Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical Storage

•Chemical cost analyzed for 40 rechargeable couples developed over the past 60 years•Aqueous sulfur/sodium/air system identified with ultralow chemical cost of ∼US$1/kWh•Air-breathing flow battery architecture demonstrated at laboratory scale•Techno-economic analysis shows installed cost is comparable with PHS and CAES Wind and solar generation can displace carbon-intensive electricity if their intermittent output is cost-effectively re-shaped using electrical storage to meet user demand. Reductions in the cost of storage have lagged those for generation, with pumped hydroelectric storage (PHS) remaining today the lowest-cost and only form of electrical storage deployed at multi-gigawatt hour scale. Here, we propose and demonstrate an inherently scalable storage approach that uses sulfur, a virtually unlimited byproduct of fossil fuel production, and air, as the reactive components. Combined with sodium as an intermediary working species, the chemical cost of storage is the lowest of known batteries. While the electrical stacks extracting power can and should be improved, even at current performance, techno-economic analysis shows projected costs that are competitive with PHS, and of special interest for the long-duration storage that will be increasingly important as renewables penetration grows. The intermittency of renewable electricity generation has created a pressing global need for low-cost, highly scalable energy storage. Although pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES) have the lowest costs today (∼US$100/kWh installed cost), each faces geographical and environmental constraints that may limit further deployment. Here, we demonstrate an ambient-temperature aqueous rechargeable flow battery that uses low-cost polysulfide anolytes in conjunction with lithium or sodium counter-ions, and an air- or oxygen-breathing cathode. The solution energy density, at 30–145 Wh/L depending on concentration and sulfur speciation range, exceeds current solution-based flow batteries, and the cost of active materials per stored energy is exceptionally low, ∼US$1/kWh when using sodium polysulfide. The projected storage economics parallel those for PHS and CAES but can be realized at higher energy density and with minimal locational constraints. The intermittency of renewable electricity generation has created a pressing global need for low-cost, highly scalable energy storage. Although pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES) have the lowest costs today (∼US$100/kWh installed cost), each faces geographical and environmental constraints that may limit further deployment. Here, we demonstrate an ambient-temperature aqueous rechargeable flow battery that uses low-cost polysulfide anolytes in conjunction with lithium or sodium counter-ions, and an air- or oxygen-breathing cathode. The solution energy density, at 30–145 Wh/L depending on concentration and sulfur speciation range, exceeds current solution-based flow batteries, and the cost of active materials per stored energy is exceptionally low, ∼US$1/kWh when using sodium polysulfide. The projected storage economics parallel those for PHS and CAES but can be realized at higher energy density and with minimal locational constraints. The rapidly dropping cost of wind and solar electricity generation, as illustrated by levelized costs of electricity (LCOE) that are now competitive, or nearly so, with fossil fuel generation,1US Department of Energy (2015). Revolution now: the future arrives for five clean energy technologies–2015 update. https://energy.gov/eere/downloads/revolution-now-future-arrives-five-clean-energy-technologies-2015-update.Google Scholar highlights the need for low-cost electrical storage that can transform intermittent renewable power into predictable and dispatchable electricity generation, and potentially even baseload power. Such a revolutionary outcome will require energy storage with costs well below the trajectory of current technology, while also being safe, scalable, long-lived, and sufficiently energy dense for widespread deployment, including in space-constrained environments. Emerging use-case studies suggest that installed costs of <$50/kWh, operating over multi-day or longer durations, will be required for renewable-based generation to compete economically with existing fossil plants on a drop-in basis. It is unclear whether electrochemical storage can meet these challenges. Here, we first review the underlying chemical cost of energy storage for about 40 electrochemical couples representing all major classes of rechargeable batteries developed over the past 60 years. From this analysis, it is clear that the best opportunities for overcoming the above challenges reside with electrochemical couples that use ultralow-cost, highly abundant raw materials. Among these, sulfur has the 14th highest crustal abundance and is widely available as a byproduct of natural gas and petroleum refining.2Mason B. Principles of Geochemistry. John Wiley, 1958Google Scholar Sulfur also has the lowest cost per stored charge of known redox active materials, next to water and air (see Table S1; in US$/kAh, sulfur 0.15, zinc 3.66, graphite 32.27, and LiCoO2 292.14). In this work, we demonstrate an ambient-temperature, air-breathing, aqueous polysulfide flow battery that exploits sulfur's intrinsic advantages, and show using techno-economic analyses that such an approach has the potential to meet future storage needs for renewable energy. A reasonable starting point for bottom-up analysis of the economics of any battery technology is the cost of the cathode, anode, and electrolyte, normalized to the stored electrical energy. We define this quantity as the chemical cost of energy storage (abbreviated as chemical cost and given in US$/kWh), building on an earlier study3Wadia C. Albertus P. Srinivasan V. Resource constraints on the battery energy storage potential for grid and transportation applications.J. Power Sourc. 2011; 196: 1593-1598Crossref Scopus (192) Google Scholar that analyzed the elemental costs of electrochemical couples. The chemical cost for about 40 battery chemistries is plotted in Figure 1 against the year that each electrochemistry was introduced (all costs are in 2017 US$). Although we have not attempted to exhaustively list all extant electrochemical couples, exemplars of each of the major classes of bulk rechargeable batteries are included. The numerical results plotted in Figure 1 are tabulated in Table S2, and details of the calculations, including input parameters, key assumptions, and literature sources, are given in the Supplemental Information. A striking result apparent in Figure 1 is that the chemical cost of new battery chemistries has in general systematically increased rather than decreased over 60 years of battery development. We believe that to a large extent this trend can be attributed to the pursuit of higher energy density, as exemplified by the deployment of Li-ion batteries in portable device and transportation applications once dominated by NiMH, NiCd, or Pb-acid batteries. Figure 1 shows that Li-ion technology itself spans nearly a 3-fold range of chemical cost, from ∼US$35/kWh (C6/0.3Li2MnO3-0.7LiMn0.5Ni0.5O2) to ∼ US$100/kWh (C6/LiCoO2). Given that energy density has generally increased over time, the rising chemical cost implies that the high cost of new materials has more than compensated. Figure 1 also shows that while several aqueous electrochemical couples have chemical cost below US$10/kWh, the lowest cost of which is Zn-air, which as a primary chemistry dates to the year 1878, several aqueous electrochemical couples have chemical costs greater than low-cost Li-ion. This again reflects the high cost of synthesized active materials relative to stored energy. The case of Na/S is instructive; while it has the lowest chemical cost in our plot, excluding present results, it is known that high-temperature Na/S batteries are among the most expensive at system level (∼US$800/kWh). This is due to the high cost of supporting components and balance of plant. Conversely, an ambient-temperature sodium-sulfur chemistry has the potential for exceptionally low system cost, given a starting chemical cost of ∼US$1/kWh. These considerations led us to explore new ambient-temperature alkaline-sulfur chemistries, culminating in the air-breathing aqueous polysulfide couples, denoted in Figure 1 as Li2Sx/air and Na2Sx/air, the lowest-cost members of which have