From K-O2 to K-Air Batteries: Realizing Superoxide Batteries on the Basis of Dry Ambient Air

Article abstract of DOI:10.1002/anie.202003481

Although using an air cathode is the goal for superoxide-based potassium-oxygen (K-O₂) batteries, prior studies were limited to pure oxygen. Now, the first K-air (dry) battery based on reversible superoxide electrochemistry is presented. Spectroscopic and gas chromatography analyses are applied to evaluate the reactivity of KO₂ in ambient air. Although KO₂ reacts with water vapor and CO₂ to form KHCO₃, it is highly stable in dry air. With this knowledge, rechargeable K-air (dry) batteries were successfully demonstrated by employing dry air cathode. The reduced partial pressure of oxygen plays a critical role in boosting battery lifespan. With a more stable environment for the K anode, a K-air (dry) battery delivers over 100 cycles (>500 h) with low round-trip overpotentials and high coulombic efficiencies as opposed to traditional K-O₂ battery that fails early. This work sheds light on the benefits and restrictions of employing the air cathode in superoxide-based batteries.

Full text of DOI:10.1002/anie.202003481

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Accepted Article
Title: From K-O2 to K-Air Batteries: Realizing Superoxide Batteries on
the Basis of Dry Ambient Air
Authors: Lei Qin, Neng Xiao, Songwei Zhang, Xiaojuan Chen, and
Yiying Wu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202003481
Angew. Chem. 10.1002/ange.202003481
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COMMUNICATION
From K−O to K−Air Batteries: Realizing Superoxide Batteries on
2
the Basis of Dry Ambient Air
Lei Qin, Neng Xiao, Songwei Zhang, Xiaojuan Chen, and Yiying Wu*
[*]
Dr. L. Qin, Dr. N. Xiao, S. Zhang, X. Chen, Prof. Y. Wu
Department of Chemistry and Biochemistry
The Ohio State University
100 West 18th Avenue, Columbus, OH 43210 (USA)
E-mail: wu@chemistry.ohio-state.edu
Supporting information for this article is given via a link at the end of the document.
Abstract: Superoxide-based potassium-oxygen (K−O
2
) batteries
atmosphere with a large round-trip overpotential (1.3~1.62 V).[6]
have demonstrated enormous potential to realize high-energy-density
battery chemistry with low round-trip overpotentials. Although using
air cathode is the ultimate goal to unlock the full potential of
Lately, similar studies have been brought to sodium (Na)−air
electrochemistry. Sun et al. discovered that CO
dramatically alter the electrochemical pathway of Na−air batteries
·2H O,
.[7] The continuous accumulation of peroxide
2
and moisture
superoxide batteries, prior studies were limited to pure oxygen. Herein, and promotes side products formation, including Na
we report the first K−air (dry) battery based on reversible superoxide
2
O
2
2
NaOH, and Na
and carbonate largely limits the battery cyclability. Since the
demonstration of potassium (K)−O battery by our group in 2013,
2 3
CO
[
8]
electrochemistry. Spectroscopic and gas chromatography analyses
are applied to evaluate the reactivity of KO
that KO reacts with water vapor and CO to form KHCO
to be highly stable in dry air. With this knowledge, rechargeable K−air
dry) batteries were successfully demonstrated by employing dry air
2
in ambient air. Despite
2
2
2
3
, it is found
great progress has been made, including anode stability
improvement, electrolyte formula optimization, and cathode
design.[ One major advantage is that KO
9]
is the only product in
(
2
battery without the interference of peroxide species.[
1b]
cathode. We further prove that the reduced partial pressure of oxygen
plays a critical role in boosting the battery lifespan. With a more stable
environment for K anode, a K−air (dry) battery delivers over 100
cycles (> 500 hours) with low round-trip overpotentials and high
a K−O
Moreover, pure KO
to quantitatively investigate its reactions with moisture/CO
2
2
is commercially available. This facilitates us
and
2
understand the role of these reactions in a K−air (dry) battery.
coulombic efficiencies as opposed to traditional K−O
2
battery that fails
Herein, our work explores for the first time the highly reversible
at an early stage. This work sheds light on the benefits and restrictions
of employing air cathode in superoxide-based batteries, and will be of
use for the development of practical metal-air batteries.
2
KO electrochemistry under dry ambient air. The K−air (dry)
battery delivers a small round-trip overpotential of 74 mV and
+
offers a flat discharge voltage output at 2.40 V (vs. K /K). Based
on chemical quantifications and spectroscopic characterizations,
the superoxide chemistry is identified as the reaction mechanism
and such reaction route is barely affected by air components
Non-aqueous alkali metal-air batteries are deemed as energy
storage alternatives to the current lithium-ion batteries due to their
large energy densities and low costs.[1] To date, these gas-open
systems have been largely limited to pure oxygen (O ) conditions
2
due to the cell contamination and degradation caused by
unwanted atmospheric components, including nitrogen, carbon
(
especially CO
cycles (above 500 hours) with a high coulombic efficiency (CE) of
9.0% have been achieved. In the light of the feasibility of utilizing
2
) in the absence of moisture. As a result, over 100
9
dry ambient air and the realization of superb reaction kinetics with
an impressive battery lifespan, the “open” K−air (dry) battery
demonstrated here motivates the development of superoxide
batteries as a promising high-energy-density alternative to current
lithium-ion batteries technology.
dioxide (CO
for the air purification device and O
2
), and moisture. In order to eliminate additional needs
cylinder, it is crucial to design
2
a practical metal-air battery operating in ambient air.
A few groups have investigated the chemistry of metal−O
2
To explore the effects of different gas components, the
batteries in an air-like atmosphere. Note that CO
contaminant in metal-air batteries, which adversely affects the O
electrochemistry. Takechi et al. noticed that O -
can be captured
by CO to form a series of intermediates (e.g., peroxycarbonate
and peroxydicarbonate).[2] The final product of Li
CO results in a
2
is an inevitable
reactivity of KO
powder in air-tight vials filled with dry CO
saturated with water vapor), dry air, and dry O
2
was first examined by aging the commercial KO
2
2
2
, wet CO (namely CO
2
2
2
2
, respectively
Supporting Information, Figure S1a-c). The yellow KO powder
within 24 hours, while no obvious
2
(
2
2
3
turns to white only in wet CO
2
sluggish decomposition kinetics and requires electrocatalysts to
assist the removal of carbonate, leading to an intractable issue in
color change is observed in other vials. As shown in X-ray
diffraction (XRD) patterns, the white product is identified as
KHCO (JCPDS No. 12-0292) in wet CO , while the KO (JCPDS
3 2 2
No. 43-1020) does not change in other atmospheres (Supporting
Information, Figure S1d). Raman spectrum further confirms the
metal−CO
Li CO , by the reaction between LiO
Achilles’ heel of lithium (Li)−air batteries. Zhou et al. applied
2
batteries.[3] Peng et al. revealed that the build-up of
intermediate and CO , is the
2
3
2
2
[4]
ionic liquid (IL)-based gel to prevent electrolyte degradation when
-
1
dominant KO
while only KHCO
Figure S1e). Therefore, the presence of water vapor is critical for
CO to react with KO . It is also revealed that the KO powder is
highly sensitive to the moisture in ambient air (relative humidity of
2
(1142 cm ) in dry CO
2
(same as dry O
2
and dry air),
operating in ambient air.[5] Nevertheless, Li
CO
and undermines the reversibility of Li−O
still accumulates
electrochemistry.
2
3
3
is present in wet CO
2
(Supporting Information,
2
Curtiss and Salehi-Khojin et al. reported a long-life Li−O
2
battery
2
2
2
based on IL−dimethyl sulfoxide electrolyte in an air-like
1
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Angewandte Chemie International Edition
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COMMUNICATION
~
30%), and the final byproducts are identified as carbonate and
carbon paper cathode. These KO
2
crystals exhibit a similar cubic
hydrated hydroxide after aging (Supporting Information, Figure
S2).
morphology just as that formed in K−O
density of KO crystals increases at a higher DOD. After charging
to 1 mAh, the cubic KO crystals exhibit a volume shrinkage.
2
batteries.[9d] The area
2
2
When the cell is charged to 3.0 V, most cubes are removed and
only a film-like residue is left on the surface. This is ascribed to
the side products based on our prior study.[
10]
Figure 1. Stability test of commercial KO
in dry/wet CO based on 3 parallel titrations. (b) Schematic illustration of the
gas-analysis process. (c) Gas evolution from aged KO powder in dry/wet CO
for 72 hours. Commercial KO powder was put inside an air-tight vial, which is
connected to a balloon filled with high-purity CO . A smaller vial with deionized
water provides moisture with a relative humidity of 100% if necessary. The
evolved gas was analyzed by gas chromatography with N as the carrier gas.
2 2
powder. (a) Amounts for residual KO
2
2
2
2
2
2
To conduct a quantitative analysis on the detrimental effect of
CO on the KO stability, a specific amount of KO powder (~10
mg) was placed in both dry CO and wet CO atmospheres for
comparison. The amount of KO residue was determined by UV-
vis spectroscopy following its reaction with TiO(SO ) aqueous
solution (Supporting Information, Figure S3). It is found that most
KO (96.6 wt%) is left in dry CO , while there is only 2.3 wt% KO
that remains in wet CO after 45 hours (Figure 1a). Gas
chromatography (GC) measurement was carried out to
investigate possible gas evolution (Figure 1b). No O signal is
observed in dry CO , confirming the superb chemical stability of
KO (Figure 1c). In stark contrast, the significant O signal is
identified in wet CO . Through the spectroscopic and GC analysis
on the formed product, the side reactions between KO and wet
CO can be summarized as follows:
KO + 4CO + ꢀH O → 4KHCO + ꢁO
2
2
2
2
2
2
4
2
2
2
Figure 2. (a) Typical discharge-charge curves of K−air (dry) batteries. (b)
Cycling performance and coulombic efficiency of K−air (dry) batteries (DOD =
2
0
.5 mAh, I = 0.2 mA). (c) Voltage profiles of K−air (dry) batteries cycling at the
2
+
current of 0.2 mA with different DODs (cycled between 2-3 V vs. K /K). The
2
corresponding CEs (left) and energy efficiencies (right) are shown in the inset.
(
d) XRD pattern of the discharged cathode and charged cathode in K−air (dry)
2
2
o
batteries (DOD = 1 mAh). The XRD peak at 54.4 is ascribed to the carbon
2
paper substrate.
2
2
The voltage profile of a K−air (dry) battery is similar to a K−O
2
4
2
2
2
3
2
battery, except for the slightly lower voltage plateau (Figure 3a).
Given substantial water vapor in ambient air, the aforementioned
Based on the Nernst equation, the potential is decreased by 40
side reaction can easily take place in a K−air (dry) electrochemical
2
mV due to the lower partial pressure of O in the air (~1/5 by
cell and produce KHCO
evolution.
3 2
as discharge products together with O
volume), which results in an equilibrium potential of 2.446 V.
Cyclic stability of the K−air (dry) batteries is further compared with
As a proof of concept, a K−air (dry) battery consisting of
metallic K anode and dry air cathode was assembled. The
conventional carbon paper was used as the gas diffusion layer,
and a moisture trap was needed to remove the water from the air
cylinder. The cyclic stability of K−air (dry) batteries was evaluated
with shallow cycling (capacity of 0.5 mAh at the rate of 0.2 mA).
K−O
potassium hexafluorophosphate (KPF
DME) electrolyte exhibits sudden death after 40 cycles (~200
hours). With a shallow cycling capacity of 0.5 mAh, the K−O
battery sustains 35 stable cycles (Supporting Information, Figure
S5a-b). Then its round-trip overpotential rapidly increases after
the 35th cycle due to serious anode corrosion (Figure 3c). Finally,
the pristine K metal anode turns to a yellow disc, and Raman
spectrum reveals that the main components on the surface of
2
batteries in Figure 3b. A common K−O
2
battery using
6
)/1,2-dimethoxyethane
(
2
2
Without employing the O shield membrane or solid electrolyte,
the K−air (dry) battery is demonstrated to deliver over 100 stable
cycles (above 500 hours) with a low round-trip overpotential of 74
mV and a high average CE of 99.0% (Figure 2a-b). When cycled
with different depths of discharge (DODs) of 1, 2, and 3.92 mAh,
highly reversible charge processes can still be achieved (CEs%:
2 2 3
cycled K are KO , KOH, and K CO , etc. (Figure 3d). Note that
same byproduct components with relatively lower intensities are
identified on the corroded K anode from K−air (dry) batteries.
To eliminate the influence of cathode degradation on battery
performance, the K anode and separator were replaced and the
electrolyte was refilled after the battery failure. The same battery
soon recovers to show normal voltage curves after replacing the
9
(
8.9%-99.3%) with merely 110 mV round-trip overpotential
energy efficiencies%: 94.2%-96.1%) (Figure 2c). The XRD
pattern confirms that the discharge product is still crystalline KO
in a K−air (dry) battery and these KO crystals would disappear
2
2
after charge, suggesting that the various components in dry air do
not alter the essential superoxide-based electrochemistry of
batteries (Figure 2d). For experimentally studying the
morphology change upon discharge/charge, a series of K−air
O
2
-corroded K anode (Supporting Information, Figure S5c-d).
The first charge curve (with dash line) shows a relatively larger
capacity, likely due to the accumulation of undecomposed KO in
previous cycles. Based on the above observations, the anode
degradation due to O crossover is mainly responsible for the
failure of K−O batteries. A similar phenomenon is also observed
2
(
(
dry) batteries were assembled and discharged to 1 and 2 mAh
Supporting Information, Figure S4). Scanning electron
2
2
microscopy (SEM) images of the discharge cathode reveal that
the cubic-shaped KO crystals are formed on the surface of
in K−air (dry) batteries, which dies after the 120th cycle. The cell
voltage and cycling completely recover with a new K anode
2
2
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COMMUNICATION
(
Supporting Information, Figure S6). A longer K anode lifetime is
spectra of the cathode reveal a limited growth of carbonyl species
and carbonate from decomposition after the first discharge
(Supporting Information, Figure S10a-b). The formation of stable
realized in a K−air (dry) battery owing to the lower O
2
permeation/crossover rate in air, which eventually contributes to
the enhanced lifespan of K−air (dry) batteries. Details on the
kinetics influence are subject to further investigation in the future.
To mimic the K anode corrosion under various gas components, S10c), is crucial for improving the reversibility of KO
a metallic K was placed in a DME solution saturated with dry CO
wet CO , dry air, and dry O , respectively. Raman and nuclear
magnetic resonance (NMR) were adopted to analyze the aged K
samples. Figure S7a (Supporting Information) shows that none
of the aged samples remain shiny metal luster, white byproduct
layers are found on the surface instead. Raman spectra confirm
that the KOH component is formed on the K surface in O
environment, while K CO is identified in dry CO and wet CO
atmospheres (Supporting Information, Figure S7b). By dissolving
KO
indicated by the Raman result (Supporting Information, Figure
-based
superoxide batteries. Titration results reveal that 37.1 μmol KO
is formed (Supporting Information, Table S1). After charging to
3.0 V, Raman analysis confirms the disappearance of KO , and
quantitative titration shows that some dead superoxide/peroxide
is left in the air electrode that is equivalent to 0.7 μmol dead KO
and contributes to an irreversible capacity of 1.9%. We propose
that this is due to KO reacting with trace H O in the electrolyte by
forming titratable KHO (See the detailed analysis in Supporting
Information, Figure S11). This indicates that a total amount of
2
without the interference of peroxide and other byproducts, as
2
2
,
2
2
2
2
2
2
and air
2
2
2
3
2
2
2
in D
2
O, the surface layer components were further characterized
37.1 μmol KO
2
forms during the discharge process, and 36.4 μmol
1
-
by NMR (Supporting Information, Figure S8). It is found that H
NMR shows peaks attributed to formate (HCOOK, δ = 8.44 ppm)
is reversibly charged in the first cycle. In addition, high e /KO
ratios of 1.02 and 1.01 are estimated for the O
2
2
reduction and
in dry CO
dimethyl ether (CH
K in wet CO
.[11] In comparison, none of the aforementioned
components are detected (except for residual DME) on K surface
in dry O and air. Despite this, the parasitic reactions between K
anode and the electrolyte/O are much increased with more side
products accumulation during long-term cycling (Figure 3d). An
-blocking membrane and a stable solid electrolyte interphase
are critical to protect the K anode from O crossover and
2
, and extra acetate (CH
3
COOK, δ = 1.90 ppm) and
evolution processes, respectively. Notably, a slight portion of KO
2
3
OCH , δ = 3.20 ppm) are found on the aged
3
could still react with DME electrolyte and form non-titratable side
products (e.g., formate, acetate, and methoxyacetate) on the
surface, which account for another irreversible capacity portion
(0.2 μmol, 0.5%). The related mechanism is discussed in our prior
work.[10] The detailed information on the different fractions of
electrons during the first cycle is summarized in Figure 4b. 97.6%
2
2
2
O
2
of electrons are reversible through KO
in a K−air (dry) battery, while 2.4% loss of electrons is ascribed to
the side reactions between KO and electrolyte/H O, which
consists of dead KO (1.9%) and non-titratable side products
0.5%).
2
formation/decomposition
2
electrolyte decomposition, and further boost the K−O
2
/air (dry)
2
2
batteries performance.
2
(
Figure 4. (a) Raman spectra of the pristine, discharged, and charged cathodes
in K−air (dry) batteries. The typical Raman peaks related to carbon paper
substrate are also labeled. (b) The fraction of total electrons (with 1 mAh
capacity) attributed to the formation of reversible KO
titratable side products in the first cycle.
2
, “dead” KO2, and non-
In summary, our study demonstrated for the first time a K−air
Figure 3. Comparison between K−O
2
and K−air (dry) batteries in (a) first voltage
(dry) battery on the basis of superoxide chemistry. The high
profiles, (b) cyclic stability, and (C) round-trip overpotentials. The round-trip
overpotentials are estimated by measuring the middle potential in the
discharge/charge processes. The batteries are tested at the same conditions
reversibility of the O
dry ambient air. Quantitative titration verifies the high-efficiency
one-electron transfer processes in K−air (dry) battery.
Meanwhile, spectroscopic and GC analyses reveal that KO does
not react with dry CO . Considering the small amounts of CO
0.04% in air) and H O (removed by moisture trap) in a K−air (dry)
battery, pure O is not required for KO -based superoxide
2 2
/KO redox couple is well-maintained in the
a
(rate at 0.2 mA with the curtailed capacity at 0.5 mAh). (d) Raman spectrum of
the corroded K in K−O batteries (cyan) and K−air (dry) batteries (red) after 50
2
2
cycles. The inset shows the optical images of corroded K. The KPF
comes from salt residue.
6
component
2
2
(
2
2
2
As shown in Figure 2(a-b) that the first cycle shows the lowest
CE compared to other cycles. The titration method was then
applied to quantify the reversible KO amounts on the cathode in
2
the first cycle (Supporting Information, Figure S9a). A K−air (dry)
batteries. As a result, the cost can be lowered and the gas-open
K−air (dry) battery technology is practically feasible. Further
Raman analysis determines that
(
K
anode corrosion
accumulation of side products) resulted from O crossover is the
battery failure. The reduced O partial
2
battery was first discharged to a DOD of 1 mAh, which is equal to
main reason for K−O
2
2
3
7.3 μmol electrons (Supporting Information, Figure S9b). From
the Raman analysis, KO dominates the discharge product on the
air cathode (Figure 4a). X-ray photoelectron spectroscopy (XPS)
pressure benefits the lifetime of K anode, which accounts for the
enhanced lifespan of K−air (dry) battery. At a controlled DOD (0.5
mAh), K−air (dry) batteries system demonstrates the reversible
2
3
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COMMUNICATION
O
2
/KO
superb cycling (over 100 cycles) is achieved with a high CE of
9.0%. We hope these findings would facilitate a better
2
conversion (with 74 mV round-trip overpotential), and a
9
understanding of superoxide chemistry and motivate the
development of practical air cathode for superoxide battery
technologies.
Acknowledgements
The authors acknowledge funding support from the U.S.
Department of Energy (Award No. DE-FG02-07ER46427) and the
Ohio State University for partial support of this work. Electron
microscopy was performed at the Center for Electron Microscopy
and Analysis (CEMAS) at the Ohio State University. The first
author especially thanks his wife Tingting Sun for drawing the
graphical abstract and the schemes. The authors also
acknowledge Jieren Shao for the use of nuclear magnetic
resonance and thank Jiaonan Sun for many fruitful discussions.
Conflict of interest
The authors declare no conflict of interest.
Keywords: air battery • anode degradation • oxygen crossover •
potassium-oxygen batteries • superoxide chemistry
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Entry for the Table of Contents
2
Getting rid of O cylinder: Dry ambient air is utilized for the first demonstration of K−air (dry) battery (see picture), which sheds light
on the reversible potassium superoxide electrochemistry. This motivates the development of high-efficiency superoxide battery
technology and represents an important step towards the direct use of air in metal-air batteries.
5
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