Emerging Materials for Sodium-Ion Hybrid Capacitors: A Brief Review

The demand for energy storage is exponentially increasing with growth of the human population, which is highly energy intensive. Batteries, supercapacitors, and hybrid capacitors are key energy storage technologies, and lithium and sodium ions are critical influencers in redefining the performances of such devices. Batteries can store energy with high density, and capacitors can deliver a high power density. In addition, hybrid capacitors bridge the energy and power gap between a battery and supercapacitor by combining reactions from a battery-type electrode and a capacitor-type electrode. Sodium-ion hybrid capacitors (NICs) can combine the benefits of high power capacitors and high energy batteries at a cost potentially lower than that of Li analogues. However, research on NICs is in the nascent stage and requires significant attention to enable their use in practical applications. This review presents a comprehensive summary of the development of Na-ion hybrid capacitors based on carbon materials, a sodium superionic conductor NASICON, and metal oxide or sulfide-type anodes, with a particular emphasis on the performance metrics. Furthermore, design strategies and unsolved issues in emerging capacitor systems, such as pseudocapacitive electrodes, organic electrodes, MXenes, and flexible capacitors, which could be trend setters for next-generation applications, are the focus. The revolving issues with each system and the strategies to overcome such issues are also briefly discussed. A perspective and outlook on the future of NICs will help the scientific community direct their future studies.