Transition Metal Oxides as the Electrode Material for Sodium-Ion Capacitors

The research of energy-storage systems has been encouraged in the last ten years by the rapid development of portable electronic gadgets. Hybrid-ion capacitors are a novel kind of capacitor-battery hybrid energy storage device that has earned a lot of interest because of their high power density while maintaining energy density and a long lifecycle. Mostly, lithium-based energy storage technology is now being studied for use in electric grid storage. But the price increment and intermittent availability of lithium reserves make lithium-based commercialization unstable. Therefore, sodium-based technologies have been proposed as potential substitutes for lithium-based technologies. Sodium-ion capacitors (SICs) are acknowledged as potential innovative energy storage technologies which have lower standard electrode potentials and lower costs than lithium-ion capacitors. However, the large radius of the sodium ion also contributes to unfavorable reaction kinetics, low energy density, and brief lifespan of SICs. Recently, transition metal oxide (TMO)-based candidates have been considered potential due to the large theoretical capacity, environmental friendliness, and low cost for SICs. This brief study summarizes current advancements in research of TMOs and sodium-based TMOs as electrode candidates for SIC applications. Also, we have covered in detail the state of the exploration and upcoming prospects of TMOs for SICs.