Constructing a stable 3 V high-energy sodium ion capacitor using environmentally benign Na2FeSiO4 anode and activated carbon cathode

A novel and highly stable 3 V sodium hybrid supercapacitor is designed with an intercalation type ecofriendly carbon-coated Na2FeSiO4 (CNFS) and activated carbon (AC) with an organic electrolyte. The CNFS/AC cell shows potential to outperform present mature lithium-based capacitors. This novel sodium-based capacitor produces an energy density of 70 Wh kg(-1) at a specific power of 400 W kg(-1), and still maintains an energy density of 45 Wh kg(-1) at a higher specific power of 5400 W kg(-1). The constructed cell also exhibits a remarkable cyclic stability of similar to 94% at a current density of 0.6 Ag-1 after 30,000 cycles. The presence of the carbon coating and stable Si-O bonding in CNFS improves its stability and conductivity while the porous nature of the support eases ion access at high current density and helps maintain high activity of the material. The presented CNFS/AC cell possesses high conductivity and reverse-polarity characteristics, which enhances the safety of the cell under high voltage. This ecofriendly, low cost and extremely safe NIC bridges the gap between batteries and capacitor and make it attractive for application in low maintenance energy storage devices. (C) 2019 Published by Elsevier Ltd.