Nonaqueous hybrid redox flow energy storage with a sodium–TEMPO chemistry and a single-ion solid electrolyte separator

Abstract

There has been increasing interest in recent years in exploring and implementing organic electrode materials for electrochemical energy storage. Liquid phase electrodes with an electrochemically active organic material dissolved in a nonaqueous solvent are of particular interest for developing nonaqueous redox flow batteries (RFBs). In contrast to traditional aqueous RFBs, nonaqueous systems offer two eminent benefits in terms of being able to operate at low temperatures and being able to access a high working voltage. By incorporating an anode chemistry of sodium, we present in this study a nonaqueous hybrid flow battery (HFB) with a (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) liquid cathode. To prevent the oxidative TEMPO species from entering the sodium anode, a sodium-based solid-electrolyte membrane, Na₃Zr₂Si₂PO₁₂, is incorporated as a single-ion solid electrolyte separator. The Na₃Zr₂Si₂PO₁₂ membrane selectively allows the transport of Na⁺ ions but avoids the crossover of other species between the two electrodes. At a medium current density, the nonaqueous Na–TEMPO cell delivers a high voltage of ca. 2.6 V. By managing the TEMPO catholyte with a reasonably high concentration, the cell provides a remarkable volumetric energy density of ca. 32 W h L⁻¹.