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 (C/0.3LiMnO-0.7LiMnNi) to ∼ US$100/kWh (C/LiCoO). 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 Li/air and Na/air, the lowest-cost members of which have

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$/kWh), building on an earlier studythat analyzed the elemental costs of electrochemical couples. The chemical cost for about 40 battery chemistries is plotted in Figure 1 against the year that each electrochemistry was introduced (all costs are in 2017 US$). 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

Chemical cost represents a floor on the cost of the complete battery and is computed here from the unit costs (e.g., US$/kg) of the cathode-active material, anode-active material, and electrolyte, normalized to the stored energy (e.g., Wh/kg). Exemplars of each of the major rechargeable battery families are included. Numerical values and electrochemical details are given in Table S2 , and details of the calculations appear in Supplemental Information . For the present approach, Li/air and Na/air, the range of costs shown represents varying utilization of the theoretical sulfur capacity from 25% to 50%.

In the remainder of the paper, we first demonstrate key performance characteristics of the proposed electrochemical couples. Since both catholyte and anolyte are fluids, our electrochemical couples lend themselves to a flow battery design, which is also demonstrated. An attribute of flow batteries is the ability to independently size the power stack and chemical storage capacity to meet desired energy to power ratios. In such architecture, the contribution to system cost of the power stack can be minimized by sizing it only as large as is necessary to meet the maximum power requirement. Using the methodology in a recent techno-economic analysis of flow batteries,we show that the proposed electrochemical storage system has attractive performance and cost attributes very similar to those of pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES), which are currently the lowest-cost energy storage technologies.Finally, the exceptionally low chemical cost allows electrochemical storage to address a new domain of long-duration discharge that was not previously accessible in a cost-effective manner.

In order to determine the chemical cost, the stable speciation range of the polysulfide must be taken into account. Although polysulfide solubilities in aqueous solutions can reach as high as 12 M sulfur concentration,stability issues (see below) may limit the practical capacity to less than that for complete reduction of sulfur according to the reaction 2A + S → AS (where A is Li or Na). However, even with a more limited range of sulfur reduction, exceptionally low chemical cost is attainable while reaching energy densities higher than many previous flow batteries. This is shown in Tables S4 and S5 , where energy density and chemical cost for catholytes and anolytes having 5 M of Li or Na and 5 M of S, respectively, have been calculated. If the entire sulfur reduction capacity is utilized, the energy densities are 121 and 58 Wh/L for acidic and alkaline catholyte, respectively. Increasing the anolyte S concentration to 10 M raises the upper bound to about 145 Wh/L and 70 Wh/L for acidic and alkaline catholyte, respectively. If cycling is restricted to just 25% of the full sulfur reduction capacity, the corresponding energy densities are 60 and 29 Wh/L when using acidic and alkaline catholyte, respectively. This capacity range corresponds to the solution range Li-Lior Na-Na, for which we later demonstrate stable cycling over ∼1,000 hr. For sodium-polysulfide chemistry, the chemical cost is remarkably low, only US$0.4–1.7/kWh (using acidic catholyte at 5 M S), depending on the utilization of sulfur theoretical capacity (100%–25%). At 50% utilization or higher, one reaches the lowest chemical cost to our knowledge of any rechargeable battery ( Figure 1 ). When using lithium polysulfide rather than sodium polysulfide, the chemical cost is US$2.0–5.0/kWh, still lower than existing flow batteries.

In the present approach, cells may be assembled in the discharged state, in which case the alkali-metal working ion is provided by one of several possible low-cost salts dissolved in the catholyte. In the examples below we use Li 2 SO 4 and Na 2 SO 4 . Cells may also be assembled in the charged state, in which case the working ion is provided by lithium or sodium sulfide dissolved in the anolyte. To avoid H 2 S formation at the anolyte, adequately high pH (≥12) is maintained by the addition of a suitable base (here, LiOH or NaOH). The catholyte may be either acidic or alkaline, providing cell voltages of ∼1.68 V and ∼0.85 V, respectively, at standard state equilibrium, as explained via the cell reactions.

Specifically, during operation, Liand Na(or other metal ions) shuttle between the electrolytes. In the anolyte, these working ions participate in the polysulfide redox reactions. In the catholyte, in order to maintain electroneutrality, ions are generated or consumed through oxygen electrochemistry, using water as both solvent and reactant. The reversible capacity of the cell is determined by either the total concentration of alkali-metal ions, or by the sulfur concentration in the anolyte, whichever is limiting. The generation and consumption of protons (for acid catholyte) or hydroxyls (for alkaline catholyte) lead to pH swings in the catholyte. In the alkaline catholyte case, hydroxyl crossover is not anticipated to be detrimental to performance, while in the acid catholyte case, protons must be confined to the catholyte chamber to prevent mixing with the alkaline anolyte. Here, we use ceramic membranes as the separators. Note that this scheme is not a sulfur-air battery, as there is no direct reaction between any sulfur species and oxygen. We searched for related concepts in the literature, and found a recently proposed Zn-air battery that also utilizes acidic catholyte and alkaline anolyte separated by a solid electrolyte (LiSICON) membrane,but in a stationary (non-flowing) design. We also found a published patent applicationthat proposes an all-alkaline flow cell in which an air cathode is paired with a solid sulfur-polysulfide anode; here the working ions are hydroxyl ions and the system requires an anion exchange membrane.

Whereas nonaqueous lithium-sulfurand high-temperature sodium-sulfur batteriesuse sulfur as the cathode, an all-aqueous system must use sulfur as the anode material to preserve aqueous stability while reaching a meaningful cell voltage. Solubilized aqueous sulfur electrodes have previously been paired with halogenated catholytes in flow batteries,used as the catholyte versus “protected” lithium metal anodes,and used as the anolyte with lithium intercalation cathodes.(Here, “anode” and “cathode” refer to solid-phase active materials, whereas “anolyte” and “catholyte” refer to fluids with solubilized active materials.) In each of these cases, the chemical cost of storage is dominated by the nonsulfur electrode. Thus, we reasoned that if aqueous anolytes of highly soluble metal polysulfides (up to 12 M)could be matched with a similarly low-cost catholyte, unprecedented low storage economics could be achieved. Our scheme, illustrated in Figure 2 , is an aqueous flow battery that pairs a polysulfide anolyte with an oxygenated/aerated salt solution as the catholyte. At the anolyte side, charge transfer occurs to a negative current collector connected to the external circuit. At the catholyte side, a single or dual oxygen evolution/reduction cathode configuration is used. This configuration pairs two half-reactions, namely polysulfide oxidation/reduction and oxygen evolution reaction (OER)/oxygen reduction reaction (ORR), which unlike conventional rechargeable couples, do not share a common working ion.

The anolyte (left) is an aqueous polysulfide solution within which the working ions (here, Li + or Na + ) carry out the sulfur redox reaction. The catholyte (right) is an acidic or alkaline solution containing the working ion as a dissolved salt, e.g., Li 2 SO 4 or Na 2 SO 4 .

Using H cells ( Figure 3 A), we confirmed that OER/ORR occurs at the catholyte side by observing the changes in cell voltage induced by changing the gas composition or by cycling the gas flow on and off. An immediate response of the cell voltage to a change in gas flow conditions is clearly seen in Figures 4 A and 4B for acidic and alkaline catholyte, respectively. Then, to determine whether the charge-storing capacity of the catholyte corresponds to the concentration of added salt (LiSOor NaSO), cells with an excess of anolyte were prepared using the modified H-cell design in Figure 3 B. Results in Figures 4 C and 4D show that the measured capacity of these cells corresponds almost exactly to the theoretical capacity calculated from the Lior Naconcentration. In addition, the cell voltage is the same for both alkali ions, as expected from the cell reactions. In the anolyte, polysulfide redox activity was readily discerned from changes in color with state of charge.

(A–D) Confirmation of ORR as the catholyte discharge reaction for cells with (A) acidic catholyte (1 M Li 2 SO 4 + 1 M H 2 SO 4 ) and (B) alkaline catholyte (2 M LiOH). Cell voltage responds immediately as oxygen/argon flow conditions are changed. Unmodified platinum mesh was used as the cathode current collector and carbon foam on stainless steel as the anode current collector. The anolyte solution is 2 M Li 2 S 4 + 0.25 M LiOH in water. Discharge current density for both experiments is 0.065 mA/cm 2 . Confirmation of Li- and Na-salt limited capacity in modified H cells using 4 M S anolyte with (C) 0.5 M Li 2 SO 4 catholyte, and (D) 0.5M Na 2 SO 4 catholyte. Each was cycled over the entire capacity range afforded by the salt concentration in the starting catholyte.

Another important cost factor is the cost of catalyst required to support a given power density. Several non-platinum group metal (PGM) catalysts for OER and ORR in acid media have been proposed,which we are in the process of evaluating. However, we have also investigated the impact of Pt black loading on the charge-transfer resistance of the experimental flow cell ( Figure 3 D), using galvanostatic electrochemical impedance spectroscopy (GEIS). The US Department of Energy's (DOE) future PEM fuel cell total PGM target for Pt loading is 0.15 mg/cmbut this loading accommodates about 100 times higher power density (800 mW/cm) than is assumed in our model. Experiments were conducted using two Pt black loadings, one higher (0.21 mg/cm) than the DOE target and one a factor of five lower (0.03 mg/cm). We found that the impact of temperature was much greater than the impact of loading. Figure 7 A shows selected Cole-Cole plots from these measurements, in which the higher frequency arc is due to membrane impedance and the lower frequency arc is correlated with ORR kinetics.A small inductive loop at the lowest frequencies is observed in each case, consistent with the literature,which we did not attempt to fit. Figure 7 B summarizes the GEIS results as a plot of total cell area-specific resistance (ASR) versus temperature. It is seen that the curves converge with increasing temperature such that at 50°C and above, neither current density nor loading significantly changes the ASR. At lower temperature where results are more differentiated, increasing discharge current density (comparing the red and black curves in Figure 7 A) primarily decreases the ORR charge-transfer resistance, consistent with Butler-Volmer behavior whereby a larger overpotential results in faster kinetics. Increasing temperature (comparing blue and black curves in Figure 7 A) decreases both membrane and ORR impedance. Increasing the catalyst loading with other parameters held constant (comparing blue and green curves) decreases the ORR charge-transfer resistance, but only at the lower temperatures, and does not change the membrane resistance. We use these results later for T-E modeling of the flow battery. Unlike Pt black, IrOis not readily available as high surface area powder, so the impact of its loading on OER kinetics during charging was not investigated. In the T-E modeling, we assume similar loadings as for Pt black.

(A) Selected GEIS data plotted as area-specific impedance. In order of descending impedance, the data are for the following conditions. Red circles: low Pt loading, 20°C, 0.5 mA/cm 2 . Black squares: low Pt loading, 20°C, 2.5 mA/cm 2 . Blue triangles: low Pt loading, 30°C, 2.5 mA/cm 2 . Green triangles: high Pt loading, 30°C, 2.5 mA/cm 2 . The dotted lines are the corresponding fitted data for the two major semicircles based on the equivalent circuit shown in the inset.

Two Pt loadings were investigated: A high loading of 0.21 mg/cm 2 and low loading of 0.03 mg/cm 2 , at two discharge current densities (0.5 mA/cm 2 and 2.5 mA/cm 2 ) and at five temperatures (20°C, 30°C, 40°C, 50°C, and 60°C.) A LiSICON membrane of 150 μm thickness was used, the catholyte contained 15 mL 0.5 M Li 2 SO 4 + 0.5 M H 2 SO 4 and anolyte contained 15 mL 0.5 M Li 2 S 2 + 0.25 M Li 2 S 4 + 1.25 M LiOH. The electrolyte flow rate was 0.5 mL/min and the air flow rate was 10 cm 3 /min. Fixed amplitude (0.1 mA/cm 2 ) and frequency range (1 MHz to 100 mHz) were used throughout.

Further evidence that polarization and power density in the current unoptimized design are primarily limited by membrane resistance appears in Figure 6 , where the polarization and power density of H cells were measured for two LiSICON membranes with different thicknesses (both at 55°C using a single Pt mesh cathode coated with Pt black). With the thinner membrane of 50 μm thickness, decreased polarization is evident and a higher power density with peak value of 5.1 mW/cmat 7.1 mA/cmwas reached. In comparison, a 150-μm-thick membrane of the same composition yielded a peak power density of 3.4 mW/cmat 5.5 mA/cm. These values of power density are low compared with those for polymer-membrane-based redox flow batteries or proton exchange membrane (PEM) fuel cells, and may be significantly improved if membrane resistance is lowered. Nonetheless, we use these experimentally observed values as starting input parameters in the techno-economic (T-E) model for the flow battery, discussed later.

The cell with the 50-μm-thick membrane shows reduced polarization and a higher power density (peak value of 5.1 mW/cm 2 at 7.1 mA/cm 2 ), compared with the cell with the 150-μm-thick membrane of the same composition (peak power density of 3.4 mW/cm 2 at 5.5 mA/cm 2 ).

With increasing current density, the voltage efficiency decreases since additional contributions to impedance become significant. The results in Figure 5 A were obtained using a 150-μm-thick LiSICON membrane, with a measured ionic conductivity of 0.28 mS/cm at room temperature and 0.6 mS/cm at 55°C, whereas the results in Figure 5 B were obtained using a NaSICON membrane of higher ionic conductivity, 2 mS/cm at room temperature,but also much greater thickness of 1,000 μm. We conducted most of our experiments using Li chemistry simply due to the greater availability of LiSICON membranes. To separate the membrane resistance from other contributions, an experiment was conducted using two different reference electrodes, an alkaline Hg/HgO (in 1 M LiOH) reference electrode at the anolyte side and a Hg/HgSO(in saturated KSO) reference electrode at the catholyte side. A stainless-steel mesh anode current collector and dual cathodes with IrOand Pt black catalysts were used. Step-galvanostatic scans consisting of sequential 5 min galvanostatic charge or discharge at various current densities were performed while measuring voltage. The overall polarization is obtained from the voltage difference between cathode and anode, the cathode contribution from the voltage between cathode and Hg/HgSOreference electrode, and the anode contribution from the voltage between anode and Hg/HgO reference electrode. Figure 5 C plots the total polarization and the cathode, anode, and membrane contributions as a function of current density, using the 150-μm-thick LiSICON membrane. It is seen that the cathode and anode polarization remain relatively constant, with the anode polarization being lower by at least a factor of five over the measured current density range (up to 7 mA/cm). The membrane polarization, on the other hand, increases with current density, and is about 50% of the total impedance at current density of 3 mA/cm

Galvanostatic step charges and discharges produced voltage efficiency results shown in Figures 5 A and 5B for Li and Na chemistry, respectively. Values plotted are the round-trip voltage efficiency, i.e., the discharge voltage divided by the charge voltage. Using the acidic catholyte and dual catalysts (IrOfor OER and Pt black for ORR), the voltage efficiency reaches 71%–74% for both Li and Na solutions at low current density (0.065 mA/cm). Here, efficiency is limited by the OER/ORR reaction. Using a less efficient, Pt mesh, single cathode rather than the dual cathodes with IrO/Pt black catalysts, and holding all other cell parameters constant, the voltage efficiency at 70°C is about 20% lower, as shown in the Supplemental Information Figure S2 ).

(C) With increasing current density, the ceramic membrane resistance becomes limiting. Sources of polarization determined by a cell using dual reference electrodes, an alkaline Hg/HgO (in 1 M LiOH) reference electrode at the anolyte side and an Hg/Hg 2 SO 4 (in saturated K 2 SO 4 ) reference electrode at the catholyte side at 55°C. Polarization due to membrane resistance dominates over that due to the catholyte and anolyte redox reactions at current densities higher than about 2 mA/cm 2 . All cells used a stainless-steel mesh anode and dual cathodes with catalysts (IrO 2 for OER and Pt black for ORR). Gas flow at the catholyte side was oxygen gas during ORR and argon gas during OER. The anolyte was 1 M Li 2 S 4 + 1 M LiOH and the catholyte was 0.5 M Li 2 SO 4 + 0.1 M H 2 SO 4 .

The voltage efficiency of the cells is determined by the OER/ORR reaction as well as other contributions to cell impedance. To separate these, we first measured the voltage efficiency as a function of current density and temperature, using the standard H cells as well as a cell with separate reference electrodes for the cathode and anode. Cell tests were conducted in the temperature range 25°C–70°C, within which increasing the temperature was observed to dramatically reduce cell impedance. Increasing temperature also sharply increases NaSOsolubility in the catholyte beginning at about 35°C.

Taking into account these prior studies, we prepared starting polysulfide solutions containing nominal stoichiometry Liand 1 M or 3 M LiOH at room temperature, and operated cells from room temperature up to 70°C, such that the starting predominant sulfur species is Sand the major degradation reactions are minimized. To date, we have found that the most stable cycling is obtained when we constrain the composition range to Lito Lior Nato Na; see Figure 8 B for cycling results at 5 M S concentration obtained over 720 hr. In the interest of increasing energy density and further lowering cost, the ability to reversibly cycle beyond the LiS or NaS solubility limit is desirable. Our group has previously shown that percolating nanocarbon suspensions can be used to improve charge transfer in fluid electrodes to the extent of realizing reversible precipitation of LiS.A similar strategy could be applied here to extend the polysulfide capacity range.

Cycling stability of the polysulfide anolyte was tested in cells with two gas-tight chambers of differing volume containing the same anolyte, thus deeply cycling the smaller chamber ( Figure 3 C). LiSICON membrane and stainless-steel electrodes were used, and an alkaline Hg/HgO in 1 M LiOH reference electrode was placed in the larger chamber. Selection of the appropriate speciation range over which to cycle the anolyte requires consideration of complex equilibria.Aqueous alkali-metal polysulfide solutions contain a wide range of species, including the alkali-metal cations (Li, Na, or K), HO, OH, H, HS, HS, S, S, S, S, and S. Polysulfide solubility and stability are strongly dependent on pH,alkali-metal cation,nominal polysulfide speciation, and concentrationas well as temperature.At low pH (<7), HSis the primary species, and the HS molecule is the predominant reduced product in the polysulfide solution.At intermediate alkalinity (pH 9–14), the primary polysulfide species are Sand Sinstead of HSalthough without good sealing HS may still be generated at pH ≈ 12, especially during the reduction reaction.In a recently reported lithium polysulfide/LiMnbattery, adding porous SBA-15 silica adsorbent to the polysulfide anolyte was found to reduce irreversible capacity loss and to improve capacity retention.The improvements were attributed to the suppression of gaseous HS release, preventing continuous loss of sulfur. In highly alkaline polysulfide solution (e.g., >3 M OH), the predominant species are Sand SA Lisolution under these conditions may disproportionate into Sand S/Sduring long-term storage.At temperatures above 80°C, thiosulfate is readily formed via the reaction S+ mOH→ S+ mHSThis parasitic disproportionation reaction is detrimental to the stability of polysulfide anolyte, but slow at moderate temperature.

In the Supplemental Information Figure S4 , we show that extreme variations in either water loss or water excess (dilution of the catholyte), or interruptions in gas flow, cause large deviations in coulombic efficiency, but also that the cells are tolerant of such variations and return to stable operation once the water content and gas flow at the cathode are restored to their initial values.

The ability of the catholyte and anolyte to undergo sustained deep cycling, and the durability of the cell components in contact with catholyte and anolyte, were tested using modified H-cell designs ( Figures 3 B and 3C). To test the catholyte, cells were assembled in which the catholyte capacity was lower than the anolyte capacity (provided by a 4 M S solution). Using a single cathode (Pt mesh), galvanostatic cycling (0.325 mA/cm) was conducted at room temperature, with the capacity during each cycle being limited to 96% of the catholyte theoretical capacity, based on the starting alkali ion concentration. Results are shown in Figure 8 A, plotted as voltage-capacity curves for the 1, 10, 20, and 30galvanostatic charge/discharge cycles, spanning a total test time of over 1,600 hr. Dry air was continuously flowed into the catholyte chamber via a dispersion tube during discharge, and water was periodically added to the catholyte to compensate for evaporation. Initially, we chose to use capacity-limited cycling out of concern that proton intercalation into the LiSICON might occur upon the exhaustion of Liwithout a clear voltage indication. However, as seen in Figure 8 A, and in expanded scale in Figure S3 , the charge curves are almost invariant after the first charge, indicating that the results would be nearly identical if a voltage cutoff were used. The slope of the voltage profile clearly deviates toward the end of the charge ( Figure S3 ) as expected from an increased mass transfer resistance resulting from the depletion of working ion (Li). The minor variations in polarization for the discharge curves were found to be correlated with fluctuations in catholyte water level and air flow rate, which are expected to affect ORR kinetics. Over the >2 month duration of the test, the cell impedance did not grow detectably, pointing to good stability of the LiSICON membrane in contact with the acidic catholyte as well as stability of catholyte and anolyte with their respective electrodes, Pt mesh and stainless steel. (The voltage efficiency in these tests is low since they are conducted at room temperature and only use the single Pt mesh cathode for OER and ORR.)

(A and B) Long-duration cycling tests of a typical (A) catholyte and (B) anolyte using modified non-flowing H cells shown in Figures 2 B and 2C, respectively. (A) Air-purging cell cycles with 1 M Liacidic catholyte over 96% of the total capacity against an excess of polysulfide anolyte. Stable cycling is observed during 30 cycles accumulated over 1,600 hr total test time at room temperature. Observed polarization is predominantly due to the resistance of the 150-μm-thick LiSICON membrane. (B) Symmetric cell test of 5 M S anolyte over Li-Licomposition range, using a 150-μm-thick LiSICON membrane. Active sulfur concentration was 1.25 M S (2.5 M electrons). The Hg/HgO (1 M NaOH, ∼0.1 V versus standard hydrogen electrode) was placed in the counter electrode side, and thus the potential obtained on the working electrode (versus Hg/HgO as shown in the figure) refers to the electrode potential plus the iRbetween the working electrode and the reference electrode. Stable cycling is observed over 30 cycles corresponding to 720 hr total test time at room temperature.

Having independently demonstrated each component of the system in a non-flowing cell, we then tested the air-breathing flow cell system with a continuously pumped supply of both catholyte and anolyte ( Figure 9 A). Design details of the cell are given in Experimental Procedures and shown in Figure 3 D. In order to have a true air-breathing cell, an acidic LiSO-based catholyte was used; for an alkaline catholyte, it is expected that some form of COexclusion or trapping would be necessary to prevent trace carbon dioxide present in air from reacting with the base to precipitate lithium or sodium carbonate during long-term operation. For ease of handling and assembly, we used the 150-μm-thick LiSICON membrane, although it was recognized that its resistance would dominate the ASR of the cell at room temperature. Figure 9 B shows the polarization and power density of the flow cell measured at 55°C with an air stream feeding the air chamber. On discharge, the flow cell delivers a peak power of 3.2 mW/cmat 6.0 mA/cm, which is consistent with the result from the static counterpart in Figure 6 (3.4 mW/cmat 5.5 mA/cm, under pure oxygen flow). Extended cycling of the flow cell system was then performed at room temperature. Figure 9 C shows charge-discharge curves for cycling under air-breathing conditions at 0.325 mA/cmfor 40 cycles, spanning a total test time of 960 hr. The relatively stable charge/discharge curves reflect a stable membrane impedance, which was separately validated by impedance measurements of the cell after 720 hr, shown in Figure S5 . We did observe sulfur precipitation (verified by energy-dispersive X-ray analysis of the precipitate) in the anolyte reservoir after about every 10 cycles (240 hr) and replaced the anolyte at those intervals. Since the sealed non-flowing cell ( Figure 8 B) did not exhibit detectable sulfur precipitation, whereas a deliberately air-exposed anolyte did (in a separate experiment), we attribute the precipitation in the flow cell to an air leak in the anolyte circuit. The round-trip voltage efficiency of the air-breathing flow cell at 55°C and a current density of 0.325 mA/cmis ∼55% ( Figure 9 C). Results in Figure 7 show that at this temperature, the ASR is no longer dependent on the Pt loading and is dominated by the membrane resistance. At room temperature, the voltage efficiency drops to ∼43%, and operating the cell at a higher current density of 2 mA/cmat 50°C resulted in a round-trip voltage efficiency of ∼33% ( Figure S6 ), again limited by membrane resistance as shown by Figure 5 C. These tests demonstrate proof-of-concept operation of the proposed air-breathing aqueous sulfur flow battery. Clearly, decreased membrane resistance is necessary to reach higher round-trip efficiency and at higher current densities. Several possible paths to more highly conductive membranes for this application are currently under study.

(C) Select cycles of voltage-time curves measured at room temperature and 0.325 mA/cm 2 for 40 cycles, corresponding to 960 hr of continuous operation. The 1 st and 3 rd cycles of voltage-time curves at 55°C at the same current density are shown as the dotted lines. The catholyte contained 10 mL of 0.5 M Li 2 SO 4 + 0.5 M H 2 SO 4 and the anolyte contained 10 mL of 1 M Li 2 S 4 + 1 M LiOH. The electrolyte flow rate is fixed at 1 mL/min. During charge, no gas was supplied to the cell. During discharge, dry air was supplied at 10 cm 3 /min to the air chamber for convective air breathing. The anolyte was replaced every 10 cycles (240 hr) due to sulfur precipitation, attributed to accidental air ingress in the anolyte circuit.

Techno-economic Analysis and Comparison with Other Approaches

16 Darling R.M.

Gallagher K.G.

Kowalski J.A.

Ha S.

Brushett F.R. Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries. Figure 10 Installed Cost versus Storage Duration for Several Energy Storage Technologies Show full caption 2 2− to S 4 2− corresponding to 25% of theoretical capacity. The peak power density of the stack ranges from 4.3 mW/cm2 at US$4,000/kW and US$2,000/kW to 28.6 mW/cm2 at US$150/kW; details are given in the Curves for the present air-breathing aqueous sulfur flow battery approach using Na and Li chemistry are shown in green and gray, respectively. The chemical costs for Na and Li are shown as dashed lines. Curves of constant power cost show that the power stack dominates the system cost at short storage durations, whereas at long duration the cost asymptotically approaches the energy cost due to chemical constituents plus storage tank and related costs. 5 M concentrations of both Na and S are assumed, with cycling of the sulfur over the speciation range Sto Scorresponding to 25% of theoretical capacity. The peak power density of the stack ranges from 4.3 mW/cmat US$4,000/kW and US$2,000/kW to 28.6 mW/cmat US$150/kW; details are given in the Supplemental Information . The projected costs for air-breathing aqueous sulfur compare favorably with those for pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES), while also having several-fold higher energy density and being free of the locational constraints faced by each. eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiI0M2I1Y2VhM2ZkMzhmMWYxNDkwYTY1ODJkM2ExMWIxMSIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjAwODQzNTcxfQ.EcFYHC3h6_VRZdBfWkrLilNHreV1MYzMRawHUqgGO5r65Uz07sYxt1nzAFpVZPc4S1MR2J4N72WnVo8Zid9VP8NdAA-5ibCcv8qupo1UDWEV6qDA47_4UrjSWAj2ftT-LKayabpkXVVjn19ULEOsm7UrYpSy5AxmQbEnHdGpma0CX_XRC1phiHJ0hweAFlrU-CZz8V9mZGHCWbQvHE36TVFrWUoQ-k4DBrZ38zPNbR8w87W2uyz6Zl693W-hQD17V3pZZxS8UnX3B_CCVulLN4xE4-TDYOBDefOO--4P7jeyUz8vFjhijut1DzVIn8h4TG6-BBwgPXvfigyAgp04PA Flow batteries, by virtue of their design allowing independent scaling of power and energy, have a cost structure similar to that of PHS and CAES. The total cost of these technologies can be separated into costs for the power-generating reactor and the energy-storing reservoirs, plus certain additional costs. For PHS and CAES, the cost of power (e.g., US$/kW) is primarily determined by the cost of power-generating turbines and associated equipment, whereas for flow batteries, it is primarily the cost of the power-generating stack. The cost of energy (e.g., US$/kWh) is primarily determined by size of the storage reservoir in the case of PHS and CAES, and by the chemicals and tanks in the case of flow batteries. At system level, the cost of energy (US$/kWh) is the ratio of power cost (US$/kW) to total storage duration (hr), the former being defined by the cost of the storage reservoirs or tanks and the working fluids within. Thus, a useful way to display the cost of storage is as a log-log plot of installed cost (US$/kWh) versus E/P ratio (hr) ( Figure 10 ). This plot allows the cost of storage technologies to be compared on an equal basis. In the limit of long storage duration (>1 month), the cost asymptotically approaches the cost of energy alone, as the cost of the power-generating stacks or turbines becomes negligible. As the storage duration shrinks, the power cost constitutes an increasing proportion of the total cost. Our T-E model is based on the flow battery model for future-state battery price developed by Darling et al.,to which adjustments were made to arrive at an installed cost that can be directly compared with the installed costs of PHS and CAES facilities. The methodology for calculating each of the cost contributions is explained in Experimental Procedures

+ or Na+ (added as Li 2 SO 4 or Na 2 SO 4 ) is 5.2 M, the excess salt relative to the nominal anolyte concentration (5 M) being added to avoid the onset of mass transfer limitations in the catholyte as it approaches the fully charged state. The resulting chemical costs for Li and Na chemistry are shown as horizontal dashed lines in 2 2− to S 4 2−, corresponding to the experimentally demonstrated speciation range in In arriving at the energy cost component of the cost curves in Figure 10 , the anolyte sulfur concentration is assumed to be 5 M (the experimental flow cell in Figure 9 used 4 M), and the total catholyte concentration of Lior Na(added as LiSOor NaSO) is 5.2 M, the excess salt relative to the nominal anolyte concentration (5 M) being added to avoid the onset of mass transfer limitations in the catholyte as it approaches the fully charged state. The resulting chemical costs for Li and Na chemistry are shown as horizontal dashed lines in Figure 10 , and assume cycling only between Sto S, corresponding to the experimentally demonstrated speciation range in Figures 8 B and 9 C. The Na chemical cost is US$1.7/kWh, and after accounting for tank cost (US$0.15/L) and other costs, the resulting energy cost for Na chemistry is a factor of 4–6 higher than the chemical cost alone. Thus, as with PHS and CAES, the energy-storing fluids have lower cost than the structures used to contain them.

2. 39 U.S. Department of Energy (2016). ARPA-E funding opportunity Announcement IONICS DE-FOA-0001478. https://arpa-e-foa.energy.gov/. 2 and US$100/m2. Alternative low-cost membranes such as polymer-ceramic composites 40 Aetukuri N.B.

Kitajima S.

Jung E.

Thompson L.E.

Virwani K.

Reich M.-L.

Kunze M.

Schneider M.

Schmidbauer W.

Wilcke W.W.

et al. Flexible ion-conducting composite membranes for lithium batteries. 2; the experimentally validated value in 2 for the Pt black alone. The largest contributions to power cost come from the membrane and catalyst. Our current experiments use ceramic membranes, for which the US DOE has projected a cost, at high production volumes, of less than US$10/mIn the cost model, we conservatively assume costs of US$10/mand US$100/m. Alternative low-cost membranes such as polymer-ceramic compositescould also be developed for this application. The total PGM catalyst loading is assumed to be 0.05 mg/cm; the experimentally validated value in Figure 7 is 0.03 mg/cmfor the Pt black alone.

2 in the current laboratory flow cell, this is for an unoptimized lab cell, and improvements are to be expected. Thus, we use R ∼100 Ω·m2 as the highest input value in the model. In 2 peak power) to US$150/kW (28.6 mW/cm2 peak power). The values for membrane cost, catalyst loading, and ASR corresponding to each curve are tabulated in 2 (more than ten times the DOE projection), and R = 100 Ω·m2. This power cost drops by half to US$2,000/kW if a membrane cost of US$10/m2 is assumed. An additional factor of two reduction, to US$1,000/kW, is obtained assuming a relatively modest reduction in ASR, to R = 55 Ω·m2. Further reduction of power cost, to US$500/kW and US$150/kW, would be obtained if the ASR were reduced to R = 30 Ω·m2 and 15 Ω·m2, respectively. Based on literature values for the resistivity of LiSICON and NaSICON 21 Hayashi A.

Noi K.

Sakuda A.

Tatsumisago M. Superionic glass-ceramic electrolytes for room-temperature rechargeable sodium batteries. , 41 Ohara Corp. (2016). Lithium-Ion Conducting Glass-Ceramics. http://www.oharacorp.com/lic-gc.html. 2, respectively, and the ORR reaction according to literature 27 Antoine O.

Bultel Y.

Durand R. Oxygen reduction reaction kinetics and mechanism on platinum nanoparticles inside Nafion®. 2. Our measurements ( A remaining adjustable parameter in the power cost calculation is the ASR (R). Although we measured values of R ∼100 Ω·min the current laboratory flow cell, this is for an unoptimized lab cell, and improvements are to be expected. Thus, we use R ∼100 Ω·mas the highest input value in the model. In Figure 10 , cost curves are shown in which the power cost ranges from US$4,000/kW (4.3 mW/cmpeak power) to US$150/kW (28.6 mW/cmpeak power). The values for membrane cost, catalyst loading, and ASR corresponding to each curve are tabulated in Table S7 . The highest power cost, US$4,000/kW, corresponds to a membrane cost of US$100/m(more than ten times the DOE projection), and R = 100 Ω·m. This power cost drops by half to US$2,000/kW if a membrane cost of US$10/mis assumed. An additional factor of two reduction, to US$1,000/kW, is obtained assuming a relatively modest reduction in ASR, to R = 55 Ω·m. Further reduction of power cost, to US$500/kW and US$150/kW, would be obtained if the ASR were reduced to R = 30 Ω·mand 15 Ω·m, respectively. Based on literature values for the resistivity of LiSICON and NaSICON, 150-μm-thick membranes should contribute bulk ASR at 55°C of only 24.5 and 1.5 Ω·m, respectively, and the ORR reaction according to literatureshould contribute only 6 Ω·m. Our measurements ( Figure 5 C) show that membrane and ORR resistance are indeed the two largest contributions, so the assumed R values may well be achievable with further development. For example, the membrane resistance includes interfacial contributions that could be reduced by flow channel design.

16 Darling R.M.

Gallagher K.G.

Kowalski J.A.

Ha S.

Brushett F.R. Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries. 42 Nykvist B.

Nilsson M. Rapidly falling costs of battery packs for electric vehicles. 43 Task Committee on Pumped Storage of the Hydropower Committee of the Energy Division of ASCE Compendium of Pumped Storage Plants in the United States. 44 Akhil A.A.

Huff G.

Currier A.B.

Kaun B.C.

Rastler D.M. DOE/EPRI 2013 Electricity Storage Handbook in Collaboration with NRECA. 44 Akhil A.A.

Huff G.

Currier A.B.

Kaun B.C.

Rastler D.M. DOE/EPRI 2013 Electricity Storage Handbook in Collaboration with NRECA. 45 Ingersoll E. Long Duration Storage for Baseload Wind. To arrive at the installed cost, balance-of-plant costs and allowances for sales, administration, depreciation, warranty, R&D, and profit margin are included. Figure 10 compares the projected installed cost for the present approach with several other storage technologies suitable for large-scale stationary storage. The cost range of vanadium redox flow batteries (VRFB) is shown with lower and upper bound cost factors as calculated by Darling et al.The cost of Li-ion batteries is also shown as a band that represents the range of 2030 projected costs for EV battery packs.Given the relative inflexibility of the power to energy ratio in Li-ion battery design compared with flow batteries, we plot Li-ion costs as being independent of storage duration; this is a reasonable representation except at the shortest durations on this scale (<1 hr) where high-power and high-energy Li-ion batteries are differentiated. The PHS data are obtained from the Compendium of Pumped Storage Plants in the United Statesfor existing installations, and the DOE/EPRI Electricity Storage Handbookfor four new “greenfield” sites, as listed in Table S9 . Note that the costs of existing PHS installations have been adjusted for inflation, an important correction without which the cost of PHS can be underestimated by up to a factor of >5 for the oldest US plants. Only United States PHS facilities and only those in which storage capacity is generated by pumping alone are included; we have excluded those benefiting from riverine flows, which may have artificially low costs of energy. The cost of CAES is taken from Appendix B of the DOE/EPRI Electricity Storage Handbookand an analysis of long-duration storage (100–300 hr) storage for levelizing the output of wind farms.

3, of which 2.1 m3 is the electrolyte volume (calculated for an energy density of 60Wh/L). In Figure 10 one sees, firstly, that the projected cost of air-breathing aqueous sulfur flow batteries has a similar dependence on storage duration to PHS and CAES. This shared characteristic is due to the low baseline energy cost of all three approaches. The installed cost is dominated by the power stack cost at short discharge times and declines with increasing storage duration to asymptotically approach, at long discharge times, the energy cost alone. Beyond cost-competitiveness with PHS and CAES, the proposed battery has numerous other advantages, chief among which are higher energy density and locational flexibility. For the solution concentrations assumed in Figure 10 , the chemical energy density (catholyte + anolyte) is 60 Wh/L; at system scale the energy density may be 20–40 Wh/L. This is a factor of 500–1,000 higher than the energy density of a typical PHS facility. Such electrochemical storage is also free of the highly restrictive geographical and environmental constraints that have limited growth of PHS and CAES. The batteries could be deployed as large systems of many MWh, or as smaller systems in a highly distributed manner, allowing a broader range of use models (e.g., residential storage) than is possible with PHS or CAES. To illustrate the likely size of distributed storage units, we have modeled in the Supplemental Information (see Figure S7 ) the installed cost of a 12 kW system. With increasing energy, the system cost declines, reaching US$100/kWh at 125 kWh of storage, corresponding to a 10.5 hr discharge time. The volume of such a system is 4.6 m, of which 2.1 mis the electrolyte volume (calculated for an energy density of 60Wh/L).

45 Ingersoll E. Long Duration Storage for Baseload Wind. 46 Hand M.M.

Baldwin S.

DeMeo E.

Reilly J.M. Renewable Electricity Futures Study. 47 Budischak C.

Sewell D.

Thomson H.

Mach L.

Veron D.E.

Kempton W. Cost-minimized combinations of wind power, solar power and electrochemical storage, powering the grid up to 99.9% of the time. 48 Steward D.M. Scenario Development and Analysis of Hydrogen as a Large-Scale Energy Storage Medium. Electrochemical storage approaches with ultralow energy cost, which best express their advantages in systems designed for long storage durations, may become increasingly attractive as the percentage of renewable generation on the electric grid increases. Figure 10 shows that below about 10 hr duration, multiple technologies can compete on a cost-of-energy basis; these may be differentiated on the basis of other factors not shown here, such as energy density and round-trip efficiency at high charge/discharge rates. Beyond 10 hr duration, where heretofore PHS and CAES have had the cost advantage, the air-breathing aqueous sulfur flow battery offers a new low-cost option. Consider the long-duration CAES case study included in Figure 10 , in which 100–300 hr storage was found to be necessary to fully smooth the output of a single wind farm and produce baseload power.At