Enabling High-Voltage Membrane-Free Nonaqueous Redox Flow Battery through Eutectic Lithium Chemistry

Although, membrane-free redox flow batteries have a simple design and are cost effective, but their electrochemical performance remains limited. Here a high-voltage nonaqueous membrane-free redox flow battery was constructed using a biphasic electrolyte. A deep eutectic solvent composed of lithium hexafluorophosphate and 2,2,2-trifluoroacetamide is used as the bottom-phase anolyte, which provides a wide electrochemical stability window of 4.81 V, low viscosity of 13.8 mPa·s, high ionic conductivity of 8.59 mS·cm⁻¹, and excellent 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. Under flow conditions, the 0.5 M battery maintained over 92% capacity after 10 days of continuous cycling with minimal self-discharge. Compared to traditional vanadium redox flow batteries, a preliminary cost estimation yields 134.8 $ kWh -1 , which highlights the cost effective system. Furthermore, both the electrolyte phases demonstrate strong flame resistance. This study provides a viable design strategy to enable membrane-free NORFBs by leveraging eutectic lithium chemistry and immiscible solvent interfaces. overall, this approach offers a safer, cost-effective, and sustainable solution for large-scale energy storage systems.