An Aqueous Phase Transition Electrolyte Retaining High Ionic Conductivity after Solidification for Low-Temperature Zinc–Iodine Batteries

Abstract Conventional low-temperature aqueous electrolytes are primarily designed to suppress electrolyte freezing and thereby preserve ion transport in the liquid state, but such strategies become ineffective once electrolyte solidification becomes unavoidable. Here, we report an aqueous phase-transition electrolyte (PTE) that preserves intimate electrode–electrolyte contact and exhibits high ionic conductivity after solidification. Systematic characterization reveals that the solidified PTE consists of an interconnected ice framework permeated by a continuous liquid electrolyte, whereas a conventional ZnSO4-based electrolyte (ZSBE) forms isolated salt domains within the ice matrix, resulting in poor interfacial contact and limited ionic transport. This unique architecture establishes continuous ion-transport pathways throughout the solidified electrolyte, enabling an ionic conductivity of 1.870 mS cm–1 at −60 °C, compared to only 0.065 mS cm–1 for ZSBE. Using an aqueous Zn||I2 battery as a model system, PTE affords exceptional cycling stability and rate capability at low temperatures, achieving a record-high cycling lifespan of 430 000 cycles at 40 A g–1 and −40 °C. An Ah-level pouch cell remains operational in liquid nitrogen, powering 107 light-emitting diodes. This work marks a paradigm shift from preventing electrolyte freezing to sustaining high ionic conductivity after solidification for low-temperature aqueous batteries.