High-Energy,\nSingle-Ion-Mediated Nonaqueous Zinc-TEMPO\nRedox Flow Battery

Abstract

Two\ndistinct advantages of nonaqueous redox flow batteries (RFBs)\nare the feasibility of building a high cell voltage (without a constraint\nof the water-splitting potential) and the operability at low temperatures\n(without a concern of freezing below 0 °C). However, electrochemically\nactive organic redox couples are usually selectively soluble in specific\nnonaqueous solvents, and their solubility is relatively low (in contrast\nto that in aqueous solutions). The selective and low solubility of\nredox couples seriously constrict the practical energy density of\nnonaqueous RFBs. Herein, we present a hybrid nonaqueous RFB with a\nsolid zinc anode and a liquid (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl\n(TEMPO) cathode. Toward accessing a high solubility of the TEMPO cathode\nand to sufficiently accommodate the discharge products of a Zn anode,\nasymmetric electrolyte solvents, <i>viz</i>., propylene\ncarbonate (PC) and acetonitrile (ACN), have, respectively, been employed\nat the cathode and anode. To prevent a mixing of the two asymmetric\nelectrolyte solvents, a NASICON-type Na⁺-ion conductive\nsolid-state electrolyte (SSE, Na₃Zr₂Si₂PO₁₂) is employed to serve as a mediator-ion separator.\nThe shuttling of Na⁺ ions through the Na₃Zr₂Si₂PO₁₂ SSE sustains the ionic charge\nbalance between the two electrodes. The Zn-TEMPO nonaqueous cell with\na stable energy density of ca. 12–18 Wh L^–1 over 50 cycles was demonstrated.