Article

High-Voltage, Room-Temperature LiquidMetal Flow Battery Enabled by Na-KjK-b00-Alumina Stability

Antonio C. Baclig, Geoff

McConohy, Andrey

Poletayev, ..., Joon-Hyung Lee,

William C. Chueh, Jason Rugolo

wchueh@stanford.edu (W.C.C.)

jason.rugolo@gmail.com (J.R.)

HIGHLIGHTS

K-b00-alumina solid electrolyte is

stable in contact with Na-K liquid

metal

We demonstrate Na-KjK-b00-alumina batteries with aqueous

and nonaqueous posolytes

Resistance rise in K-b00-alumina

associated with aqueous

posolytes can be tuned

The maximum power density of 65

mW cm�2 at 22�C is limited by K-

b00-alumina

Na-K is a room-temperature liquid metal that could unlock a high-voltage flow

battery. We show that K-b00-alumina solid electrolyte is stable to Na-K and

selectively transports K+. We report the cycling of cells with OCVs of 3.1–3.4 V

employing aqueous and nonaqueous posolytes, and maximum power densities of

65 mW cm�2 at 22�C, ohmically limited by 330-mm K-b00-alumina membranes.

Baclig et al., Joule 2, 1287–1296

July 18, 2018 ª 2018 Elsevier Inc.

https://doi.org/10.1016/j.joule.2018.04.008

Article

High-Voltage, Room-TemperatureLiquid Metal Flow Battery Enabledby Na-KjK-b00-Alumina StabilityAntonio C. Baclig,1 Geoff McConohy,1 Andrey Poletayev,1 Aaron Michelson,1 Nathan Kong,1

Joon-Hyung Lee,1,2 William C. Chueh,1,5,* and Jason Rugolo1,3,4,*

Context & Scale

Flow batteries are a compelling

grid-scale energy storage

technology because the stored

energy is decoupled from the

system power. Conventional flow

batteries have aqueous solutions

on both sides, and thus are

constrained in voltage by water

splitting (�1.5 V). Replacing the

negative side with a liquid metal

would yield a much higher voltage

flow battery, benefiting energy

density, power density, and

efficiency. As a room-temperature

SUMMARY

Flow batteries are a compelling grid-scale energy storage technology because

the stored energy is decoupled from the system power. Aqueous redox flow

batteries (RFBs), however, are limited by low open-circuit voltages (OCVs).

Replacing the aqueous negative electrolyte (negolyte) with liquid alkali

metals—of which Na-K, a room-temperature liquid metal alloy, is attractive—

would increase the OCV considerably. However, a suitable solid electrolyte

has not been reported for Na-K. Here we show that K-b00-alumina is a selective

and robust K+ ion conductor in contact with Na-K, to which it is stable with min-

imal exchange of Na. We report the cycling of cells with OCVs of 3.1–3.4 V em-

ploying aqueous and nonaqueous positive electrolytes (posolytes), and power

density tests showing promising maximum power densities of 65 mW cm�2 at

22�C and >100 mW cm�2 at 57�C, ohmically limited by 330-mm K-b00-alumina

membranes. Further development of Na-KjK-b00-alumina batteries could unlock

cost-effective energy storage.

liquid metal, Na-K is attractive.

However, a suitable solid

electrolyte has not been reported

for Na-K since the well-studied

Na-b00-alumina ceramic fractures

on contact. Here we show that

K-b00-alumina is a selective and

robust K+ ion conductor in contact

with Na-K. We report the cycling

of cells with open-circuit voltages

of 3.1–3.4 V. Our maximum power

densities are limited, as expected,

by the K-b00-alumina membranes,

but can be improved with thin

membranes. Further

development of Na-KjK-b00-alumina batteries could unlock

cost-effective energy storage.

INTRODUCTION

Aqueous redox flow batteries (RFBs) are limited by energy density (10–50 Wh L�1)

and round-trip efficiency (65%–85%).1 A contributing factor to these limitations is

their low open-circuit voltage (OCV), which is constrained by the electrochemical

stability window of water—the solvent often employed for both the posolyte and

negolyte. The pursuit of a higher OCV has spurred investigations of nonaqueous

flow batteries,2,3 semi-solid flow batteries,4 organic redox-active materials,5

redox-targeting flow batteries,6 and other concepts. Liquid alkali metals, with their

highly negative redox potentials, present an alternative route to high-voltage RFBs if

they are paired with a suitable redox couple in the posolyte.

Liquid sodium negative electrodes have been used in high-temperature NajS and

NajNiCl2 batteries, which have OCVs of 2.0–2.3 V. However, their operating temper-

ature (270�C–350�C) excludes aqueous and most nonaqueous posolytes, and

presents challenges for cell sealing and construction. The high operating tempera-

ture is necessary to maintain the liquid phase of sodium polysulfides in NajS and to

achieve high current densities with the Na-b/b00-alumina solid electrolyte. Recent

work has lowered the temperature of these batteries to 100�C–200�C through the

use of solvent additives to dissolve the polysulfides,7 sodium alloys to increase

the wetting of sodium on the solid electrolyte,8 and planar designs to reduce the

solid electrolyte thickness.9 However, achieving room-temperature operation and

flow configuration requires a new approach.

Joule 2, 1287–1296, July 18, 2018 ª 2018 Elsevier Inc. 1287

1Department of Materials Science andEngineering, Stanford University, 496 LomitaMall, Stanford, CA 94305, USA

2School of Material Science and Engineering,Kyungpook National University, Daegu 41566,

Previously, room-temperature Na-Hg batteries10 and Na-Cs batteries11,12 have

been investigated. However, the chronic toxicity of mercury, high freezing point of

Na-Hg (eutectic temperature of 21�C), and rarity of cesium makes these alloys less

attractive for grid-scale batteries. By contrast, sodium-potassium (Na-K) alloy, which

has recently been investigated for potassium-ion batteries,13 contains earth-abun-

dant elements; has a low eutectic temperature (�13�C), giving it a wide liquid region

at room temperature; has low viscosity; and is used and pumped at scale, such as for

a liquid coolant in industrial nuclear reactors.14 Whereas the negolyte capacities in

aqueous RFBs are <100 mAh g�1, a Na-K negolyte has an extremely high theoretical

capacity of 580 mAh g�1 at room temperature (using the full range of potassium in

the liquid region, 30–85 mol% K),13 and 320 mAh g�1 at 0�C (55–74 mol% K). These

capacities are equivalent to 19 M and 11 M concentrations, respectively, of cyclable

potassium. Indeed, Na-K appears to be a promising liquid metal for a high-voltage,

high-energy-density, room-temperature flow battery.

Na-K flow batteries have not been realized, however, because a suitable solid

electrolyte has remained elusive.11 All of the liquid sodium batteries discussed

above use Na-b/b00-alumina as a solid electrolyte due to its high ionic conductivity,

mechanical strength, and chemical stability.15 However, Na-b/b00-alumina is incom-

patible with a Na-K negative electrode. Specifically, the potassium in Na-K

exchanges with the sodium in the solid due to K-b/b00-alumina having a lower free

energy than Na-b/b00-alumina, possibly from a smaller distortion in the Al tetra-

hedra.16 Since K-b/b00-alumina has a larger c-axis lattice constant (by 1% in b phase,

2% in b00 phase), the exchange of Na with K, which occurs from even small impurities

of K, causes polycrystalline Na-b/b00-alumina ceramic to fracture.17 Furthermore, the

presence of substantial fractions of both Na and K in the b/b00-alumina lattice creates

a mixed alkali effect that decreases ionic conductivity.16 Interestingly, K-b00-alumina

is, like Na-b/b00-alumina, also a superionic conductor (s = 1–2 mS cm�1 at 25�C[Figure S7]), and has received interest for KjS batteries,18,19 alkali metal thermal-

to-electric conversion devices,20 potassium purification processes,21 and as a

precursor material.22

In this work, we demonstrate that K-b00-alumina can be used as a stable K+-selective

ion conductor with Na-K at room temperature. This electrolyte-electrode combina-

tion enables a new type of Na-K flow battery that exhibits high OCV (3.1–3.4 V in this

work) with pathways to high energy density and high cell power density, while using

earth-abundant materials and chemicals. Figure 1A shows the schematic of the bat-

tery. During discharge, potassium atoms are oxidized from theNa-K liquidmetal and

travel through the K-b00-alumina to the posolyte, where the electron that migrates

through the external circuit reduces the redox-active species at the positive

electrode. Having established the compatibility of Na-K with K-b00-alumina, we

demonstrate that this battery can be used with multiple solvents and redox couples

on the positive side to yield high-voltage flow batteries (Figure 1B).

Korea

3Advanced Research Projects Agency for Energy,United States Department of Energy, 1000Independence Avenue, S.W., Washington, D.C.20585, USA

4Present address: 135 East Water Tank Road,Gilbert, AZ 85296, USA

5Lead Contact

*Correspondence:wchueh@stanford.edu (W.C.C.),jason.rugolo@gmail.com (J.R.)

https://doi.org/10.1016/j.joule.2018.04.008

RESULTS AND DISCUSSION

The Compatibility of Na-K and K-b00-Alumina

The first question that arises for pairing a Na-K negative electrode with a K-b00-alumina solid electrolyte is the stability of the K-b00-alumina to contact with Na-K.

In full- and half-cell testing, we observed no evidence of K-b00-alumina mechanically

degrading from contact with Na-K of various compositions in its liquid region (30–85

mol% K), in stark contrast to the rapid pulverization of Na-b00-alumina in contact with

Na-K. The microstructure of a K-b00-alumina membrane immersed in Na-K at room

1288 Joule 2, 1287–1296, July 18, 2018

BA

Figure 1. Cell Schematic and Voltage

(A) Schematic of the room-temperature Na-K battery with multiple options for the posolyte.

(B) The full-cell voltage of these various combinations compared with a range of voltages for conventional redox flow batteries.

temperature remains stable over a period of 7,200 hr, exhibiting no microscopic or

macroscopic fracture (Figure 2A). Furthermore, X-ray diffraction patterns of a K-b00-alumina pellet pressed against 30 and 80 mol% K Na-K at room temperature over

a period of 8 weeks confirmed that the K-b00-alumina lattice constants changed by

less than 0.02% (Figure S1).

Since the ion exchange of Na+ to K+ is the cause of fracture of polycrystalline Na-b/

b00-alumina in contact with Na-K, we investigated the K+-to-Na+ ion exchange in

K-b00-alumina. We characterized the ratio of Na to K in K-b00-alumina via electron

microprobe analysis (EPMA) of a polished K-b00-alumina surface that was exposed

to Na-K of varying compositions and at varying temperatures. We found very small

(<1.5%) exchange of Na+ for K+ in K-b00-alumina exposed to Na-K of intermediate

composition at temperatures up to 300�C, consistent with previous thermodynamic

studies at 810�C16 (Figure 2B). We also measured the nonequilibrium Na-K ex-

change properties by measuring the transference number of Na+ for the mem-

branes. This was done by monitoring the concentration of Na+ in the posolyte of a

full battery cell, where the posolyte consisted of 0.1 M K3[Fe(CN)6], 0.1 M

K4[Fe(CN)6], and 1 M KOH. The Na+ concentration was determined by inductively

coupled plasma optical emission spectrometry (ICP-OES) at different times during

the discharge of the cell, finding that the Na+ ion concentration remained essentially

at baseline levels as the cell discharged (Figure 2C). We conclude that the K+ trans-

ference number is >>0.99. As another test of transference number and exchange, we

characterized a K-b00-alumina pellet cross-section in EPMA after passing 100 mAh of

charge in a Na-KjNa-K symmetric cell, finding a similarly low presence of sodium

(1.6% G 1.8%) in this K-b00-alumina as in the K-b00-alumina that was in static contact

at 300�C (Figure 2B). The negligible ion exchange in K-b00-alumina is consistent with

the lower free energy of K-b00-alumina than Na-b00-alumina (65 kJ mol�1 difference16).

A second question for the use of a Na-K negative electrode is the effect of the changing

Na/K ratio of Na-K as the battery is charged or discharged. Previouslymeasured activity

values of K in Na-K23 yield a standard reduction potential between +3 mV (85 mol% K)

Joule 2, 1287–1296, July 18, 2018 1289

B

D

A

E

C

Figure 2. Stability and Compatibility of K-b00-Alumina to Na-K

(A) Scanning electron micrographs of a K-b00-alumina surface (top) as received and (bottom) after

immersion in Na-K (30 mol% K) for 7,200 hr.

(B) Electron probe microanalysis measurements of the ratio of Na/K measured in K-b00-alumina that

has been immersed in Na-K of varying concentrations at 300�C for 60–140 hr. Error bars showG1 SD

of measurements taken at different points on each sample’s surface. The data are compared with a

previous study16 on K-b00-alumina immersed in NaCl/KCl at 810�C for 96 hr.

(C) Concentration of Na in the aqueous posolyte of a Na-K (�50 mol% K) cell upon constant

discharge, as measured by ICP-OES. These are lower than the expected concentration for Na+

transference numbers (TNa+) of 0.01 and 0.02 (dashed lines). The baseline concentration of Na in the

solution is �10 ppm.

(D) As a function of Na/K ratios in Na-K as measured by ICP-OES, (top) voltage of Na-K versus a

Ag/AgCl reference electrode compared with the expected voltage (dotted lines). The posolyte is

0.1 M K3[Fe(CN)6], 0.1 M K4[Fe(CN)6], 1 M KOH in water. (Bottom) Area-specific resistance between

the Na-K and the Ag/AgCl reference electrode. The expected resistance of a s = 2 mS cm�1

membrane is shown, with the difference attributed to the waterjK-b00-alumina interface.

(E) As a function of absolute pressure in a Na-K cell, resistance of the K-b00-alumina seen in

electrochemical impedance spectroscopy measured between the Na-K and a Cr/Au coating on the

K-b00-alumina. Shown are a sample that had been exposed to air and a sample that after being

exposed to air had been held in contact with Na-K at 300 kPa for 16 hr.

1290 Joule 2, 1287–1296, July 18, 2018

and +18 mV (30 mol% K) versus potassiummetal at room temperature, which is consis-

tent with the OCVs we measured between Na-K of different compositions and a Ag/

AgCl reference electrode (Figure 2D), as well as in Na-KjNa-K symmetric cells (Figures

S2 and S3; Table S1). The resistance measured across the K-b00-alumina does not

depend on the Na/K ratio (Figure 2D) and is consistent with the conductivity of the

K-b00-alumina, 1–2 mS cm�1. Finally, we note that the viscosity of Na-K is similar to

that of water and does not vary substantially with the composition of Na-K (e.g., at

58�C the viscosity is 0.62 cP at xK = 0.68 and 0.78 cP at xK = 0.2124).

A third question that arises for pairing a Na-K negative electrode with a K-b00-alumina

solid electrolyte is the well-documented poor wetting of liquid metals to ceramics at

low temperatures.8 However, in our full cells the resistance across the K-b00-alumina

shows no significant effect on transport from poor wetting; for example, in Figure 2D

the resistance is consistent with what we expect from a s z 2 mS cm�1 membrane.

We explain these results by a slight pressurization within the Na-K compartment that

causes the Na-K to completely contact the K-b00-alumina regardless of wetting at

ambient pressure, similar to how metal-ceramic composites are fabricated through

pressure-driven infiltration of liquid metal.25 To measure the pressure required, we

conducted impedance spectroscopy (Figure S4) on a Na-KjK-b00-aluminajAu/Crcoating half cell in which the pressure on the Na-K side was varied from 100 kPa

to 500 kPa. The resistance associated with K-b00-alumina decreased as the pressure

increased, showing that the pressure overcomes the poor wetting (Figure 2E). When

the Na-K is left in contact with the K-b00-alumina at 300 kPa for 16 hr, the resistance

decreases to a baseline value that does not depend on pressure in the range we

measured (Figure 2E). Another route to achieving the baseline resistance is heat

treatment of the K-b00-alumina at 1,000�C immediately before being placed in the

cell, which was not possible in the pressure tests due to the Au/Cr coating. We

performed this heat treatment before all of our full-cell tests, which showed baseline

levels of Na-KjK-b00-alumina resistance that did not change over time. We speculate

that in the pressure test, over time the Na-K reacts with a surface species and

removes it as an impediment to intimate contact. The surface species is possibly a

carbonate species from contact with air, since we have found heat treatment at

1,000�C to be effective, whereas water should be removed from the surface by

200�C.26 When Na-K comes into intimate contact with K-b00-alumina, the area of

the contact can also be confirmed visually. The Na-K leaves a visible metallic-hued

film across the entire exposed surface area of the K-b00-alumina. X-ray photoelectron

spectroscopy confirmed that this film contains both Na and K (Figure S5).

The Electrochemical Performance of Na-KjK-b00-Alumina Batteries

Having confirmed that the K-b00-alumina is compatible with Na-K and is potassium-

ion conducting, we tested cells having the form Na-KjK-b00-aluminajposolyte, wherethe posolyte falls into three categories: (1) aqueous, basic pH, (2) nonaqueous, and

(3) aqueous, near-neutral pH. The advantage of a high pH solution will be discussed

below. Well-understood, facile redox couples were used to investigate the behavior

of K-b00-alumina in contact with the posolytes. Figure 3A demonstrates the voltage

curves and coulombic efficiencies of ten cycles each of full cells with the posolyte

being (i) ferri-/ferrocyanide at pH 14, (ii) ferrocenium/ferrocene in propylene carbon-

ate, (iii) triiodide/iodide in propylene carbonate, and (iv) ferri-/ferrocyanide at pH 8.5

with a carbon coating on the K-b00-alumina (see below for a rationale for testing a car-

bon coating). In all cells, Na-K was in excess on the negative electrode. We will focus

on cycles 2 and later, due to some anomalies in the first cycles. High (>99.7%)

coulombic efficiency was observed in the propylene carbonate cells. Slightly lower

(98%–99%) coulombic efficiency was observed in the cell with ferri-/ferrocyanide

Joule 2, 1287–1296, July 18, 2018 1291

Power d

ensity (mW

cm-2)

Current density (mA cm-2) Time (hr)

E0 = 3.4 V

22°C

57°C

DischargeCharge

41°C

BA

Volt

age

(V)

103/T (K-1)2.9

-0.8

-0.2

3.5

log

(T

(S c

m-1 K

))

Ea = 0.17 eV

Coulom

bic E

fficiency (%)

Volt

age

(V)

i) aq, pH 14 (ferri/ferrocyanide)

iii) PC (I3-/I-)

ii) PC (ferrocenium/ferrocene)

iv) aq, pH 8.5/carbon coat (ferri/ferrocyanide)

0.3 mm thick K- ’’-alumina2 mm thick K- ’’-alumina

Figure 3. Cell Performance

(A) Voltage curves and coulombic efficiencies for ten cycles of a Na-KjK-b00-aluminajposolyte battery, where the posolytes were (i) 0.02 M K3[Fe(CN)6],

1 M KOH in water, (ii) 50 mM Fe(C5H5)2, 0.4 M KPF6 in propylene carbonate, (iii) 67 mM KI, 0.5 M KPF6 in propylene carbonate, (iv) 0.004 M K3[Fe(CN)6],

10�5 M KOH in water. The K-b00-alumina in (iv) has a carbon coating. Constant-current cycling was used with voltage cutoffs as depicted and current

densities of (i) 24 mA cm�2, (ii) 100 mA cm�2, (iii) 50 mA cm�2, and (iv) 5 mA cm�2. Cycles 1–10 are shown for each. The cells used carbon felt positive

electrodes and copper or brass negative electrode current collectors.

(B) Voltage and power density of cell with Na-K (64 mol% K)jK-b00-aluminaj0.1 M K3[Fe(CN)6], 0.1 M K4[Fe(CN)6], 1 M KOH in water using a �330-mm K-b00-alumina membrane at 22�C, 41�C, and 57�C. The inset shows an Arrhenius plot of the conductivities (in S cm�1) determined by a linear fit of the full-cell

voltage-current density curves at each temperature, showing an activation energy of 0.17 eV.

in pH 14 aqueous solution, possibly due to impurities in the carbon felt or to solution

being reduced. In the cell with ferri-/ferrocyanide in pH 8.5 aqueous solution with

carbon coating, low coulombic efficiency (60%) in the first cycle followed by

>100% coulombic efficiency in subsequent cycles was observed, possibly due to a

large and evolving resistance from the carbon coating preventing the posolyte

from being fully charged or discharged.

We further investigated themaximumpower density of the pH 14 aqueous chemistry

with a thinner K-b00-alumina membrane (�330 mm) and higher current densities. To

seal the thinner K-b00-alumina membrane, we used an epoxy seal instead of o-rings.

This caused some oxidation of the Na-K via transport of water or oxygen over

periods of days, preventing long-term testing, but did not show oxidation of the

Na-K over the period of 4 hr during which the power density tests were conducted

(ceramic seals could be used in commercial applications). Amaximum power density

of 65mW cm�2 was obtained at 22�C, which increased to 115 mW cm�2 at 57�C (Fig-

ure 3B). These cells displayed ohmic current-voltage relationships with the majority

of the overpotential attributable to ohmic resistance from the K-b00-alumina as

determined by a Ag/AgCl reference electrode in the positive electrode compart-

ment (Figure S6). The activation energy inferred from that overpotential, 0.17 eV,

is consistent with that expected from the K-b00-alumina alone based on previous

studies of K-b00-alumina18–20 and on K-b00-alumina that we measured in Na-KjNa-K

1292 Joule 2, 1287–1296, July 18, 2018

A B

Figure 4. Effect of Water on K-b00-Alumina

(A and B) The rate of change of K-b00-alumina area-specific resistance (ASR) in a symmetric cell with

ferri-/ferrocyanide solutions and various concentrations of KOH and KBr (A). The ASR was

determined by impedance spectroscopy measurements (dotted lines are guides to the eye), which

are shown in (B) at different times of K-b00-alumina immersion for (i) a 0.05 M K3/4 Fe(CN)6 aqueous

solution and (ii) a 0.05 M K3/4 Fe(CN)6, 1 M KOH aqueous solution.

symmetric cells (Figure S7). These results imply that the resistance of these cells was

primarily due to the bulk and interfacial properties of K-b00-alumina. A cell with a

2-mm K-b00-alumina membrane was tested for 300 hr of shallow cycling at a current

density of 1.9 mA cm�2 (Figure S8), confirming that the limiting factor is the K-b00-alumina bulk and grain boundary resistance in addition to a resistance that grows

with time in the presence of water, which we now discuss.

The Effect of Water on K-b00-Alumina Can Be Tuned

If an aqueous solution is used on the positive side, an additional question that must

be answered is the effect of water on the resistance of the K-b00-aluminajwater inter-face. It is well known that the resistance of b/b00-alumina membranes increases upon

exposure to water,27–30 likely due to ion exchange between cations in the solid and

hydrated protons in solution.27,31,32 Previous studies operating Na-Cs and Na-Hg

batteries with Na-b/b00-alumina used low current densities (0.01–0.1 mA cm�2)10–12

and in some cases were unable to be recharged10 due to operation in water. We

examined the effect of water on resistance by conducting impedance spectroscopy

experiments using aqueous symmetric cells. The Nyquist plot of the as-prepared cell

(Figure 4) shows a high frequency arc that is consistent with the grain boundary

feature of pristine K-b00-alumina and does not change during immersion. However,

during immersion a second arc emerges at lower frequencies, which grows approx-

imately linearly with time for the time scales measured (�50 hr), causing the area-

specific resistance (ASR) of the cell to rise. Changing the chemical composition of

the aqueous solution dramatically alters the rate of change of the ASR. Using 10�6

M KOH, the ASR increases at a rate of �102 U cm2 hr�1 (Figure 4). However, using

a basic pH solution of 1 M KOH solution lowers the ASR rate of change by three

orders of magnitude to �10�1 U cm2 hr�1. Adding high concentrations of

potassium-containing salts, such as KBr, also decreases the ASR rate of change

significantly. Both of these trends are consistent with a mechanism of ion exchange

for hydrated protons causing the resistance rise. In addition, the ASR rise is reversed

after immersing the sample in molten KNO3.

Joule 2, 1287–1296, July 18, 2018 1293

To further study the water/K-b00-alumina interaction, we measured Fourier transform

infrared (FTIR) spectra for K-b00-alumina membranes immersed in various aqueous

solutions for different lengths of time (Figure S9). Peaks associated with O-H and

C-O bond stretching were far less intense in samples immersed for 100–300 hr in

2 M KOH than in samples immersed in neutral water. These spectra are qualitatively

consistent with our impedance spectroscopy results and similar to previous

studies.29 These data also suggest the exchange of K+ for hydrated protons as the

cause of ASR rise. However, these results could also be consistent with other possi-

bilities, such as reaction with an amorphous impurity phase at grain boundaries, and

a follow-up study on this mechanism is in progress. One strategy to prevent the ASR

rise of the K-b00-alumina in contact with aqueous solutions is to use a thin coating to

prevent any undesirable reaction at the interface. Our preliminary results on carbon

coatings show ASR change at pH 8.5 similar to that we would expect at pH 13–14

(Figure 4A). Optimization of a coating material could enable K-b00-alumina to be

used with acidic aqueous solutions.

Safety

Regardless of performance, safety is of the utmost importance for battery design. If

the posolyte is nonaqueous, such as in the propylene carbonate cells we demon-

strated here, it can be safe with Na-K. A drop of Na-K in anhydrous propylene car-

bonate develops a solid electrolyte interphase, but is otherwise stable for the period

of weeks during which we observed it. If the posolyte is aqueous, safety consider-

ations come to mind due to the well-known exothermic reaction when alkali metals

are dropped in water in the presence of air. There are multiple routes to achieve a

safe Na-K-based flow battery with aqueous solutions. Safety can be engineered

with methods (isolation from air, pressure relief valves) similar to those used by

Na-Kjwater heat exchangers,33 which have been developed for nuclear power plants

such as the Dounreay fast reactor. Furthermore, we note that only recently has the

atomistic mechanism of the Na-K/water reaction been examined,34 which may

lead to novel ways to engineer safety. The addition of a small amount of surfactant

to water, such as 0.5 wt% hexanol, is found to greatly lower the rate of reaction.34 We

find that adding this surfactant to the aqueous ferrocyanide solution does not

increase the interfacial resistance of K-b00-alumina (Figure S10).

In conclusion, we demonstrated that K-b00-alumina can be used as a solid electrolyte

with a Na-K negative electrode solution in a Na-K-based flow battery at room

temperature. The use of Na-K enables a high-energy-density battery as a result of

its negative reduction potential and large capacity. Even though the ferri-/ferrocya-

nide redox couple is limited by its solubility, the flow battery we demonstrated with

ferri-/ferrocyanide in aqueous solutions has an energy density of �20 Wh L�1, com-

parable with existing RFBs. The use of other redox couples with higher solubilities in

aqueous or nonaqueous solutions on the positive electrode could achieve substan-

tially higher energy densities. For example, Na-K with an aqueous bromine/bromide

posolyte of 1 M Br2 would have an OCV of �4 V and an energy density of

�170 Wh L�1. Such a posolyte would have low pH, so developing a method to sta-

bilize the K-b00-alumina for chemistries at acidic or neutral35 pH is critical for expand-

ing the redox options for this new type of flow battery. Another route to optimize this

type of flow battery is the exploration of nonaqueous solutions such as deep eutectic

liquids. Fabricating thin (10–100 mm), supported K-b00-alumina membranes is

expected to achieve desired power densities, and system-level design to

take advantage of the high energy density and high electrical conductivity of

Na-K is expected to improve system material costs. The ability to use Na-K as a

1294 Joule 2, 1287–1296, July 18, 2018

room-temperature liquid metal electrode by pairing it with K-b00-alumina enables a

new type of flow battery with promising characteristics for grid-scale energy storage.

EXPERIMENTAL PROCEDURES

For full details please refer to Supplemental Experimental Procedures.

SUPPLEMENTAL INFORMATION

Supplemental Information includes Supplemental Experimental Procedures, ten fig-

ures, and one table and can be found with this article online at https://doi.org/10.

1016/j.joule.2018.04.008.

ACKNOWLEDGMENTS

We acknowledge funding support from the TomKat Center for Sustainable Energy,

the Anthropocene Institute, and the State Grid Corporation of China. A.C.B. was

supported by an NSF Graduate Research Fellowship and a Stanford Graduate

Fellowship through Stanford Energy 3.0 Corporate Affiliate Program. G.M. was

supported by an NSF Graduate Research Fellowship. A.P. was supported by an

NSF Graduate Research Fellowship and a Stanford Interdisciplinary Graduate

Fellowship. Part of this work was performed at the Stanford Nano Shared Facilities

(SNSF), supported by the National Science Foundation under award ECCS-

1542152. Part of this work was supported by Basic Science Research Program

through the National Research Foundation of Korea funded by the Ministry of

Education (2017R1D1A1B03035336 and 2017R1A4A1015022). We thank K. Lim

for assistance with Rietveld refinement. We thank Y. Li for assistance with carbon

coating. We thank E. Carlson, J. Blackburn, and R. Huggins for insightful discussions.

AUTHOR CONTRIBUTIONS

J.R., A.C.B., and W.C.C. conceived the project. A.C.B. and J.-H.L. performed

aqueous full-cell measurements and K-b00-alumina characterization. G.M. per-

formed aqueousjaqueous cell measurements and FTIR measurements. A.P. per-

formed nonaqueous full-cell measurements. A.M. performed stability tests and

EPMA measurements. N.K. performed Na-K pressure tests. A.C.B. and G.M. wrote

the manuscript with contributions from all authors.

DECLARATION OF INTERESTS

We have filed a patent related to this work: Application No. PCT/US2017/046156,

dated August 9, 2017. J.R. previously served as a Program Director at ARPA-E,

although this work was performed outside of his official duties.

Received: February 13, 2018

Revised: March 29, 2018

Accepted: April 11, 2018

Published: May 2, 2018

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