Air-stable Membrane-free Magnesium Redox Flow Batteries

Membrane-free biphasic self-stratified batteries (MBSBs), utilizing aqueous/nonaqueous electrolyte systems, have garnered significant attention for their flexible manufacturing and cost-effectiveness. In this study, we present an ultrastable high-voltage Mg MBSB based on an aqueous/nonaqueous electrolyte system. The engineered aqueous electrolyte has a wide electrochemical stability window of 3.24 V. The Mg metal anode features an Mg2+-conductive protective coating. Two metal-free redox compounds, 2,2,6,6-tetramethyl piperidinyl oxy (TEMPO) and N-propyl phenothiazine (C3-PTZ), in dichloromethane serve as the catholyte. The Mg||TEMPO and Mg||C3-PTZ MBSBs exhibited high cell voltages of 2.07 and 2.12 V, respectively, and were studied under static, stirred, and flow conditions. The Mg MBSB was initially evaluated at different cathode concentrations (0.1, 0.3, and 0.5 M) under static conditions. Notably, Mg||TEMPO (0.5 M) and Mg||C3-PTZ (0.5 M) static batteries maintained exceptional performance over 500 cycles at 8 mA/cm2, with capacity retention rates of 97.84% and 98.87%, Coulombic efficiencies of 99.17% and 99.12%, and capacity utilization of 70.2% and 71.3%, respectively. Under stirred and flow conditions, the Mg||TEMPO (0.5 M) and Mg||C3-PTZ (0.5 M) batteries cycled 500 times at 12 mA/cm2, showing capacity retention rates of 99.82% and 99.88% (stirred), 93.58% and 92.16% (flow), respectively. The Mg||TEMPO (0.5 M) and Mg||C3-PTZ (0.5 M) batteries under flow conditions demonstrated power densities of 195 and 191 mW/cm2, respectively, surpassing those of 139 and 144 mW/cm2 under static conditions. These cost-effective Mg MBSBs offer remarkable performance, advancing Mg chemistry applications in organic flow batteries.