High-performance aqueous zinc ion hybrid capacitors obtained by Na₂SO₄ additive and dense pore structure

In this study, the electrochemical performance of zinc ion hybrid capacitors (ZICs) was improved by employing carbon-based materials and electrolyte together. First, we prepared pitch-based porous carbon HC-800 as the electrode material, which possessed a large specific surface area (3607 m² g^-1) and a dense pore structure. This provided abundant adsorption sites for zinc ions, and thus stored more charges. Subsequently, 0.5 M Na₂SO₄ was added to 1 M Zn(CF₃SO₃)₂ electrolyte via the cationic additive strategy, and the adsorption energy of sodium and zinc ions on the zinc electrode was calculated. The results showed that sodium ions would preferentially be adsorbed on the surface of the zinc electrode, which would inhibit the growth of zinc dendrites, and thus prolong the service life of the zinc electrode. Finally, the presence of solvated zinc ions in the narrowly distributed pores of HC-800 was studied, and the results showed that Zn(H₂O)₆²⁺ underwent a desolvation process, resulting in the removal of two water molecules to form a tetrahedral structure of Zn(H₂O)₄²⁺, which made the central surface of the zinc ions closer to the surface of HC-800, and thus the more capacitance achieved. Furthermore, the uniform distribution of Zn(H₂O)₄²⁺ in the dense and neat pores of HC-800, improved the space charge density. Consequently, the assembled ZIC exhibited a high capacity (242.25 mA h g^-1 at 0.5 A g^-1) and ultra-long cycle stability (capacity retention at 87% after 110 000 charge/discharge cycles at a high current density of 50 A g^-1 and a coulombic efficiency of 100%) and an energy density of 186.1 W h kg^-1 and power density of 41 004 W kg^-1.