Air-Stable Membrane-Free Magnesium Redox Flow Batteries

Membrane-free biphasic self-stratified batteries (MBSBs) utilizing aqueous/nonaqueous electrolyte systems have garnered significant attention owing to 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 had a wide electrochemical stability window of 3.24 V. The Mg metal anode features a Mg²⁺-conductive protective coating. Two metal-free redox compounds, 2,2,6,6-tetramethylpiperdinyl oxy (<b>TEMPO</b>) and <i>N</i>-propyl phenothiazine (<b>C3-PTZ</b>), were used as catholytes. The Mg||<b>TEMPO</b> and Mg||<b>C3-PTZ</b> MBSBs exhibited high cell voltages of 2.07 and 2.12 V, respectively, and were studied under static, stirred, and flow conditions. The Mg MBSBs were initially evaluated at different catholyte concentrations (0.1, 0.3, and 0.5 M) under static conditions. Notably, the Mg||<b>TEMPO</b> (0.5 M) and Mg||<b>C3-PTZ</b> (0.5 M) static batteries maintained exceptional performances 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||<b>TEMPO</b> (0.5 M) and Mg||<b>C3-PTZ</b> (0.5 M) batteries cycled 500 times at 12 mA/cm² demonstrated capacity retention rates of 99.82% and 99.88% (stirred), 93.58% and 92.16% (flow), respectively. Under flow conditions, the Mg||<b>TEMPO</b> (0.5 M) and Mg||<b>C3-PTZ</b> (0.5 M) batteries 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 exhibit remarkable performance and advance the application of Mg chemistry in organic flow batteries.