Revealing capacity fading in Sb-based anodes using symmetric sodium-ion cells

Abstract The electrochemical performance of negative active materials employed in sodium-ion batteries is dependent on the amount of Na + available in the test cells. As such, electrodes that exhibit long cycle-life and high coulombic efficiency (CE) in half-cells could suffer from fast capacity fading in full-cells as a result of unstable solid electrolyte interphase (SEI) and mechanical degradation leading to loss of active materials. In this work, the performance of Sb–graphite composite active materials prepared by extended ball-milling was evaluated in sodium half-cells and various types of symmetric cells (SCs). In half-cell tests, the composite electrodes provided specific capacities in the range 350–600 mAh g −1 at C/20 with initial CE of 82%. A stable capacity of 380 mAh g −1 was observed in the subsequent 100 cycles with the CE increasing to nearly 99%. However, self-discharge tests on half-cells and galvanostatic cycling of SCs revealed poor capacity retention as a result of parasitic reaction occurring through the SEI layer. Contrary to half-cells, the SCs revealed that Sb electrodes suffered from sharp capacity losses when a limited amount of Na + ions was available in the cells. This is also characteristic of full-cells in which the sodium ions are supplied by the positive electrode.