Rethinking zinc-air flow batteries for stationary energy storage

To reach Europe's 2050 zero emissions goal amid the energy crisis triggered by Russia's invasion of Ukraine, Europe is expediting its shift to renewable energy. This shift requires increased focus on long-duration energy storage (LDES) to ensure reliable power supply, grid resilience, and cost reductions. Flow batteries are promising candidates to address the needs of grid-scale energy storage. The technology comprises two electrolyte tanks, pumps, and a stack of cells where chemical reactions take place, involving the exchange of electrons between two electrolytes separated by an ion-exchange membrane. Their energy capacity can be adjusted by simply increasing the size of the electrolyte tanks, making them suitable for both small- and grid-scale applications. However, while flow batteries offer a discharge duration (12 hours) longer than that of Li-ion batteries (typically 4-6 hours), their capacity may still fall short of meeting the requirements for LDES (> days). Rethinking the way flow batteries operate could unlock the “Holy Grail” of energy storage. ReZinc will “rethink” the operations and design of conventional flow batteries by developing and demonstrating at lab-scale a completely new hybrid zinc-air flow battery. Differently from conventional metal-based flow batteries (where the metal is plated on the anode during charge), the Zn/ZnO redox reaction is confined in the negative reservoir to eliminate the electroplating process inside the cell. This can be achieved by developing novel electrochemical mediators, necessary for enhancing the charge transfer processes and building a dendrite-free battery. Additionally, the use of non-toxic and non-corrosive materials, like vanadium, is a major advantage of the technology. ReZinc will enable long-duration discharge (>days) and long-term storage capabilities (months) with consequent commercialization of the first sustainable and economically viable electrochemical energy storage technology for LDES applications.

The penetration of renewable energies into the electric grid increases the demand for energy storage to ensure reliable power supply, grid resiliency, and cost reductions. Long-duration and long-term energy storage can bridge the intermittency of renewable sources and reduce the risks incurred by diminished fossil-fuel baseload generation. Electrochemical energy storage (EES), or Li-ion batteries, are considered for short-duration energy storage (4-6 hours). When talking about seasonal storage, chemical energy storage (CES) could potentially be the preferable option because of the long-discharge duration sustained and the relatively long lifetime. However, low round trip efficiency, and the problems associated with direct incorporation of hydrogen into the gas grid and/or the combustion of fossil fuels can still be a challenge. The goal of ReZinc is to fill the gap between short-term EES and long-term CES by developing and demonstrating at lab-scale (TRL 4) a completely new zinc-air flow battery technology with a round trip DC efficiency = 70%, daily self-discharge < 0.1%, expected lifetime = 10 years (or 10’000 cycles). A disruptive redox-mediated strategy for enhanced charge transfer processes is employed with the goal of confining the Zn/Zn2+ redox reaction in the negative reservoir (filled with a semi-solid zinc solution) and eliminating the electroplating process inside the cell (no dendrites) to improve battery lifetime. This will allow discharge times beyond days, contrary to conventional zinc-based batteries where long discharge is hampered by the formation of a cm-thick zinc anode. If successful, the technology has disruptive potential in terms of both extremely low levelized-cost-of-storage (<0.5 €/kWh/cycle, based on 100 kW/1000 kWh system, 1 week discharge duration), extended storage time, recyclability, and use of non-critical-raw-materials. An innovative concept design of the cell (including tanks) will be conceived for demonstration of the technology.