Multi-electron zinc–iodine batteries stabilized by acid-durable selective framework membranes

Abstract

Aqueous zinc–iodine batteries (AZIBs) are regarded as promising energy storage systems owing to their cost-effectiveness and intrinsic safety. To improve energy density, recent studies have activated high-voltage multi-electron iodine redox couples (I−/I0/I+) by introducing Cl−-containing electrolyte additives. However, intermediate polyiodide species are inevitably formed during the two-step reactions and migrate toward the Zn anode through the porous separator, resulting in active iodine loss and reduced reversibility of I−/I0/I+ reactions. Therefore, beyond electrolyte optimization, separator modification is also essential to achieve high-voltage operation. In this study, we introduce a metal–organic framework (MOF)-integrated separator that functions as a molecular sieve to block polyiodide crossover and enhance the reversibility of I−/I0/I+ reactions in AZIBs. Specifically, a MOF-808 layer with an internal pore size of approximately 0.48 nm and high stability in acidic electrolyte is uniformly coated onto a glass fiber membrane, forming a well-ordered nanoporous structure. The nanochannels facilitate selective ion transport, effectively suppressing polyiodide migration to the anode, as demonstrated by ultraviolet-visible spectroscopy. Moreover, self-discharge tests of [Zn || iodine-impregnated activated carbon] full cells confirm the efficacy of the MOF layer in mitigating polyiodide-induced parasitic reactions. Consequently, a full cell incorporating the MOF-integrated separator achieves a high areal discharge capacity of 2.9 mAh cm−2 and stable cycling performance under high-voltage operation involving the I0/I+ redox couple. Ex-situ Raman spectroscopy further reveals a more intense I+ band for the MOF/GF cell than for the GF cell, indicating stronger retention and stabilization of I+ species in the cathode owing to suppressed polyiodide migration. These results highlight a separator-based strategy for stabilizing reversible I−/I0/I+ redox reactions, marking a new direction for achieving high-voltage AZIBs.