Manganese Dioxide Composite

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Preparation of carbon nanotubes/manganese dioxide composite catalyst with fewer oxygen-containing groups for Li-O2 batteries using polymerized ionic liquids as sacrifice agent Wenpeng Ni, Shimin Liu, Yuqing Fei, Yude He, Xiangyuan Ma, Liujin Lu, and Youquan Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16531 • Publication Date (Web): 13 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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ACS Applied Materials & Interfaces

Preparation of Carbon Nanotubes/Manganese Dioxide Composite Catalyst with Fewer Oxygen-Containing Groups for Li-O2 Batteries Using Polymerized Ionic Liquids as Sacrifice Agent Wenpeng Ni,[a,b] Shimin Liu,[a] Yuqing Fei,[a,b] Yude He,[a] Xiangyuan Ma,[a] Liujin Lu,[a] and Youquan Deng*[a] a

Centre for Green Chemistry and Catalysis, State Key Laboratory for Oxo Synthesis

and Selective Oxidation, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b

University of Chinese Academy of Sciences, Beijing 100039, China

Abstract: Considering the significant influence of oxygen-containing groups on the surface of carbon involved electrode, carbon nanotubes (CNTs) based MnO2 composite catalyst was synthesized following a facile method while using polymerized ionic liquids (PIL) as sacrifice agent. Herein, the PIL, polymerized hydrophobic

1-vinyl-3-ethylimidazolium

bis

((trifluoromethyl)sulfonyl)imide,

wrapped CNTs were prepared. The composite was applied to support MnO2 by the treatment of KMnO4 solution taking advantage of the reaction between PIL and KMnO4 which exclude or suppress the oxidation of CNTs, and the as-synthesized material with fewer oxygen-containing groups acted as cathode catalyst for Li-O2 batteries directly avoiding the application of binders. The catalyst shows enhanced activity compared with the samples without PIL as verified by the lower overpotential during discharging and charging (0.97 V at the current density of 100 mA g-1).

ACS Paragon Plus Environment


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Meanwhile, the performance, such as coulombic efficiency and rate capability, is also improved for the Li-O2 battery utilizing this catalyst. What is more, the formation of confined Li2O2 particles could be responsible for the reduction of charge potential of Li-O2 batteries due to the synergy effect of the intrinsic catalytic activity of MnO2 and the fewer oxygen functional groups on the surface of catalyst.

KEYWORDS: manganese dioxide, polymerized ionic liquids, carbon nanotubes, oxygen-containing groups, Li-O2 batteries

1. INTRODUCTION There is growing interest in rechargeable lithium-oxygen (Li-O2) batteries in recent years due to its superior practical gravimetric energy density which is 3 to 5 times higher than those of conventional Li-ion batteries

1-6

. Lithium-metal, as the anode in

this system, is oxidized to Li ions which migrate across the electrolyte to the cathode where the oxygen reduction reaction (ORR) occurs during battery discharge. Then, the lithium peroxide (Li2O2) is formed as the final discharge product in a reaction between the Li+ and the intermediate superoxide O2•- (overall: 2Li+ + O2 +2e- → Li2O2). The reactions are reversed in the charge procedure involving the dissociation of Li2O2 and the oxidation of peroxide anions. Unfortunately, several critical challenges, such as the blocking of the porous cathode, the by-products produced by the decomposition of organic electrolyte and carbon electrode, hinder the realizing of the high performance of Li-O2 batteries

7-10

. Especially, due to the sluggish oxygen

evolution reaction (OER) kinetics, the higher overpotential has drawn a lot of interests


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of researchers around the world. Thus, many studies focused on developing highly efficient catalysts for the air cathode in order to significantly reduce the overpotential. It is reasonable to conclude that an appropriate catalyst could increase not only the round-trip efficiency but also cycle performance of Li-O2 batteries based on the previous works

11-13

. Up to now, the materials that could be used as cathode catalyst

are carbon materials 14-15, metal 16-17 and metal oxides 18-19 and so on. Noble metal and oxides often exhibit the best activity in these catalysts. For instance, the charge potential could be reduced to lower than 3.8 V when RuO2 was introduced in the cathode

6,20-21

. However, it is not cost-effective considering the price of noble metal

and the higher mass loading of noble metal in these catalysts (it is usually higher than 20 %). Therefore, non-precious metal and oxides based catalysts are still the hot-point in the research of Li-O2 batteries 22-24. MnO2, one of the common used non-precious metal oxides, has been studied as cathode catalyst for Li-O2 batteries. MnO2 samples with different morphologies, including nanoparticles 25, nanowires 26, nanorods 27, nanosheets 28, and nanoboxes

29

are all considered to be effective ORR or OER catalyst for Li-O2 batteries. What is more, MnO2 with different crystals are also involved in the previous reports, such as α-

26

, δ- 29, ε-

13

and γ-MnO2

30

. Moreover, the MnO2 could be supported on carbon

substrate (including porous carbon

25

, carbon nanotubes

31

, carbon paper

32

and

graphene 33) or deposited on nickel foam directly 34. However, the charge potentials of these catalysts are often higher than 4 V. As a result, the investigations about the Li-O2 batteries based on MnO2 cathode catalyst are still focused by the researchers.



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Carbon nanotubes (CNTs) had been widely investigated as a promising cathode material for Li-O2 batteries, due to its superior mechanical strength and the higher electrical conductivity.

Furthermore, the surface properties of CNTs have great

influence on the performance of Li-O2 batteries based on CNTs cathode, especially for the content of oxygen-containing groups. For example, the electrical conductivity and catalytic activity could be optimized by tuning the content of oxygen-containing groups on the surface which enhanced the cycling stability of CNTs electrode While coincidence, according to the research of Byon’s group

35

.

36

, the presence of

oxygen functional groups on CNTs could alters the morphology of discharge products which impacts the charging potential. However, for the CNTs based composite catalyst,

such

as

CNTs

supported

MnO2

(MnO2/CNT),

the

effect

of

oxygen-containing groups on the catalytic activity for ORR and OER has not been concerned. Some particular methods were developed to prepare materials based on MnO2/CNT, such as electrodeposition

[37]

. However, for the synthesis of MnO2/CNT

with a chemical approach, the reaction between CNTs and potassium permanganate (KMnO4) was often utilized due to the convenience, or CNTs after pretreatment was acts as supporter which could afford enough anchor points for MnO2

[38-40]

. It is

inevitable to introduce a lot of oxygen-containing groups on the surface of CNTs when these two routes were adopted. Predictably, these exposed oxygen functional groups also affect the realization of intrinsic activity of MnO2. Therefore, it is necessary to develop a facile method to synthesize CNTs based MnO2 composite catalyst with fewer oxygen-containing groups in order to investigate the implications


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of these groups. On the other hand, according to the previous literatures 41-42, ILs with imidazolium cation could be oxidized by KMnO4 which results in the formation of MnO2. Based on this procedure, it inspires us to prepare CNTs supported MnO2 composite catalyst by a new method without oxidizing of CNTs which decreases the content of oxygen-containing groups on the surface of catalyst. IL containing vinyl group and imidazolium cation (1-vinyl-3-ethylimidazolium, [VEIm]+) was used as monomer which would disperses near the surface of CNTs owing to the strong interaction between imidazolium cations and π-electrons of the carbon surface. After the adding of initiator, the PIL are generated as wrapping agent for CNTs (denoted as PILCNT). Therefore, under the protection of PIL, it could be presumed that the oxidation of CNTs would be replaced by the oxidation of PIL under the treatment of KMnO4 solution when this PILCNT acts as supporter for MnO2 (MnO2/PILCNT). This will result in the formation of composite catalyst with fewer oxygen-containing groups while using PIL as sacrifice agent. In addition, owing to the sensitivity of lithium metal and Li2O2 to the moisture, IL based on bis ((trifluoromethyl)sulfonyl)imide (NTf2–) anion which is one common hydrophobic ILs, was selected in this work. Besides, this catalyst with appropriate viscosity can be used to make cathode for Li-O2 batteries directly avoiding the application of any binders. The catalyst was characterized by FTIR, XRD, XPS, TEM and nitrogen adsorption-desorption isotherm measurements. Galvanostatic discharge-charge measurements show that the catalyst exhibit superior OER catalytic activity compared to the pure CNTs and MnO2/CNTs.



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The charging potential is about 3.7 V at the current density of 100 mA g-1. Additionally, the enhanced performance in terms of rate capability and coulombic efficiency is also observed.

2. EXPERIMENTAL SECTION 2.1 Material preparation All the reagents are analytical grade without further purification. Synthesis of [VEIm]NTf2 was carried out as reported previously 43. In brief, 1-ethylbromide (0.15 mol) and 1-vinylimidazole (0.13 mol) were mixed in methanol under vigorous stirring. The mixture was refluxed for 16 h, and a white solid was obtained after the solution pour into diethyl ether. The product, [VEIm]Br, was dried in a vacuum oven after washed several times with ethyl acetate. After that, the obtained product was dissolved in distilled water, and the [VEIm]NTf2 was produced after the lithium bis ((trifluoromethyl)sulfonyl)imide (LiNTf2) was added. In the synthesis of polymerized ionic liquids and CNTs composites, 200 mg CNTs (Shenzhen Nanotech Port Co., Ltd. Special surface area 40-300 m2 g-1, purity ≥ 95 %, main range of diameter < 10 nm) was added to 30 mL ethanol containing 1 g [VEIm]NTf2. The amount of ionic liquids monomer is excessive due to the reaction between KMnO4 and ionic liquids. In addition, this ratio of ionic liquids in raw materials gives the favourable viscosity to prepare cathode. The dispersion was ultrasonicated for 30 min, and then 35 mg 2,2ˊ-azobisisobutyronitrile (AIBN) was added to the solution. The solution was refluxed under N2 atmosphere for 24 h. The obtained products were separated by centrifugal and washed with water and ethanol


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for several times. The material is denoted as PILCNT in this work. After this, 200 mg PILCNT was dispersed in ethanol (20 mL), and then an ethanol solution containing 0.1 g KMnO4 was added dropwise. The mixture kept at ice water bath for 24 h under stirring. MnO2/PILCNT was obtained after washing and drying. For comparison, the MnO2/CNT was also prepared by the same method except the PILCNT was replaced by CNTs. 2.2 Material characterization FTIR tests were performed on a Thermo Nicolet 5700 spectrometer using the samples dispersed in KBr pellets. The phase purity of the materials was characterized by X-ray powder diffractometer (Siemens D/mas-RB powder X-ray diffractometer) using Cu Kα radiation (40 mA and 40 kV). N2 adsorption/desorption curve was determined by Brunauer-Emmett-Teller (BET) measurements using a Micromeritics ASAP 2020 volumetric adsorption analyzer at 76.2 K. The pore size distribution was determined from the adsorption branch of the isotherms based on the density functional theory (DFT). The X-ray photoelectron spectroscopy (XPS) was recorded on a VG ESCALAB 210 instrument using Mg Kα radiation (1253.6 eV) and the XPS spectra were referred to a C 1s value of 284.6 eV. The detailed morphologies of the as prepared samples were determined by transition electron microscopy (JEM 2010) operated at 200 keV. The morphologies of discharge products were measured by scanning electron microscope (JEOL-6701F) at an acceleration voltage of 5 kV. The amount of Mn was measured using Atomic Absorption Spectrometry (AAS, Varian AA240). TGA was conducted with a Thermogravimetric/Differential Thermal



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Analyzer (Mettler TGA SDTA851) at a heating rate of 10 oC min-1 in flowing N2. 2.3 Electrochemical characterization All the electrochemical measurements were carried out at room temperature (about 25 oC). The performance of Li-O2 batteries was tested in a swagelok type cell with an air hole on the cathode side. For the cathode based on pure CNTs and MnO2/CNT, the ink composed of catalyst (90%) and PVDF (10%) binder was coated on carbon paper disc with a diameter of 20 mm by using a scraper. However, the binder was not used in the sample of MnO2/PILCNT. Then, the electrode was dried at 100 oC for 12 h under vacuum conditions to remove any residual solvent, and the total loading of the cathode material was approximately 1.4 ± 0.3 mg cm-2. Generally speaking, the batteries composed of a pure lithium disk anode, a glass fiber separator (GF/D, Whatman) with a saturated electrolyte (about 130 µL) and a cathode. The electrolyte is 0.1 M LiClO4 in DMSO which was dried by freshly activated 4A molecular sieve. The water content is less than 15 ppm as verified by Karl Fisher titration. The building of Li-O2 batteries was conducted in a glove box with a water and oxygen level of less than 1 ppm. After assembly, the cells were placed in a glass bottle and purged with a flow of pure O2 for at least 10 min. The batteries were typically rested for at least 5 h before the electrochemical measurements were taken. All measurements were conducted under an oxygen atmosphere to avoid the negative effects of humidity and CO2. The galvanostatic discharge/charge tests were performed on a LAND battery testing system with the voltage between 2.0 V and 4.25 V versus Li/Li+ at the current density of 100 mA g-1. The cycling experiments were carried out


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at a current density of 100 mA g-1 with a capacity-limited method. The capacity and current density were calculated by the mass of catalyst (PILCNT and MnO2/PILCNT), or the total mass of catalyst and binder (CNT and MnO2/CNT). The LSV results were acquired by the CHI660A electrochemical workstation at a voltage sweep rate of 0.1 mV s-1. The morphology and the component of the cathode at different stages were studied by the SEM and XPS measurements. The battery was disassembled in the glove box after testing, and then the cathode was rinsed with acetonitrile (dried using 4A molecular sieve until the water level was less than 20 ppm) for several times. The electrode was dried in vacuum at 70 oC. The samples were transferred to the equipment with the help of a sealed bottle.

3. RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the preparation of MnO2/CNT and MnO2/PILCNT. As illustrated in Fig. 1, CNTs supported MnO2 nanoparticles was synthesized following the redox reaction: 4MnO4- + 3C + H2O → 4MnO2 + CO32- + 2HCO3-. However, the oxidation of CNT is popular under oxidizing conditions. In general, the



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defects on the surface of CNT are considered as the reaction sites for oxidation [44-45]. After the oxidative treatment, the CNT was modified by some oxygen-containing groups, including carboxyl, carbonyl and hydroxyl, even though the amouts of each group were varied with the identity of the oxidant. Thus, it should be possible that the formation of MnO2 occurred simultaneous with the oxidation of CNT. In order to exclude the influence of these oxygen functional groups, PIL was integrated with CNTs as a wrapping agent. As a result, the KMnO4 contacted and reacted with PIL first. It is feasible that all or most of KMnO4 were consumed in the oxidation of PIL when excessive PIL was introduced. In this way, the oxidation of CNTs could be suppressed due to the protection of PIL which acts as sacrifice agent. Thus, it could be presumed that the fewer oxygen-containing groups will be introduced in the sample of MnO2/PILCNT. Finally, the effect of oxygen-containing groups on the performance of Li-O2 batteries based on CNTs involved composite catalyst could be investigated.

Figure 2. (a) FTIR spectra of [VEIm]NTf2 and PILCNT. (b) SEM image of


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MnO2/PILCNT. (c) TEM image and SEAD pattern (inset), (d) HRTEM image of MnO2/PILCNT.

Firstly, the FTIR was conducted to monitor the polymerization process of ILs monomer, as shown in Fig. 2a. For the [VEIm]NTf2, the peaks at 3150 cm-1, 3030 cm-1 and 1660 cm-1 could be ascribed to the vibration from =C-H and C=C groups. At the same time, the responses for imidazolium group were also observed in the spectrum (the peaks at 1550 cm-1 and 1420 cm-1). In accordance with previous reports 46

, the peaks attribute to the carbon-carbon double bonds disappeared in the sample of

PILCNT as displayed in the green block which implies the monomers are polymerized successfully in this step. What is more, the CNTs were totally wrapped by PIL as verified by the comparison of SEM images of pristine CNTs and PILCNT (Fig. S1). Otherwise, the TEM image of PILCNT shows the nanotubes structure which is similar to the morphology of pristine CNTs (Fig. S2). These results confirm that the CNTs were buried in PIL while keeping the tubular structure. To prepare MnO2-based catalyst, the as-synthesized material was treated with KMnO4 solution. Some particles with rod shape were exposed which is dissimilar from the aggregative particles of PILCNT, as shown in Fig. 2b. This implied that partial CNTs were exposed, indicating quite a few PILs were consumed in this process. For comparison, the MnO2/CNT was also synthesized and characterized. Then, TEM was applied to research the morphology of these two MnO2-based catalysts. It was found that MnO2 nanoparticles were dispersed well on the PILCNT,



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and their average diameters were in the range of 20-40 nm (Fig. 2c). The corresponding selected area electron diffraction pattern showed the diffraction ring for the (201), (111) and (020) planes of MnO2 (the inset of Fig. 2c). Additionally, an interplanar spacing of 0.24 nm which corresponds to the distance of (111) plane of MnO2 appeared in the HRTEM image of MnO2/PILCNT (Fig. 2d). But, the nanoparticles without lattice planes were also observed in this image which reveals that these particles could be the aggregation of ultrafine nanoparticles. The similar results were also achieved for MnO2/CNT as presented in Fig. S3a and Fig. S3b. The results conclude that the PIL does not have a influence on the morphology of MnO2 in this study.

Figure 3. (a) XRD patterns of CNT, PILCNT, MnO2/CNT and MnO2/PILCNT. (b) Nitrogen adsorption-desorption isotherms and the pore size distribution (inset) of MnO2/PILCNT. Subsequently, the crystalline structures of the catalysts were investigated by XRD (Fig. 3a). The peaks at 25.84° and 42.79° were indexed to the graphite carbon for the CNTs and PILCNT samples (JCPDS: 26-1079). Furthermore, in the case of MnO2/CNT and MnO2/PILCNT, the marked peaks at 37.28° (d=2.38) and 65.36° (d=1.43) corresponded to the (400) and (020) facet of MnO2 (JCPDS: 42-1316).


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Besides, the wide peaks indicate that the MnO2 is poorly crystallized or consisted of smaller nanoparticles. The diffraction peaks belonging to other phases were not detected in the XRD patterns. Moreover, it is widely accepted that the surface area and porosity of cathode materials play a vital role in improving the performance of Li-O2 batteries, especially for initial discharge capacity. Therefore, the N2 adsorption-desorption measurements were performed to investigate the pore structure of the samples. For the pure CNTs (Fig. S4a), the specific surface area was 346.8 m2 g-1, and the pore volume was 1.11 cm3 g-1. In addition, the pore size distribution was calculated from the Barrett-Joyner-Halenda (BJH) method which shows that the pore size was major less than 10 nm. After integration with PIL, as delivered by Fig. S4b, the specific surface area for PILCNT was only 7.39 m2 g-1 while the pore volume was 0.029 cm3 g-1. These results were far less than the pure CNTs due to the wrapping of PIL. However, after the treatment of KMnO4 solution, the higher pore volume (0.687 cm3 g-1) and specific surface area (108.96 m2 g-1) were observed as shown in Fig. 3b. The pore size distribution had a major peak at about 18 nm. This conclusion also verifies the occurrence of reaction between PIL and KMnO4 which results in the exposure of some CNTs. As noted earlier, the groups on the surface of these two MnO2-based catalysts prepared by different routes can vary greatly. Consequently, XPS was applied to give insight into the surface properties of these catalysts. As can be seen in Fig. S5, the PILCNT contains peaks for C, N, O, S and F while the signal for Mn element appeared in the spectra of MnO2/PILCNT. As a result, the high-resolution XPS



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spectrum of Mn, C, N and O were recorded as shown in Fig. 4. Like MnO2/CNT (Fig. S6), the oxidation state for Mn was +4 based on the binding energy of Mn 2p3/2 and

Figure 4.

(a) Mn 2p XPS spectrum of MnO2/PILCNT. High-resolution (b) C 1s, (c)

N 1s and (d) O 1s XPS spectrum of different catalysts.

Mn 2p1/2 coupled with the spin-energy separation of 11.7 eV (Fig. 4a). There are five peaks at 284.7, 285.1, 286.2, 287 and 293 eV in the fitting results of XPS C1s spectra for PILCNT, corresponding to the carbon atoms of C-C, C-N, N-C=C-N, N-C=N and –CF3 groups, respectively (Fig. 4b)

47

. However, the intensity of the peaks arising

from PIL decreased in the case of MnO2/PILCNT owing to the decomposition of PIL and the deposition of MnO2 particles. Moreover, this conclusion is also confirmed by the N 1s spectra which consist of two peaks attributed to the two N atoms from cation and anion (Fig. 4c). Furthermore, the results of O 1s after devolution were compared as displayed in Fig. 4d. Only one peak at about 532.9 eV was observed in the case of


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PILCNT which may ascribe to the anion NTf2-. On the other side, the O 1s main peak at 529.7 eV which attributed to the lattice oxygen bonded to Mn atoms appeared in the spectra of MnO2/CNT and MnO2/PILCNT. The wide peak at 531-533 eV might be assigned to a mixture of hydroxyl groups and other oxygen-containing groups on the surface of catalysts. It should be noted that the groups not only arise from the surface of MnO2 but also the substrate including CNTs and PIL. However, for MnO2/PILCNT, the fraction of peak area for oxygen-containing groups was only 24.4 % which is significant lower than MnO2/CNT (75.9 %). As with forecasts, the PIL wrapping CNTs suppresses the oxidation of CNTs through the decomposition of PIL. It should be mentioned that the by-products from the decomposition of PIL were removed after the washing procedure since the absence of other species on the surface of this catalyst according to the characterizations. Indeed, it was thought that the imidazolium cation was ultimately decomposed to CO2 and H2O

42

. Therefore, it

could be excluded that the influence of the by-products on the catalytic activity of this material. Last, the content of MnO2 was detected by AAS measurement, and the results gave that the fraction of Mn was about 11% and 14.6% for MnO2/PILCNT and MnO2/CNT, respectively. In order to investigate the activity of these catalysts for ORR and OER, LSV of the as-prepared materials based cathode from 2.1 to 4.4 V (vs. Li/Li+) under a scan rate of 0.1 mV s-1 was conducted firstly. Under O2-saturated electrolyte, the curve of pure CNTs and PILCNT electrode showed an ORR peak at about 2.4 V (Fig. 5a). Meanwhile, the ORR potential for MnO2/CNT is 2.43 V which is slight higher than



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that of CNT and PILCNT. In contrast, for the MnO2/PILCNT electrode, the ORR potential increased to 2.5 V. For the OER procedure, there was only one ambiguous peak at about 4.0 V in the profile of CNTs which means the poor activity of pure CNTs. Nevertheless, the oxidation peak for PILCNT electrode shift to 3.7 V. Similar to MnO2/CNT, two well defined OER peak at 3.2 and 4.25 V appeared in the curve of MnO2/PILCNT which exhibits higher current density than MnO2/CNT electrode. The lower potential of OER peak and the increased current density all confirm the superior catalytic activity of MnO2/PILCNT electrode. On the contrary, the two peaks disappeared when the test was conducted in N2 atmosphere which indicates that the material is stable enough in this condition. This also demonstrates that the peaks mentioned above are really the responses of ORR and OER. So, this material has a potential to act as bifunctional catalyst for Li-O2 batteries.

Figure 5.

(a) LSV curves of CNT, PILCNT, MnO2/CNT and MnO2/PILCNT in

O2-saturated DMSO containing 0.1 M LiClO4. The gray line is the curve of


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MnO2/PILCNT in N2-saturated electrolyte. (b) The first discharge-charge curves of Li-O2 batteries with CNT, PILCNT, MnO2/CNT and MnO2/PILCNT electrodes at the current density of 100 mA g-1 in the range of 2.0-4.25 V. (c) Discharge-charge profiles with different catalysts measured at 100 mA g-1 with a 500 mAh g-1 cutoff specific capacity. (d) The capacity retention capability of Li-O2 batteries with CNT, PILCNT, MnO2/CNT and MnO2/PILCNT electrodes at various current densities. The variation in the discharge capacity (e) and energy efficiency (f) with cycles for CNT, PILCNT and MnO2/PILCNT electrodes.

And then, the electrochemical performance was determined by galvanostatic discharge-charge at a potential range of 2.0 to 4.25 V versus Li/Li+ with the current density of 100 mA g-1 (Fig. 5b). Due to the highest specific surface area and pore volume, the CNTs electrode exhibited the highest discharge capacity (2414 mAh g-1) while the other three cases were 1380 mAh g-1 (PILCNT), 1900 mAh g-1 (MnO2/CNT) and 1815 mAh g-1 (MnO2/PILCNT), respectively. The coulombic efficiency was 80.45 % for CNTs, 7.24 % for PILCNT and 100 % for MnO2/CNT and MnO2/PILCNT electrode, meaning the discharge capacity of the MnO2/CNT and MnO2/PILCNT electrode could be totally recharged under this condition. Meanwhile, the overpotential between discharge and charge was 0.97 V which is much lower than CNTs (1.58 V), MnO2/CNT (1.42 V) and PILCNT electrode. These results imply the superior activity for OER of MnO2/PILCNT compared to the other catalysts. In order to investigate the nature of the better catalytic activity of MnO2/PILCNT,



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the comparisons between different catalysts were performed, and the result is shown in Fig. 5c. The discharge potential was 2.72 V for MnO2/PILCNT electrode which was slight higher than that of the other samples (2.63 V). Nevertheless, the catalysts had a great influence on the charge potential. The CNTs and PILCNT electrodes, for instance, had a potential of 4.4 V at the end of charging process which means that the improvements can not be fulfilled by the adding of PIL sole. The lower potential (at about 4.15 V) for MnO2/CNT proves that MnO2 is capable of catalyzing the decomposition of discharge products. Furthermore, the MnO2/PILCNT electrode possessed the lowest charge potential (at about 3.92 V). Therefore, the excellent catalytic activity of MnO2/PILCNT may get benefit from the synergy effect of the intrinsic activity of MnO2 and the fewer oxygen-containing groups on the composite catalyst. Otherwise, based on the results metioned above, it is hard to evaluate the effect of PIL in MnO2/PILCNT on the performance of the battery. So, it is necessary to remove the PIL in order to explore the variation of the catalytic activity of this catalyst. According to the characterizations of TGA and FTIR, the most of PIL was removed after washing with toluene at high temperature (the sample was denoted as MnO2/PILCNT-W, Fig. S8a and Fig. S8b). It could be observed that the discharge potential of the battery based on MnO2/PILCNT-W electrode is similar to MnO2/PILCNT while the charge potential increased slightly. The overpotential was about 1.1 V which is 0.13 V higher than MnO2/PILCNT, but it is still much lower than CNT and PILCNT electrode. So, the PIL might play some role in the enhancement of catalytic activity of MnO2/PILCNT, but this effect should not be the main source for


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the high performance of this catalyst. The electrochemical performance of the batteries at different current densities was also measured to characterize the rate capability of the catalysts (Fig. 5d), and initial discharge curves are also shown in Fig. S7. The retention of capacity at 400 mA g-1 was 71.9 % for MnO2/PILCNT which is far higher than CNTs (33.8 %), MnO2/CNT (51.6 %) and PILCNT (55.1 %). In addition, compared to CNT and PILCNT, the cycling stability of the cathode is also enhanced by using the MnO2/PILCNT electrode. The capacity-limited method (400 mAh g-1) was adopted to investigate the cycling performance of the Li-O2 batteries, as illustrated in Fig. 5e. The results indicated that a reversible cycling of the CNTs and PILCNT based batteries can be maintained 18 and 34 cycles, respectively. The prolonged life of PILCNT electrode may derives from the protective effect of PIL which prevent the direct contact between CNT and electrolyte or Li2O2. However, the MnO2/PILCNT based cathode can be sustained over 42 cycles (the discharge-charge profiles of cycles were presented in Fig. S9). But, for MnO2/PILCNT electrode, it could be observed that the variations of the discharge-charge profiles with the increase of cyclic numbers were more significant than CNT and PILCNT electrodes. As we know, the decomposition of electrolyte in Li-O2 batteries is inevitable which results in the accumulation of by-products in cathode, such as Li2CO3. On the other hand, the charging potential for MnO2/PILCNT was about 3.85 V in early cycles which is lower than the decomposition potential of Li2CO3. Thus, the charging potential increased with the accumulation of by-products. However, partial by-products would be decomposed due



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to the higher charging potential of CNT and PILCNT electrode (higher than 4.15 V). Based on this, the variations of the discharge-charge profiles are more significant than the other two samples. Remarkably, the energy efficiency of the MnO2/PILCNT cathode was about 75 % in the earliest cycles while the values were lower than 65 % after 35 cycles (Fig. 5f). In contrast, for the sample of pure CNT and PILCNT, the energy efficiencies were only about 65 % in the initial cycles. Additionally, the energy efficiency of MnO2/PILCNT electrode was always higher than the two others. Clearly, the MnO2/PILCNT is a more excellent cathode catalyst for Li-O2 cells in view of the energy efficiencies.

Figure 6. SEM image of (a) CNTs, (b) PILCNT and (c) MnO2/PILCNT electrode after discharging. (d) The high-resolution Li 1s spectrum of the MnO2/PILCNT electrode at different stages.


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The size and morphology of discharge products are essential to the performance of Li-O2 batteries. As we all know that the Li2O2 with small particles often exhibit a sloping profile coupled with low overpotential 48. The similar result was also obtained by Chen’s group who tested the charge performance of Li2O2 with different particle sizes prepared by ball-milling

49

. It was found that the average voltage of charge

plateaus reduced as the particle size decreases which owing to the diminished electrode polarization and enhanced kinetics of the oxidation reaction of Li2O2. Therefore, the variations in the morphology and the species on the surface of cathodes at different stages were detected by SEM and XPS. For the three samples, the electrodes before discharge are all clean and possessing porous structure including PILCNT and MnO2/PILCNT which were mainly the pores packed with large particles. Inevitably, there were no signs corresponding to Li-containing species in the XPS spectra of Li 1s in all the electrodes (Fig. S10, Fig. S11 and Fig. 6d). After discharging, there were some particles about 1-2 µm observed in the electrode of CNT and PILCNT which totally clogged the pores (Fig. 6a and Fig. 6b). Unexpectedly, the size of Li2O2 particles was reduced to less than 0.5 µm when the MnO2/PILCNT electrode was used as illustrated by Fig. 6c and Fig. S12b. The XPS results of the electrodes after discharging indicated that the main product was Li2O2 according to the peak at 54.5 eV (Fig. S10c, Fig. S11c and Fig. 6d). However, there were some Li2CO3 remained on the electrodes as the residues even though their morphologies were recovered on the basis of the SEM images (Fig. S10b, Fig. S11b and Fig. S12c). Actually, the Li2CO3, as the by-products in nonaqueous Li-O2 batteries, is often



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detected due to its higher decomposition potential. However, it should be mentioned that the peak area of LiCO3 for CNT electrode was smaller that the other two samples. Obviously, the reduction of the size of Li2O2 is beneficial to the decrease of charge potential. On the basis of the characterizations presented above, the MnO2/PILCNT, which possesses fewer oxygen-containing groups, shows superior activity for OER. It could be concluded that the intrinsic catalytic ability of MnO2 and the formation of confined Li2O2 particles all facilitate the decomposition of Li2O2. It was found that PIL could not reduce the size of Li2O2 particles solely. Besides, the size is still large for MnO2 based electrode when a lot of oxygen containing groups exist. Therefore, even though the mechanism is still unclear, it could be speculated that the fewer oxygen-containing groups on the surface and the activity of MnO2 contribute to the formation of confined Li2O2 particles, as the conclusion made by Zhang’s group

35

. As a result, for carbon

materials involved catalysts, tuning of the oxygen functional groups is vital to optimize the catalytic activity for ORR and OER.

4. CONCLUSIONS In summary, polymerized [VEIm]NTf2 was integrated with CNTs which was wrapped by PIL completely. Then, the composite was used to support MnO2 utilizing KMnO4 solution. The as-prepared material with fewer oxygen-containing groups severed as a cathode catalyst for Li-O2 batteries in this work. The enhanced performance including lower charge potential, higher coulombic efficiency, good rate capability and superior cycling life with higher energy efficiency was realized


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employing this catalyst. What is more, the formation of confined Li2O2 particles could be responsible for the improvement of performance of Li-O2 batteries due to the synergy effect of the intrinsic catalytic activity of MnO2 and the fewer oxygen containing groups on the surface. This work provides a new strategy to develop superior cathode catalysts for high performance Li-O2 batteries.

ASSICIATED CONTENT Supporting Information Available SEM and TEM images of CNTs and PILCNT. Mn 2p XPS spectra and TEM images of MnO2/CNT. BET tests of CNTs and PILCNT. The survey XPS spectra of PILCNT and MnO2/PILCNT. The initial discharge curves and discharge-charge profiles of cycles for CNTs, PILCNT, MnO2/CNT and MnO2/PILCNT at different current density. SEM images and Li 1s spectrum of CNTs, PILCNT and MnO2/PILCNT electrodes at different stages. Author information Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21403259, 21373247).

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CNTs based MnO2 composite catalyst with fewer oxygen containing groups was used as cathode for Li-O2 battery and shows excellent catalytic activity for OER.


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Manganese Dioxide Composite

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