Reversible and fast Na-ion storage in MoO2/MoSe2 heterostructures for high energy-high power Na-ion capacitors

Abstract Sodium ion capacitors (SICs) combine the advantages of electric double layer capacitors and sodium ion batteries. Molybdenum diselenide (MoSe2) holds promise for sodium ion storage because of fast sodium ion transport. However, the formation of irreversible Na2Se species consequentially leads to capacity decay and limited life-span. This work introduced MoO2 nanoclusters immobilized on two-dimensional (2D) MoSe2-graphene interfaces to promote the reversible conversion of MoSe2. The adsorption of Na2Se species on MoO2 during the discharge process was confirmed by direct observation of the separator and Na chip, in-situ bottle-cell process, and ex-situ XRD/TEM techniques. The combined merits of each component displayed a high rate capacity with a dominant pseudocapacitive contribution of 85% at 1 mV s−1 and excellent cycling stability without degradation at 5 A g−1 for 800 cycles in sodium half cells. A hybrid sodium ion capacitor device delivered a maximum energy density and power output of 71 Wh kg−1 and 14316 W kg−1, respectively, and an excellent cycling life-span with 8% capacitance loss after 7000 cycles at 6 A g−1, outperforming other reported hybrid SIC devices. The superior energy-power behavior bridges the performance gap between batteries and capacitors, holding promise for next-generation high-energy and high-power devices.