Sodium-Ion Hybrid Capacitor of High Power and Energy Density

Sodium-ion hybrid capacitors (NHCs) have been attracting research interest in recent years. However, NHCs suffer from slower redox reaction kinetics of electrodes as compared to non-Faradaic capacitive counterparts. Herein, a high-performance NHC using porous NaBi as anode, activated carbon (AC) as cathode, and 1.5 M of NaPF₆ in diglyme as electrolyte is reported. In a charging process, Na⁺ is inserted into NaBi to form Na₃Bi, and PF₆ ^- is stored in the electric double layers of the AC cathode; in a reverse process, the Na₃Bi is desodiated to NaBi and eventually Bi, and the adsorbed PF₆ ^- is released into the electrolyte in the first cycle. The NHC exhibits a capacity of ∼298 mA h g_Bi ^-1, capacity retention of 98.6% after 1000 cycles at 2 A g_Bi ^-1, and Coulombic efficiency of >99.4%. The achievable power and energy density are as high as 11.1 kW kg_total ^-1 and 106.5 W h kg_total ^-1, respectively. The superior electrochemical performance is ascribed to the gradually formed three-dimensional (3D) porous and stable networks of the anode, ensuring its comparable fast reaction kinetics and cycle stability to the AC cathode.