Development of an all-aqueous membrane-free inorganic redox flow battery

• Inorganic membrane-less RFB based on Ti 3+ /Ti 4+ and V 4+ /V 5+ has been demonstrated. • Battery setup delivers >90% CE%, and stable operation over 2000 cycles. • Active species crossover inhibition by retaining OCP for >20 hours. The intermittency of renewable energy sources demand a scalable and durable energy storage technology capable of long-term operation with safety and reliability. Redox flow batteries (RFBs) are well suited for grid-scale energy storage; however, their reliance on ion-exchange membranes introduces a significant capital cost (>30%), resistance, and long-term stability challenges. On the other hand, membrane-less batteries based on biphasic electrolyte design offer a promising alternative by exploiting their liquid–liquid interface as an intrinsic separator, eradicating the need for an ion-exchange membrane. Herein, we report a fully soluble, all-inorganic aqueous biphasic membrane-less redox flow battery employing ‘Ti 3+ /Ti 4+ ’ and ‘V 4+ /V 5+ ’ redox couples. Phase separation is induced by LiTFSI, forming two immiscible aqueous phases with a stable liquid–liquid interface that inherently suppresses crossover. Comprehensive physical characterizations confirm the density-driven stratification and high viscosity contrast. Electrochemical studies demonstrated efficient cycling over 2000 cycles with coulombic efficiencies (CE%) exceeding 90% at a maximum discharge current density of 3.18 mA∙cm −2 , and a stable potential of 0.8 V for >20 hours at zero current condition, indicating effective crossover suppression. These results underscore the potential of aqueous biphasic inorganic systems as an alternative to ion-exchange membranes for chemically robust redox flow batteries.