Adv. Energy Mater. http://doi.org/f3kn3b (2016)

Lithium-ion batteries can store a large amount of energy but often don't deliver the energy quickly. Supercapacitors, on the other hand, have high power densities but suffer from limited energy densities. Lithium- or sodium-ion capacitors, which combine the advantages of the two devices, have emerged as a promising way to achieve both high energy and high power performance. Although sodium-ion capacitors are cost-effective compared with their lithium counterparts, the slow ionic diffusion and reaction kinetics are the main challenges. Yun-Sung Lee and colleagues at Chonnam National University, University of Western Ontario and Seoul National University have now reported a sodium-ion capacitor that mitigates the problem of the sluggish kinetics and exhibits an energy density of 118 W h kg−1 and a power density of 850 W kg−1 that even outperforms the lithium-ion capacitors.

The sodium-ion capacitor was built with a carbon-coated sodium conductor (Na3V2(PO4)3) and a carbon material derived from cinnamon sticks as electrodes. The porous characters of Na3V2(PO4)3 and the bio-derived carbon are key in enhancing the sodium insertion/de-insertion in Na3V2(PO4)3 as well as the adsorption/desorption on the electrode surfaces, both of which contribute to the high energy and power densities. In addition, Na3V2(PO4)3 can retain its framework structure well during the electrochemical reactions, which is mainly responsible for the observed high stability in cycling experiments.