Reduced water activity in co-solvent electrolyte enables 2 V zinc-ion hybrid capacitors with prolonged stability and high energy density

Abstract

Aqueous Zn-ion hybrid capacitor (ZIC) is a promising energy storage device owing to the cost-benefit and intrinsic safety conferred by the energy-dense Zn anode and capacitor-type cathode in aqueous electrolytes. However, the cycling stability under realistic conditions and high energy density of aqueous ZICs has yet to be demonstrated before ZICs can be commercialized. In this work, we introduce a co-solvent hybrid electrolyte, with Zn(CF₃SO₃)₂/water/acetonitrile molar ratio of 1 : 3.5 : 3.5, which not only yields enhanced stability of ZIC but also allows extending the operating voltage of ZICs from 1.8 V to 2 V. Using a hierarchically porous carbon as the model cathode and Zn metal anode, maximum capacity of 181.6 mA h g⁻¹ (72.2% increase compared to 1.8 V) and energy density of 162.1 W h kg⁻¹ (68.3% increase) are achieved for hybrid ZICs charged to 2 V. The hybrid ZICs display excellent reversibility with a coulombic efficiency above 99.7% when cycled in the range of 0.2–2 V at 0.5, 1, 2, and 5 A g⁻¹. Such remarkable performance of ZICs is attributed to the altered solvation structure of this 4.8 m hybrid electrolyte with fewer free and Zn²⁺-bound water molecules than the aqueous electrolyte as well as the participation of acetonitrile and CF₃SO₃⁻ in the primary solvation shell of Zn²⁺. This mitigates deprotonation of Zn²⁺-bound water and facilitates dendrite-free Zn plating thanks to the increased Zn nucleation/growth overpotential. The decrease of water activity in this nonflammable hybrid electrolyte allows bypassing the cathodic challenge associated with aqueous ZICs and shifting the threshold of hydrogen evolution reaction below −0.5 V (vs. Zn/Zn²⁺). In contrast, organic ZICs experience instability when the cell voltage is increased from 1.8 V to 2 V, due to the decomposition of trace water absorbed from the ambient atmosphere. This work opens opportunities for designing co-solvent electrolytes for stable ZICs with high energy density.