Revealing the Self-Doping Defects in Carbon Materials for the Compact Capacitive Energy Storage of Zn-Ion Capacitors

Zn-ion capacitors are attracting great attention owing to the abundant and relatively stable Zn anodes but are impeded by the low capacitance of porous carbon cathodes with insufficient energy storage sites. Herein, using ball-milled graphene with different defect densities as the models, we reveal that the self-doping defects of carbon show a capacitive energy storage behavior with robust charge-transfer kinetics, providing a capacitance contribution of ca. 90 F g^-1 per unit of defect density (A_D/A_G value from Raman spectra) in both aqueous and organic electrolytes. Furthermore, a simple NaCl-assisted ball-milling method is developed to prepare novel graphene blocks (BSG) with abundant self-doping defect density, enriched pores, balanced electric conductivity, and high compact density (0.83 g cm^-3). The optimized ion and electron transfer paths promote efficient utilization of the self-doping defects in BSG, contributing to improved gravimetric and volumetric capacitance (224 F g^-1/186 F cm^-3 at 0.5 A g^-1) and remarkable rate performance (52.2% capacitance retention at 20 A g^-1). The defect engineering strategy may open up a new avenue to improve the capacitive performance of dense carbons for Zn-ion capacitors.