Tailoring hard carbon interfaces in carbonate-based electrolytes for sodium-ion hybrid capacitors

The poor rate performance of hard carbon (HC) in carbonate electrolytes limits its applicability in hybrid capacitors, primarily due to the low working potential and the slow Na+ transport kinetics within the potential plateau region. The slow desolvation of Na+ at the electrode surface and sluggish transport of Na+ through the solid electrolyte interface are the critical factors contributing to this issue. In this study, Co3O4 nanoparticles are uniformly self-grown on the HC surface to modulate the surface chemistry of HC. The introduction of Co3O4 not only facilitates the desolvation of Na+ and reduces internal resistance, but also provides additional active sites for Na+ storage as an active material. As a result of these dual effects, HC125@Co3O4 (a composite with an optimal Co3O4 loading on HC surfaces) exhibits superior rate performance and reversible capacity compared to pure HC. The sodium-ion hybrid capacitor assembled with the HC125@Co3O4 anode and activated carbon cathode demonstrates high energy density (129.5 Wh kg-1 at 583 W kg-1) and high power density (26.5 Wh kg-1 at 11,650 W kg-1), along with excellent long-time cycling stability. This study offers an effective solution to the poor rate performance and slow kinetics of HC in carbonate-based electrolytes, addressing the issue from the perspective of the electrode-electrolyte interface.