Sodium borohydride (NaBH4) as a high-capacity material for next-generation sodium-ion capacitors

Energy storage is an integral part of the modern world. One of the newest and most interesting concepts is the internal hybridization achieved in metal-ion capacitors. In this study, for the first time we used sodium borohydride (NaBH ) as a sacrificial material for the preparation of next-generation sodium-ion capacitors (NICs). NaBH is a material with large irreversible capacity of ca. 700 mA h g at very low extraction potential close to 2.4 vs Na /Na . An assembled NIC cell with the composite-positive electrode (activated carbon/NaBH ) and hard carbon as the negative one operates in the voltage range from 2.2 to 3.8 V for 5,000 cycles and retains 92% of its initial capacitance. The presented NIC has good efficiency >98% and energy density of ca. 18 W h kg at power 2 kW kg which is more than the energy (7 W h kg at 2 kW kg ) of an electrical double-layer capacitor (EDLC) operating at voltage 2.7 V with the equivalent components as in NIC. Tin phosphide (Sn ) as a negative electrode allowed the reaching of higher values of the specific energy density 33 W h kg (ca. four times higher than EDLC) at the power density of 2 kW kg , with only 1% of capacity loss upon 5,000 cycles and efficiency >99%.