Halogen-bonded covalent confinement enables solid-solid four-electron transfer in aqueous zinc-iodine batteries.

Aqueous Zn-I2 batteries are hindered by I+ instability and polyiodide shuttling. We introduce pyridinium tribromide (PTB) as an interphase regulator that establishes halogen-bonded covalent confinement at the cathode, enabling solid-solid four-electron transfer and suppressing both I+ hydrolysis and polyiodide shuttling.