Carbonaceous Anode Materials for Non-aqueous Sodium- and Potassium-Ion Hybrid Capacitors

Sodium- and potassium-ion (Na-/K-ion) hybrid capacitors are promising electrochemical energy storage systems that are more cost-effective than corresponding lithium-based alternatives. Their hybrid configuration integrates a battery-type anode and a capacitor-type cathode and affords high energy density, high power density, and good cycling stability. However, the primary issue encountered in Na-/K-ion hybrid capacitors is a lack of reliable anodes because of the sluggish reaction kinetics of large Na-/K-ions. In recent years, significant advancements have been achieved in carbonaceous anodes because of their high Na-/K-ion storage feasibility, natural abundance, low cost, and non-toxicity. This review encompasses the fundamental electrochemical principles of Na-/K-ion hybrid capacitors and provides insights into the intimate structure–performance relationship of carbonaceous anodes. The existing challenges and alternative strategies for improving the electrochemical performance of the carbonaceous anodes are emphasized. Finally, future prospects and possible directions for further improving carbonaceous anodes for Na-/K-ion hybrid capacitors are presented.