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

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.