Unraveling the origin of enhanced safety in capacitive-type carbon electrodes for 20C sodium-ion capacitors

Abstract

Sodium-ion capacitors (SICs) combine battery-type and capacitive-type electrodes to achieve both high energy density and high power density. However, the capacity of conventional capacitive carbon materials is typically limited to below 120 mA h g⁻¹, resulting in inferior energy density for SICs. Therefore, the development of high-capacity capacitive electrode materials is considered critical. Herein, capacitive soft carbon (SC) is synthesized through in situ phosphorus crosslinking strategies applied to coal tar, with the resulting material exhibiting a high tap density of 1.26 g cm⁻³, which is larger than that of other carbon materials (<0.81 g cm⁻³). Furthermore, the obtained carbon materials deliver a high capacitive capacity of 308 mA h g⁻¹ at 0.03 A g⁻¹. Impressively, in situ Raman characterization reveals that the P–O–C/P–C bonds introduced by in situ crosslinking can provide abundant reversible capacitive sodium storage sites, which effectively suppress Na–Na bonding, thereby inhibiting sodium dendrite formation and enhancing safety, according well with DFT calculations. When assembled into SICs, an ultrahigh energy density of 196.81 Wh kg⁻¹ is achieved at 2891.90 W kg⁻¹, greatly surpassing previous reports (<120 Wh kg⁻¹). In a pouch cell (PC) configuration, a high energy density of 56.04 Wh kg⁻¹ can be achieved at a high rate of 20C, with full charge accomplished within 96 seconds. This work provides essential guidance for constructing SICs with both ultrahigh energy and power densities.