ZIF-8 coating on graphite: a high-rate and long-term cycling anode for sodium-ion capacitors

Abstract

Sodium-ion capacitors (SICs) have been designed to combine the advantages of high-energy batteries and high-power capacitors as well as low-cost sodium resources. However, anode materials usually exhibit sluggish diffusion of Na⁺, which results in kinetics imbalance with the capacitive cathode. Herein, the zeolitic imidazolate framework-8 (ZIF-8) layer is uniformly grown on the graphite surface (ZIF-8@Gr) to promote solvated-Na⁺ co-intercalation reactions in an ether electrolyte. The ZIF-8 coating can act as a multifunctional protection layer to inhibit electrolyte decomposition in the initial cycle, and also withstand volume expansion of graphite during the long-term co-intercalation process. The initial coulombic efficiency (ICE) of the ZIF-8@Gr electrode can be improved to 86%, much higher than that of the pristine graphite electrode (58%). More importantly, the ZIF-8@Gr electrode possesses ultrafast-charging sodium storage (20 A g⁻¹, a full charge time within 16.2 s) and ultralong cycle life (96% and 94% capacity retention after 15 000 and 20 000 cycles at 5 A g⁻¹ and 10 A g⁻¹, respectively). By coupling the ZIF-8@Gr electrode with the activated carbon (AC) as the positive electrode, the as-fabricated SIC device demonstrates impressive energy//power densities (82 W h kg⁻¹ at 518 W kg⁻¹, 47 W h kg⁻¹ at 10 370 W kg⁻¹). These results indicate that ZIF-based surface modification enables graphite as a fast-charging and long-term cycling sodium-storage material.