Sodium sulfide cathodes superseding hard carbon pre-sodiation for the production and operation of sodium-sulfur batteries at room temperature

This study demonstrates for the first time a room temperature sodium-sulfur (RT Na-S) full cell assembled based on a pristine hard carbon (HC) anode combined with a nanostructured Na2S/C cathode. The development of cells without the demanding, time‐consuming and costly pre‐sodiation of the HC anode is essential for the realization of practically relevant RT Na-S prototype batteries. New approaches for Na2S/C cathode fabrication employing carbothermal reduction of Na2SO4 at varying temperatures (660 to 1060 °C) are presented. Initial evaluation of the resulting cathodes in a dedicated cell setup reveals 36 stable cycles and a capacity of 740 mAh gS−1, which correlates to ≈85% of the maximum value known from literature on Na2S‐based cells. The Na2S/C cathode with the highest capacity utilization is implemented into a full cell concept applying a pristine HC anode. Various full cell electrolyte compositions with fluoroethylene carbonate (FEC) additive have been combined with a special charging procedure during the first cycle supporting in situ solid electrolyte interphase (SEI) formation on the HC anode to obtain increased cycling stability and cathode utilization. The best performing cell setup has delivered a total of 350 mAh gS−1, representing the first functional full cell based on a Na2S/C cathode and a pristine HC anode today.