High-Voltage, Room-Temperature Liquid Metal Flow Battery ...
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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
[email protected] (W.C.C.)
[email protected] (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).
Korea3Advanced 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:[email protected] (W.C.C.),[email protected] (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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