Quasi-Solid Cathode Additive Enables Highly Reversible Four-Electron I^-/I⁰/I⁺ Conversion in Aqueous Zn-I₂ Batteries

Aqueous zinc-iodine (Zn-I₂) batteries with four-electron (4e) I^-/I⁰/I⁺ conversion (4eZIBs) offer high energy density but face both-step I^-/I⁰ and I⁰/I⁺ challenges, including the polyiodide shuttle effect, sluggish I⁰/I⁺ conversion kinetics, and severe I⁺ hydrolysis. To mitigate these issues, a quasi-solid additive composed of 1-butyl-3-methylimidazolium chloride (BMICl) and carbon nanotubes (CNTs) is introduced into the cathode. Specifically, by co-grinding BMICl with CNTs, a homogeneous quasi-solid additive is formed due to the π-π stacking interactions between CNTs and imidazole rings. This additive not only suppresses the shuttle effect by binding with polyiodides in the first-step I^-/I⁰ conversion, but also enhances I⁺ conversion kinetics by immobilizing Cl^- inside the electrode and curbs I⁺ hydrolysis through forming a BMI-ICl₂ complex in the second-step conversion. This innovative approach enables the 4eZIBs to achieve a near-theoretical specific capacity of 418.9 mA h g^-1 at 0.5C, while maintaining a robust lifespan of over 600 cycles with a capacity retention of 93.4% at 1C. Moreover, pouch cells under a high areal capacity of 7.1 mA h cm^-2 for each side of the cathode demonstrate a high-capacity retention of 95.8% after 150 cycles at 6.3 mA cm^-2 (≈0.5C).