A High‐Voltage Membrane‐Free Li–Organic Hybrid Flow Battery Using Eutectic Lithium Chemistry

Abstract

ABSTRACT Membrane‐free redox flow batteries offer simplified design and reduced cost, but their electrochemical performance remains limited. Here, a high‐voltage nonaqueous membrane‐free Li–organic hybrid flow battery was constructed using a biphasic electrolyte. The bottom‐phase anolyte is a deep eutectic solvent formed by lithium hexafluorophosphate and 2,2,2‐trifluoroacetamide, which provides a wide electrochemical stability window of 4.81 V, low viscosity of 13.8 cP, high ionic conductivity of 8.59 mS·cm −1 , and stable compatibility with lithium metal. The top‐phase catholyte is dichloromethane containing a long‐alkyl‐chain phenothiazine derivative, which exhibits excellent redox reversibility and strict confinement in the upper phase. The biphasic system achieved an open‐circuit voltage of approximately 3.6 V and stable operation at 0.5 M concentration, retaining 85.6% of theoretical capacity with a Coulombic efficiency of 97.6% after 100 cycles. In flow configuration, the 0.5 M battery maintained over 92% capacity after 10 days of continuous cycling with minimal self‐discharge. A preliminary cost estimation yielded $134.8 kWh −1 , which is lower than vanadium redox flow batteries, and both electrolyte phases displayed strong flame resistance. This study provides a viable design strategy to enable membrane‐free nonaqueous Li–organic hybrid flow battery by leveraging eutectic lithium chemistry and immiscible solvent interfaces.