Pseudocapacitive Jarosite Cathode Enabling High-Energy-Density Sodium-Ion Capacitors

Sodium-ion capacitors (SICs), consisting of a high-rate capacitive cathode and a high-capacity battery-type anode, are considered as one of the most promising energy storage devices due to their high energy/power densities as well as abundant sodium resources. However, the relatively low capacitance of the conventional carbon-based cathode materials significantly limits the energy density update of SICs. Herein, we develop a platelike jarosite (KFe3(SO4)2(OH)6) with large pseudocapacitance as the cathode for constructing high-energy-density SICs. The open channels formed by the linked FeO4(OH)2 octahedra and SO4 tetrahedra in the layered hexagonal crystal structure of KFe3(SO4)2(OH)6 provide rapid ion diffusion pathways, thus enabling pseudocapacitive-dominant charge storage of KFe3(SO4)2(OH)6. A self-supporting KFe3(SO4)2(OH)6 cathode with a three-dimensional electronically conductive architecture is designed, exhibiting high reversible capacities, excellent cycling stability, and superior rate performance. Additionally, the SIC assembled using the pseudocapacitive KFe3(SO4)2(OH)6 cathode and Na2Ti3O7 battery anode delivers a maximum energy density of 123.3 Wh kg–1 and a maximum power density of 3625.9 W kg–1 (still with an energy density of 51.2 Wh kg–1 at this high power density). This work unveils the great potential of the jarosite family to serve as a type of high-performing intercalation pseudocapacitive cathode material for next-generation sodium-ion storage devices.