A Unique Single‐Ion Mediation Approach for Crossover‐Free Nonaqueous Redox Flow Batteries with a Na⁺‐Ion Solid Electrolyte

Abstract

Among the diverse set of secondary batteries, redox flow batteries (RFBs) have long been considered as a highly promising, affordable system for grid-scale energy storage in facilitating electricity generation from naturally sustainable sources. [8] Traditionally, research and development of RFBs have been mainly focused on aqueous systems in which redox active electrode materials are dissolved in water. [9] However, there are two major limitations associated with the use of aqueous electrolytes: 1) the cell voltage of an aqueous RFB is strictly constrained by the electrochemical window of water electrolysis; and 2) aqueous RFBs are not able to work over a broad range of temperatures due to the limitation of the freezing point and boiling point of water. [10] Therefore, RFBs with nonaqueous electrolytes have drawn much interest in the electrochemical energy storage community. [11] In contrast to water-based systems, nonaqueous electrolytes can offer a relatively broader potential window for operating RFBs. [12] Moreover, nonaqueous electrolytes can usually offer a lower freezing point and a higher volatile temperature (in contrast to water), allowing the operation of nonaqueous RFBs below 0 °C in cold-weather regions and at high temperatures in hot-weather regions. [13] Most of the research on nonaqueous RFBs has been focused on the exploration of organic redox couples and the screening of suitable nonaqueous electrolytes. The relevant cell chemistries and systems have usually been tested/demonstrated with traditional porous polymer membranes to separate the positive and negative electrolytes. [14] However, the use of porous polymeric membranes usually poses undesirable crossover of chemical species between the anode and cathode. [15] Research into membranes that are especially for nonaqueous RFBs is rare. In this study, we present a mediator-ion approach for the advancement of nonaqueous RFBs by employing a Na +-ion solid electrolyte. The anode and cathode electrolytes are separated with a solid electrolyte, which can totally circumvent the cross-mixing of the two liquid electrode materials. Electrochemical reactions at the anode and cathode are ionically linked by the shuttling of Na +ions through the solid electrolyte membrane. The proposed mediator-ion RFB concept is validated and demonstrated with a chromium (III) acetylacetonate symmetric redox chemistry and nitrobenzene (NB)-bromine redox couple. Nonaqueous redox flow batteries (RFBs) provide significant advantages over aqueous RFBs because the use of nonaqueous electrolytes offers the possibilities of operating RFBs at higher voltages (above the water breakdown voltage) and low temperatures (below 0 °C, the freezing point of H 2 O). However, the advancement of RFBs has long been plagued by the chemicalcrossover problem due to a lack of reliable separators. Herein, a single-ion mediation strategy is presented for the advancement of nonaqueous RFBs by employing a Na +-ion solid electrolyte. The catholyte and anolyte in a single cell are physically and elect...

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

“…After a calibration, the Ag + /Ag RE is determined to be ~ 0.45 V versus the standard hydrogen electrode (SHE). 18,23 The CV profile in Fig. 1b indicates that the average reduction/oxidation potential of TEMPO is ≈ 0.30 V vs. the lab-made nonaqueous Ag + /Ag RE, which is(~ 0.75 V vs SHE).…”

“…1b shows the cyclic voltammogram (CV) of a glassy carbon working electrode in a nonaqueous PC-solvent-based electrolyte containing 0.5 M NaClO 4 and 0.1 M TEMPO. The experiments were conducted in a threecompartment cell with a lab-made Ag + /Ag nonaqueous reference electrode (RE, prepared according to our previous report, 18,23 and as depicted in Fig. S2 in the Supporting Information) and a Pt mesh counter electrode.…”

“…S1 . 22,23 To make a hermitic seal between the cell body and the ceramic solid-electrolyte pellet, a strong marine epoxy glue was applied. A piece of sodium foil (1.0 × 1.0 cm) was used as the anode.…”

“…With a NASICON-type Na-SSE (Na 1+x Zr 3 P 3-x Si x O 12 with 0 < x < 3), we have previously proposed and validated a "mediator-ion" concept for the development of crossover-free RFBs. 17,18 The volumetric energy density is a highpriority parameter to be considered when developing an RFB system, which is determined by the storage capacity of active electrodes, cell voltage of the redox couples, and the solubility of liquid electrodes. Due to the solubility limitations of electrode materials in organic solvents, the energy density of nonaqueous RFBs is currently not able to surpass aqueous-based RFB systems.…”

Recent Advances and Perspectives on the Polymer Electrolytes for Sodium/Potassium‐Ion Batteries

“…Besides, aqueous electrolytes are favorable due to their environmental benignity and low cost. Nevertheless, the insufficient voltage window of aqueous electrolytes bestows aqueous SIBs and KIBs with low working voltages, and resultantly limited energy densities, even though “water‐in‐salt” strategy is able to obviously widen the electrochemical stability windows of aqueous electrolytes, allowing upper voltage of 2.5 V. [ 78–80 ] Besides, the dissolution of transition metal elements in aqueous electrolytes is also undesirable for improving the cycling performance of batteries. Differing from aqueous electrolytes, ionic liquid electrolytes are endued with strong tolerance to high voltage up to 5.0 V. Consequently, the application of ionic liquid electrolytes has attracted growing attention, [ 81 ] particularly in the DIB community.…”