I⁺/I₂/I^- conversion toward energy-dense aqueous Zn-I₂ batteries: progress and perspective

Aqueous zinc-iodine batteries (ZIBs), exploiting reversible conversion among various iodine species, have drawn significant research interest due to their fast redox kinetics and capability for multi-electron transfer. Although significant progress has been made in ZIBs based on the two-electron I₂/I^- redox pathway (2eZIBs), their inherently limited energy density impedes practical deployment. Achieving the additional reversible conversion of high-valence iodine species, particularly the I⁺/I₂ redox chemistry, offers substantial potential for improving energy density up to 630 Wh kg^-1 based on the mass of I₂. Nonetheless, Zn-I₂ batteries based on this four-electron I⁺/I₂/I^- conversion (4eZIBs) suffer from severe reversibility issues due to the shuttle of iodide intermediates and the detrimental hydrolysis of I⁺ species during the conversion process. In this perspective, we comprehensively elucidate the fundamental principles of the I₂/I^- and I⁺/I₂ redox chemistry, while critically evaluating the merits and limitations of diverse strategies for enhancing the performance of 4eZIBs. Significantly, we propose specific methodological approaches from multiple angles to improve the reversibility of I⁺/I₂/I^- conversion. These findings aim to provide valuable insights for the development of advanced metal-halogen battery energy storage systems.