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

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.