Peanut shaped MnCo2O4 winded by multi-walled carbon nanotubes as an efficient cathode catalyst for Li-O2 batteries

Accepted Manuscript Peanut shaped MnCo2O4 winded by multi-walled carbon nanotubes as an efficient cathode catalyst for Li-O2 batteries Faquan Zhu, Jianbo Zhang, Bingqian Yang, Xueru Shi, Changjian Lu, Jiguang Yin, Yawei Yu, Xiulan Hu PII:

S0925-8388(18)31171-X

DOI:

10.1016/j.jallcom.2018.03.295

Reference:

JALCOM 45525

To appear in:

Journal of Alloys and Compounds

Received Date: 24 November 2017 Revised Date:

27 February 2018

Accepted Date: 23 March 2018

Please cite this article as: F. Zhu, J. Zhang, B. Yang, X. Shi, C. Lu, J. Yin, Y. Yu, X. Hu, Peanut shaped MnCo2O4 winded by multi-walled carbon nanotubes as an efficient cathode catalyst for Li-O2 batteries, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.03.295. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

ACCEPTED MANUSCRIPT [Title page] Peanut shaped MnCo2O4 winded by multi-walled carbon nanotubes as an

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efficient cathode catalyst for Li-O2 batteries

Faquan Zhu1,2,3, Jianbo Zhang1,2,3, Bingqian Yang1, Xueru Shi1, Changjian

College of Materials Science and Engineering, Nanjing Tech University, China 2

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Lu1,2,3, Jiguang Yin1,2,3, Yawei Yu1,2,3, Xiulan Hu1,2,3,*

The Synergetic Innovation Center for Advanced Materials， ，China

Jiangsu National Synergetic Innovation Center for Advanced Materials

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(SICAM), China

Correspondence information: Xiulan Hu, College of Materials Science and

Engineering, Nanjing Tech University, Xin-Mo-Fan Road No. 5, 210009, Nanjing,

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Jiangsu, China, [email protected], +86 152 4022 7230

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ACCEPTED MANUSCRIPT Peanut shaped MnCo2O4 winded by multi-walled carbon nanotubes as an efficient cathode catalyst for Li-O2 batteries

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Faquan Zhu1,2,3, Jianbo Zhang1,2,3, Bingqian Yang1, Xueru Shi1, Changjian Lu1,2,3, Jiguang Yin1,2,3, Yawei Yu1,2,3, Xiulan Hu1,2,3,* 1

College of Materials Science and Engineering, Nanjing Tech University, China The Synergetic Innovation Center for Advanced Materials，China

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Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), China

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Correspondence information: Xiulan Hu, College of Materials Science and

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Engineering, Nanjing Tech University, Xin-Mo-Fan Road No. 5, 210009, Nanjing,

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Jiangsu, China, [email protected], +86 152 4022 7230

(Our preference for color artworks: online only)

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ACCEPTED MANUSCRIPT Abstract: Rechargeable Li-O2 batteries have great potential on achieving large-capacity electrochemical energy storage. However, the poor cyclic stability seriously limits its

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commercial application. In this paper, peanut shaped MnCo2O4 which winding by multi-walled carbon nanotubes (MCO/MWCNTs) were synthesized through a facile solvothermal method. As-prepared MCO/MWCNTs have a unique hierarchical

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mesoporous of three-level pore sizes and show good ORR and OER electrocatalytic

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activity. When MCO/MWCNTs served as cathode catalysts in Li-O2 batteries, the batteries exhibited long cycle life of 120 times at 100 mA g-1 with a limited capacity of 500 mAh g-1 and high discharge capacity up to 8849 mAh g-1 with a restrict voltage of 2 V at 100 mA g-1.

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Keywords: Li-O2 batteries; Peanut shaped MnCo2O4; Hierarchical mesoporous; Electrocatalyst

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1. Introduction

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Because of the increasingly energy and environment crisis, conventional fossil-fueled vehicles are gradually being replaced by electric vehicles (EVs). Although Li ion batteries have been used as main power source for EVs, their lower energy density largely limits driving distance of EVs. In recent years, rechargeable Li-O2 batteries have become dramatically for its extremely high theoretical energy density of 3600 Wh kg-1 based on the reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of 2 Li + O2 ↔ Li2O2 [1], which is much higher 3

ACCEPTED MANUSCRIPT than that of Li ion batteries [2, 3]. Thus, Li-O2 batteries are considered as a promising candidate power source for EVs. Since Li-O2 batteries was first studied by Abrahamin in 1996 [4], several breakthrough progresses have been achieved [5-8]. However,

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Li-O2 batteries still suffer from poor cycle life and fast capacity fading, which seriously limit its commercialization. The major reasons are that the insoluble and nonconductive discharge products such as Li2O2, which usually deposits on the

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cathode and hence blocks the further contact of Li ion and O2. Current researches

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have proved that cathode catalysts can promote the electrochemical kinetics of ORR and OER, which can effectively improve the cycle stability of Li-O2 batteries [8, 9]. Some transition metal oxides have shown high electrocatalytic activity in Li-O2 batteries, especially the Co3O4 [10, 11] and MnO2 [12, 13]. And a kind of mixed spinel

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transition metal oxide, MnCo2O4, is also used as catalysts for Li-O2 batteries and performs higher electrocatalytic activity [14-19]. As is well known, the morphology and structure usually have significant influence on the performance of catalysts.

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MnCo2O4 prepared by solvothermal method usually have rich mesoporous, which can

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provide efficient tunnel to transmit Li ion and O2, and expose more active sites [15-17, 19]. Moreover, MnCo2O4 has lower price than noble metal and more stable chemical property than carbon materials. Multi-porous MnCo2O4 microspheres inducing by solvothermal method were studied as cathode catalysts in Li-O2 batteries and performs high electrocatalytic activity [15]. And porous MnCo2O4 hollow nanocages synthesized by solvothermal method delivered higher electrocatalytic performance with a long cycle life of 70 times [17]. Although excellent electrocatalytic activity of 4

ACCEPTED MANUSCRIPT MCO has been proven, the inherent poor electronic conductivity may seriously limit the further electrochemical performance of MCO in Li-O2 batteries. In general, multi-walled carbon nanotubes (MWCNTs) have great value on

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electrochemical energy storage due to its high electronic conductivity, superior resilience and better mechanical strength [20-22]. Herein, in order to improve the electronic conductivity of MCO, the peanut shaped MCO winded by MWCNTs

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(MCO/MWCNTs) with hierarchical mesoporous of three-level sizes were synthesized

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through a facile solvothermal method. The electrochemical and electrocatalytic performance of MCO/MWCNTs were investigated in details in this study.

2． ．Experimental section

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2.1 Chemicals

MnSO4·H2O, CoSO4·7H2O and CO(NH2)2 were purchased from XILONGD SCIENTIFIC Co., Ltd (Shantou, China). Ethylene glycol was purchased from

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Shanghai No.4 Reagent & H V Chemical Co., Ltd (Shanghai, China). H2SO4 was

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purchased from Shanghai Ling feng chemical reagent, Co., Ltd (Shanghai, China). Multi-walled carbon nanotubes (MWCNTs) and acetylene black (AC) were purchased from Nanjing XFNANO Materials Tech Co., Ltd (Nanjing, China). All the chemical reagents were analytical grade and used without further purification. 2.2 Synthesis of MCO/MWCNTs To prepare functionalized MWCNTs, 120 mg commercial MWCNTs were treated by 50 ml H2SO4 (95%) and 2 M KOH solution. After washing with ethanol and water, 5

ACCEPTED MANUSCRIPT the treated MWCNTs were dried for use. For synthesis of MCO/MWCNTs, 2 mmol MnSO4·H2O and 4 mmol CoSO4·7H2O were dissolved into a mixture solvent of ethylene glycol (60 ml) and

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distilled water (5 ml), the mixture was continually stirred for 1 hour. 30 mmol CO(NH2)2 were then dispersed into the above solution keeping stirring until it became transparent. Next, 60 mg functionalized MWCNTs were dispersed into the above

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mixture solution. After stirring for another 1 hour, the result suspension was

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transferred into a 100 ml Teflon-lined stainless steel autoclave and heated in an electric oven at 200 °C for 24 h. After cooling to room temperature naturally, the precipitates were washed and dried at 60 °C overnight. Finally, MCO/MWCNTs were obtained after heating treatment of precipitates in argon atmosphere at 600 °C for 2 h

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(2 °C min-1). For comparison, pure MCO was also prepared by the same method. 2.3 Materials characterization

Phase compositions of all products was analyzed by powder X-ray diffraction

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(XRD, Rigaka Smartlab) with Cu Kα (λ=1.5418 Å) incident radiation at 30 kV

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voltage and 40 mA current. XRD patterns were recorded from 10 ° to 70 ° (2 θ) with a scanning step of 10 °/min. The size and morphology of products were examined by a field emission scanning electron microscopy (FE-SEM HITACHI S4800). The specific surface area of MCO/MWCNTs was measured by a Brunauer–Emmett–Teller (BET, ASAP 2020) method using nitrogen adsorption and desorption isotherms on a micromeritics instrument corporation sorption analyze. X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTR DLD) with an Al X-ray source was carried 6

ACCEPTED MANUSCRIPT out to detect the chemical element valence of the sample. 2.4 Electrochemical measurements The electrochemical performance was tested with a three-electrode system in O2

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saturated 0.1 M KOH solution on CHI 750 electrochemical workstation. For preparing catalyst ink, 10 mg MCO/MWCNTs and AC (weight ratio of 1:1) were ultrasonically dispersed in 2 ml deionized water containing 20 µl Nafion solution (5

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wt %, DuPont, USA) for at least 30 min to form a homogeneous ink. Then, 10 µl of

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as-prepared ink was pipetted onto the rotating disc electrode (RDE, φ=5 mm). After drying at room temperature, the catalyst-modified RDE was used as working electrode. Ag/AgCl electrode and platinum electrode were separately used as reference electrode and counter electrode. The cyclic voltammetry (CV) was tested

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between -0.6−0.4 V (vs Ag/AgCl) at a sweep rate of 10 mV s-1. The liner sweep voltammetry was tested at a sweep rate of 10 mV s-1 between -0.6−0.4 V (ORR) and 0.4−1 V (OER). The rotating speed is fixed to 1600 rpm. For comparison,

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electrochemical properties of MWCNTs, AC and MCO/AC were measured, too.

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2.5 Fabrication of Li-O2 batteries and cell measurements The electrocatalytic performance of MCO/MWCNTs was performed by

CR2032-type coin cells, consisted of a lithium metal disc, a piece of glass fiber that soaked with electrolyte containing 1 M LiTFSI in tetraethylene glycol dimethylether and a working electrode. For preparing the working electrode, MCO/MWCNTs, AC and PVDF (polyvinylidene fluoride) were mixed in NMP (N-methyl-2-pyrrolidone) solution with a weight ratio of 4.5:4.5:1. The slurry was then brushed onto a piece of 7

ACCEPTED MANUSCRIPT carbon cloth (15 mm in diameter) and dried in a vacuum oven at 80 °C for 24 h. The mass loading of each was 0.8 mg cm-2. For comparison, pure AC and MWCNTs mixed PVDF with a weight ratio of 9:1 were also separately prepared. All the Li-O2

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batteries were assembled in an argon filled glovebox with water and O2 contents below 0.1 ppm. The batteries were rested for 24 h in glovebox before testing. Galvanostatic charge-discharge measurements were conducted on a battery tester

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(LAND CT2001A) at room temperature in a pure/dry O2 atmosphere. The specific

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capacity was speculated based on the total mass of brushed on the carbon cloth. Cyclic voltammetry (CV) were conducted on a Zennium electrochemical workstation (Germany) at a sweep rate of 0.5 mV s-1 between 2.0−4.5 V (vs Li/Li+).

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3. Results and discussions

3.1 Component and morphology characterization Fig. 1 is the XRD pattern of the as-prepared sample, several main diffraction peaks

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at 2 θ of 18.5 °, 30.5 °, 36.0 °, 37.6 °, 43.8 °, 54.3 °, 57.9 ° and 63.6 ° are observed,

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which can be respectively indexed to the (111), (220), (311), (222), (400), (422), (511) and (440) crystal plane of MnCo2O4 (JCPDS Card No. 23-1237). There is an obvious bun peak from 10 ° to 30 °, which suggests the presence of MWCNTs. These results of XRD demonstrate the products are primarily composed by MCO and MWCNTs.

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Fig. 1 XRD pattern of MCO/MWCNTs.

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The morphology of products were investigated by FE-SEM. Fig. 2a−c are the FE-SEM images of precursors directly obtained by co-precipitation during solvothermal process, which consist of MnCO3 and CoCO3 with good crystallinity and MWCNTs by XRD, respectively (Fig. S1). Fig. 2d−f are the FE-SEM images of

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MCO/MWCNTs obtained after heating treatment. These results clarified the morphology of MCO winded by MWCNTs. Compared with the Mn and Co-carbonate

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precursors, MCO was porous which may be caused by the gas escaping inducing by heating treatment (Fig. 2c and 2f). As can be seen from the Fig. 2f, the as-prepared

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MCO show approximate shape of a peanut with length of 1µm and diameter of 500 nm (spheres at both ends). As shows in Fig. 2b and 2e, the peanuts were winded by MWCNTs, which may effectively improve the electronic conductivity of MCO. Fig. 2f clearly shows that the peanut was assembled by numerous nanoparticles, and many mesoporous existed among the nanoparticles. Therefore, a large number of pores may provide tunnels for fast ion transport and O2 transport, and more attachment sites for discharge products. 9

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Fig. 2 FE-SEM images of products. (a−c) Precursors of MCO/MWCNTs obtained by co-precipitation during solvothermal process; (d−f) MCO/MWCNTs obtained after heating treatment.

The synthesis procedure of MCO/MWCNTs can be illustrated as follows (Fig. 3).

Firstly, peanut shaped Mn and Co-carbonate precursors formed and were winded by MWCNTs during the solvothermal process. In this process, CO(NH2)2 served as the growth promoter. In the assistance of MWCNTs, the forming of peanut shaped Mn 10

ACCEPTED MANUSCRIPT and Co-carbonate precursors can be assumed as a heterogeneous nucleation process. Secondly, porous MCO/MWCNTs were obtained by further heating treatment. Part of carbon came from the MWCNTs joined in the solid phase reaction of Mn,

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Co-carbonate precursors in argon atmosphere at 600 °C during the heating treatment. Based on the above analysis, the chemical reactions process may be speculated as

CO (NH2)2 + 2H2O → 2NH4+ + CO32-

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Mn2+ + CO32- → MnCO3 ↓

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follows:

(1)

(2) (3)

MnCO3 + 2CoCO3 + 2C → MnCo2O4 + 5CO ↑

(4)

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Co2+ + CO32- → CoCO3 ↓

Fig. 3 Schematic illustration for the synthesis process of MCO/MWCNTs.

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To investigate the specific surface area and pore size of MCO/MWCNTs,

Brunauer–Emmett–Teller (BET) gas-sorption measurements were performed. The N2 adsorption–desorption isotherm of MCO/MWCNTs at 77 K is shown in Fig. 4. A specific surface area of 79.1 m2 g-1 was obtained, which is between MCO (16.1 m2 g-1, Fig.S2a) and MWCNTs (228.4 m2 g-1, Fig.S2b). According to the BJH plots (insert in Fig. 4), the pore size distribution ranges mainly from 4 nm to 100 nm, and the average pore size is classified to three levels of 13 nm, 24 nm and 32 nm, respectively. Noted 11

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different from the previous reports [15, 17-19, 23]. These results suggests the hierarchical mesoporous should be attributed to the morphology of MCO winded by MWCNTs. This different pores are connected as channels which may accelerate the

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filtration of electrolyte and provide more efficient diffusion path for Li ion and O2 and

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more space for discharge products.

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Fig. 4 N2 adsorption-desorption isotherm at 77 K of MCO/MWCNTs and pore size distribution (insert).

The information about the chemical element valence of MCO/MWCNTs was

investigated by X-ray photoelectron spectroscopy (XPS). The survey spectrum shows the presence of Mn, Co, O and C elements (Fig. 5a). In Co 2p spectrum (Fig. 5b), two spin-orbital of Co 2p3/2 and Co2p1/2 peaks can be clearly observed at binding energies of 780.5 eV and 795.7 eV with two apparent shakeup satellites (labelled as Sat.) 12

ACCEPTED MANUSCRIPT located at 786.3 eV and 803.3 eV, respectively [24], which indicates the coexistence of Co2+ and Co3+ [22]. Similarly, in Mn 2p spectrum (Fig. 5c), two main peaks at 642.0 eV and 653.7 eV are indexed to the Mn 3/2p and Mn 1/2p. After refined fitting,

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the two main peaks can be divided into four subpeaks: two peaks at 641.6 and 653.3 eV are assigned to the binding energy of Mn2+, while other two peaks at 643.4 and 654.2 eV are ascribed to Mn3+ [25]. These results of XPS demonstrate the coexistence

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of Mn3+/Mn2+ and Co3+/Co2+, which may provide high electrocatalytic activity toward

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ORR and OER. The deconvoluted spectra of O 1s region is shown in Fig. 5d, it shows that there are four peaks at 529.8, 531, 533.1 and 533.9 eV. The binding energy at 530.2 eV is attributed to the O2- forming oxide with Mn and Co, and the other three peaks can be ascribed to OH-, C-O and O-C=O, respectively [26, 27]. It is reported

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that acid treatment could roughen the exterior walls of the MWCNTs and thus exposes carbon atoms on the edge sites [28]. Lee et al. [29] find that edge carbon atoms could connect with oxygenated species more strongly than that in the basal

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plane, which is consist with the spectrum of C 1s (Fig. 5e). In C 1s spectrum, four

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peaks located at 284.4, 285.1, 286.6 and 289.0 eV are observed and can be assigned to the C-C, C-H, C-O and O-C=O bonds [26, 30]. These functional groups on MCO and MWCNTs may lead to some defect structure, which might provide additional reaction active sites for Li ion and O2 [28].

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Fig. 5 XPS spectra of (a) survey spectrum; (b) Co 2p; (c) Mn 2p; (d) O 1s; and (e) C 1s for MCO/MWCNTs.

3.2 Electrochemical and electrocatalytic performances

The electrochemical performance of MCO/MWCNTs was first evaluated by the

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rotating disc electrode (RDE) in O2-saturated 0.1 M KOH. Though the practical Li-O2 batteries use nonaqueous organic electrolyte as the electrolyte, the electrochemical

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activity of catalyst in aqueous solution is still meaningful to evaluate the performance of catalyst for ORR and OER in real Li-O2 batteries [10, 31, 32]. Fig. 6a shows the

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CV curves of MCO/MWCNTs/AC, MWCNTs, AC and MCO/AC. Comparing these four samples, the MCO/MWCNTs/AC exhibits more positive ORR potential and higher peak current. The linear scanning voltammogram (LSV) at 1600 rpm also illustrates

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situation

(Fig.

6b).

The

half-wave

potential

of

MCO/MWCNTs/AC is about -0.26 V, more positive than that of MWCNTs (-0.31 V), AC (-0.39 V) and MCO/AC (-0.30 V). The limiting diffusion current of MCO/MWCNTs/AC (2.37 mA cm-2) is also higher than that of MWCNTs (0.65 mA 14

ACCEPTED MANUSCRIPT cm-2), AC (0.90 mA cm-2) and MCO/AC (1.56 mA cm-2). These results reveal high ORR activity of MCO/MWCNTs. And their OER activities were also studied (Fig. 6c). According to the curves tested at 1600 rpm, the limiting diffusion current of

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MCO/MWCNTs/AC reaches to 28.59 mA cm-2, much more than that of MWCNTs (7.81 mA cm-2), AC (1.91 mA cm-2) and MCO/AC (15.78 mA cm-2). The result indicates the high electrochemical activity of MCO/MWCNTs toward OER.

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In order to further explore the electrochemical activity toward ORR and OER,

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CV curves were tested in 1 M LiTFSI/TEGDME. As can be seen from Fig. 6d, MCO/MWCNTs/AC presents more positive onset ORR potential and much higher peak current than MWCNT and AC. There is also an obvious OER peak at 3.28 V, which is also higher than MWCNTs and AC. Though the onset ORR potential of

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MCO/AC is almost same as that of MCO/MWCNTs/AC, the ORR peak current of MCO/MWCNTs/AC is much higher than that of MCO/AC. Besides, the OER peak current of MCO/AC is much lower than MCO/MWCNTs/AC. The above comparison

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results suggest the main electrocatalytic activities toward ORR/OER are attributed to

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MCO instead of MWCNTs. Meanwhile, MWCNTs are also significant for the improvement of MCO with electrochemical performances, which may be attributed to the improved electronic conductivity. All of the above results indicate that MCO/MWCNTs have good bi-functional electrocatalytic activity toward ORR/OER in both alkaline and organic electrolyte.

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Fig. 6 (a) CV curves of MCO/MWCNTs/AC, MWCNTs, AC and MCO/AC in O2-saturated 0.1 M KOH; (b−c) LSV curves of MCO/MWCNTs/ AC, MWCNTs, AC

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and MCO/AC on RDE at 1600 rpm in 0.1 M KOH; (d) CV curves of MCO/MWCNTs/AC, MWCNTs AC and MCO/AC in 1 M LiTFSI/TEGDME.

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To deeply characterize the electrocatalytic activity of MCO/MWCNTs, the Li-O2 batteries were assembled and tested between 2−4.5 V at different current densities.

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Fig. 7 shows the cycle performances of Li-O2 batteries with MCO/MWCNTs/AC, MWCNTs and AC. The capacity-limited approach is usually used to avoid larger capacity depth so that the batteries performance can be well evaluated. With a limited capacity of 500 mAh g-1 at 100 mA g-1, the Li-O2 batteries with MCO/MWCNTs/AC can effectively cycle to 120 times and no obvious voltage attenuation (Fig. 7a), which is more stable than that of the previous reports [17, 18]. Meanwhile, the Li-O2 batteries with MWCNTs and AC could only cycle 54 times (Fig. 7b) and 37 times 16

ACCEPTED MANUSCRIPT (Fig. 7c). Fig. 7d shows the comparison of cycle performance. The Li-O2 batteries with MCO/MWCNTs/AC has longer cycle life and better cycle stability than MWCNTs and AC. The excellent cycle stability of Li-O2 batteries with

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MCO/MWCNTs may owe to the good bi-functional electrocatalytic activity of MCO and the improved electronic conductivity inducing by the winded MWCNTs. However, the terminal discharge voltage gradually comes down after 120 cycles.

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When cycling to the 125th cycle, the terminal discharge voltage has already dropped to

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below 2 V. According to the previous reported, there are three main factors which could led to the attenuation. Firstly, the gradually accumulation of insulated discharge products (Li2O2) in the surface of cathode could block the pathways for oxygen diffusion and electron transfer [33, 34]. Secondly, the by-products (such as Li2CO3)

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formed in the discharge process need high charge potential, which results in electrode passivation and capacity fading [35, 36]. Besides, the decomposition of the electrolyte caused the decrease of the electrolyte level and the corrosion of the lithium anode may

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be possible factors for the declined performances [19].

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Fig. 7 Charge-discharge curves of Li-O2 batteries with (a) MCO/MWCNTs/AC; (b)

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MWCNTs; (c) AC with a limited capacity of 500 mAh g-1 at 100 mA g-1 and (d) the comparison of cycle performance.

Further, the Li-O2 battery with MCO/MWCNTs/AC also performs higher

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discharge capacity. The first full discharge capacity reaches to 8849 mAh g-1 with a

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restrict voltage of 2 V at 100 mA g-1 (Fig. 8a), which is much higher than that of MWCNTs (5363 mAh g-1, Fig. S3a) and AC (5175 mAh g-1, Fig. S3b), and also higher than that of the previous reports about MCO [14, 15, 17, 18, 23]. The excellent discharge capacity may be attributed to the good ORR and OER electrocatalytic activity of MCO/MWCNTs. When the current density increases to 200 mA g-1, 400 mA g-1 and 800 mA g-1, the discharge capacity reduces to 7172 mAh g-1 (81%), 4884 mAh g-1 (55%) and 4278 mAh g-1 (48%), respectively. It’s worth noting that the 18

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property of the batteries with MCO/MWCNTs/AC at large current densities, which may be attributed to the connected channels inducing by three-level pore of

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MCO/MWCNTs and therefore providing fast transport channels for Li ion and O2.

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Fig. 8 (a) First full discharge curves of Li-O2 batteries with MCO/MWCNTs/AC with a restrict voltage of 2 V at current densities of 100 mA g-1, 200 mA g-1, 400 mA g-1

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and 800 mA g-1; (b) The comparison of first full discharge capacity for Li-O2 batteries with MCO/MWCNTs/AC, MWCNTs and AC at different current densities.

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4. Conclusion

In summary, peanut shaped MCO/MWCNTs with unique hierarchical mesoporous

of three-level pore sizes were synthesized through a facile solvothermal method. The MCO/MWCNTs shown good bi-functional electrocatalytic activity toward ORR and OER in both aqueous and nonaqueous solution. Correspondingly, the Li-O2 batteries with MCO/MWCNTs exhibited a long cycle life of 120 times with a limited capacity of 500 mAh g-1 at 100 mA g-1 and high discharge capacity of 8849 mAh g-1 with a 19

ACCEPTED MANUSCRIPT restrict voltage of 2 V at 100 mA g-1. The enhanced electrochemical performance of the MCO/MWCNTs could be attributed to the improved electronic conductivity by MWCNTs and the unique hierarchical mesoporous which promotes electrolyte

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infiltration and accelerates the transport for Li ion and O2. It can be believed that MnCo2O4 is a promising cathode catalyst for Li-O2 batteries by mixing with MWCNTs. This work also offers an example of composites synthesis strategy for

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transition metal oxide and MWCNTs, which performs wide application prospect. Acknowledgements

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This work was supported by the National Natural Science Foundation of China (Grant No. 51772148, 51372113), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP, PPZY2015B128), and the Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions

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ACCEPTED MANUSCRIPT Figure Captions: Fig. 1 XRD pattern of MCO/MWCNTs. Fig. 2 FE-SEM images of products. (a−c) Precursors of MCO/MWCNTs obtained by

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co-precipitation during solvothermal process; (d−f) MCO/MWCNTs obtained after heating treatment.

Fig. 3 Schematic illustration for the synthesis process of MCO/MWCNTs.

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Fig. 4 N2 adsorption-desorption isotherm at 77 K of MCO/MWCNTs and pore size

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distribution (insert).

Fig. 5 XPS spectra of (a) survey spectrum; (b) Co 2p; (c) Mn 2p; (d) O 1s; and (e) C 1s for MCO/MWCNTs.

Fig. 6 (a) CV curves of MCO/MWCNTs/AC, MWCNTs, AC and MCO/AC in

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O2-saturated 0.1 M KOH; (b−c) LSV curves of MCO/MWCNTs/ AC, MWCNTs, AC and MCO/AC on RDE at 1600 rpm in 0.1 M KOH; (d) CV curves of MCO/MWCNTs/AC, MWCNTs AC and MCO/AC in 1 M LiTFSI/TEGDME.

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Fig. 7 Charge-discharge curves of Li-O2 batteries with (a) MCO/MWCNTs/AC; (b)

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MWCNTs; (c) AC with a limited capacity of 500 mAh g-1 at 100 mA g-1 and (d) the comparison of cycle performance. Fig. 8 (a) First full discharge curves of Li-O2 batteries with MCO/MWCNTs/AC with a restrict voltage of 2 V at current densities of 100 mA g-1, 200 mA g-1, 400 mA g-1 and 800 mA g-1; (b) The comparison of first full discharge capacity for Li-O2 batteries with MCO/MWCNTs/AC, MWCNTs and AC at different current densities.

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ACCEPTED MANUSCRIPT Highlights


Peanut shaped MnCo2O4 winded by multi-walled carbon nanotubes (MCO/MWCNTs) were facile synthesized MCO/MWCNTs have hierarchical mesoporous of three-level pore sizes




The assembled Li-O2 batteries exhibit a long cycle life of 120 times

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