Two distinct advantages of nonaqueous redox flow batteries (RFBs) are the feasibility of building a high cell voltage (without a constraint of the water-splitting potential) and the operability at low temperatures (without a concern of freezing below 0 °C). However, electrochemically active organic redox couples are usually selectively soluble in specific nonaqueous solvents, and their solubility is relatively low (in contrast to that in aqueous solutions). The selective and low solubility of redox couples seriously constrict the practical energy density of nonaqueous RFBs. Herein, we present a hybrid nonaqueous RFB with a solid zinc anode and a liquid (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) cathode. Toward accessing a high solubility of the TEMPO cathode and to sufficiently accommodate the discharge products of a Zn anode, asymmetric electrolyte solvents, viz., propylene carbonate (PC) and acetonitrile (ACN), have, respectively, been employed at the cathode and anode. To prevent a mixing of the two asymmetric electrolyte solvents, a NASICON-type Na+-ion conductive solid-state electrolyte (SSE, Na3Zr2Si2PO12) is employed to serve as a mediator-ion separator. The shuttling of Na+ ions through the Na3Zr2Si2PO12 SSE sustains the ionic charge balance between the two electrodes. The Zn-TEMPO nonaqueous cell with a stable energy density of ca. 12-18 Wh L-1 over 50 cycles was demonstrated.
Keywords: asymmetric electrolyte; mediator-ion solid electrolyte; nonaqueous battery; organic catholyte; redox flow battery.