Dual-carbon Na-ion capacitors: progress and future prospects

The abundance of sodium and the absence of costly transition metals in electrodes are the significant strongholds of dual carbon sodium-ion capacitors (DC-NICs) due to which they are cheaper and readily available compared to other prominent energy storage devices. A perfect amalgamation of energy and power density is the aim of DC-NICs, which is achieved by combining carbon-based battery type and capacitor type electrodes and using a suitable electrolyte. An optimum combination of surface area, the volume of pores, and ordering of the structure applied to both the anode and the cathode enable the efficient fusion of energy density and power density. Battery-type electrodes are mainly ordered carbon structures (graphite, hard carbon, or layered structures), which promote the faradaic mechanism-based energy storage that imparts high energy density. Structural modification aimed towards providing a higher number of pores and surface area has improved the high rate performance and power density of such structures. A majority of the capacitor-type electrodes are fabricated with a very large surface area (activated carbon or highly porous structures) to bolster the power density, which is promoted via the surface charge-storage process. Through material engineering including defects and functional groups, additional charge storage sites are created that can improve the energy density. DC-NICs fabricated from biomass precursors are promising and exhibit performance on par with that of lithium-ion batteries. High power and energy densities make DC-NICs a suitable candidate for electric vehicle applications. Though DC-NIC is a novel concept, the progress within a short time is immense, with the capability to provide clean, green, and cost-effective energy. Reported works have been studied and factors that played a crucial role in improving the performances have been highlighted. Some parameters that govern the performance of DC-NICs and can help future research works have been discussed.