Sodium amide as a “zero dead mass” sacrificial material for the pre-sodiation of the negative electrode in sodium-ion capacitors

Abstract

The galvanostatic oxidation/reduction of a sodium amide (NaNH2)-activated carbon (AC) composite electrode has been studied vs. sodium counter/reference electrode with 1 mol L−1 NaClO4; EC:PC (ethylene carbonate:propylene carbonate, 1:1 in volume) electrolyte. The irreversible oxidation capacity of NaNH2 reached a value of 680 mAh g−1 at ~ 3.8 V vs. Na+/Na, which is close to the theoretical capacity of 686 mAh g−1. The internal pressure increase during the positive polarization of the NaNH2-AC electrode, as well as the ex-situ electrode gravimetric measurements after the polarization, confirmed the evolution of gaseous by-products (N2 and H2). Owing to its high irreversible capacity, NaNH2 could be used as low amount (only 25 wt%) sacrificial material added to AC (55 wt%) in a NaNH2-AC//HC (hard carbon) cell with 1 mol L−1 NaClO4; EC:PC electrolyte. During the positive polarization of the NaNH2-AC electrode, sodium was irreversibly extracted and inserted into HC, giving rise to an AC//HC-Na sodium-ion capacitor (NIC). In the voltage range from 2.2 V to 3.8 V, the obtained NIC demonstrated a discharge capacitance of 35 F g−1 (with a capacitance retention of 92% after 5000 cycles) and a value of specific energy of 35 Wh kg−1 at 1 kW kg−1 (per total active mass of the two electrodes), which is almost four times higher than the specific energy (9 Wh kg−1 at 1 kW kg−1) of an electrical double-layer capacitor (EDLC) with the same AC used for electrodes and operating in the same electrolyte up to 2.7 V.