Na2S sacrificial cathodic material for high performance sodium-ion capacitors

Abstract

Sodium sulfide (Na2S) has been used as sacrificial material for the presodiation of a Sn4P3 negative electrode in order to realize a high-performance sodium-ion capacitor (NIC). In two-electrode cells with Na counter/reference electrode and 1 mol L−1 NaClO4 electrolyte, sodium could be irreversibly extracted from Na2S at potential lower than 3.8 V vs. Na/Na+, with a capacity close to the theoretical value of 687 mAh g−1. In the realization of the composite positive electrode for a NIC, the relatively high capacity of Na2S allows to reduce its amount to 40 wt%, whereas the other materials are activated carbon (AC, 40 wt%), carbon soot (15 wt%) and binder (5 wt%). Once the pre-sodiation of Sn4P3 is completed, activated carbon being a part of the positive electrode stores charges in the electrical double-layer (EDL), while reversible sodium insertion occurs in the Sn4P3 negative electrode. The full NIC demonstrates stable performance in the voltage range 2.0 V–3.8 V with high specific energy ca. 48 Wh kg−1 at specific power of 1 kW kg−1 (per total mass of electrodes). Hence, Na2S is an excellent sacrificial material, which allows the NICs construction to be simplified and consequently the manufacturing costs to be reduced.