Pseudocapacitive\nJarosite Cathode Enabling High-Energy-Density\nSodium-Ion Capacitors

Sodium-ion capacitors (SICs), consisting\nof a high-rate capacitive\ncathode and a high-capacity battery-type anode, are considered as\none of the most promising energy storage devices due to their high\nenergy/power densities as well as abundant sodium resources. However,\nthe relatively low capacitance of the conventional carbon-based cathode\nmaterials significantly limits the energy density update of SICs.\nHerein, we develop a platelike jarosite (KFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>) with large pseudocapacitance as the\ncathode for constructing high-energy-density SICs. The open channels\nformed by the linked FeO<sub>4</sub>(OH)<sub>2</sub> octahedra and\nSO<sub>4</sub> tetrahedra in the layered hexagonal crystal structure\nof KFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub> provide\nrapid ion diffusion pathways, thus enabling pseudocapacitive-dominant\ncharge storage of KFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>. A self-supporting KFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub> cathode with a three-dimensional electronically\nconductive architecture is designed, exhibiting high reversible capacities,\nexcellent cycling stability, and superior rate performance. Additionally,\nthe SIC assembled using the pseudocapacitive KFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub> cathode and Na<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub> battery anode delivers a maximum energy density\nof 123.3 Wh kg<sup>–1</sup> and a maximum power density of\n3625.9 W kg<sup>–1</sup> (still with an energy density of 51.2\nWh kg<sup>–1</sup> at this high power density). This work unveils\nthe great potential of the jarosite family to serve as a type of high-performing\nintercalation pseudocapacitive cathode material for next-generation\nsodium-ion storage devices.