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Ultrahigh Capacity Lithium-Oxygen Batteries Enabled by Dry-Pressed
Holey Graphene Air Cathodes
Yi Lin,1,* Brandon Moitoso,2 Chalynette Martinez-Martinez,2 Evan D. Walsh,2 Steven D. Lacey,2,3
Jae-Woo Kim,1 Liming Dai,4,* Liangbing Hu,3,* and John W. Connell5,*
1National Institute of Aerospace, 100 Exploration Way, Hampton, Virginia 23666-6147; 2NASA
Interns, Fellows, and Scholars (NIFS) Program, NASA Langley Research Center, Hampton,
Virginia 23681; 3Department of Materials Science and Engineering, University of Maryland,
College Park, MD 20742; 4Department of Macromolecular Science and Engineering, Case
School of Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland,
Ohio 44106; and 5Mail Stop 226, Advanced Materials and Processing Branch, NASA Langley
Research Center, Hampton, Virginia 23681-2199.
Keywords: holey graphene, catalyst-free, lithium-oxygen batteries, dry processing,
areal performance
https://ntrs.nasa.gov/search.jsp?R=20190025748 2020-06-07T05:34:51+00:00Z
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Abstract
Lithium-oxygen (Li-O2) batteries have the highest theoretical energy density of all the Li-
based energy storage systems, but many challenges prevent them from practical use. A major
obstacle is the sluggish performance of the air cathode, where both oxygen reduction (discharge)
and oxygen evolution (charge) reactions occur. Recently there have been significant advances in
the development of graphene-based air cathode materials with a large surface area and high
catalytic activity for both oxygen reduction and evolution reactions. However, most studies
reported so far have examined air cathodes with a limited areal mass loading rarely exceeding 1
mg/cm2. Despite the high gravimetric capacity values achieved, therefore, the actual (areal)
capacities of those batteries were far from sufficient for practical applications. Here, we present
the fabrication, performance, and mechanistic investigations of high mass loading (up to 10
mg/cm2) graphene-based air electrodes for high-performance Li-O2 batteries. Such air electrodes
could be easily prepared within minutes under solvent-free and binder-free conditions by
compression molding holey graphene because of the unique dry compressibility of this graphene
structural derivative with in-plane holes. High mass loading Li-O2 batteries prepared in this
manner exhibited excellent gravimetric capacity and thus ultrahigh areal capacity (as high as ~40
mAh/cm2). The batteries were also cycled at a high curtailing areal capacity (2 mAh/cm2), with
ultrathick cathodes showing a better stability during cycling than thinner ones. Detailed
postmortem analyses of the electrodes clearly revealed the battery failure mechanisms under both
primary and secondary modes, which were the oxygen diffusion blockage and the catalytic site
deactivation, respectively. The results strongly suggest that the dry-pressed holey graphene
electrodes are a highly viable architectural platform for high capacity, high performance air
cathodes in Li-O2 batteries of practical significance.
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1. Introduction
The demand for high performance energy storage systems is increasing in order to
provide electrical energy for portable electronics, grid-scale applications, and electric
transportation. Although performance requirements can vary significantly for these different
applications, an ideal advanced battery system should, in general, have a small footprint (i.e.,
lightweight and small volume), high capacity, high energy density, good cyclability, high
power/rate capability, and ensured safety. In electric transportation, lithium (Li) ion battery
technology is currently being used in the electric vehicles (EVs), but their energy densities are
intrinsically limited and insufficient for the need of more powerful future EVs.1-3 In aeronautics,
energy storage systems for future electric aircrafts will demand even higher energy densities.4
Li-air batteries are an advanced Li-based energy storage system that utilize Li metal and
oxygen (O2) as the anode and cathode reactants, respectively.1-3 Li metal has the highest energy
density value among the solids (3860 mAh/g), while O2 is sustainable with an unlimited supply
from the air. Thus, the Li-air battery system is expected to have an extraordinarily high
theoretical energy density (11700 Wh/kg), which significantly surpasses that of typical Li ion
battery systems (< 500 Wh/kg) and can potentially revolutionize the energy storage technology if
successfully developed.5
Li-O2 batteries using pure O2 atmosphere are the most widely studied Li-air batteries.
However, many challenges remained to be addressed in order to advance their readiness level
toward practical applications. In an ideal non-aqueous Li-O2 battery, the discharge reaction is an
oxygen reduction reaction (ORR) involving the formation of lithium peroxide (Li2O2) from Li
and O2. The charge reaction is an oxygen evolution reaction (OER), in which Li2O2 decomposes
into O2 and Li metal back to the anode surface. One of the major bottlenecks in Li-O2 battery
technology has been the sluggish performance of air cathodes where ORR and OER occur.6,7
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Most of current research has focused on the development of cathode materials that can alleviate
this problem,8 although some recent studies have also focused on other aspects such as the use of
redox mediators as “soluble catalysts”.9 Carbon nanomaterials, such as carbon nanotubes10-15 and
graphene,16-18 have received special attention as the air cathodes because of their low weight,
high surface area, excellent electrical conductivity, and chemical stability. In addition, carbon
nanomaterials are excellent catalytic substrates, which, upon heteroatomic doping, 19 - 24 or
incorporation of metal-containing catalysts,25-29 or combinations thereof,30-32 become even more
effective in catalyzing ORR/OER reactions.
While significant progress has been made in the air cathode material development, very
few studies have been devoted to the preparation of high capacity Li-O2 batteries with energy
densities that are practically meaningful. Carbon nanomaterial-based air cathode materials with
reported ultrahigh gravimetric performance were often of very small areal loadings that rarely
surpassed 1 mg/cm2 with only very few exceptions (no more than ~2 mg/cm2).17,33 In a recent
study, Zhou et al. reported an outstanding graphene-based air electrode material design with
hierarchical porosity and controlled surface functionality.34 Such Li-O2 cells exhibited a deep
discharge capacity as high as 19800 mAh/g and were cycled 50 times with a curtailing capacity
of 1000 mAh/g at a current density of 1000 mA/g. However, the areal loading of the active
material was only 0.25 mg/cm2. Thus, the true total and cyclable areal capacities are calculated to
be ~5 and 0.25 mAh/cm2, respectively. In comparison, the typical cyclable capacity of current
commercial Li ion batteries is in the range of ~2 – 4 mAh/cm2.35-38 Scaling up the air cathode for
practical applications, therefore, requires significant increase in the electrode areal loading and
thickness. Meanwhile, the desirable porous architecture needs to be retained to ensure the
effectiveness of the battery reactions that are already intrinsically kinetically sluggish. It remains
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unknown whether the reported ultrahigh gravimetric performances could still be achieved at high
cathode mass loading.
While the cathode reactant is gaseous oxygen obtained from external atmosphere, the air
cathode of a Li-O2 battery is the true physical location for the battery reactions. Unlike their
aqueous counterparts, non-aqueous Li-O2 batteries often contain discharge products that are
insoluble in the electrolyte.3 In order to achieve high capacity, the cathode compartment needs
sufficient space, i.e., pore size and volume, to host a large amount of solid discharge products.
The total surface area of the cathode material, typically measured by gas adsorption/desorption, is
less relevant in Li-O2 batteries in comparison to other energy storage systems, such as
supercapacitors. 39 , 40 To ensure rechargeability, the insoluble discharge products need to be
attached to the conductive substrate for electrochemical decomposition during charging.
Therefore, an ideal high capacity air electrode should have an optimized substrate-pore
architecture to allow for the growth/filling of discharge products and the subsequent
decomposition/emptying during charging while maintaining electrical contact at all time.
It was recently demonstrated in our laboratories that graphene-based articles of arbitrary
shapes could be prepared via direct compression of holey graphene (hG) powder under solvent-
free and additive-free conditions at room temperature. 41 The procedure was facilely conducted
using a common pressing die and a hydraulic press, and could be completed within minutes in
comparison to the typical lengthy multi-step procedure involving slurry preparation, solvent
evaporation, and often the use of parasitic binders to prepare carbon-based electrodes. The hG
material is the key to the procedure because the holes through the lateral surface of the graphene
sheets allow air molecules to readily escape from the interstitial space between adjacent sheets
during dry compression. Also, because of the presence of the in-plane holes that allow facile
through-thickness mass transport, hG has been proven to be a superior electrode material to intact
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graphene for supercapacitors,42,43 Li ion batteries,44,45 and Li-O2 batteries.46 The dry compression
properties of hG therefore provide a unique opportunity to facilely achieve binder-free graphene-
based electrodes of ultrahigh areal loading with high performance. Indeed, supercapacitors using
dry-pressed hG electrodes with areal loading as high as 30 mg/cm2 were achieved with little
reduction in gravimetric performance even in comparison to devices with loadings as low as 1
mg/cm2.41 With high mass loading, the devices exhibited outstanding areal capacitances of >1
F/cm2. It was also demonstrated that the hG powder could be used as the host matrix to fabricate
complex electrode architectures with precise catalyst placements for Li-O2 batteries using the dry
compression approach.47
Here, we report that the binder-free, dry-pressed hG discs with areal loadings as high as
10 mg/cm2 exhibited high performance as air cathodes for non-aqueous Li-O2 batteries. These
electrodes enabled ultrahigh areal capacity when operated in the primary (i.e., full discharge)
mode. The thick hG electrodes were also operated in the secondary (i.e., rechargeable) mode at a
high curtailing capacity and exhibited excellent cyclability. The postmortem analysis of thick
electrodes was studied in great detail, and the mechanism for the progress of battery reactions in
these high capacity Li-O2 batteries was proposed.
2. Results and Discussion
2.1. Preparation of Dry-Pressed hG Air Cathodes
hG was prepared using a previously reported facile one-step controlled air oxidation
process.43,48 The resultant hG sheet exhibited holes with diameters in the range of ~5 – 15 nm
throughout the lateral surface, as shown in a typical scanning electron microscopy (SEM) image
in Figure 1a. For Li-O2 batteries, the air cathodes were prepared using hG by a facile, dry
compression molding process enabled by the unique compressibility of hG without the use of any
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polymer binder or solvent.41,47 In the process, the as-produced hG powder was directly loaded in
a pressing die and compressed at room temperature using a laboratory hydraulic press unit
(Figure 1b). Two precisely-cut separation layers (such as glossy Celgard membrane discs) were
used to encase the hG powder in order to prevent its adherence to the die surfaces. The separation
layers were readily peeled off from the hG disc to form a free-standing electrode (Figure 1c).
The air electrode was then incorporated into a Li-O2 coin cell, in which a perforated cathode cap
was used to allow for oxygen passage (Figure 1d; see details in Methods).
Figure 1. High capacity Li-O2 batteries with dry-pressed holey graphene (hG) air cathodes. (a) A
typical SEM image of a hG sheet. (b) A schematic drawing (not to scale) on using the hydraulic
compression of hG powder to directly prepare hG air cathodes. (c) A photograph of dry-pressed
hG air cathodes with stainless steel (SS) woven mesh support (left), with Al mesh support
(middle), and freestanding (right). (d) A schematic drawing (not to scale) of a high capacity Li-
O2 coin cell battery with an ultrathick air cathode. (e-g) Side-view SEM images of an hG air
cathode (mA ~ 10 mg/cm2) with SS woven mesh supports. Most hG sheets were horizontally
aligned as shown in (f) and area 1 labeled in (e). However, compression from the interwoven SS
wires resulted in different localized orientations of hG sheets, even those near vertical to external
compression force as shown in (g) and area 2 labeled in (e).
When using perforated Al meshes as the separation layers, the top mesh could be left
attached to serve as the current collector to ensure a good electrical contact, with perforation
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allowing sufficient oxygen access (Figure 1c). As expected, higher compression pressure resulted
in more compact hG sheet packing and higher density.41 At a compression pressure of 200 MPa
(used in this work unless otherwise specified), the thicknesses of the hG discs for areal mass
loadings (mA) of 5 and 10 mg/cm2 were ~50 and ~100 µm, respectively, as measured directly by a
thickness gauge or SEM (Figure S1a in Supporting Information). The density values of these
hG discs were therefore ~1 g/cm3.
Air cathodes were also prepared by sandwiching the hG powder using two precisely cut
stainless steel (SS) woven meshes as the support and current collector (Figures 1c & 1e-g). These
electrode discs were thicker than those using flat separation layers because the ~150-µm thick SS
wires were embedded in the architecture. hG sheets in the electrode discs with flat separation
layers were horizontally aligned (Figure S1a and b in the Supporting Information). However, in
a SS woven mesh-supported hG electrode disc, the intertwined SS wires caused the hG sheets to
align in various orientations in the final architecture. The sheets were not only in the direction
parallel to disc surfaces (Figure 1f), but also nearly vertically aligned at some locations (Figure
1g), the latter of which is the most preferred graphene sheet orientation for mass transport
through the electrode thickness. The use of woven wire meshes was the key for such unique
architecture, because the hG sheets compressed between two close-neighbored (< 100 µm in
distance) SS wires became aligned more along the wire interfaces within the confined space than
the direction of the external compression force (see area 2 in Figure 1e). Consistently, although
air electrodes with different separation layers or support structures all performed well, those with
sandwiching SS woven meshes provided the highest specific capacity per gram carbon at the
same mA in similarly assembled Li-O2 cells (Figure S1c in Supporting Information). Therefore,
most hG air electrodes in this work used SS woven wire mesh supports unless otherwise
specified. It is recognized, however, that the current SS woven meshes are much heavier than the
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perforated Al meshes (~40 vs. 3 mg/cm2). For practical Li-O2 cells, dry-pressed hG electrode
support with similar interwoven wiring architecture but much lighter in material weight would be
more desirable.
2.2. Full Discharge Properties
As shown in Figure 2a, the full discharge voltage profile for a Li-O2 battery with a dry-
pressed hG air cathode with mA of 5 mg/cm2 at an areal current density (IA) of 0.1 mA/cm2 was
stable and flat during majority of the discharge time. The discharge potential measured at half
total capacity (VC/2) was ~2.74 V, with a discharge overpotential of only 0.22 V (considering E0 =
2.96V for the Li2O2-based discharge reactions3), suggesting good ORR properties of the packed
hG materials in the air cathode. The discharge lasted more than 160 h, corresponding to a
specific capacity (Cm) of ~7700 mAh/g and a specific energy (Em) of ~21,000 Wh/kg. These
weight-averaged values are on par with most graphene-based air cathodes reported (without
additional catalyst), but the mass loadings used here are much higher (Table S1 & S2 in
Supporting Information). As expected, increasing the current density to 0.2 and 0.5 mA/cm2
resulted in decrease in specific capacity (6540 and 2824 mAh/g) and discharge potential (2.59
and 2.36 V). Because of the high mass loading, the cells exhibited outstanding areal capacities
(CA), which were calculated to be as high as 38, 33, and 14 mAh/cm2 at 0.1, 0.2, and 0.5
mA/cm2, respectively. Areal capacity is a critical parameter in the evaluations of practical
batteries with large mass loadings. The CA values calculated from most of the previous literature
reports with carbon-based air electrodes rarely surpassed 10 mAh/cm2 while the mA values were
usually smaller than 1 mg/cm2 (Table S2).
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Figure 2. Deep discharge properties of Li-O2 batteries with dry-pressed hG air cathodes: (a)
discharge profiles at areal current densities of 0.1, 0.2, and 0.5 mA/cm2 for hG air cathodes with
the same mass loading of 5 mg/cm2; (b) discharge profiles for hG air cathodes with mass
loadings of 2, 5, and 10 mg/cm2 at the same areal current density of 0.2 mA/cm2; (c) dependence
of specific capacity (left y-axis) and total capacity (right y-axis) of the batteries on the areal mass
loading; and (d) a schematic drawing showing the utilization efficiency and the discharge product
accumulation in air cathode with various mass loadings.
At the same areal current density, varying the mass loading resulted in changes in specific
capacity. As shown in Figures 2b and 2c, at 0.2 mA/cm2, the hG air cathode with mA of 5
mg/cm2 exhibited a higher specific capacity than those with either smaller or higher mass
loadings. For example, the specific capacities of hG air cathodes with mA of 2, 5, and 10 mg/cm2
were 3819, 6540, 3530 mAh/g, respectively (Figure 2b and Table S1 in the Supporting
Information). The full discharge time (or total capacity) of the 10 mg/cm2 electrode was ~12%
higher than that of the 5 mg/cm2 electrode under otherwise identical conditions (run side-by-side
in the same oxygen chamber; Figure 2b inset). The two electrodes also exhibited almost identical
discharge profiles up till ~150 h of discharge, when the cell with the 5 mg/cm2 hG electrode
started to fail while that with the 10 mg/cm2 electrode was stable for another 20 h.
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The above findings can be understood by considering two competing factors. First, at a
constant areal current density IA, the smaller the mA, the higher the specific current density in
terms of unit mass (Im, in mA/g). Im has been very commonly used in the evaluations of low mass
loading air electrodes.1-3 It is known that higher Im usually results in smaller total discharge
capacity, reduced cyclability, and lower discharge potential. At IA of 0.2 mA/cm2, the Im values
for hG electrodes with mA of 2, 5, and 10 mg/cm2 were 100, 40, 20 mA/g, respectively.
According to the data shown in Figure 2b, the VC/2 values for these hG electrodes were 2.52,
2.59, and 2.67 V, respectively. Such VC/2 – mA dependence is consistent with the expected effect
from the respective change of Im. Second, it can be expected that higher mA, or thicker electrodes,
may exhibit reduced cathode material utilization efficiency across the entire thickness, especially
considering the depth-wise concentration gradient of dissolved oxygen in the electrolyte.
Consistent with this hypothesis is the finding that the areal capacity of hG air cathodes exhibited
little increase beyond ~35 mAh/cm2 with mA ≥ 5 mg/cm2 at 0.2 mA/cm2 (Figure 2c). In other
words, the entire thickness of a dry pressed hG air cathode can only be effectively utilized up to
~5 mg/cm2 (“thick” electrodes) under current deep discharge conditions.
To fully illustrate the discharge product distribution in the thick electrode configurations,
multi-layered “analytical” hG air cathodes with extra Al mesh insertions were designed so that
postmortem analyses of electrode materials at different depths could be conveniently identified,
isolated and evaluated. In an analytical air cathode with the total hG mass loading of 7 mg/cm2
and one Al mesh insertion, the layered architecture from top (air side) to bottom (separator/Li
side) was: SS wire mesh || 3.5mg/cm2 hG || Al mesh || 3.5 mg/cm2 hG || Al mesh. After full
discharge (~21 mAh/cm2), the volume of the hG air electrode expanded to form a thick layer
outside the top SS wire mesh in the wave spring-induced space between the electrode and the
perforated cathode cap. Therefore, the postmortem analysis of the cathode exhibited three distinct
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discharge product layers (Figure 3a). The materials in each layer were thus facilely collected and
analyzed.
Figure 3. (a) Schematics of the architecture of a fully discharged “analytical” hG air cathode
with a SS wire mesh support, an Al mesh insertion to separate the middle and the bottom layers,
and an Al mesh to allow facile separation of the electrode from the glass fiber membrane
separator. (b) XRD and (c) FT-IR results of the three layers from a fully discharged hG air
cathode with mA of 7 mg/cm2 and a similar architecture to (a). SEM results of the top, middle,
and bottom layers of the same electrode are shown in (d), (e), and (f), respectively.
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Results from XRD patterns shown in Figure 3b suggested the discharge product
contained mostly Li2O2 (ICDD# 09-0355), but both LiOH (ICDD# 85-1064) and Li2CO3 (ICDD#
22-1141) were also present. The corresponding FT-IR spectra shown in Figure 3c are consistent
with the above findings, where the peaks for Li2O2 (~490 cm-1), LiOH (~3700 cm-1), and Li2CO3
(~1400 cm-1) all appeared. In addition, lithium carboxylates (RCOOLi, R- = H- or CH3-, ~1600
cm-1) were also found. Both XRD and FT-IR data showed higher signal intensities of various
discharge products in the top layer than those in the middle layer, while signal intensities from
the bottom layer were comparably much weaker. Consistently, SEM results showed that the top
layer contained a large amount of toroid-like discs that were of ~1 µm in diameter and ~400 nm
in peripheral thickness (Figure 3d). Toroid-like discs are typical morphology for Li2O2 discharge
products in Li-O2 batteries.1-3,49 The toroids were also populated in the middle layer with similar
diameters but somewhat thinner in thicknesses (~300 nm) (Figure 3e). In the bottom layer,
however, the hG sheets appeared to be mostly free from deposits, however some small toroids
(400 – 800 nm in diameter, less than 200 nm in thickness) were occasionally found (Figure 3f).
The combination of battery performance and analytical data therefore conclusively
supported the mechanism that the initial discharge products preferentially formed in the top layer
that was in the closest proximity to O2 atmosphere, and then gradually populated the middle
layer. With the progress of discharge, the oxygen concentration in the bottom layer became
critically low due to the blocking of the mass transport from the top and middle layer. When no
more discharge products could be effectively formed, the cell failed.
The amount of cathode materials that were effectively utilized from the above layer-by-
layer analyses (combined for ~4 mg/cm2) was consistent with the results deducted from the
battery performance with various air cathode mass loadings (Figure 2c). The “fully” discharged
ultrathick hG electrodes (mA ~ 10 mg/cm2 shown in Figure 2b & c) were likely not saturated with
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discharge products, but exhibited a concentration gradient that gradually decreased from the air
side through the electrode depth. The air side had the most product accumulation, while the
separator/Li side was minimally utilized (Figure 2d). In thin electrodes with mA of 5 mg/cm2 or
less, all electrode materials were effectively used under the same current density (0.2 mA/cm2).
For high capacity Li-O2 batteries, the amount of Li anode used can become more relevant.
The weight of Li chips used in this work was ~40 mg, which corresponded to ~90 mAh/cm2, a
safe excess in comparison to the highest areal capacity achieved by the ultrathick hG air cathodes
(~40 mAh/cm2). To confirm that the battery failure was indeed due to the blocking of air
electrodes and not over-usage of Li, a fully discharged cell (mA ~ 10 mg/cm2, Im = 0.2 mA/cm2,
CA ~ 40 mAh/cm2) was disassembled. The hG air electrode was removed and assembled into a
new cell with a fresh Li chip. As shown in Figure S2 in Supporting Information, the re-
assembled cell exhibited a very small (< 3 mAh/cm2) full discharge capacity before failure,
confirming the above suggested hypothesis.
A possible solution to alleviate the blocking of oxygen diffusion channels is to prepare a
more porous air electrode. To achieve this, an hG electrode was dry pressed at a lower pressure
(60 MPa) under otherwise identical conditions. The stacking of hG sheets was less densified, as
suggested by the reduced Barrett–Joyner–Halenda (BJH) pore volume (0.673 vs. 0.544 cm3/g)
measured by nitrogen adsorption-desorption experiments despite very similar Brunauer–Emmett–
Teller (BET) surface area (309 vs. 304 m2/g). Less densified hG sheet stacking could thus allow
mass transport deeper into the electrode thickness for better electrode utilization. Indeed, at mA of
5.3 mg/cm2, the air electrode prepared at 60 MPa exhibited a full areal capacity of ~40 mAh/cm2
(specific capacity ~7450 mAh/g), ~14% higher than the air electrode prepared at 200 MPa with
similar mA (Figure S3 in Supporting Information). More in-depth studies are needed to correlate
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the porosity and optimized electrode utilization. Most hG air electrodes in this work were still
prepared with 200 MPa of compression pressure.
2.3. Discharge Products
A useful method to confirm the main discharge product was through the use of
galvanostatic intermittent titration technique (GITT).7,50 In a typical GITT experiment, a cell that
is being discharged is intermittently relaxed with no current flow for a prolonged period of time
to allow the cell potential to gradually reach the equilibrium potential corresponding to the main
discharge product. Li-O2 cells with dry-pressed hG air cathodes with mA of both 5 and 10 mg/cm2
were subjected to GITT experiments, in which the cells were discharged for 5 h and relaxed for 5
h, and the process was repeated multiple times. As shown in Figure S4 in the Supporting
Information, the relaxed cell potentials with hG air electrodes with mA of both 5 and 10 mg/cm2
were ~2.96 V, thus confirming that the main discharge product was indeed Li2O2.
A major side product found in discharged hG air cathodes was LiOH, which has been
attributed to trace moisture present in the system (see Figure S5 and discussions in the
Supporting Information).51 There were also small amounts of Li2CO3 and RCOOLi identified in
the discharge product. A common source for these side products in Li-O2 cells was from
electrolyte decomposition.3 Minor oxidative corrosion of the high mass loading hG cathodes
might have also occurred during ORR, but the majority of the cathode materials were stable.
2.4. Charge and Cycling Properties
Reversing the current flow after discharge resulted in the charging of the Li-O2 cells,
during which the discharge products decomposed to restore the battery, in an ideal case, to the
original state via OER reactions. Our previous report46 showed that hG materials without doping
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or additional metallic catalysts are reasonable (although improvable) OER catalysts in Li-O2
batteries with low cathode mass loading (mA < 0.5 mg/cm2). With high mass loading dry pressed
hG air cathodes (mA > 5 mg/cm2), the Li-O2 cells became unstable when the charge Colombic
efficiency reached ~70%. To investigate a fully charged cell, the discharge capacity was thus
limited to 20 mAh/cm2 (100 h at 0.2 mA/cm2), and the cell with mA of 10 mg/cm2 was able to
charge at near 100% efficiency with a charge-discharge overpotential (ΔVC/2; measured at half
discharge capacity) of ~1.2 V (Figure 4a). The voltage instability at the end of charge was likely
a combined effect from electrolyte exhaustion and deep stripping-plating of Li metal anode. FT-
IR results (Figure 4b) showed that most discharge products (Li2O2, LiOH, RCOOLi, and most
Li2CO3) were removed after charging. Low magnification SEM imaging (Figure 4c) confirmed
the hG sheets in the charged electrode were free from large discharge products such as Li2O2
toroids. At higher magnification (Figure 4d), the individual hG sheet surfaces appeared clean
and largely free from deposits, very similar to the morphology of pristine hG sheets (Figure 1a).
The surface holes of hG sheets were readily visible; no obvious diameter enlargement was found.
These results strongly suggested that the dry pressed hG air electrodes are stable and an excellent
platform for rechargeable Li-O2 cells.
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Figure 4. (a) Voltage profiles and (b) the corresponding FT-IR spectra of (i) a fully discharged
cell and (ii) a fully charged cell after a 100 h discharge-100 h charge cycle both with hG air
electrodes with mA of 10 mg/cm2 at 0.2 mA/cm2. (c) and (d) are SEM images at different
magnifications of the above charged hG air electrode.
The cycling performance of Li-O2 cells with dry-pressed hG air cathodes was further
investigated at curtailing capacities, with discharge and charge potential limits set at 2.0 and 4.5
V, respectively. As shown in Figure 5a and b, when the curtailing areal capacity was set at 2
mAh/cm2 (projected lower limit of practical EV batteries)1-3 at 0.2 mA/cm2, the cycling of a Li-
O2 cell with a hG air cathodes with mA of 5 mg/cm2 could persist 15 cycles with no reduction in
the curtailing discharge capacity. However, the charging started to reach the upper voltage limit
as early as the 7th cycle, with the efficiency deteriorating after 13 cycles. In comparison, the
cyclability of a hG air cathode with mA of 10 mg/cm2 was much enhanced under the same
experimental conditions. As shown in Figure 5c and d, the voltage profiles of the higher mass
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loading electrode was relatively stable for nearly 20 cycles. At cycle 20, the discharge and charge
areal capacities for the air cathode with mA of 5 mg/cm2 significantly dropped to 1.2 and 0.8
mAh/cm2, respectively, while both of those maintained at 1.7 mAh/cm2 when using the higher
mass loading electrode (mA of 10 mg/cm2). The overpotential comparison of the two cells
exhibited a similar trend (Figure S6 in the Supporting Information). The overpotential values
(measured at 1 mAh/cm2) for the 10 mg/cm2 electrode were slightly smaller than those for the 5
mg/cm2 one for the first 10 cycles (1.45-1.55 vs. 1.50-1.65 V). The former remained relatively
stable for 20 cycles, while the latter significantly deteriorated after 13 cycles. In the literature,
there were reports on high cycle numbers (50 or more) for Li-O2 batteries with undoped
graphene-based air cathodes, but they were usually obtained with a low curtailing capacity of
0.25 mAh/cm2 or less at an areal mass loading not exceeding 0.5 mg/cm2 (Table S3 in the
Supporting Information).
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Figure 5. (a,c) Voltage profiles and (b,d) the corresponding cycling dependence of discharge (D)
and charge (C) areal capacities and Colombic efficiency of Li-O2 cells with dry pressed hG air
electrodes. The mA values were (a,b) 5 and (c,d) 10 mg/cm2, respectively. The current density
was 0.2 mA/cm2 and the curtailing capacity was 2 mAh/cm2.
Several factors might have contributed to the enhanced cyclability of the cells with higher
mass loading cathodes. First, at the same areal current density IA, the higher mass loading
cathodes experienced lower gravimetric current density Im, which is more favorable for stable
discharge and charge events. Second, with total areal capacity kept the same (2 mAh/cm2), the
same amount of discharge products were distributed in a larger architecture in the higher mass
loading cathodes (despite the preferential location near the air side; see below). Repeated
filling/emptying the pores during cycling thus had less of a mechanical effect on the electrode
structure.
20
In order to investigate the cell failure mechanism, similar cycling experiments were
stopped at different reaction stages, and the postmortem analyses of the air cathodes were
conducted. Because the total capacity of the thick (~5 mg/cm2) and ultrathick (~10 mg/cm2)
electrodes could be as high as ~40 mAh/cm2, the above depth of cycling was only ~5% of the
total capacity. By analyzing the top (air side) and bottom (separator/lithium side) of the
electrodes at the end of discharge during cycling (Figure S7 in the Supporting Information), it
was confirmed that most discharge products formed on the air side of the air electrode facing the
perforated opening of the cathode cap. For convenience of discussion, all data shown are results
from specimens carefully isolated from the top layer of the electrodes. FT-IR and SEM were
chosen as the representative spectroscopic and microscopic tools, respectively, for the product
characterization.
With a shallow discharge, the amount of the battery reaction products formed were much
less than those from deep discharge shown previously in Figure 3. For example, an hG air
electrode with mA ~ 5 mg/cm2 after the first discharge of 2 mAh/cm2 at 0.2 mA/cm2 exhibited
weak FT-IR signals of Li2O2 and LiOH with very small amount of Li2CO3 (Figure 6a).
Consistently, the SEM image of the electrode material showed Li2O2 toroids of ~1 µm in
diameter and ~200 nm in thickness decorating the surfaces of the hG sheets (Figure 6b). After
the subsequent 10 h charge to complete the first cycle, all FT-IR product peaks became much
weaker (Figure 6a), and the hG sheet surface appeared free from coating of discharge products
(Figure 6c). While both Li2O2 and LiOH were still observed in the FT-IR spectra of the
discharge products from the 3rd and 9th cycle, a significant difference is that the signals from
Li2CO3 and RCOOLi became much stronger. The shapes of the discharge products were less
defined with the presence of some amorphous coating (Figure 6d). FT-IR of the charged
products from later cycles (e.g., 10th charge) showed the presence of Li2CO3 and RCOOLi
21
byproducts, indicating incomplete removal of some of the discharge products (Figure 6a).
Consistently, the hG sheet surfaces exhibited an amorphous coating most likely from the
byproduct residue (Figure 6e). The hG sheet integrity during cycling experiments was
investigated using Raman spectroscopy. As shown in Figure S8 in the Supporting Information,
the D/G ratios and the peak widths for the cathode materials at different stages of cycling were
very similar, suggesting that there was no significant degradation of hG sheets in the air
electrodes under current cycling conditions.
Figure 6. Characterization of dry-pressed hG air electrodes after cycling (mA ~ 5 mg/cm2; IA =
0.2 mA/cm2): (a) FT-IR spectra for products (from bottom to top) after 1st discharge, 1st charge,
3rd discharge, 3rd charge, 9th discharge, and 10th charge. SEM images were shown for products
after (b) 1st discharge, (c) 1st charge, (d) 9th discharge and (e) 10th discharge.
The above results strongly suggested that the cell failure after multiple charge/discharge
cycles was arising from the deactivation of the catalytic centers on the hG sheet surfaces at the air
side by the accumulation of less reversible discharge products such as Li2CO3. 52 , 53 The
22
deactivated hG sheets on the top electrode surface, although only consisting of a small amount of
total electrode material, likely prevented the mass transport into deeper electrode architecture
after a number of cycles. It is therefore imperative to incorporate more active OER catalysts to
reduce the charge overpotential and alleviate such blocking, thereby further improving the
cycling performance of these ultrathick hG air cathode platforms.
3. Conclusions
The unique dry compressibility of hG enabled the facile preparation of high mass loading
electrodes under solvent-free and binder-free conditions. These electrodes were directly used as
air cathodes for Li-O2 batteries that exhibited unprecedented ultrahigh areal capacity values (as
high as ~40 mAh/cm2) ever reported for carbon-only electrodes under deep discharge conditions.
The battery reactions were reversible, rendering excellent cyclability even at a high curtailing
capacity (2 mAh/cm2). Characterizations of postmortem electrodes showed that the discharge
products preferentially accumulated first on the air side and exhibited a gradient into the thick
electrode architecture. Under deep discharge conditions, the total areal capacity saturated at a
medium mass loading (~ 5 mg/cm2 under current experimental conditions) when the discharge
product accumulation at the air side blocked the further oxygen entry. The failure mode for
battery cycling was more likely due to the deactivation of catalytic centers from the accumulated
coating of less reversible discharge products such as Li2CO3 over multiple cycles. Ultrathick
electrodes (e.g., mass loadings of 10 mg/cm2) were found advantageous in improved cyclability,
most likely because of the reduced gravimetric current density and better mechanical support
during cycling.
This work was only focused on the hG air cathode platform itself. It is highly anticipated
that further optimization of cathode material (e.g. heteroatomic doping onto hG structure and
23
catalytic decoration onto hG surface), cathode architecture (e.g. precise catalyst placement), and
electrolyte (e.g. the use of a redox mediator in combination with a stable electrolyte) will further
improve the Li-O2 battery performance. Research in these various directions are currently
underway in our laboratories.
4. Methods
hG Synthesis. In a typical experiment, the starting graphene powder (~1.5 g; Vorbeck
Materials; Vor-X; grade: reduced 070; lot: BK-77x) was placed in a quartz boat and heated to
430 °C in static air with an open-ended tube furnace (MTI Corporation OTF-1200X-80-II) at a
ramp rate of 10 °C/min and held isothermally for 10 h. The final hG product (~70% yield) was
directly collected after cooling down the reaction.
hG Air Electrode Preparation. hG air electrodes were all prepared using a facile solvent-
free compression approach. In a typical experiment, a piece of precisely cut (15 mm in diameter)
separation layer (i.e., a piece of SS woven wire mesh, Al perforated mesh, or Celgard membrane;
all obtained from MTI Corporation) was first loaded in a stainless steel pressing die with an inner
diameter of 14.85 mm (MTI Corporation; Model EQ-Die-15D), followed by a measured amount
of hG powder (~3 – 20 mg in this work), and then another piece of separation layer. The die was
then placed in a hydraulic press (Carver Hydraulic Unit Model #3925) and a load of ~8000 lbs
(equivalent to ~200 MPa) was applied. After 15 minutes, the die was unloaded, and the pellet
was removed from the die. Electrodes prepared with Celgard membrane separation layers were
made freestanding by gently peeling off both plastic discs. Those with SS woven wire meshes or
Al perforated meshes were used as fabricated.
Multilayer “Analytical” Air Cathodes. To enable facile isolation of electrode materials at
different depths with minimal added physical and electrical impedance, one or more extra
24
perforated Al mesh insertions were used. In a typical preparation procedure to prepare an
analytical air cathode with one Al mesh (bottom piece; for facile separation of the wet electrode
material from the glass fiber membrane separator), half of the hG electrode material, another Al
mesh (insertion), the other half of the hG electrode material, and finally a SS wire mesh (top
current collector and structural support) were sequentially loaded into the same pressing die (MTI
Corporation; Model EQ-Die-15D) and subjected to hydraulic compression under the same
conditions as the regular air cathodes. The resultant disc was one robust piece that was directly
used as the cathode in a Li-O2 battery following the typical assembling procedure (see below).
Li-O2 Battery Assembly. The Li-O2 batteries were assembled in the format of CR2032
coin cells in an Ar-filled glove box. 1 M lithium bis(trifluoromethane)sulfonimide (LiTFSI; Sigma-
Aldrich, 99.95%) in tetraethylene glycol dimethyl ether (TEGDME; Sigma-Aldrich, ≥99%) was
used as the electrolyte. A “bottom-to-top” assembly process was used to avoid losing cathode
materials through the perforated cathode cap. In such a process, a piece of precisely cut copper
foil (15 mm in diameter; as the anode current collector) and a Li chip (15.6 mm in diameter; 0.45
mm in thickness; as the anode; MTI Corporation, EQ-Lib-LiC45) were placed in a SS anode cap
with a built-in isolation ring (MTI Corporation), followed by addition of 90 µL of the electrolyte.
A piece of glass fiber membrane (19 mm in diameter; as the separator) was then placed on top of
the wet Li chip to cover the entire anode cap opening. Another 90 µL of electrolyte was added to
wet the membrane. The hG air electrode was then centered on top of the membrane, followed by
the addition of 90 – 120 µL of electrolyte to completely wet the electrode. After placing a SS
wave spring (MTI Corporation) on top of the wet air electrode, a perforated cathode cap (MTI
Corporation) was carefully placed on the very top and gently pushed down to encase the anode
cap. The entire assembly was placed in a hydraulic crimping device (MTI Model MSK-110) and
crimped at a pressure of ~800 psi into a functional coin cell battery.
25
Li-O2 Battery Testing. The Li-O2 coin cells were tested in a custom made air-tight box
with continuous flow (30 – 100 mL/min) of pure oxygen using either a multi-channel battery
analyzer (MTI Corporation; Model BST8-MA) or a multi-channel electrochemical station (Bio-
Logic; Model VMP3). The discharge and charge voltage limits were set at 2.0 and 4.5 V,
respectively. Areal current densities of 0.1, 0.2, and 0.5 mA/cm2 were used for the deep
discharge experiments, while 0.2 mA/cm2 was used for the cycling experiments with 10 h each
for discharge and charge events, respectively. The areal current density values were calculated
based on the area of the air electrode (1.73 cm2). Therefore, the actual current used for 0.1, 0.2,
and 0.5 mA/cm2 were 0.173, 0.346, and 0.865 mA, respectively.54
Other Measurements. Scanning electron microscopy (SEM) images were acquired using a
Hitachi S-5200 field emission SEM (FE-SEM) system at an acceleration voltage of 30 kV. A
Rigaku SmartLab X-ray Diffractometer with a Cu Kα radiation source was employed for X-ray
diffraction (XRD) analyses. FT-IR measurements were conducted on a Thermo Scientific Nicolet
iS5 FT-IR spectrometer equipped with an iD7 ATR diamond optic accessory. Raman spectra
were acquired with an excitation wavelength of 532 nm on a Thermo-Nicolet-Almega Dispersive
Raman Spectrometer. Nitrogen adsorption-desorption isotherms and the corresponding BET
surface area values and BJH pore characteristics were acquired on a Quantachrome Nova 2200e
Surface Area and Pore Size Analyzer system using a 9-mm bulbless cell.
Acknowledgements. Financial support from the NASA Langley Internal Research and
Development (IRAD) Program is gratefully acknowledged. We also thank Hoa Luong and Joel
Alexa for experimental assistance. C.M.M., B.M., E.D.W. and S.D.L. (partially) were NASA
Interns, Fellows, and Scholars (NIFS) Program interns supported by IRAD.
26
Supporting Information Available. This material is free of charge from the journal website or
from the authors.
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S1
--Supporting Information—
Ultrahigh Capacity Lithium-Oxygen Batteries Enabled by
Dry-Pressed Holey Graphene Air Cathodes
Yi Lin,1,* Brandon Moitoso,2 Chalynette Martinez-Martinez,2 Evan D. Walsh,2 Steven D.
Lacey,2,3 Jae-Woo Kim,1 Liming Dai,4,* Liangbing Hu,3,* and John W. Connell5,*
1National Institute of Aerospace, 100 Exploration Way, Hampton, Virginia 23666-6147; 2NASA Interns,
Fellows, and Scholars (NIFS) Program, NASA Langley Research Center, Hampton, Virginia 23681; 3Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742; 4Department of Macromolecular Science and Engineering, Case School of Engineering, Case Western
Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106; and 5Mail Stop 226, Advanced
Materials and Processing Branch, NASA Langley Research Center, Hampton, Virginia 23681-2199.
*To whom correspondence should be addressed:
Y.L.: [email protected]; L.D.: [email protected];
L.H.: [email protected]; J.W.C.: [email protected]
S2
Figure S1. (a,b) Typical SEM images of dry-pressed hG air cathodes with perforated Al mesh
supports. (c) Full discharge voltage profile comparison for Li-O2 batteries with dry-pressed hG
air cathodes with SS woven mesh support, with Al mesh support, and with no support and thus
freestanding. The mass loadings were all ~10 mg/cm2; IA = 0.2 mA/cm2.
S3
Figure S2. Full discharge profile (black) of a typical Li-O2 cell with a dry-pressed hG air
cathode with mA ~ 5 mg/cm2 at IA = 0.2 mA/cm2. The cell was then dissembled, and the
discharged air electrode was incorporated into a new cell with a fresh Li chip, a new glass fiber
membrane separator and electrolyte. The voltage profile for the reassembled cell at the same
current density is shown in red.
S4
Figure S3. Full discharge profiles at IA = 0.2 mA/cm2 of two Li-O2 cells with dry-pressed hG air
cathodes (mA ~ 10 mg/cm2 for both) prepared under compression pressures of 200 (black, solid)
and 60 MPa (green, dashed), respectively.
S5
Figure S4. GITT experiments (5 h discharge, 5 h relax) of two Li-O2 cells with dry-pressed hG
electrode mass loading of (a) 5 mg/cm2 and (b) 10 mg/cm2, respectively, at IA = 0.2 mA/cm2.
S6
Figure S5. XRD patterns of Li-O2 cells with (top) a carefully dried dry-pressed hG air cathode in
a well-sealed oxygen chamber and (bottom) a dry-pressed hG air cathode in an oxygen chamber
with moisture contamination. The mass loading in both cases were 5 mg/cm2; the current density
was 0.2 mA/cm2. Although no intensive drying procedure was applied to the electrolyte solution
used in this work, fresh lithium chips immersed in the electrolyte but stored in an Ar-filled
glovebox retained their shiny surface for over three months, indicating that no moisture is
present. Thus, the moisture that resulted in LiOH formation was unlikely from the electrolyte.
The data shown in Figure S5 above therefore suggested the moisture contamination in the
oxygen chamber was the main cause for the LiOH formation.
S7
Figure S6. Overpotential comparison for cycled Li-O2 cells with mass loadings of 5 and 10
mg/cm2. The discharge and charge voltages were both measured at 1 mAh/cm2.
S8
Figure S7. FT-IR spectra of the top (red) and bottom (blue) sides of a dry-pressed hG air
electrodes after 3rd discharge (mA ~ 5 mg/cm2; IA = 0.2 mA/cm2), showing that the majority of
the discharge products formed on the top.
S9
Figure S8. Raman spectra for a hG air cathode after various discharge and charge cycles: (from
bottom to top) 1st Discharge, 1st Charge, 3rd Discharge, 3rd Charge, 9th Discharge, and 10th
Charge.
S10
Table S1. Full discharge characteristics of Li-O2 cells with dry-pressed hG cathodes of various
mass loadings.
Mass Loading (mg/cm2)
Specific Capacity (mAh/g) Areal Capacity (mAh/cm2) Specific Energy (kWh/kg)
0.1 mA/cm2
0.2 mA/cm2
0.5 mA/cm2
0.1 mA/cm2
0.2 mA/cm2
0.5 mA/cm2
0.1 mA/cm2
0.2 mA/cm2
0.5 mA/cm2
2 3819 6.2 9.5
5 7667 6540 2824 37.3 33.1 14.4 20.8 16.8 6.6
10 3737 3529 545 41.1 37.1 5.8 10.1 9.1 1.3
S11
Table S2. Full discharge properties of previously reported Li-O2 batteries with graphene-based (no doping or additional catalyst) air
cathodes in comparison to this work. Underlined numbers were calculated or estimated based on available data in the respective
reports.
Material Electrode
Preparation
Mass Loading (mg/cm2)
Electrolyte
Current Density Capacity Publication
Yearref Gravimetric (mA/g)
Areal (mA/cm2)
Gravimetric (mAh/g)
Areal (mAh/cm2)
Slurry or Slurry-like Approaches
Thermally Expanded Graphene
Slurry 2.1 1M LiTFSI in TEGDME 48 0.1 15000
(at 2 atm O2) 31
(at 2 atm O2) 20111
Graphene Sol-gel Slurry 0.8 mg
(total weight) 1M LiTFSI in DMSO
~62.5 0.05 10600 ~8.5 20122
~125 0.1 7200 ~5.8
Porous Graphene Slurry 0.5 0.1M LiClO4 in DMSO 200 0.1 29375 14.7
20143 2000 1 6187 3.1
rGO Slurry Not Specified 1M LiTFSI in TEGDME 50 5140 20144
Thermally Reduced Graphene
Slurry Not Specified 1M LiTFSI in TEGDME 200 8000 20155
Graphene with Mesoporous Carbon
Slurry 1 1M LiTFSI in TEGDME 100 0.1 9000 9 20156
rGO Slurry Not Specified 1M LiTFSI in TEGDME 200 9535 20157
rGO
Slurry 0.25 0.1M LiClO4 in DMSO 300 0.075
1780 0.45
20158 Vacuum-promoted, thermal-expanded graphene
19800 5
rGO Slurry 0.3 1M LiTFSI in TEGDME 100 0.03 10000 3 20169
rGO Slurry 0.8-1 1M LiClO4 in DMSO 100 ~0.1 3200 ~3.2 201610
rGO Slurry 0.3-0.5 mg
(total weight) 1M LiTFSI in TEGDME 200 ~0.1 9535 ~4.8 201611
rGO Slurry 0.1 1M LiCF3SO3 in DME 1000 0.1 30000 3 201612
S12
Table S2 (continued)
Material Electrode
Preparation
Mass Loading (mg/cm2)
Electrolyte Current Density Capacity
Publication Yearref Gravimetric
(mA/g) Areal
(mA/cm2) Gravimetric
(mAh/g) Areal
(mAh/cm2)
Binder-Free Electrodes
GNP/GO Paper Vacuum filtration
0.45 1M LiNO3 in DMAc 200 0.09 6910 3.1 201513
Graphene Aerogel
Aerogel templated with
polystyrene nanoparticles
0.83 1M LiPF6 in TEGDME 200 0.17 21507 17.9 201514
Porous Graphene Hydrothermal self-assembly
2 1M LiCF3SO3 in
TEGDME 100 0.2 13000 26 201515
Vertically aligned carbon nanosheets
Solution growth
Not Specified 0.5M LiCF3SO3 in
TEGDME 75 -- 1248 -- 201616
CVD Graphene on Ni CVD growth 0.6 1M LiClO4 in TEGDME 200 0.12 89 0.05 201617
Vertically grown graphene sheets
CVD growth 0.4 1M LiNO3 in DMSO 200 0.08 14461 5.8 201618
Graphene Aerogel Hydrothermal self-assembly
Not Specified 1M LiTFSI in TEGDME -- 0.05 4142 -- 201619
3-Dimensional Graphene Gel
Hydrothermal self-assembly
0.8 mg 1M LiCF3SO3 in
TEGDME 50 -- 2250 -- 201620
Porous graphene paper
Vaccum filtration with nanoparticle templates
followed by annealing
Not Specified 1M LiNO3 in DMAc 200 -- 10176 -- 201621
Graphene Foam Hydrogel self-
assembly 0.0375 1M LiTFSI in TEGDME 100 0.004 3120 0.12 201622
Graphene Foam Hydrogel self-assembled on
Al foam < 1.3 1M LiClO4 in DMSO 100 < 0.13 > 24000 31.5 201623
Holey Graphene Dry
Compression
5
1M LiTFSI in TEGDME
20 0.1 7667 37.3
This Work 40 0.2 6540 33.1
10 10 0.1 3737 41.1
20 0.2 3529 37.1
S13
Table S3. Cycling properties of previously reported Li-O2 batteries with graphene-based air cathodes (no doping or additional
catalyst) in comparison to this work. Underlined numbers were calculated based on available data in the respective reports.
Material Electrode
Preparation
Mass Loading (mg/cm2)
Electrolyte
Current Density Curtailing Capacity (mAh/g) Number of
Cycles
Publication Yearref Gravimetric
(mA/g) Areal
(mA/cm2) Gravimetric
(mAh/g) Areal
(mAh/cm2)
Slurry or Slurry-Like Approaches
Graphene Sol-gel Slurry 0.8 mg
(total weight) 1M LiTFSI in DMSO ~125 0.1
1000
~0.8 10 20122
Porous Graphene Slurry Not Specified 0.1M LiClO4 in DMSO 200 -- 1000 -- >40 201424
Porous Graphene Slurry 0.5 0.1M LiClO4 in DMSO 200 0.1 500 0.25 >20 20143
rGO Slurry 0.02 0.25M LiTFSI
- 0.05M LiI in DME 5000 0.1 5000 0.1 >300 201525
Graphene with Mesoporous Carbon
Slurry 1 1M LiTFSI in TEGDME 100 0.1 1000 1 28 20156
Vacuum-promoted, thermal-expanded graphene
Slurry 0.25 0.1M LiClO4 in DMSO 1000 0.25 1000 0.25 >50 20158
rGO Slurry 0.3-0.5 mg
(total weight) 1M LiTFSI in TEGDME 200 ~0.1 600 ~0.3 ~20 201611
rGO Slurry 0.1 1M LiCF3SO3 in DME 500 0.05 1000 0.1 100 201612
S14
Table S3 (continued)
Material Electrode
Preparation
Mass Loading (mg/cm2)
Electrolyte Current Density Curtailing Capacity (mAh/g) Number
of Cycles
Publication Yearref Gravimetric
(mA/g) Areal
(mA/cm2) Gravimetric
(mAh/g) Areal
(mAh/cm2)
Binder-Free Electrodes
Graphene Foam
Electro-chemically exfoliated
graphite paper
Not Specified 1 M LiCF3SO3 in
TEGDME 100-300 -- 1000 -- >20 201326
GNP/GO Paper Vacuum filtration
0.45 1M LiNO3 in DMAc 200 0.09 1000 0.45 16 201513
Graphene Aerogel
Aerogel templated with
polystyrene nanoparticles
0.83 1M LiPF6 in TEGDME 200 0.17 1000 0.83 42 201514
Porous Graphene Hydrothermal self-assembly
2 1M LiCF3SO3 in
TEGDME 200 0.4 1000 2 <18 201515
Graphene Aerogel Hydrothermal self-assembly
Not Specified 1M LiTFSI in TEGDME 0.1 500 -- 16 201619
3-Dimensional Graphene Gel
Hydrothermal self-assembly
0.8 mg (total weight)
1M LiCF3SO3 in TEGDME
100 -- 500 -- <10 201620
Porous graphene paper
Vaccum filtration with nanoparticle templates
followed by annealing
Not Specified 1M LiNO3 in DMAc 500 mA/g -- 1000 -- ~100 201621
Graphene Foam Hydrogel self-
assembly 0.0375
1 M LiTFSI in TEGDME.
100 mA/g 0.004 500 0.02 10 201622
Graphene Foam Hydrogel self-assembled on
Al foam < 1.3 1M LiClO4/DMSO 100 mA/g < 0.13 1000 < 1.3 ~35 201623
Holey Graphene Wet Filtration 0.2 1M LiTFSI in TEGDME 200 0.04 800 0.16 ~30 201627
Holey Graphene Dry
Compression
5 1M LiTFSI in TEGDME
40 0.2
400 2
~15 This Work
10 20 200 ~20
S15
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