Advanced sodium-ion pseudocapacitor performance of oxygen-implanted hard carbon derived from carbon spheres

Sodium-ion capacitors (NICs) are rising as a promising candidate for next-generation energy storage systems owing to the abundance of sodium resources and the better performance than that of their lithium-ion capacitor counterparts. The design of the pseudocapacitive electrode materials is of great significance for an integrated device to have high energy and power densities. In this work, activated hard carbon (AHC) with a high surface area and abundant oxygen-containing functional groups is synthesized via the KOH activation of controllable carbon spheres that act as a special type of precursors. The AHC can be used as a supercapacitor-type material because it exhibits excellent properties as anode or cathode of sodium-ion batteries. The integrated symmetrical NIC with the AHC as both anode and cathode is superior to most previously reported all-carbon-based capacitors in terms of electrochemical performance. Benefiting from the oxygen-containing groups on the surface of the AHC, the pseudocapacitor shows an energy density of 43 Wh kg−1 at a power density of 3724 W kg−1 after 5000 cycles. This study not only provides a novel electrode material for NICs but also deepens the understanding of all-carbon-based capacitors.