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REVIEW Li and Liu 95 [29–31]. The potential for precipitation of V2O5 in the catholyte when electrolyte temperature exceeds 40◦C for an extended period of time may reduce re- liability and battery life. Recently, a mixed-acid VRB system was reported, in which chloride ions were added to vary the solvation structure of vanadium ions in the solution, thus increasing the solubility to over 2.5 M. The energy density of mixed-acid VRB was increased by more than 70% relative to tradi- tional VRB while expanding the temperature oper- ation window by 80% [7]. The mixed-acid system has been successfully scaled up and is commercially produced [32]. Still, as mentioned earlier, vanadium is expensive. The ultimate solution is to find highly soluble, low-cost redox materials for aqueous sys- tems. In addition, in VRBs, perfluorosulfonic acid ionomer (Nafion) membranes are used [33–35]. Nafion membranes consist of a perfluorinated back- bone with pendant vinyl ether side chains termi- nated with sulfonic acid (SO3H) groups. The ion conductivity is limited, not selective enough and de- grades over time. It also happens to be the second most expensive component in the system [8]. Ca- pacity fading originating from active-ion crossover might not be avoided because of the poor ion se- lectivity or the chemical instability of the mem- branes. To retard capacity fading, chemically stable and highly ion-selective membranes are required. Hybrid flow battery systems In traditional RFB systems, such as VRBs and Fe/Cr RFBs, energy-density calculations include two liq- uid electrolytes on both sides (Fig. 2b). However, as shown in Fig. 2c [36], for a hybrid flow battery de- sign, energy density is determined only by the liquid volume on one side. In this design, one half-cell fea- tures a solid electrode and the electrolyte possesses ambipolar and bifunctional characteristics. In such electrolytes, cationic and anionic ions from a single soluble compound are both energy-bearing redox- active species, thus eliminating the need for non- active counter ions such as Cl– and SO42– that are commonly used in VRB and Fe/Cr RFB systems. This design minimizes the amount of electrolyte in one half-cell and achieves a high active species con- centration in the other half-cell [37]. The traditional aqueous zinc/bromine RFB (ZBR) [38], ZIB RFB [37] and Li metal-based hybrid flow battery that are discussed below all follow the above design strate- gies. When ZBR systems are being charged, zinc metal is plated on the anode side of a carbon-based electrode. Meanwhile, bromide ions (Br–) are oxi- dized to bromine (Br2) at the cathode side. During discharge, the reverse process occurs [39]. This sys- tem has a high cell voltage and energy density, and expectations are that low-cost materials can be used. However, demonstration of ZBR systems has been limited because of material corrosion, dendrite for- mation and electrical shorting, high self-discharge rates, low energy efficiencies and short cycle life. The existence of corrosive bromine at the cathode side is the cause of some of these problems. Expensive cell electrodes, membranes and fluid-handling com- ponents are needed to withstand the chemical con- ditions. In addition, bromine has limited solubility in water and is toxic. Therefore, it is critical that or- ganic agents be used to complex the bromine to mit- igate the crossover contamination and toxicity of bromine when it sinks to the bottom of the catholyte tank. In 2014, Li et al. reported a new environmen- tally friendly flow battery system [37]. They devel- oped a novel ZIB system in which bromine was re- placed with soluble I3 – /I– redox couples. A high en- ergy density of 167 Wh/L, which approaches that of Li-ion batteries, is achievable (Fig. 3c) based on the high solubility of zinc iodide electroytes (>7 M). The ambipolar and bifunctional designs of the ZIB use zinc ions as both the redox-active species and charge carriers, eliminating the need for non-active counter ions. Together with the high selectivity of the Nafion membrane against iodide anions, the ZIB system delivers a very high coulombic efficiency (CE) value (∼99%). The energy efficiency (EE) val- ues decrease from 90.9% to 76.0% as the concen- tration of ZnI2 increases from 0.5 to 3.5 M when operated at a current density of 20 mA/cm2. The cell shows excellent cycling performance. Figure 4a shows the typical charge/discharge curves at 1.5 M ZnI2. The addition of an alcohol (ethanol) induces ligand formation between oxygen on the hydroxyl group and the zinc ions, which expands the stable liquid electrolyte temperature range from –20◦C to 50◦C. As we know, dendrite growth is one of the issues to be addressed for most metal anode-based batteries, such as Zn- and Li-based systems, which would cause serious short-circuit and safety con- cerns. Here, the addition of an alcohol was found to effectively ameliorate the zinc dendrite propagation. Aqueous lithium flow battery The similar metal anode concept has been in- vestigated in Li flow systems. Taking advantage of the high solubility and low viscosity of aque- ous solutions and the low potential of Li/Li+, Goodenough’s group [40] and Zhou’s group [41] proposed a cell that uses Li metal with a non-aqueous electrolyte at the anode side and Downloaded from https://academic.oup.com/nsr/article/4/1/91/2866462 by guest on 11 January 2023PDF Image | Progress in low cost redox flow batteries energy storage
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