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

Abstract Aqueous zinc‐iodine (Zn‐I 2 ) batteries with four‐electron (4e) I − /I 0 /I + conversion (4eZIBs) offer high energy density but face both‐step I − /I 0 and I 0 /I + challenges, including the polyiodide shuttle effect, sluggish I 0 /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 − /I 0 conversion, but also enhances I + conversion kinetics by immobilizing Cl − inside the electrode and curbs I + hydrolysis through forming a BMI‐ICl 2 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).