Tailored Polyimide-Graphene Nanocomposite as Negative Electrode and Reduced Graphene Oxide as Positive Electrode for Flexible Hybrid Sodium-Ion Capacitors

Redox-active polyimide materials hold a great promise for electrochemical energy storage applications, especially for flexible energy storage devices. However, the low utilization efficiency due to poor electrical conductivity of the materials remains one of the greatest challenges. In this work, we designed and prepared polyimide-graphene composite materials and tested their electrochemical properties in sodium-ion capacitors. By manipulating the interfacial chemistry and interactions between the polyimide and graphene, composite electrode materials with different polyimide particle sizes and morphologies were obtained. Sodium-ion storage capacity was significantly improved, from ∼50 mAh g^-1 for pure polyimide to 225 mAh g^-1 for a polyimide-graphene composite. A hybrid sodium-ion capacitor fabricated with freestanding polyimide-graphene composite as the negative electrode and reduced graphene oxide as the positive electrode delivered energy densities of 55.5 and 21.5 Wh kg^-1 at power densities of 395 and 3400 W kg^-1, respectively. A flexible sodium-ion capacitor with outstanding mechanical properties was also demonstrated.