Polarity coupling in biphasic electrolytes enables iodine/polyiodide co-extraction for portable Zn–iodine batteries following a liquid–liquid conversion route

Abstract

The shuttle effect of polyiodides and aggregation of solid iodine on the cathode surface in aqueous Zn–iodine batteries are considered the main issues for their unsatisfactory cycling stability and slow charge transfer kinetics, respectively. Herein, we develop a biphasic (BP) electrolyte composed of immiscible organic (ethyl acetate, EA) and aqueous solvents for the co-extraction of iodine/polyiodides. The underlying mechanism is clarified by the principle of polarity coupling between iodine species and solvent molecules. Notably, distinct from the formation of solid iodine in an aqueous electrolyte, the electrochemical redox reactions of iodine/polyiodides at the cathodic side (organic phase) investigated by rotating ring electrodes follow the liquid–liquid conversion route. Accordingly, the diffusion of polyiodides is effectively suppressed at the interface of the BP electrolyte and the absence of solid iodine deposition significantly enhances charge transfer kinetics. Moreover, the quasi-solid-state Zn–iodine batteries featuring a gravity-independent stratified architecture are demonstrated, enabled by a BP system consisting of microspace-confined EA and PAM-CMC hydrogel. The fabricated portable device exhibits an areal capacity of 1.40 mA h cm⁻² at 1 mA cm⁻², improved rate performance and stable cycling performance over 22 000 cycles at 10 mA cm⁻², indicating extraordinary reliability for wearable applications.