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 (NaBH4) as a sacrificial material for the preparation of next-generation sodium-ion capacitors (NICs). NaBH4 is a material with large irreversible capacity of ca. 700 mA h g(-1) at very low extraction potential close to 2.4 vs Na+/Na-0. An assembled NIC cell with the compositepositive electrode (activated carbon/NaBH4) 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. 18Wh kg(-1) at power 2 kWkg(-1) which is more than the energy (7Whkg(-1) at 2 kWkg(-1)) of an electrical double-layer capacitor (EDLC) operating at voltage 2.7 V with the equivalent components as in NIC. Tin phosphide (Sn4P3) as a negative electrode allowed the reaching of higher values of the specific energy density 33Wh kg(-1) (ca. four times higher than EDLC) at the power density of 2 kWkg(-1), with only 1% of capacity loss upon 5,000 cycles and efficiency >99%.