Developing an oxygen-induced capacitive hard carbon anode for fast sodium ion storage through pore-scale engineering

Sodium-ion capacitors are highly promising for bridging the gap between high-power-density supercapacitors and high-energy-density batteries. However, developing appropriate battery-type anodes to boost Na+ transfer remains a major challenge. Herein, we manufacture a capacitive hard carbon anode with multiple pore-scale and oxygen-functionality surfaces by a sulfonation-assisted etching technique, featuring a large capacity (372.8 mAh g-1 at 0.035 A g-1), superior rate capability (129.6 mAh g-1 at 20 A g-1) and decent long-cycle stability. The synergistic effect of meso-macropores and C=O facilitates fast Na-ion migration and surface pseudocapacitive reaction, achieving 70% capacitive contribution even at a low sweep rate of 0.2 mV/s1. These mechanisms are validated by complementary EIS, GITT, ex-situ XPS, and DFT analyses. Outstandingly, the assembled sodium-ion capacitor delivers very high energy (53.5 Wh kg-1) and power densities (17.5 kW kg-1) simultaneously. This work provides a new approach to tune the oxygen-containing functional groups in hard carbon for enhancing Naion storage performance toward practical applications.