism is apparently SEI growth on the HC anode consuming active sodium. Furthermore, the electrode specific aging currents above a cell voltage of 2.8 V indicate a side reaction at the cathode, which increases significantly towards higher cell voltages. This is associated with a possible conductive salt decomposition at the cathode that continuously increases internal resistance and adds active sodium to the cell. Therefore, the cathode side reaction partly mitigates the capacity loss caused by SEI growth. The combination of both effects results in a stagnating increase of capacity loss rate towards higher cell voltages.