An Ultra‐Low Self‐Discharge Aqueous|Organic Membraneless Battery with Minimized Br2 Cross‐Over

Batteries dissolving active materials in liquids possess safety and size advantages compared to solid-based batteries, yet the intrinsic liquid properties lead to material cross-over induced self-discharge both during cycling and idle when the electrolytes are in contact, thus highly efficient and cost-effective solutions to minimize cross-over are in high demand. An ultra-low self-discharge aqueous|organic membraneless battery using dichloromethane (CH₂ Cl₂ ) and tetrabutylammonium bromide (TBABr) added to a zinc bromide (ZnBr₂ ) solution as the electrolyte is demonstrated. The polybromide is confined in the organic phase, and bromine (Br₂ ) diffusion-induced self-discharge is minimized. At 90% state of charge (SOC), the membraneless ZnBr₂ |TBABr (Z|T) battery shows an open circuit voltage (OCV) drop of only 42 mV after 120 days, 152 times longer than the ZnBr₂ battery, and superior to 102 previous reports from all types of liquid active material batteries. The 120-day capacity retention of 95.5% is higher than commercial zinc-nickel (Zn-Ni) batteries and vanadium redox flow batteries (VRFB, electrolytes stored separately) and close to lithium-ion (Li-ion) batteries. Z|T achieves >500 cycles (2670 h, 0.5 m electrolyte, 250 folds of membraneless ZnBr₂ battery) with ≈100% Coulombic efficiency (CE). The simple and cost-effective design of Z|T provides a conceptual inspiration to regulate material cross-over in liquid-based batteries to realize extended operation.