Mechanistic insights of Mg²⁺-electrolyte additive for high-energy and long-life zinc-ion hybrid capacitors

An electrolyte cation additive strategy provides a versatile route for developing high-energy and long-life aqueous zinc-ion hybrid capacitors. However, the mechanisms of energy storage and Zn anode protection are still unclear in Zn-based systems with dual-ion electrolytes. Here, a dual charge storage mechanism for zinc-ion hybrid capacitors with both cations and anions adsorption/desorption and the reversible formation of Zn4SO4(OH)6·xH2O enabled by the Mg2+ additive in the common aqueous ZnSO4 electrolyte are proposed. Theoretical calculations verify that the self-healing electrostatic shield effect and the solvation-sheath structure regulation rendered by the Mg2+ additive account for the observed uniform Zn deposition and dendrite suppression. As a result, an additional energy storage capacity of ≈50% compared to that in a pure 2 m ZnSO4 electrolyte and an extended cycle life with capacity retention of 98.7% after 10 000 cycles are achieved. This work highlights the effectiveness of electrolyte design for dual-ion carrier storage mechanism in aqueous devices toward high energy density and long cycle life.Ministry of Education (MOE)Submitted/Accepted versionThis work was supported by National Natural Science Foundation of China (Grant Nos. 51932011, 51972346), the Program of Youth Talent Support for Hunan Province (2020RC3011), Innovation-Driven Project of Central South University (No. 2020CX024), and the Fundamental Research Funds for the Central Universities of Central South University (No. 202321024). H.J.F. acknowledges the financial support from Ministry of Education by Tier 1 grant (RG157/19)