High-Rate and High-Areal-Capacity Air Cathodes with Enhanced

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High-Rate and High Areal Capacity Air Cathodes with Enhanced Cycle Life Based on RuO2/MnO2 Bifunctional Electrocatalysts Supported on CNT for Pragmatic Li-O2 Batteries Young Joo Lee, Se Hwan Park, Su Hyun Kim, Youngmin Ko, Kisuk Kang, and Yun Jung Lee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00248 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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High-Rate and High Areal Capacity Air Cathodes

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with Enhanced Cycle Life Based on RuO2/MnO2

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Bifunctional Electrocatalysts Supported on CNT for

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Pragmatic Li-O2 Batteries

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Young Joo Lee, †, ‡ Se Hwan Park, †, ‡ Su Hyun Kim, † Youngmin Ko§, Kisuk Kang§, and Yun Jung Lee*,†.

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†

Department of Energy Engineering, Hanyang University, Seoul, 133-791, Republic of Korea

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§

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro,

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Gwanak-gu, Seoul, 151-742, Republic of Korea

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Abstract

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Despite their potential to provide high energy densities, lithium-oxygen (Li-O2) batteries are

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not yet widely used in ultrahigh energy density devices like electric vehicles, owing to various

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challenges, including poor cyclability, low efficiency, and poor rate capability, especially at high

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areal mass loading. Even the most promising Li-O2 cells are unsuitable for practical applications,

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owing to a limited areal mass loading below 1 mg cm−2, resulting in low areal capacity. Here, we

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demonstrate air cathodes of unprecedentedly high areal capacity at a high rate with sufficient

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cycle life for pragmatic operation of Li-O2 batteries. A separator-carbon nanotube (CNT)

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monolith-type cathode of massive loading is prepared to achieve high areal capacity, but the

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cycle life and round-trip efficiency of CNT only-separator monolith cathodes are limited. The

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reversible and energy-efficient operation at high areal capacity and a high rate is enabled by

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adopting RuO2/MnO2 solid catalysts on the CNT (RMCNT). RMCNTs show bifunctional

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catalytic effect in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)

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and also completely decompose LiOH and Li2CO3 by-products that may exist in discharged

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electrodes. This separator-RMCNT monolith offers beneficial features such as high mass

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loading, binder-free, intimate contact with the separator, and most importantly, catalysts for

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reversibility. Together, these features provide remarkably long cycle life at unprecedentedly high

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capacity and high rate: 315, 45, and 40 cycles, with areal capacity limits of 1.5, 3.0, and 4.5 mAh

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cm−2, respectively, at a rate of 1.5 mA cm−2. Cycling is possible even at the curtailing capacity of

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10 mAh cm−2.

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KEYWORDS: lithium-oxygen batteries, RuO2/MnO2, practical applications, monolith cathode,

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high areal capacity


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1. Introduction

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In the development of rechargeable battery-based electric vehicles for a cleaner future, one of

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the biggest hurdles has been the limited energy density of current lithium-ion batteries (LIBs),

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which has prevented sufficient driving range from being achieved.1-4 For this reason, the

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development of battery systems surpassing the performance of current LIBs, such as Li-sulfur,1,

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5-6

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these candidates, Li-air batteries represent a promising Li-based energy storage system, which

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achieve a higher energy density (about 3.5 kWh kg−1) compared with existing Li-ion batteries

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(about 500 Wh kg−1) by utilizing Li metal and oxygen (O2) as the anode and cathode reactants,

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respectively.13-15 The discharge capacities of Li-O2 batteries are primarily determined by the

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amount of the main discharge product Li2O2 in the O2-breathing cathode. Thus, the properties

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and structures of cathodes could have a major influence on the discharge capacity. With their

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huge surface areas, which can accommodate large amounts of discharge products (resulting in

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large discharge capacities), some nanocarbons such as Super P (SP), Ketjenblack (KB),16-17

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graphene or graphene oxide (GO),18-21 carbon fibers, and carbon nanotubes (CNTs)22-23 have

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been suggested as air electrode materials. Some of these light materials show significantly high

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specific capacities, of about 10000 mAh g−1.1-4, 12 Despite the high specific capacities, the active

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material loadings in previous research have mostly been below 1 mg cm−2, resulting in areal

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capacities below 10 mAh cm−2. To achieve a large, practically meaningful areal capacity, the

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loading level would have increase immensely. Recently, ultrahigh discharge capacities were

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achieved of above 30 mAh cm−2, which is 10 times higher than the discharge capacity of LIBs,

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using massive CNT sheets 24 and pressed holey graphene air cathodes.25 However, these previous

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studies did not show reversible operation of cells with high round-trip efficiency and long cycle

multivalent ion,7-9 and metal-air1, 10-12 batteries, has been an area of active research. Among


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life. The highest achieved reversible capacity was about 2 mAh cm−2, with capacity retention of

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only 10 cycles.25 Moreover, these Li-O2 cells with massive loading operated at relatively low

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rates of 0.05 mA cm−2 24 and 0.2 mA cm−2, respectively.25 A high rate capability and a high

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power performance are as important as energy density for powering, and particularly

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accelerating electric vehicles.

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The need for higher energy efficiency and reversibility of Li-O2 cells has spurred researchers

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to develop catalytic systems for the facile formation and decomposition of discharge products.26-

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28

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with diverse types of structures, have been studied as solid catalysts. As the form of soluble

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catalysts, redox mediators such as LiI,37 LiBr,38 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO),39

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tetrathiafulvalene (TTF),26 5,10-dimethylphenazine (DMPZ),40 and Fe-phthalocyanine (Fe-PC)26,

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37-39, 41

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Furthermore, even when reactions related to Li2O2 are facilitated, the formation of by-products

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such as Li2CO3 and LiOH is unavoidable in many cases, owing to side reactions,1, 42-44 and the

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poor chargeability of such by-products has been theoretically and experimentally demonstrated.43,

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45

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cycling, especially under high rates. Hence, the decomposition kinetics of these inevitable by-

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products should be addressed to achieve highly reversible Li-O2 batteries. Several reports have

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discussed the possibility of catalytic decomposition of the by-products. NiO plates on a CNT

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promoted the oxidation of carbonate and carboxylate species, leading to improved cyclability in

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Li-O2 batteries.46 The ability of RuO2-coated NiO47 and Ir/B4C48 to decompose Li2CO347-48 and

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LiOH47 has been proved. To realize the substantive potential of Li-O2 batteries, not only should

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high areal capacity be achieved, but, more importantly, long cycle life and high rate capability

Noble metals,10, 29-31 transition metals23, and transition metal oxides

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and carbides35-36,

have also been investigated to boost the formation and decomposition kinetics of Li2O2.

The accumulation of these by-products causes deterioration of reversibility of operation during


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should be simultaneously ensured with effectual catalysts by decomposing insoluble discharge

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products.

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Here, we report air cathodes of Li-O2 batteries that operate at unprecedentedly high areal

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capacity and high rates with sufficient cycle life. The reversible operation at high areal capacity

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and high rates was enabled by RuO2/MnO2 bifunctional catalysts and separator-CNT monolith-

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type electrodes of massive loading. Firstly, the operation at high capacity and a high rate was

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achieved by separator-CNT monolith-type electrodes of massive loading. The separator-CNT

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monolith electrodes were fabricated by a simple vacuum-assisted filtration method. Through this

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facile method to prepare highly loaded cathodes, we could deposit CNT, and even KB and SP,

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with a loading mass of 7–8 mg cm−2, and up to 14–15 mg cm−2 on a glass fiber separator, without

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any polymeric binder. The separator-monolith electrode allows thinner CNT layer deposition and

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better contact between the separator and the active layer than free-standing CNT sheets at the

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same mass loading. These features enabled better kinetics for higher discharge capacity at this

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high areal mass loading level. In the case of CNT with loading mass of 15 mg cm−2, the areal

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capacity of full discharge reached 102.48 mAh cm−2. This binder-free highly loaded separator-

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monolith electrode has an intimate interface between the separator and the active layer, which is

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also beneficial to high rate operation. However, the cycle life and round-trip efficiency of CNT

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only-separator monolith electrodes were limited. To ensure cyclability and energy-efficiency at

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high areal capacity and high rates, we introduced a bi-functional catalyst composed of RuO2 and

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MnO2 which are known to have OER49-50 and ORR33, 51 activities, respectively. RuO2/MnO2

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catalyst-coated CNT (RMCNT) was successfully synthesized and integrated with the separator,

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to produce a separator-RMCNT monolith. The resulting separator-RMCNT monolith electrodes

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demonstrated not only superior catalytic activity in Li2O2 formation and decomposition, but also


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perfect decomposition abilities for Li2CO3 and LiOH. The decomposition of Li2CO3 and LiOH

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was demonstrated with Li2CO3- or LiOH- preloaded electrodes. To the best of our knowledge,

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this is the first report on the quantitative assessment of the decomposability of Li2CO3 and LiOH

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by RuO2/MnO2. The separator-RMCNT monolith combines unique features for ensuring high

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areal capacity at a high rate with superior cyclability, to achieve practically meaningful

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performance of Li-O2 cells. The features include (i) high mass loading, (ii) binder-free

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construction, (iii) better contact with the separator, and most importantly, (iv) a catalyst of

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superior activity. RuO2/MnO2 facilitated complete decomposition of persistent Li2CO3 and LiOH

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by-products as well as Li2O2 during charging, enabling highly reversible operation at high

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capacity and high rates. Moreover, CNTs in electrodes are electrically well connected and the

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formed films showed good wettability with TEGDME based electrolytes so that the whole CNT

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film was utilized effectively despite the relatively thick active layer. Together, these features

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allowed us to achieve excellent cyclability with an unprecedentedly high capacity limit and high

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rate; that is, 4.5 mAh cm−2 at a rate of 1.5 mA cm−2 for 40 cycles. The highly loaded thick

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electrodes also provided new insight on the morphology control of discharge products. In the

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thick electrodes, the morphology of discharge products depended on the activity of oxygen,

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suggesting that the morphology of discharge products can be tuned by changing activity of

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oxygen and there could be a strategy to increase discharge capacities by tuning the way of

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supplying oxygen to the electrodes.

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2. Results and Discussion

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As a simple method for massive loading of active material, we fabricated separator-active

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material monolith-type air cathodes by vacuum-assisted filtration. The carbon-based active

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materials were deposited directly on a GF/D glass fiber filter that could be used as a separator in

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Li-O2 cells. The colloids of active materials were filtered through the GF/D membrane, and this

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active material integrated with the GF/D filter was used as a monolith electrode. This monolith

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type has an advantage in terms of processability compared with the previous free-standing

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sheet24 made by filtration. Figure 1a shows a comparison between the free-standing CNT paper

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(CNT paper, hereafter) produced by vacuum filtration on an anodic aluminum oxide (AAO) filter

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with a pore size of 200 nm, and the separator-CNT monolith (CNT monolith, hereafter) produced

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by the same vacuum filtration on a GF/D glass fiber separator. The intimate contact formation

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between the CNT sheet and the separator in the CNT-monolith electrode was verified by peel-off

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tests in Figure 1b. Peel-off tests were conducted with the CNT-paper electrodes assembled with

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the GF/D separator and CNT-monolith electrodes. The CNT paper was completely removed

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from the GF/D separator by 3M tape, but only few active materials were detached in the CNT-

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monolith electrode. Most CNT materials remained attached to the separator after the peel-off test

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by 3M tape. The interfaces between the CNT sheet and the GF/D were also examined after

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forced separation between them. The CNT paper and GF/D were completely separated showing a

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clean interface on each layer as in the peel-off test by 3M tape; however, after the forced

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separation of the CNT sheet and GF/D in CNT-monolith electrodes, some GF/D residues were

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found in the removed CNT sheet and also the interface of GF/D was darkened with attached

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carbon materials, indicating intimate contact formation with the GF/D separator in CNT-

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monolith electrodes. Besides CNT, we have tried to make free-standing papers and separator-

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monoliths using SP and KB carbon through a vacuum filtration method. Unlike CNT, which


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forms a neat film on AAO that is detachable as a free-standing paper (Figure S1c, Supporting

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Information; and Figure 1a), neither the SP nor the KB could be made in a free-standing form

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(Figure S1a and b). Common nanocarbon materials, such as SP and KB, which do not stack,

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cannot form a sheet by a vacuum filtration. Consequently, it is impossible to realize a free-

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standing electrode of high areal mass loading without an additional binder. On the other hand,

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filtration of carbon colloids through the GF/D glass microfiber separator allowed fabrication of a

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neat separator-carbon sheet monolith with CNT, and even with SP and KB, although a small

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crack was found on the KB surface (Figure S1d, e, and f). The areal loading masses of all

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monoliths were similar, at 7–8 mg cm−2. Owing to facile transport through the network, CNT

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outperforms nanocarbons such as SP and KB, especially at high rates.52 Hence, we focused on

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CNT in this work.

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In addition to the better processability on GF/D for forming a binder-free sheet by vacuum

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filtration, the thickness of CNT on GF/D was less than that of the free-standing CNT paper. The

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thickness of CNT on GF/D was measured at 160 and 360 µm, when the mass loadings of CNT

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were 7–8 and 14–15 mg cm−2, respectively. The thickness of free-standing CNT paper was 180

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µm and 420 µm at the corresponding CNT loading masses. This thickness difference might have

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originated from the different vacuum pressures exerted through AAO and GF/D filters, probably

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due to the different microstructures, porosity, and thickness of the two filters. Figure 1c presents

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the full galvanostatic discharge profiles of the CNT paper and CNT monolith with two different

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mass loading levels (7–8 and 14–15 mg cm−2) at a current density of 0.5 mA cm−2, with a voltage

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cutoff of 2.3 V. The discharge capacities of the CNT paper and CNT monolith were 19.83 and

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31.80 mAh cm−2, respectively, when the loading mass was 7–8 mg cm−2. The monolith type

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showed a higher discharge capacity than the CNT paper, with same mass loading. We ascribed


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this higher capacity of the separator-monolith electrodes to two factors: the reduced thickness,

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and the separator-monolith integration of the active CNT layer. The reduced thickness of highly

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massive electrodes may provide a shorter distance for mass transport and more efficient

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utilization of active materials across the entire thickness, which could lead to higher discharge

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capacities.25 The intimate integration of separator and CNT in the monolith electrodes could also

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enable better contact between separator and active materials, thereby providing facile transport

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and better kinetics. When the loading mass was doubled to 14–15 mg cm−2, the discharge

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capacities increased to 83.37 and 102.48 mAh cm−2 for the CNT paper and CNT monolith,

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respectively. Since we applied the same areal current density of 0.5 mA cm−2 to electrodes with

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different mass loadings, the electrodes with higher mass loading experienced higher specific

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current density. Lower specific currents result in higher capacity.25 Moreover, SP and KB loaded

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monolith type electrodes with the same areal loading levels were discharged at the same current

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rate to 2.3 V (Figure S2). SP and KB monolith electrodes showed typical discharge profiles and

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exhibited high areal capacities, but lower than those of CNT monolith electrodes; the discharge

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capacities of KB monolith at loading of 7-8 mg cm-2, KB monolith at loading of 14-15 mg cm-2,

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SP monolith at loading of 7-8 mg cm-2, and SP monolith at loading of 14-15 mg cm-2 are 65.8,

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52.16, 36.6, and 26.7 mAh cm-2 respectively. Considering that the typical capacity of current

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commercial Li ion batteries is in the range of 2–4 mAh cm−2,3, 53-55 these high areal loading

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electrodes exhibited practically meaningful capacities.

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Apart from the total electrode thickness, the vertical distribution of CNT might be a matter of

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interest. In our CNT-based electrodes, we assume that there is vertical non-uniformity in the

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loading density. When the loading amount increased about twice from 7-8 mg cm-2 to 14-15 mg

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cm-2, the thickness increased more than double (from 160 to 360 µm in CNT monolith and from


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180 to 420 µm in CNT paper). We thereby surmise that those electrodes have lower loading

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density in the top side (gas inlet side) presumably due to the lower filtration rate caused by the

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blockage of pores of the filtration filter with the deposited materials at the bottom of electrode

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(separator side). However, the effect of vertical uniformity in loading density on the cathode

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performance is not verified yet in Li-O2 batteries. The exact assessment of the effect of vertical

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uniformity needs further extensive study including controls over filtration speed and this could

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be our next study. In the massively loaded electrodes, the utilization efficiency throughout the

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whole electrode thickness is also important. Generally, the areal capacity has a trade-off relation

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with the specific capacity partly because of the possibility of reduced active material utilization

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efficiency across the entire thickness in thicker cathodes. To evaluate the utilization efficiency of

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CNTs across the whole thickness in our CNT based electrodes, we have examined the vertical

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distribution of discharge products from top to bottom of air cathodes after discharge (Figure S3).

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The cell configuration is described in Figure S3a. In Figure S3b, the discharged CNT-separator

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electrode was analyzed in SEM at three locations, top (contact with O2 flow), middle, and bottom

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(separator and Li anode side). First, the amounts of discharge products were different in three

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vertical locations: the top layer showed the largest amount of discharge products compared to the

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middle and bottom side of electrode. Moreover, the changes in the amount accompanied by the

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morphological changes in the discharge products. Thin disc-like products with the size of about

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200 nm and film-like products co-existed in the top, while there were only a few toroidal Li2O2

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at the middle and the film-like products were fully dominant at the bottom side. It is clear that

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the entire electrode participated in discharging process, and therefore high efficiency of cathode

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utilization would be expected. Furthermore, excellent electrolyte wettability into our electrodes

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was verified by measuring the mass of absorbed electrolyte in CNT films of different mass


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loadings. In Figure S4, the amount of electrolyte absorption into CNT increased linearly with the

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loading mass of the CNT-based electrode.

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discharge products at the top of the electrodes indicate excellent wettability and complete

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impregnation of TEGDME based electrolyte into the CNT film toward.

This linear relationship and the formation of

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From the vertical distribution of the discharge products, more discharge products were formed

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in the gas inlet side rather than in the electrolyte (separator) side. Therefore, it appears that O2

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migration kinetics is the determining factor for forming discharge products in the air cathodes.

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More importantly, toroidal discharge products were preferentially formed in the gas inlet side,

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rather than in the electrolyte side. Toroids are known to be formed through a solution mediated

10

deposition. In the bottom of the electrodes, the amount of soluble O2 or O2- radical is relatively

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lower than in the gas inlet side. The vertical morphology distribution of the discharge products

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in this thick electrode thus implies that the morphology of the discharge products not only

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depends on the property of electrolyte (donor number), but also on the activity of oxygen.

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Oxygen pressure changes presumably accompany the morphological changes of the discharge

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product. This dependence become apparent in our thick electrodes. From the vertical distribution

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of discharge products, a new way of increasing discharge capacity may be suggested: increase

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the amount of discharge products at the bottom side by supplying more oxygen to the bottom

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side, which could be presumably achieved by oxygen bubbling through the electrolyte.

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In Figure S5, the CNT paper and CNT monolith were compared with CNT electrodes

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fabricated by conventional slurry casting methods. A CNT slurry consisting of 20%

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polytetrafluoroethylene (PTFE) binder was casted on a carbon gas diffusion layer (GDL) and Ni

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foam. The four different types of electrode (CNT GDL, CNT Ni foam, CNT paper, and CNT

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monolith) were discharged to 2.3 V with a current rate of 100 mA g−1. The mass loading of these


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casted electrodes was around 1 mg cm−2. Despite the massive loading of the CNT paper, its

2

specific capacity was similar to those of the casted electrodes with low mass loading. This could

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be explained by the better porous network compared with the casted electrodes, and the lack of

4

detrimental contact with the current collector for vacuum-filtered CNT paper. The CNT monolith

5

exhibited the largest specific capacity (Figure S5a, Supporting Information), which was

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calculated based on the mass of active carbon materials only. If the mass of the binder and

7

current collector are included in the total mass of the electrode, the difference becomes even

8

greater. With the heavy current collectors of Ni foam (40 mg cm−2) and GDL (9 mg cm−2), the

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specific capacities of casted electrodes dropped to only 50.42 and 289.14 mAh g−1 (Figure S4b).

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In this regard, the binder-free and current collector-free CNT paper and CNT monolith have

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advantages in gravimetric terms as well as in areal capacity perspectives (Figure S5c).

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For practical applications in powering devices such as electric vehicles, the performance of Li-

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O2 batteries at high current densities is of great importance, as is large areal capacity. The

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presence of an electrically insulating binder can abate the conductivity of the electrode. It is

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highly likely that a binder-free electrode will exhibit more stable performance at a high current

16

rate than an electrode with electrical resistance caused by a polymeric binder. Figure 1d-f shows

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a comparison between the discharge capacities of a free-standing PTFE-bound CNT sheet

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(PTFE-CNT sheet) electrode (Figure 1d) and those of the binder-free CNT paper (Figure 1e) and

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CNT monolith (Figure 1f), at current densities from 0.5 to 2.0 mA cm−2. The CNT mass loading

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was 7–8 mg cm−2 for all electrodes. The PTFE-CNT sheet and CNT paper showed similar

21

discharge capacities at a rate of 0.5 mA cm−2 (19.10 and 19.83 mAh cm−2, respectively);

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however, the discharge capacity decayed much faster in the PTFE-CNT sheet as the current

23

density increased. At a current of 2.0 mA cm−2, the discharge capacity of the PTFE-CNT sheet


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was 0.80 mAh cm−2, which is only 12% that of the CNT-paper (6.5 mAh cm−2). The CNT

2

monolith showed even better rate performances, owing to the improved kinetics described above.

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We conclude here that the unique structure of the CNT-monolith is particularly beneficial for

4

high-rate operation at high areal capacity.

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In addition to high areal capacity at high rates, reasonable energy efficiency and reversibility

6

should be guaranteed in order to achieve efficient and continuous use of rechargeable Li-O2

7

batteries. Common carbon materials such as CNT, GO, KB, and SP do not have sufficient

8

oxygen evolution reaction (OER) activities for complete decomposition of the main discharge

9

product, Li2O2. Furthermore, by-products such as LiOH and Li2CO3 can be formed by water

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exposure, decomposition of organic electrolytes, and degradation of the carbon electrode.1, 42-44

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To achieve a high round-trip efficiency and long cycle life with high areal capacity at high rates,

12

we added bifunctional catalysts to the CNT and fabricated separator-catalyzed CNT monolith.

13

We synthesized RuO2/MnO2-coated CNT (RMCNT) as a solid bifunctional catalyst and filtered

14

on a GF/D glass separator, producing a separator-RMCNT monolith. The CNT and RMCNT

15

were examined by scanning electron microscopy (SEM), transmission electron microscopy

16

(TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR) (Figure

17

2). A large number of void spaces could be clearly observed for both CNT and RMCNT (Figure

18

2a and b), which are beneficial for the storage of Li2O2 particles in a discharging process. Upon

19

coating with RuO2 and MnO2, the surface became rough; however, the two SEM images

20

exhibited a similar morphology of entangled CNT networks, suggesting that the overall structure

21

of catalysts was maintained during the formation of MnO2 and RuO2. Each element that

22

constitutes RMCNT was clearly identified and well dispersed in SEM/energy-dispersive X-ray

23

spectroscopy (SEM/EDS) mapping (Figure 2c). In TEM images of RMCNT (Figure 2d and e),


14

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ACS Catalysis

1

the roughened CNT surface was coated with very small nanoparticles. This surface roughening

2

could have originated from a reaction between CNT and permanganate ions to produce MnO2,

3

and also from the nanoflake shapes of MnO2. Nano-crystalline RuO2 with a size of 4–5 nm was

4

found on the surface of MnO2 in the high-resolution TEM (HR-TEM) image (Figure 2f). The

5

crystalline nature of RuO2 was also confirmed from the selected area electron diffraction

6

(SAED) pattern (Figure 2g). SAED showed faint rings indicating (101) of CNT, and (101) and

7

(002) of RuO2. In the XRD pattern of RMCNT (Figure 2h), a broad shoulder corresponding to

8

nanocrystalline rutile RuO221 and a weak peak of CNT were identified. Reaction between CNT

9

carbon and permanganate ions might have induced the lower crystallinity of CNT. The XRD

10

patterns of RMCNT did not reveal the crystalline phase of MnO2, indicating that MnO2

11

nanoflakes formed on CNT are either amorphous or poorly crystalline. In the FT-IR spectra of

12

RMCNT (Figure 2i), the presence of birnessite-type MnO2 was confirmed. The main bands of

13

MnO2 at 1635 and 3433 cm−1 were clearly seen in the IR absorption spectrum of RMCNT. The

14

absorption peak at 517 cm−1 is the main characteristic band corresponding to Mn-O stretching

15

modes of octahedral layers in the birnessite-type MnO2 structure. The oxidation state of Ru and

16

Mn in RMCNT was analyzed by XPS. Figure 2j shows the high resolution Ru 3p and Mn 2p

17

XPS spectra of the synthesized RMCNT. The main oxidation state of Ru and Mn in RMCNT is

18

Ru4+ and Mn4+, respectively with minor Ru0 and Mn3+. The XPS results complement HRTEM,

19

SAED, XRD and FT-IR results above confirming that the Ru and Mn in RMCNT exist as RuO2

20

and MnO2, respectively. In the N2-sorption measurements (Figure 2k), RMCNT maintained a

21

high surface area, similar to that of the CNT. The nanoflake nature of birnessite MnO2 might be

22

the reason for this high surface area, despite the dense coating with RuO2 and MnO2.


15

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 36

1

To investigate electrocatalytic activities of each catalyst, we carried out linear sweep

2

voltammetry (LSV) measurements in O2 saturated 0.1 M KOH solution using the rotating disk

3

electrode (RDE), at a rotation speed of 1600 rpm. In ORR region (Figure S6a), the ORR onset

4

potentials of MCNT and RMCNT electrode were similar at about 0.88 and 0.83 V, which are

5

much higher than that of non-catalyzed CNT electrode. RuO2 also exhibited the ORR catalytic

6

effect though weaker than MnO2. Importantly, the ORR onset potential of RMCNT at

7

approximately 0.88 V is almost the same with that of MCNT, which means that ORR activity of

8

RMCNT was more strongly enhanced by MnO2 than by RuO2. The absolute values of kinetic

9

current density (IK, mA cm-2) at 0.6 V (vs RHE) increased with the following order, CNT (1.76),

10

RCMT (5.99), RMCNT (6.93), and MCNT (7.48), which also supports the best ORR activity in

11

MnO2. Figure S6b presents the OER catalytic activities by catalysts under study. The electrodes

12

containing RuO2, RCNT and RMCNT, demonstrated negatively shifted OER onset potentials

13

compared to MCNT and non-catalyzed CNT electrode. In addition, the OER Ik at 1.8 V were

14

10.2, 20.9, 31.1, and 35.6 mA cm-2 for non-catalyzed CNT, MCNT, RMCNT, and RCNT,

15

respectively. This indicates RuO2 possess the highest catalytic activity in OER region. In Figure

16

S5a and b, RMCNT provided ORR activity close to MCNT and OER activity close to RCNT,

17

suggesting that RMCNT synergistically integrate ORR activity of MnO2 and OER activity of

18

RuO2 showing the bi-functional catalytic activity. The catalytic activity of the catalysts in Li-O2

19

system was evaluated by cyclic voltammetry (CV) measurements in a three-electrode system

20

(Figure S6c and d). The Li foil and 0.01 M of Ag/Ag+ in Acetonitrile were used as the counter

21

and reference electrode each. In Figure S6c, pristine CNT showed only cathodic current below

22

2.8 V. MnO2-coated CNT (MCNT) exhibited cathodic onset potential at 3.2 V; however, it is

23

unclear if this indicates the onset of the oxygen reduction reaction (ORR) at this point, because


16

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ACS Catalysis

1

we could observe Li intercalation/deintercalation or capacitive current from MCNT under Ar

2

atmosphere in the same potential range (Figure S6d). Since pristine CNT showed a different

3

cathodic onset potential, the cathodic onset potential at 3.2 V might reflect the characteristic of

4

MnO2. RuO2-coated CNT (RCNT) showed similar onset potential of 2.8 V in the cathodic scan.

5

In the case of RMCNT, the cathodic current began at the same point as for MCNT. Therefore,

6

RMCNT showed the cathodic characteristic of MnO2 in the cathodic scan. The catalytic activity

7

of RuO2 was more obvious in the anodic scan. The peak current occurs at 3.8 V in the RCNT

8

electrode and that of RMCNT was about 4.0 V, which was much lower than that of CNT or

9

MCNT at above 4.3 V, indicating activity for OER. Thus, RMCNT exhibits anodic characteristic

10

close to that of RCNT rather than MCNT in anodic scan. We therefore surmised that RMCNT is

11

a suitable catalyst for reversible Li-O2 batteries with bifunctional catalytic activity from MnO2

12

during discharge (ORR activity) and RuO2 during charge (OER activity).

13

In addition to the activity of catalysts for Li2O2, an equally important factor for stable cycling

14

performance might be whether the catalyst can decompose any Li2CO3 and LiOH by-products in

15

the electrodes. Li2CO3- and LiOH-preloaded CNT, MCNT, and RMCNT electrodes were tested

16

to confirm the feasibility of by-product decomposition by MnO2 or RuO2. The charge

17

performance of LiOH-containing cells in Ar is shown in Figure 3a. Each electrode was charged

18

to 4.4 V. The charge capacities of MCNT and RMCNT correspond to 100% theoretical capacity

19

(1.8–1.9 mAh cm−2) of pre-loaded LiOH with a charge plateau of about 4.2–4.3 V, while the

20

CNT delivered charge capacity corresponding to only 6.39% of the theoretical capacity of LiOH.

21

The inset of Figure 3a shows the charge profiles of CNT, MCNT, and RMCNT electrodes

22

without LiOH, which were obtained to determine the capacity from the decomposition of

23

electrolyte below 4.4 V. The capacities of electrolyte decomposition were about 0.04, 0.11, and


17

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 36

1

0.23 mAh cm−2 for CNT, MCNT, and RMCNT, respectively. This implies that MnO2 and RuO2

2

might slightly decompose the TEGDME electrolyte; however, the capacities contributed by

3

electrolyte decomposition were negligible compared to the theoretical capacity of LiOH. XRD

4

patterns and SEM images of each electrode before and after charging are presented in Figure 3b–

5

h. The sharp peaks of crystalline LiOH disappeared in MCNT and RMCNT electrodes after

6

charging, while they remained unchanged in the CNT electrode (Figure 3b). In the SEM images,

7

LiOH with a size of 200–300 nm could still be observed clearly in CNT (Figure 3c and f). On the

8

other hand, no LiOH was detected in the MCNT (Figure 3d and g) or RMCNT (Figure 3g and h

9

for RMCNT) electrodes. All these results suggest the complete decomposition of LiOH by MnO2

10

and/or RuO2, while CNT hardly catalyzed the oxidation of LiOH.

11

The same test was applied to Li2CO3-containing electrodes. As shown in Figure 4a, the CNT

12

and MCNT electrodes exhibited charge capacities of 1.92% and 31.69% of theoretical capacity

13

(2.6–2.7 mAh cm−2), respectively. On the other hand, the RMCNT electrode showed complete

14

decomposition of Li2CO3. The CNT and MCNT electrodes after charging had peaks indexed to

15

Li2CO3, also signifying the incomplete decomposition of Li2CO3, although the intensities of

16

Li2CO3 peaks were considerably reduced (Figure 4b). In the SEM images of Figure 4c–h, some

17

residual Li2CO3 was seen distinctly in the CNT and MCNT electrodes. Thus, we concluded that

18

Li2CO3 is partially decomposed by MnO2, but completely oxidized by RuO2/MnO2. This

19

superior activity of RMCNT for removing Li2CO3 and LiOH by-products could remarkably

20

improve the cycle performance of Li-O2 cells.

21

Figure 5 shows the electrochemical properties of the separator-CNT-based monolith

22

electrodes. The areal mass loading was 7–8 mg cm−2 for all electrodes. Figure 5a presents the full

23

galvanostatic discharge and recharge profiles at current density of 0.5 mA cm−2 from 2.3 to 4.5 V.


18

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ACS Catalysis

1

Catalyst-free CNT without binders showed an ultrahigh discharge capacity of 31.5 mAh cm−2.

2

With the loading of RuO2/MnO2 (RMCNT), the discharge capacity decreased slightly to 29.47

3

mAh cm−2; however, RMCNT showed improved energy efficiency by reducing charge

4

overpotential, as expected. At the capacity mid-point, the potential gaps between discharge and

5

charge curves were 1.0 V and 1.35 V for RMCNT and CNT, respectively. The corresponding

6

round-trip efficiency was 72.67% for RMCNT, which is higher than the 66.4% obtained for CNT.

7

To assess cyclability with high areal density at the pragmatic high rate, the galvanostatic tests

8

were conducted at a rate of 1.5 mA cm−2 (Figure 5b–d), and the cycling performances of CNT

9

and RMCNT monoliths were compared. At the curtailing capacity of 1.5 mAh cm−2, the CNT

10

monolith stably cycled for 60 cycles. Compared with a previous report, which found limited

11

cyclability at the high areal capacity and low rate,25 the reversibility of CNT monolith here is

12

notable. Moreover, being further equipped with bifunctional catalysts, the RMCNT monolith

13

demonstrated a significantly improved cycle life: RMCNT stably delivered a capacity of 1.5

14

mAh cm−2 at 1.5 mA cm−2 for more than 300 cycles (Figure 5c).

15

Ex situ SEM, X-ray photoelectron spectroscopy (XPS), and XRD investigations were carried

16

out to examine the electrodes after discharging and recharging. The SEM images in Figure 6

17

show the top side of the electrodes, where the largest amount of discharge product formed in

18

Figure S2. Figure 6a and c present SEM images of discharged cathodes. The discharge products

19

of RMCNT electrode were mostly film-like shapes, with very few disk-like toroids, while both

20

shapes co-existed in CNT electrode. After recharging, there remained a film-like residue in the

21

CNT electrode (Figure 6b), while the RMCNT electrode recovered its clean surface (Figure 6d).

22

Figure S7 shows XRD patterns of electrodes after discharge and recharge. The main discharge

23

products of CNT and RMCNT cathodes were Li2O2, with the XRD peaks at 2θ = 33° and 35°,


19

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

1

although the crystallinity of Li2O2 in RMCNT was relatively poor. Besides Li2O2, the two

2

electrodes showed a minor peak at 2θ = 36°, indicative of LiOH. The Li2O2 peaks disappeared

3

after recharge for both CNT and RMCNT. There was no obvious presence of crystalline LiOH in

4

either CNT or RMCNT after recharge. Since XRD only detects crystalline materials, there could

5

have been amorphous constituents that were not identified in the XRD analysis. The chemical

6

attributes of both crystalline and amorphous products were further evaluated by XPS analysis in

7

Figure 6e-h. The main chemical in the discharged electrode of RMCNT was Li2O2 (76.35%) with

8

minor byproducts of Li2CO3 (~15.93%) and LiOH (~7.72%). After recharge, the discharge

9

product of RMCNT was almost completely removed, with negligible signals of LiOH and

10

Li2CO3. On the other hand, the discharge product of CNT was a mixture of Li2O2 (~ 59.75 %)

11

Li2CO3 (~ 34.68 %), and LiOH (~ 5.57 %). After recharge, significant amounts of Li2O2, Li2CO3,

12

and LiOH remained in the CNT electrode. The CNT electrode showed poor decomposition

13

ability toward Li2O2. Moreover, there was little change in Li2CO3 and LiOH after recharge. This

14

might be the reason for poor reversibility of CNT electrodes. The SEM, XRD, and XPS results

15

together suggested that the discharge products of CNT comprise a mixture of Li2O2 (mainly

16

crystalline), Li2CO3, and LiOH, and that CNT could not completely decompose the discharge

17

products, especially the Li2CO3 and LiOH by-products. By-product residues are accumulated

18

over cycles and eventually lead to cell failure. On the other hand, the discharge product of

19

RMCNT was Li2O2, which was largely amorphous, with minor amounts of LiOH and Li2CO3.

20

RMCNT also decomposed discharge products and by-products almost completely, as we

21

expected. Consequently, RMCNT could operate with enhanced reversibility and showed a

22

remarkably extended cycle life. Moreover, to clarify the catalytic effect of RuO2/MnO2 in our

23

thick monolith-type electrodes, we conducted an electrochemical impedance spectroscopic (EIS)


20

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ACS Catalysis

1

analysis for CNT and RMCNT electrodes at the pristine-state before discharging, after

2

discharging, and after recharging with three different loading levels: 8, 4, 2 mg cm-2. In Figure

3

S8a and d, the diameter of the semicircles increased remarkably upon discharging from 53 to

4

116 Ω for CNT, 61 to 210 Ω for RMCNT at the loading level of 8 mg cm-2, which is ascribed to

5

the poor electronic conductivity of discharge products formed on the electrode surface. The

6

larger increase in charge transfer resistance of RMCNT electrodes presumably originates from

7

film-like morphology of discharge products, which passivate larger area of the cathode surface

8

than toroid-like products of CNT electrodes do, as shown in Figure 6. After charging, the

9

resistance reduced for both CNT and RMCNT, but the CNT electrode showed remarkably lower

10

decrease in resistance presumably due to the poor decomposition of the discharge products. The

11

same is true for the different loading levels of 4 and 2 mg cm-2 in Figure S8. Moreover, the

12

semicircle diameter of RMCNT monolith electrodes showed no remarkable dependence on

13

loading level. In Figure S8d-8f, all pristine RMCNT electrodes with different loading levels

14

exhibited similar semicircle diameter of about 61 Ω. This indicates that the electrochemical

15

conductivity of RMCNT electrodes in Li-O2 systems changed little despite the increase of the

16

loading mass and thickness in this loading level range. Moreover, semicircle diameters of the

17

RMCNT electrodes with the loading level of 8, 4, and 2 mg cm-2 increased to 210, 214, and 212

18

Ω respectively, after discharging. After charging, the semicircle sizes decreased significantly to

19

90 Ω (8 mg cm-2), 88 Ω (4 mg cm-2), and 81 Ω (2 mg cm-2). RMCNT electrodes behaved

20

similarly in EIS analysis as the thickness and loading level of electrodes increase, suggesting that

21

the electrical conductivity was not compromised with the loading level increase. More notably,

22

the cell employing the RMCNT cathode was opened after 319 cycles at the initial failure, and

23

visual inspection (Figure S9a–c) confirmed that the electrode was intact, while the electrolyte


21

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

1

was fully vaporized and the Li metal was degraded. The cycled RMCNT cathode was then re-

2

assembled with fresh Li metal and electrolyte, and could run for an additional 215 cycles under

3

the same test conditions (Figure 5b and c; Figure S9d and e). This suggested that the initial cell

4

failure was not due to the RMCNT monolith, but caused by degradation of the Li metal anode

5

and/or electrolyte. The combined cycle life of the RMCNT monolith was more than 500 cycles at

6

capacity limit of 1.5 mAh cm−2 and rate of 1.5 mA cm−2, which is exceptionally long compared

7

to the CNT monolith (72 cycles under the same conditions).

8

The cycling performance of the RMCNT cathodes at higher areal capacities of 3.0 and 4.5

9

mAh cm−2 was also investigated. Figure 6d shows charge-discharge profiles of RMCNT

10

cathodes with 3.0 and 4.5 mAh cm−2 capacity limits, respectively, at 1.5 mA cm−2 for 30 cycles.

11

The end voltage vs. cycle number and capacity vs. cycle number for each capacity limit are

12

shown in Figure S10. The RMCNT monolith electrode demonstrated stable voltage profiles at

13

3.0 mAh cm−2 for 45 cycles, and at 4.5 mAh cm−2 for 40 cycles. We further evaluated the

14

feasibility of cycling RMCNT cathodes at an even higher areal capacity of 10 mAh cm−2. The

15

cell could deliver a capacity of 10 mAh cm−2 for 7 cycles (Figure S11). This capacity is one of

16

the highest reported for cycling in in Li-O2 batteries. Some recent reports addressing high areal

17

capacity Li-O2 batteries with practically meaningful energy densities are summarized in Table

18

S1 for comparison. Finally, we assessed the rate capability of the RMCNT monolith cathode

19

(Figure 5e). The current rate was sequentially varied from 0.25 mA cm−2 to 2.0 mA cm−2, and

20

this stepwise current variation was repeated. The overpotentials increased with the current

21

increase; however, the charge-discharge profiles were stably maintained for repeated current

22

changes. This result confirms the high rate capability of the RMCNT monolith.

23 24

3. Conclusion ACS Paragon Plus Environment

22

Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1 2

We successfully constructed air cathodes of high-rate and high areal capacity based on

3

RuO2/MnO2 bifunctional catalysts and separator-CNT monolith-type electrodes of massive

4

loading for pragmatic operation of Li-O2 batteries. A simple vacuum filtration enabled massive

5

active material loading to the air cathodes. The monolith cathodes have several advantages:

6

processability for forming binder-free free-standing sheets, thinner electrodes compared with the

7

paper type, and improved contact between separator and active layers. These features, together

8

with the inherently porous CNT network, allowed better transport throughout the cathodes with

9

high mass loading. The unique monolith cathode is particularly beneficial for high-rate

10

performances. The catalyst-free CNT monolith could be cycled at a high current of 1.5 mA cm−2,

11

but with limited cycleability and energy efficiency at high areal capacity. The presence of

12

bifunctional catalysts RuO2/MnO2 was critical for enhanced cyclability and energy efficiency.

13

The catalysts not only facilitated decomposition of Li2O2, but also enabled complete removal of

14

persistent by-products such as Li2CO3 and LiOH. With the improved reversibility endowed by

15

bifunctional catalysts, the RMCNT monolith exhibited superior cycle performance of 315 cycles,

16

with a capacity limit of 1.5 mAh cm−2 at 1.5 mA cm−2. The high-rate and high areal capacity

17

RMCNT air cathodes could deliver even higher curtailing capacities of 3.0 and 4.5 mAh cm−2,

18

for 45 and 40 cycles, respectively. The unprecedentedly high areal capacity of 10 mAh cm−2

19

could also be cycled. The figures are the closest to the commercialization levels for high

20

energy/high power devices. This work thus demonstrates the feasibility of pragmatic

21

development of Li-O2 batteries for applications in high energy density and high-power devices

22

such as electric vehicles. We firmly believe that this work is proposing the meaningful

23

assessment which would be highly required for the post LIB systems.

24


23

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 36

1 2

3 4

Figure 1. (a) Photographs of a CNT paper detached from the AAO filter and the GF/D separator-

5

CNT monolith. (b) Peel-off tests. Photographs of (left) a CNT-paper assembled with a GF/D

6

separator and a CNT-monolith after the peel-off test with 3M tape and (right) comparison of

7

contacts after the forced separation between the CNT sheets and the separator. (c) Full

8

galvanostatic discharge profiles of the CNT paper and CNT monolith electrodes with two

9

different mass loadings (7–8 mg cm−2 and 14–15 mg cm−2) at a current density of 0.5 mA cm−2

10

with a voltage cutoff of 2.3 V. Comparison of the discharge capacities of the CNT electrodes at

11

current densities from 0.5 mA cm−2 to 2.0 mA cm−2. (d) Free-standing PTFE-bound CNT sheet

12

electrode, (e) binder-free CNT paper electrode, and (f) binder-free CNT monolith electrode.


24

Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1 2

Figure 2. Characterization of CNT and RMCNT. SEM images of (a) CNT and (b) RMCNT,

3

with (c) EDS mapping of RMCNT. (d), (e) TEM images, (f) high-resolution TEM image, and (g)

4

SAED patterns of RMCNT. (h) XRD patterns, (i) FT-IR spectra, (j) high resolution XPS of

5

RMCNT; (top) Mn 2p and (bottom) Ru 3p, and (k) nitrogen adsorption/desorption isotherms of

6

CNT and RMCNT.


25

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

1 2

Figure 3. (a) Charge voltage profiles LiOH-preloaded electrodes of CNT, MCNT, and RMCNT

3

in Ar. Inset: charge profiles of the bare CNT, MCNT, and RMCNT electrodes without preloaded

4

LiOH. (b) XRD patterns and (c-h) SEM images of the LiOH preloaded electrodes before and

5

after charging.

6


26

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ACS Catalysis

1 2

Figure 4. (a) Charge voltage profiles of Li2CO3-preloaded CNT, MCNT, and RMCNT

3

electrodes in Ar. Inset: charge profiles of the bare CNT, MCNT, and RMCNT electrodes without

4

preloaded Li2CO3. (b) XRD patterns and (c-h) SEM images of the Li2CO3-preloaded electrodes

5

before and after charging.


27

ACS Catalysis

Voltage (V)

a.

4.5

RMCNT CNT

4.0 3.5

2.5 5

10

15

25

30

35

c.

5.0 4.5 4.0 3.5

CNT(1st - 72 nd) RMCNT(1st - 315 th)

3.0 2.5 2.0 1.5 1.0

0.0

0.6

0.9

1.2

-2

1.5

Capacity (mAh cm ) 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.0

e.

RMCNT charge

0

50

2

1.5

150

200

250

300

2

4.5 mAh/cm limited

(1st - 30 th)

1.0

100

cycle No.

3.0 mAh/cm limited

0.5

CNT charge CNT discharge

RMCNT discharge

(1st - 30 th)

2.0

2.5

3.0

-2

3.5

4.0

4.5

Capacity (mAh cm ) 5.0

-2

Voltage (V)

d.

0.3

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

Voltage Current (0.25, 0.5, 1.0, 1.5, 2.0 mA)

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

0

10

20

30

40

Current (mA cm )

Voltage (V)

20

Capacity (mAh cm-2) End voltage (V)

b.

1.35 V

1.0 V

3.0

2.0 0

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

50

60 -2

70

80

90

6 4 2 0 -2 -4 -6 -8 -10

Capacity (mAh cm )

1 2

Figure 5. Electrochemical properties of CNT-based monolith cathodes in a voltage range of 2.3–

3

4.5 V. (a) Full galvanostatic discharge and recharge profiles at current density of 0.5 mA cm−2.

4

Cycling performance with (b) voltage profiles of CNT monolith electrode (1st, 20th, 40th, 60th,

5

and 72nd cycles) and RMCNT monolith electrode (1st, 40th, 80th, 120th, 160th, 200th, 240th, 280th,

6

and 315th cycles) and (c) variation of end voltage of CNT and RMCNT electrodes at 1.5 mA

7

cm−2 with cutoff capacity 1.5 mAh cm−2, (d) voltage profiles of RMCNT monolith electrode at

8

1.5 mA cm−2 with cutoff capacities 3.0 mAh cm−2 (purple) and 4.5 mAh cm−2 (blue), and (e) rate

9

capability test of RMCNT electrode at various current densities (0.25, 0.5, 1.0, 1.5, and 2.0 mA

10

cm−2).


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ACS Catalysis

1 2

Figure 6. Characterization of discharged and recharged electrodes. SEM images of CNT and

3

RMCNT electrodes (a), (b) after discharging and (c), (d) after recharging. Chemical analysis of

4

electrode after discharging and recharging by XPS, (e), (f) of CNT and (g), (h) of RMCNT.

5


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ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

AUTHOR INFORMATION

2

Corresponding Author

3 4

*E-mail: [email protected] Author Contributions

5

‡

6

Notes

7

Page 30 of 36

These authors contributed equally to this work.

The authors declare no competing financial interest.

8

ASSOCIATED CONTENT

9

Supporting Information.

10

Experimental procedures, Additional pictures of electrodes with other kinds of carbon materials

11

by vacuum filtration and galvanostatic discharge curves, Cross sectional SEM images of

12

discharged electrodes, Wettability assessment of monolith type electrode, cyclic voltammetry

13

(CV) data, Additional electrochemical data, XRD patterns of discharged and recharged

14

electrodes, electrochemical impedance spectroscopic analysis (EIS)

15 16

ACKNOWLEDGMENT

17

This research was supported by the Basic Science Research Program and the Engineering

18

Research Center of Excellence (ERC) Program of the National Research Foundation of Korea

19

(NRF), Korean Ministry of Science & ICT (grant no. NRF-2014R1A2A1A11049801 and NRF-

20

2017R1A5A1014708).


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1 2 3

ACS Catalysis

ABBREVIATIONS Carbon Nanotube (CNT), RuO2 coated CNT (RCNT), MnO2 coated CNT (MCNT), RuO2/MnO2 coated CNT(RMCNT)

4


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REFERENCES

2 3

(1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M., Li–O2 and Li–S batteries with high energy storage. Nat. Mater. 2012, 11, 19.

4 5

(2) Lu, J.; Li, L.; Park, J.-B.; Sun, Y.-K.; Wu, F.; Amine, K., Aprotic and Aqueous Li–O2 Batteries. Chem. Rev. 2014, 114, 5611-5640.

6 7 8

(3) Christensen, J.; Albertus, P.; Sanchez-Carrera, R. S.; Lohmann, T.;Kozinsky, B.; Liedtke, R.; Ahmend, J.; Kojic. A., A critical review of Li/air batteries. J. Electrochem. Soc. 2012, 159, R1-R30.

9 10

(4) Zhang, X.; Wang, X.-G.; Xie, Z.; Zhou, Z., Recent progress in rechargeable alkali metal– air batteries. Green Energy & Environ. 2016, 1, 4-17.

11 12

(5) Mikhaylik, Y. V.; Akridge, J. R., Polysulfide shuttle study in the Li/S battery system. J. Electrochem. Soc. 2004, 151, A1969-A1976.

13 14

(6) Evers, S.; Yim, T.; Nazar, L. F., Understanding the nature of absorption/adsorption in nanoporous polysulfide sorbents for the Li–S battery. J. Phys. Chem. C 2012, 116, 19653-19658.

15 16

(7) Wang, L.; Lu, Y.; Liu, J.; Xu, M.; Cheng, J.; Zhang, D.; Goodenough, J. B., A Superior Low‐Cost Cathode for a Na‐Ion Battery. Angew. Chem. -Int. Edit. 2013, 52, 1964-1967.

17 18 19

(8) Xiao, L.; Cao, Y.; Xiao, J.; Wang, W.; Kovarik, L.; Nie, Z.; Liu, J., High capacity, reversible alloying reactions in SnSb/C nanocomposites for Na-ion battery applications. Chem. Commun. 2012, 48, 3321-3323.

20

(9) Armand, M.; Tarascon, J.-M., Building better batteries. Nature 2008, 451, 652-657.

21 22 23

(10) Lu, J.; Lee, Y. J.; Luo, X.; Lau, K. C.; Asadi, M.; Wang, H.-H.; Brombosz, S.; Wen, J.; Zhai, D.; Chen, Z., A lithium–oxygen battery based on lithium superoxide. Nature 2016, 529, 377-382.

24 25

(11) Black, R.; Adams, B.; Nazar, L., Non‐Aqueous and Hybrid Li‐O2 Batteries. Adv. Energy Mater. 2012, 2, 801-815.

26 27

(12) Cheng, F.; Chen, J., Metal–air batteries: from oxygen reduction electrochemistry to cathode catalysts. Chem. Soc. Rev. 2012, 41, 2172-2192.

28 29

(13) Lim, H.-D.; Lee, B.; Bae, Y.; Park, H.; Ko, Y.; Kim, H.; Kim, J.; Kang, K., Reaction chemistry in rechargeable Li–O2 batteries. Chem. Soc. Rev. 2017, 46, 2873-2888.

30 31

(14) Girishkumar, G.; McCloskey, B.; Luntz, A.; Swanson, S.; Wilcke, W., Lithium− air battery: promise and challenges. J. Phys. Chem. Lett. 2010, 1, 2193-2203.

32 33

(15) Abraham, K.; Jiang, Z., A polymer electrolyte‐based rechargeable lithium/oxygen battery. J. Electrochem. Soc. 1996, 143, 1-5.

34 35

(16) Beattie, S.; Manolescu, D.; Blair, S. "High-capacity lithium–air cathodes", High-capacity lithium–air cathodes. J. Electrochem. Soc. 2009, 156, A44-A47.


32

Page 33 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1 2

(17) Jung, H.-G.; Hassoun, J.; Park, J.-B.; Sun, Y.-K.; Scrosati, B., An improved highperformance lithium–air battery. Nat. Chem. 2012, 4, 579-585.

3 4

(18) Li, Y.; Wang, J.; Li, X.; Geng, D.; Li, R.; Sun, X., Superior energy capacity of graphene nanosheets for a nonaqueous lithium-oxygen battery. Chem. Commun. 2011, 47, 9438-9440.

5 6

(19) Yoo, E.; Zhou, H., Li− air rechargeable battery based on metal-free graphene nanosheet catalysts. ACS Nano 2011, 5, 3020-3026.

7 8 9

(20) Wang, Z. L.; Xu, D.; Xu, J. J.; Zhang, L. L.; Zhang, X. B., Graphene Oxide Gel‐Derived, Free‐Standing, Hierarchically Porous Carbon for High‐Capacity and High‐Rate Rechargeable Li‐O2 Batteries. Adv. Funct. Mater. 2012, 22, 3699-3705.

10 11 12

(21) Jung, H.-G.; Jeong, Y. S.; Park, J.-B.; Sun, Y.-K.; Scrosati, B.; Lee, Y. J., Rutheniumbased electrocatalysts supported on reduced graphene oxide for lithium-air batteries. ACS Nano 2013, 7, 3532-3539.

13 14 15

(22) Mitchell, R. R.; Gallant, B. M.; Thompson, C. V.; Shao-Horn, Y., All-carbon-nanofiber electrodes for high-energy rechargeable Li–O2 batteries. Energy & Environ. Sci. 2011, 4, 29522958.

16 17 18

(23) Kwak, W.-J.; Kang, T.-G.; Sun, Y.-K.; Lee, Y. J., Iron–cobalt bimetal decorated carbon nanotubes as cost-effective cathode catalysts for Li–O2 batteries. J. Mater. Chem. A 2016, 4, 7020-7026.

19 20 21 22 23 24

(24) Nomura, A.; Ito, K.; Kubo, Y., CNT Sheet Air Electrode for the Development of UltraHigh Cell Capacity in Lithium-Air Batteries. Sci. Rep. 2017, 7:45596 (25) Lin, Y.; Moitoso, B.; Martinez-Martinez, C.; Walsh, E. D.; Lacey, S. D.; Kim, J.-W.; Dai, L.; Hu, L.; Connell, J. W., Ultrahigh-Capacity Lithium–Oxygen Batteries Enabled by DryPressed Holey Graphene Air Cathodes. Nano Lett. 2017, 17, 3252-3260.

25 26

(26) Chen, Y.; Freunberger, S. A.; Peng, Z.; Fontaine, O.; Bruce, P. G., Charging a Li–O2 battery using a redox mediator. Nat. Chem. 2013, 5, 489-494.

27 28 29

(27) Lu, Y.-C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y., Lithium–oxygen batteries: bridging mechanistic understanding and battery performance. Energy & Environ. Sci. 2013, 6, 750-768.

30 31 32

(28) Lee, J.-H.; Black, R.; Popov, G.; Pomerantseva, E.; Nan, F.; Botton, G. A.; Nazar, L. F., The role of vacancies and defects in Na0.44MnO2 nanowire catalysts for lithium–oxygen batteries. Energy & Environ. Sci. 2012, 5, 9558-9565.

33 34 35 36

(29) Jeong, Y. S.; Park, J.-B.; Jung, H.-G.; Kim, J.; Luo, X.; Lu, J.; Curtiss, L.; Amine, K.; Sun, Y.-K.; Scrosati, B. Lee, Y. J., Study on the catalytic activity of noble metal nanoparticles on reduced graphene oxide for oxygen evolution reactions in lithium–air batteries. Nano Lett. 2015, 15, 4261-4268.

37 38

(30) Débart, A.; Bao, J.; Armstrong, G.; Bruce, P. G., An O2 cathode for rechargeable lithium batteries: The effect of a catalyst. J. Power Sources 2007, 174, 1177-1182.


33

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

1 2 3

(31) Lu, Y.-C.; Xu, Z.; Gasteiger, H. A.; Chen, S.; Hamad-Schifferli, K.; Shao-Horn, Y., Platinum−gold nanoparticles: a highly active bifunctional electrocatalyst for rechargeable lithium− air batteries. J. Am. Chem. Soc. 2010, 132, 12170-12171.

4 5 6

(32) Ryu, W.-H.; Yoon, T.-H.; Song, S. H.; Jeon, S.; Park, Y.-J.; Kim, I.-D., Bifunctional Composite Catalysts Using Co3O4 Nanofibers Immobilized on Nonoxidized Graphene Nanoflakes for High-Capacity and Long-Cycle Li–O2 Batteries. Nano Lett. 2013, 13, 4190-4197.

7 8 9

(33) Débart, A.; Paterson, A. J.; Bao, J.; Bruce, P. G., α‐MnO2 Nanowires: A Catalyst for the O2 Electrode in Rechargeable Lithium Batteries. Angew. Chem. -Int. Edit. 2008, 120, 45974600.

10 11 12

(34) Jung, J.; Song, K.; Bae, D. R.; Lee, S. W.; Lee, G.; Kang, Y.-M., β-FeOOH nanorod bundles with highly enhanced round-trip efficiency and extremely low overpotential for lithiumair batteries. Nanoscale 2013, 5, 11845-11849.

13 14 15

(35) Kundu, D.; Black, R.; Adams, B.; Harrison, K.; Zavadil, K.; Nazar, L. F., Nanostructured Metal Carbides for Aprotic Li–O2 Batteries: New Insights into Interfacial Reactions and Cathode Stability. J. Phys. Chem. Lett. 2015, 6, 2252-2258.

16 17 18

(36) Kwak, W.-J.; Lau, K. C.; Shin, C.-D.; Amine, K.; Curtiss, L. A.; Sun, Y.-K., A Mo2C/Carbon Nanotube Composite Cathode for Lithium–Oxygen Batteries with High Energy Efficiency and Long Cycle Life. ACS Nano 2015, 9, 4129-4137.

19 20 21

(37) Kwak, W.-J.; Hirshberg, D.; Sharon, D.; Shin, H.-J.; Afri, M.; Park, J.-B.; Garsuch, A.; Chesneau, F. F.; Frimer, A. A.; Aurbach, D., Understanding the behavior of Li–oxygen cells containing LiI. J. Mater. Chem. A 2015, 3, 8855-8864.

22 23 24

(38) Kwak, W.-J.; Hirshberg, D.; Sharon, D.; Afri, M.; Frimer, A. A.; Jung, H.-G.; Aurbach, D.; Sun, Y.-K., Li–O2 cells with LiBr as an electrolyte and a redox mediator. Energy & Environ. Sci. 2016, 9, 2334-2345.

25 26 27 28 29

(39) Bergner, B. J.; Schürmann, A.; Peppler, K.; Garsuch, A.; Janek, J. r., TEMPO: A Mobile Catalyst for Rechargeable Li-O2 Batteries. J. Am. Chem. Soc. 2014, 136, 15054-15064 (40) Lim, H.-D.; Lee, B.; Zheng, Y.; Hong, J.; Kim, J.; Gwon, H.; Ko, Y.; Lee, M.; Cho, K.; Kang, K., Rational design of redox mediators for advanced Li–O2 batteries. Nat. Energy 2016, 1, 16066.

30 31 32

(41) Sun, D.; Shen, Y.; Zhang, W.; Yu, L.; Yi, Z.; Yin, W.; Wang, D.; Huang, Y.; Wang, J.; Wang, D., A solution-phase bifunctional catalyst for lithium–oxygen batteries. J. Am. Chem. Soc. 2014, 136, 8941-8946.

33 34 35

(42) McCloskey, B.; Speidel, A.; Scheffler, R.; Miller, D.; Viswanathan, V.; Hummelshøj, J.; Nørskov, J.; Luntz, A., Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li–O2 Batteries. J. Phys. Chem. Lett. 2012, 3, 997-1001.

36 37

(43) Meini, S.; Tsiouvaras, N.; Schwenke, K. U.; Piana, M.; Beyer, H.; Lange, L.; Gasteiger, H. A., Rechargeability of Li–air cathodes pre-filled with discharge products using an ether-based


34

Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1 2

electrolyte solution: implications for cycle-life of Li–air cells. Phys. Chem. Chem. Phys. 2013, 15, 11478-11493.

3 4

(44) Gao, X.; Chen, Y.; Johnson, L.; Bruce, P. G., Promoting solution phase discharge in Li– O2 batteries containing weakly solvating electrolyte solutions. Nat. Mater. 2016, 15, 882-888.

5 6

(45) Ling, C.; Zhang, R.; Takechi, K.; Mizuno, F., Intrinsic Barrier to Electrochemically Decompose Li2CO3 and LiOH. J. Phys. Chem. C 2014, 118, 26591-26598.

7 8 9 10

. (46) Hong, M.; Choi, H. C.; Byon, H. R., Nanoporous NiO Plates with a Unique Role for Promoted Oxidation of Carbonate and Carboxylate Species in the Li–O2 Battery. Chem. Mater. 2015, 27, 2234-2241.

11 12 13

(47) Tan, P.; Wei, Z.; Shyy, W.; Zhao, T.; Zhu, X., A nano-structured RuO2/NiO cathode enables the operation of non-aqueous lithium–air batteries in ambient air. Energy & Environ. Sci. 2016, 9, 1783-1793.

14 15 16

(48) Song, S.; Xu, W.; Zheng, J.; Luo, L.; Engelhard, M. H.; Bowden, M. E.; Liu, B.; Wang, C.-M.; Zhang, J.-G., Complete Decomposition of Li2CO3 in Li–O2 Batteries Using Ir/B4C as Noncarbon-Based Oxygen Electrode. Nano Lett. 2017, 17, 1417-1424.

17 18 19

(49) Jian, Z.; Liu, P.; Li, F.; He, P.; Guo, X.; Chen, M.; Zhou, H., Core–shell‐structured CNT@ RuO2 composite as a high‐performance cathode catalyst for rechargeable Li–O2 batteries. Angew. Chem. -Int. Edit. 2014, 53, 442-446.

20 21 22

(50) Yilmaz, E.; Yogi, C.; Yamanaka, K.; Ohta, T.; Byon, H. R., Promoting Formation of Noncrystalline Li2O2 in the Li–O2 Battery with RuO2 Nanoparticles. Nano Lett. 2013, 13, 46794684.

23 24 25

(51) Park, H. W.; Lee, D. U.; Nazar, L. F.; Chen, Z., Oxygen reduction reaction using MnO2 nanotubes/nitrogen-doped exfoliated graphene hybrid catalyst for Li-O2 battery applications. J. Electrochem. Soc. 2013, 160, A344-A350.

26 27 28 29 30 31 32 33 34 35

(52) Lim, H. D.; Park, K. Y.; Song, H.; Jang, E. Y.; Gwon, H.; Kim, J.; Kim, Y. H.; Lima, M. D.; Robles, R. O.; Lepró, X., Enhanced Power and Rechargeability of a Li−O2 Battery Based on a Hierarchical‐Fibril CNT Electrode. Adv. Mater. 2013, 25, 1348-1352.

36 37 38

(55) Fang, R.; Zhao, S.; Hou, P.; Cheng, M.; Wang, S.; Cheng, H. M.; Liu, C.; Li, F., 3D interconnected electrode materials with ultrahigh areal sulfur loading for Li–S batteries. Adv. Mater. 2016, 28, (17), 3374-3382.

(53) Lv, D.; Zheng, J.; Li, Q.; Xie, X.; Ferrara, S.; Nie, Z.; Mehdi, L. B.; Browning, N. D.; Zhang, J. G.; Graff, G. L., High energy density lithium–sulfur batteries: Challenges of thick sulfur cathodes. Adv. Energy Mater. 2015, 5:1402290 (54) Zhou, W.; Guo, B.; Gao, H.; Goodenough, J. B., Low‐Cost Higher Loading of a Sulfur Cathode. Adv. Energy Mater. 2016, 6:1502059


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Page 36 of 36

1 2

BRIEFS: Assisted by superior catalytic activities of RuO2/MnO2 electrocatalysts, the massively

3

loaded CNT-monolith cathode achieved excellent cyclability at unprecedentedly high areal

4

capacity and a high rate: 40 cycles at 4.5 mAh cm-2 and 1.5 mA cm-2.

5 6

SYNOPSIS

7 8 9


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High-Rate and High-Areal-Capacity Air Cathodes with Enhanced

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