Unlike traditional batteries, flow-based electrochemical energy storage systems separate the energy storage and power generation by storing the electro-active species in externally
flowing electrolytes (i.e., the anolyte and catholyte) while maintaining the redox reactions at the electrode surface inside a stack. This unique architecture permits the redox flow batteries (RFBs) to independently scale the power and/or energy—a characteristic advantage along with high safety coveted by the energy industry for intermittent renewable energy integration and other grid services. Despite continuous progress, the energy density of traditional aqueous RFBs is considerably lower than that of low-end Li-ion batteries such as those with LiFePO4 cathodes. Traditional aqueous RFBs, such as all-vanadium (VRB) and Fe/Cr systems, are generally limited to < 25 Wh L-1 by both the low voltage owing to water electrolysis and the relatively low solubility of the active species. Recent research on the non-aqueous electrolyte has offered wider electrochemical stability windows and thus higher cell voltages (>2.0V), but the low solubility of the active redox species in the non-aqueous electrolytes has largely prevented the high energy densities from being achieved (in most cases less than 0.1 M, corresponding to an energy density of 5 Wh L-1 even at a higher cell voltage). Other challenges also include flammable nature, requirement of hermetic sealing, and low conductivity (which leads to current density at least one order of magnitude lower than the current aqueous system (< 0.5 mA cm-2)) of the non-aqueous electrolytes, which further limit the development of the non-aqueous systems. A high energy density aqueous flow battery with high safety is therefore still critically needed.
Here, as shown in Fig. 1, we demonstrate that the zinc-polyiodide electrolyte possesses the desired ambipolar and bifunctional characteristics, as well as high solubility and benign
nature, thus enabling a high-energy-density aqueous hybrid RFB. Capitalizing on the high solubility of the I-/Ix- redox species, the zinc-polyiodide flow battery (ZIB) has a theoretical
energy density of ~322 Wh L-1 at the solubility limit of ZnI2 in the water (4500 g L-1, 7.0 M). We demonstrate here a discharge energy density of 166.7 Wh L-1 with 5.0 M ZnI2 electrolytes, nearly 7 times that of the current aqueous RFBs (VRB: ~25 Wh L-1) and approaching the energy density of low-end LiFePO4 cathode-based Li-ion batteries. The ZIB system is further optimized in terms of membranes and electrodes, enabling it more competitive in energy storage market. Because of the absence of highly oxidative V5+ and Br2, low-cost hydrocarbon cation exchange membranes are applied in the ZIB thus replacing the expensive Nafion membrane, making the ZIB system more attractive from a cost prospective. In addition, the catalysts or modification of current graphite felts at the cathode side can effective increase the kinetics of I-/Ix- redox couples, which enhances the energy efficiency of ZIB cell run at high current density.