ZnFe₂O₄@Carbon Core–Shell Nanoparticles Encapsulated in Reduced Graphene Oxide for High-Performance Li-Ion Hybrid Supercapacitors

Li-ion hybrid supercapacitors (Li-HSCs) are attracting extensive attention because of their high energy/power densities. However, the performance of most Li-HSCs suffers from the limitation of the sluggish kinetics of battery-type anodes. Herein, we demonstrate that with dual protection of carbon and graphene, a three-dimensional, strongly coupled ZnFe2O4@C/reduced graphene oxide (RGO) composite anode provides an effective solution to this issue. The covalent C–O–M linkage between ZnFe2O4 nanoparticles and C/RGO promotes charge transfer and enhances structural stability. Two kinds of carbon-based buffering layers are able to well accommodate the volume change during charging/discharging, endowing the composite anode with high rate performance (692 mA h g–1 at 5 A g–1) and outstanding cycle life (98.3% of capacity retention after 700 cycles at 1 A g–1). The resulting ZnFe2O4@C/RGO//activated carbon Li-HSC shows an ultrahigh energy density of 174 W h kg–1, excellent power density of 51.4 kW kg–1 (at 109 W h kg–1), and superior cycle life (80.5% of retention capacity after 10 000 cycles at 5 A g–1).