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

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 <i>in situ</i> 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, <i>in situ</i> Raman characterization reveals that the P-O-C/P-C bonds introduced by <i>in situ</i> 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.