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for achieving high energy density.41–43 In general, a eutectic system is a homoge- neous mixture that melts at a specific temperature that is lower than the melting point of either of the components. The widely studied eutectic redox species are deep eutectic solvents (DESs) typically formed by mixing Lewis or Brønsted acids and bases at room temperature.41 Similar to ionic liquids, various anionic and cationic species with asymmetric spatial configuration and delocalized charge are generated in DESs, resulting in the reduced lattice energy and low freezing points. To obtain the high concentration of active species, the metal- and organic-based DESs are prepared via mixing corresponding metal- and organic-based electroac- tive materials with suitable inorganic or organic compounds, such as quaternary ammonium salts or hydrogen bond donor (HBD). Here, we review recent progress and generic design strategy of eutectic redox species for the development of high-energy-density RFBs. Metal-Based Eutectic Redox Species Abbott et al. reported the early study of metal-based DES, which was a eutectic mixture of quaternary ammonium salts and ZnCl2.44 Since then, a variety of DESs have been pre- pared, and they can be generally categorized into four types according to the generated cationic or anionic complex agents.41 Currently, the well-studied representatives are AlCl3,45,46 ZnCl2,47,48 and FeCl3,6H2O49 (Figure 5A), which can easily provide a concen- tration of active species over 2 M. In these DESs, the metal chloride works as both a eutectic component and redox species, and the redox reactions are dependent on both the valence state and coordination environment of the metal centers. Zhou and coworkers reported a rechargeable redox battery using an Fe-based cath- olyte containing a eutectic mixture of FeCl3,6H2O and urea paired with the Li anode.49 It showed long-term stability at ambient environment and could provide a high concentration of active species (5.4 M) (Figure 5B). With a high potential of 3.4 V, the Li-Fe hybrid battery had a gravimetric energy density of 330 Wh kg1, which was even comparable to that of advanced Li-ion batteries. During the charging and discharging, the reversible redox reactions of Fe DES are achieved with the conversion of two iron complex species. Yu and coworkers developed the AlCl3-urea DES anolyte by mixing urea and AlCl3 at room temperature in a molar ratio of 1:1.3.45 Further, 1,2-dichloroethane (DCE) was added to decrease viscosity and improve conductivity. As presented in Figure 5C, the redox reactions of Al DES/DCE involve the aluminum complex cation ([AlCl2(urea)n]+) and anion ([AlCl4]), as indicated by the 27Al NMR analysis. During the charging process, the [Al2Cl7] was reduced to Al and [AlCl4]. In light of the high concentration of active species ($3.2 M), the Al DES anolyte demonstrated an energy density of 189 Wh L1 when coupled with the I3/I catholyte. As the phys- ical properties and redox reactions of DESs can be tuned by changing organic salts, HBDs, and additives, Yu and coworkers explored the influence of ethylene glycol (EG) as the additive on the physical properties and reaction mechanism of Fe DESs.50 On the one hand, it was observed that adding EG was able to tune the con- centration, viscosity, and freezing point of Fe DESs. On the other hand, it was found that EG enabled the dissolution of LiCl salt and dissociated the iron complex species (Figure 5D). Because the coordination environment of metal ions (chloride coordi- nated cations or anions) was changed in FeCl3,6H2O/urea/EG DES, it showed a different reaction mechanism compared with FeCl3,6H2O/urea DES. Employing the newly designed Fe DES as the catholyte and Al DES as the anolyte, the authors further developed a proof-of-concept Fe-Al hybrid RFB, which delivered a high en- ergy density of 166.2 Wh L1 with a discharge voltage of 1.4 V. 1972 Chem 5, 1964–1987, August 8, 2019PDF Image | Development of Redox Flow Batteries Based on New Chemistries
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