Synergistic Cationic Shielding and Anionic Chemistry of Potassium Hydrogen Phthalate for Ultrastable Zn─I₂ Full Batteries

Electrolyte additives are investigated to resolve dendrite growth, hydrogen evolution reaction, and corrosion of Zn metal. In particular, the electrostatic shielding cationic strategy is considered an effective method to regulate deposition morphology. However, it is very difficult for such a simple cationic modification to avoid competitive hydrogen evolution reactions, corrosion, and interfacial pH fluctuations. Herein, multifunctional additives of potassium hydrogen phthalate (KHP) based on the synergistic design of cationic shielding and anionic chemistry for ultrastable Zn||I₂ full batteries are demonstrated. K cations, acting as electrostatic shielding cations, constructed the smooth deposition morphology. HP anions can enter the first solvation shell of Zn²⁺ for the reduced activities of H₂O, while they remain in the primary solvation shell and are finally involved in the formation of SEI, thus accelerating the charge transfer kinetics. Furthermore, by in situ monitoring the near-surface pH of the Zn electrode, the KHP additives can effectively inhibit the accumulation of OH⁻ and the formation of by-products. Consequently, the symmetric cells achieve a high stripping–plating reversibility of over 4500 and 2600 h at 1.0 and 5 mA cm⁻², respectively. The Zn||I₂ full cells deliver an ultralong term stability of over 1400 cycles with a high-capacity retention of 78.5%.