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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.: yi.lin-1@nasa.gov; L.D.: lxd115@case.edu;

L.H.: binghu@umd.edu; J.W.C.: john.w.connell@nasa.gov

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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