Top-Down Strategy to Synthesize Mesoporous Dual Carbon Armored

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A Top-down Strategy to Synthesize Mesoporous Dual Carbon Armored MnO Nanoparticles for Lithium-ion Battery Anodes Wei Zhang, Jiannian Li, Jie Zhang, Jinzhi Sheng, Ting He, Meiyue Tian, Yufeng Zhao, Changjun Xie, Liqiang Mai, and Shichun Mu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16576 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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ACS Applied Materials & Interfaces

A Top-down Strategy to Synthesize Mesoporous Dual Carbon Armored MnO Nanoparticles for Lithium-ion Battery Anodes Wei Zhanga, Jiannian Li a, Jie Zhanga, Jinzhi Shengb, Ting Hea, Meiyue Tian,Yufeng Zhaoc, Changjun Xied,*, Liqiang Maib and Shichun Mua,* a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China.

b

WUT-Harvard Joint Nano Key Laboratory, Wuhan University of Technology, Wuhan, 430070, China.

c

Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, China

d

School of Automation, Wuhan University of Technology, Wuhan, 430070, China.

*Corresponding author. E-mail: [email protected], [email protected]

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ABSTRACT

To overcome inferior rate capability and cycle stability of MnO-based materials as a lithium-ion battery anode associated with the pulverization and gradual aggregation during conversion process, we construct robust mesoporous N-doped carbon (N-C) protected MnO nanoparticles on reduced graphene oxide (rGO) (MnO@N-C/rGO) by a simple top-down incorporation strategy. Such dual carbon protection endows MnO@N-C/rGO with excellent structural stability and enhanced charge transfer kinetics. At 100 mA g-1, it exhibits superior rate capability as high as 864.7 mAh g-1 undergoing the deep charge/discharge for 70 cycles and outstanding cyclic stability (after 1300 cyclic tests at 2000 mA g-1, 425.0 mAh g-1 is remained accompanying merely 0.004% capacity decay per cycle). This facile method provides a novel strategy for synthesis of porous electrodes by making use of highly insulating materials.

KEYWORDS: lithium-ion battery, anode, manganese monoxide, graphene oxide, nitrogen doping

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INTRODUCTION Although rechargeable lithium ion batteries (LIBs) have been extensively used as power sources, their power and energy densities remain insufficient to power such devices.1-5 Increasing the overall capacity of LIBs requires the development of critical active electrode materials for both anodic and cathodic sides with higher specific capacities.6-7 Considering that the specific capacity of current available cathode materials is less than 200 mAh g-1, the significant improvement for total capacity of LIBs could be witnessed if the commercially-used anode (graphite, 372 mAh g-1) is replaced by the one with multiple times capacity. In this regard, as a perspective anode material toward high-energy density LIBs, transition metal oxides (TMOs) based on the conversion reaction mechanism with high rechargeable capacities have been probed.2-4,8-11 Among TMOs, MnO materials have been widely used for LIBs due to their low conversion potentials, low cost, high densities, high theoretical capacity (756 mAh g-1), environment-friendliness, and plentiful metal manganese resources.12-15 Nevertheless, like other metal oxides, MnO-based electrodes meet obstacles of unsatisfactory cyclability and rate performance because of the inferior electronic conductivity, self-aggregation

and

evident

volume

change

of

MnO

during

Li-ion

intercalation/deintercalation processes.15-18 Thus, the evolution of MnO-based materials that show splendid electrochemical performance still remains a great challenge.

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Hybridizing metal oxide materials with functional carbon provide an advanced avenue for elevating the lithium-storage property and cyclability, which can be ascribed to introduction of carbon with high electrical conductivity and alleviates pulverization of highly insulating electrode materials during lithiation/delithiation processes. On this count, carbon has been introduced

recently

on

the

surface

of

nanoscale

metal

oxides

for

practical

applications,10,12-13,18-21 and indeed exhibited enhanced electrochemical performance. However, after successive discharge-charge, the high stability of metal oxide-carbon hybrid electrode is always difficult to obtain owing to self-aggregation of metal oxides for LIBs. As reported, to date the high capacity retention is only achieved under hundreds of charge and discharge cyclic tests, which is far from enough for practical implementation. Nowadays, some new designs of metal oxides have been employed.15, 22-23 However, it is of significance to develop a simple, efficient and economical method for constructing nanoscale metal oxides with conducting carbon for promising applications, and further improvement in electrochemical performance is extremely desirable. Herein, using readily acquired one dimensional MnO2 as precursor, a facile synthesis via a solution-based method followed by a thermal decomposition process to form the mesopore MnO@N-C/rGO hybrid is developed. Among which, zero dimensional MnO nanoparticles are decorated with nitrogen doped carbon (N-C) and reduced graphene oxide (rGO). As expected, the hybrid exhibits enhanced lithium storage and superior long-cycle performances. The elevated electrochemical performance should benefit from the presence of dual conductive carbons of N-C and rGO that gain enhanced conductivity and effectively prevent the self-aggregation and volume change of MnO materials during conversion processes. 4


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Furthermore, the as-formed hybrid with relatively high surface areas and mesopore structures could achieve faster kinetics of conversion reactions. Finally, the doping carbon structures with nitrogen atoms in anode materials could offer extra active sites and then promote electron transfer, further improving the reaction activity and kinetics of lithium ion storage. This smart construction can broaden the horizon of graphene functional materials in various applications towards energy storage and other fields.

EXPERIMENTAL Synthesis of Mesopore MnO@N-C/rGO hybrids Graphene oxide (GO) was fabricated through the modified Hummer’s approach, and MnO2 nanorods were synthesized by a simple hydrothermal method (see Supplementary). First, 100 mg GO was scattered in 50 mL of DI water by ultrasonication. Second, Aniline monomer (90 µL) was introduced into GO dispersion liquid. Followed by magnetic stirring and ultrasonication, 6 mmol MnO2 nanorods was dispersed in the above mixture and stirred in an ice-water bath for 6 h. After cooling, 50 mL oxidant aqueous solution containing 4 mmol ammonium persulfate (APS, 98%) and 4 mL hydrochloric acid (HCl, 37%) , which was cooled in advance in an ice-water bath, was poured into the previously prepared mixture. The in situ polymerization reaction for 6 h took place under ice-water bath conditions. Lastly, the product was leached with ethanol and DI water many times. After drying at 60 ºC for 12h, the calcination was carried out at 650 ºC for 5 h accompanying the constant heating rate of 2 ºC /min in reducing atmosphere of 5 vol% H2 in Ar. 5



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Material characterization Microstructures were observed

by field emission scanning electron microscopy

(JEOL-7100F) and scanning transmission electron microscopy (JEM-2100F STEM/EDS microscope). The phase and crystalline properties of samples were determined by X-ray diffraction (XRD) patterns collected with a D8 Advance X-ray diffractometer (Cu Kα radiation, λ = 0.15406 nm). The Brunauer-Emmett-Teller (BET) surface area (SA) was analyzed by a nitrogen adsorption method at 77 K by means of Tristar II 3020 instrument. The valence state of elements in materials was investigated by XPS (PHI Quantera, U-P). Raman and FT-IR spectra were performed in terms of Renishaw INVIA micro-Raman spectroscopy with 633 nm laser radiation and Nexus instrument, respectively. Electrochemical measurements The mixture of as-prepared samples, binder and acetylene carbon black as an anode, and lithium foils as a counter electrode were integrated in 2016 coin cells. Before assembling cells in an argon-filled glove box, samples, polyvinylidene fluoride binder and acetylene carbon black were scattered according to the ratio of 7:2:1 in N-methyl-2-pyrrolidone. Then the resultant slurry was spread on copper foils, followed by drying at 120 ºC for 24 h. Electrolyte consisted of 1 M LiPF6 in a mixed solvent of dimethyl carbonate (50% volume) and ethylene carbonate (50% volume) as well as celgard membranes as separator. Charge-discharge tests were carried out on LAND CT2001A at room temperature. All CV curves and EIS data were obtained by a CHI660E system at room temperature.

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RESULTS AND DISCUSSION Material Design and Structure Characterization Fig. 1a shows the synthesis route of MnO@N-C/rGO hybrids. First, mix GO with aniline in water under sonication for distributing homogeneously aniline on GO, and subsequently add MnO2 nanorods under sonication. Then, the PANi and GO co-decorated MnO2 nanorods (MnO2@PANi/GO) are prepared via a copolymerization reaction by introducing oxidizer (ammonium persulphate) and hydrochloric acid solution into the above mixture under magnetic stirring in an ice bath. Finally, the composite is completely thermally decomposed at 650 ºC in a reducing atmosphere. Two main steps are proposed for evolution of the mesoporous MnO@N-C/rGO hybrid: (a) adsorbed aniline on MnO2 nanorods and/or GO, and (b) in situ synthesis of N-C encapsulated MnO nanoparticles on rGO. Particularly worth mentioning is that the thermal reduction treatment under reductive atmosphere promotes the transform of one dimensional MnO2 into zero dimensional MnO nanoparticles. To investigate the detailed information in the controllable synthesis, the scanning electron microscopy (SEM) images of products at various stages were obtained. The average diameter and length of MnO2 nanorod precursors attained by a simple hydrothermal method are ∼110 nm and about 1∼2 µm, respectively (Fig. 1b). After a copolymerization process, such nanorods are wrapped by GO with the well preserved morphology (Fig. 1c). The diffraction peaks of the precursor/intermediate samples before calcination including MnO2 nanorods and MnO2@PANi/GO composites can be well assigned to high purity β-MnO2 with a tetragonal

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structure (JCPDS no. 81-2261) (Fig. S1†). These results reveal that nanorod precursors could successfully be wrapped by GO, and the introduction of PANi has no influence upon crystal architectures of MnO2.

Figure 1

Microstructures of end products were also investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Obviously, massive particles with relatively evenly nanoscale diameter are well dispersed on the rGO sheets (Fig. 1d). In addition, nanoparticles appear to be wrapped by rGO due to the in-situ formation discussed above, as shown in Fig. 2a. From high-resolution TEM, the spacing of lattice fringes at the edge of one particle for MnO@N-C/rGO is 0.16 nm (Fig. 2b), corresponding to the distance of (220) plane of cubic MnO which is in good agreement with the fast Fourier transformed (FFT) micrograph derived from the corresponding HRTEM image (inset of Fig. 2b). Intriguingly, it can be found that MnO nanoparticles are wrapped withthin graphitized carbon layers, which can be assigned to decomposition of PANi. Specifically, the TEM image of MnO@N-C/rGO with corresponding energy dispersive spectrometry (EDS) elemental mapping (Fig. 2c) confirm that the nanoparticles were MnO, along with overlaying carbon and nitrogen signals from modified carbons, suggesting the uniform decoration of carbon for MnO nanoparticles. Under the same thermal treatment conditions, MnO/rGO (Fig. S2,4†) was obtained in the absence of aniline, and MnO/rGO/N-C mixture (Fig. S3,5†) was 8


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also fabricated by primarily mixing MnO2 nanorod precursors with GO and then aniline was dropwise added into the solution copolymerization process. Unlike what the MnO/rGO and MnO/rGO/N-C mixture structures did in the same conditions, MnO and rGO in MnO@N-C/rGO hybrids did not aggregate or accumulate after the heating treatment. It can thus be seen that the uniformly distributed aniline on GO and MnO2 nanorods before in situ copolymerization has an important role in the morphology controlling of products. The homogeneous and compact contact between particles and rGO could prohibit agglomerating of MnO and restacking of graphene. Importantly, carbon layers on MnO particles can address the issues related to MnO dissolution and aggregation. In short, the N-doped carbon encapsulated MnO particle composite with a novel multiple structure supported on rGO was designed and fabricated by a simple top-down method of in-situ polymerization in combination with the thermal decomposition.

Figure 2

Fig. 3a exhibits the XRD pattern of MnO@N-C/rGO. All diffraction peaks are assigned to a cubic MnO phase (JCPDS no. 75–1090). BET surface area (SA) with porosity was investigated by nitrogen-adsorption-desorption measurements (Fig. 3b). MnO@N-C/rGO owns specific surface area of 42.9 m2 g-1 with a mean pore size of 2.6 nm. Such a nanopore architecture increases the contact area between electrode-electrolyte and supplies mesopore channels towards rapid lithium ions diffusion. 24-25 At the same time, the relatively high SA 9



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and

porosity

are

available

to

sluggish

volume

changes

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in

lithium

ion

intercalation/deintercalation processes, leading to a good reversible capacity as well as outstanding cycling stability.

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X-ray photoelectron spectroscopy (XPS) can be used to unveil surface chemical state of materials. As can be seen from Fig. S6†, the peaks of Mn (2s, 2p1/2, 2p3/2, 3s and 3p) as well as O (1s) correspond to MnO in MnO@N-C/rGO. In addition, the C 1s peak is attributed to rGO and N-doped carbon. Furthermore, by means of high-resolution spectrum (Fig. 3c), Mn 2p peaks at 641.3 and 653.0 eV assigned to Mn (II) 2p3/2 and 2p1/2, respectively, are a feature of MnO,

12

A minor O 1s peak at 531.6 eV appears, indicating presence of remained O2-

species linked by C atoms in rGO or N-C (Fig. 3d). As presented in Fig. 3e, the evident C 1s peak can be found at 284.5 eV, which should be ascribed to graphitic carbon, and the small ones at 285.5, 286.6 and 288.8 eV correspond to C-N, C-O and O-C = O 26, demonstrating the reduction of GO and decomposition of PANi. Three Gaussian peaks belonging to N 1s signals at 401.3, 400.0 and 398.3 eV can be acquired by deconvolution, being attributed to pyridine-, pyrrole- and graphite-like nitrogen (Fig. 3f), respectively. 12,26 Moreover, it has been reported that N-doping in carbonaceous anode materials can effectively increase conductivity and reaction activity, enhancing the electrochemical performance. 12,27-28 In Raman spectrum (Fig. S7a†), for MnO@N-C/rGO two strong peaks at 1347 (labeled as D-band) and 1596 cm-1 (labeled as G-band) corresponding to A1g and E2gvibration modes, can be assigned to amorphous/disordered and graphitic carbons, respectively. 14 The intensity ratio ID/IG of 1.08 increases in comparison with 0.89 for MnO/rGO (Fig. S7b†). This is due to the existence of N-C which derived from the decomposition of PANi. The carbon content 10


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determined by TGA (Fig. S8†) is 27.73 wt% for MnO@N-C/rGO, which is at the same level as those reported in other carbon or graphene decorated MnO anode materials.

13-14

Figure 3

Under a potential scope of 0.01-3.0 V, cyclic voltammetry (CV) measurements were performed with a 0.2 mV s-1 scanning rate to disclose the electrochemical behavior of MnO@N-C/rGO

hybrid

electrodes

(Fig.

4a). As

for

CV

curves,

an

evident

oxidation/reduction pair is present. The gap of redox potentials could be induced by both large activation energy as well as low reaction kinetics between metal-oxygen. 29 During an initial cathode sweep, a spiculate peak at close to 0.1 V agrees well with evolution from Mn2+ to Mn0 by reduction reactions. In addition, the peak at 0.54 V indicates the occurrence of the solid electrolyte interface (SEI) layer on electrodes. 12,15,27 Meanwhile, the broad peak at about 1.3 V presented during the initial anode scan, is caused by oxidation of Mn0 to Mn2+. The corresponding reduction/oxidation peaks then shift to 0.38 and 1.35 V, respectively for the subsequent cathode/anode scans, possibly caused by improved kinetics and active material availability of the hybrid electrode stems from microstructural evolutions after the first insertion of lithium ions. 12,15 For the increased kinetics, relative discussions have been made in light of surface energy as well as amorphization by taking RuO2 as an example. 30 In the following scan, CV curves are apt to overlap, indicating the prepared electrode has progressively increased cyclic durability towards intercalation/deintercalation processes of Li-ions. 31 Similar phenomenon has also been observed in other anode materials. 14 11



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To prove possible applications of MnO@N-C/rGO as an anode material, its lithium storage characteristics were probed. Fig. 4b shows testing results in light of galvanostatic charge/discharge cycles. The initial discharge shows a plateau at about 0.2 V (vs. Li/Li+), agreeing well with the reported literature reflected the reduction from Mn(II) to Mn(0), which also can be observed in bare MnO samples(Fig. S).12,31 The hybrid electrode possesses initial discharge/charge capacities of 1125.2/764.6 mAh g-1. This irreversible capacity can be ascribed to a formed SEI layer. The well overlapped profiles of discharge/charge curves after the first cycle demonstrate high reversibility of electrochemical reactions. Fig. 4c reveals that the cyclability of the electrode containing MnO@N-C/rGO at 500 mA g-1 reaches 50 cyclic tests. In the first sweep, its charge capacity reaches 680.6 mAh g-1. Worth noting that in the second cycle onward, it presents outstanding cyclability. Significantly, with increased cycles about 99% Coulombic efficiency (CE) can be stably achieved. Even after 50 cycles, a reversible capacity of 634.1 mAh g-1 can be attained. As seen in Fig. 4d, the rate capability of the MnO@N-C/rGO electrode was investigated in terms of gradually increasing current densities in the range of 100 to 2000 mA g-1. The electrode shows fascinating rate performance as well. At 2000 mA g-1, the charge capacity reaches 448.9 mA h g-1 (around 59.7% charge capacity at 100 mA g-1). It is even more than the theoretical capacity of graphite as anode material (372 mAh g-1). After that, at different current densities, the deep charge/discharge for 70 cycles was carried out. The charge capacity of 864.7 mAh g-1 can be attained when back to 100 mA g-1, even higher than initial ten charge capacities at 100 mA g-1. The exceptional electrochemical performance can be ascribed to the N-doping effect that facilitates Li diffusion into the hybrid and preserves the 12


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interaction with the MnO nanorods during repeated cycling. The other reason maight be related to the strong synergistic effect between MnO core and carbon shell.15 For the sake of contrast, we also tested the rate capability of MnO/rGO electrodes without N-doped carbon and MnO/N-C/rGO mixture. The capacities of only 20.0 and 122.2 mAh g-1 can be obtained for MnO/rGO and MnO/N-C/rGO mixture at 2000 mA g-1, respectively. The capacities of two controlled samples drop rapidly with cycling as a result of aggregation and dissolution of MnO during repeated lithium ions insertion/extraction processes due to very limited elasticity protection of single rGO. Long cycling stability tests of the MnO/N-C/rGO electrode were also implemented. As presented in Fig. 4e, after 10 cycles its reversible capacity reaches 759.5 mAh g-1 at 100 mA g-1, while charge capacities for 11th and 1300th cycles correspond to 445.5 and 425.0 mAh g-1. The mean capacity loss is only 0.004% per cycle. Meanwhile, it shows decreasing capacity during first few cycles at 2000 mA g-1, and then increasing capacity. The increasing capacity shown in Figure 4e can be attributed to the activation in the electrode. It is well know that the activation process often occurs on some metal oxides electrodes. The activation phenomenon can due to that the lithium ion transport channels are gradually opened up during the lithiation/delithiation process.32 It is further pointed out that CE of MnO/N-C increases quickly to a nearly 100% after a few cycles even at high current densities. Meanwhile, the other two controlled samples demonstrate a significant decay. It is remarkable that even the MnO/N-C/rGO hybrid exhibits higher cycle stability than that of MnO/rGO without N-doped carbon. This is due to the fact that nitrogen ad-atoms in carbonaceous anode materials can effectively enhance the reaction activity and provide extra active sites, improving the Li-ion 13



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storage performance. As shown in Fig. S10a†, it can be observed that the structure remains almostunchanged after 200 cycles. These results demonstrate that the novel multiple structure of dual carbon protection can slow down the mechanical stress caused by large volume evolution in lithium insertion/extraction reactions, leading to excellent cyclability. Such remarkable electrochemical performance makes our novel MnO@N-C/rGO hybrid an advanced anodic material towards lithium ion batteries.

Figure 4

CONCLUSIONS In summary, a mesopore hybrid of MnO nanoparticle embedded N-doped carbon on reduced graphene oxide (rGO) is prepared by a facile method. The synthesized MnO@N-C/rGO hybrid has a novel structure with dual carbon (N-doped carbon and rGO) protection. As a promising anodic candidate for lithium ion batteries, MnO@N-C/rGO shows outstanding reversible capacity, cyclability and rate capability benefited from the smart function of such dual carbon, mesoporous sturture and N-doping effect. The proposed synthesis strategy provides an economical and simple approach to obtain advanced lithium ion battery electrodes by making use of abundant transition metal oxides. ASSOCIATED CONTENT Supporting Information 14


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Supporting Information is available free of charge on the ACS Publications website at DOI: Characterization

techniques.

XRD

patterns

of

MnO2

nanorods

and

MnO2

nanorods@PANi/GO. Low and high-magnification SEM images of MnO/rGO, and MnO/N-C/rGO mixtures. TEM images of MnO/rGO, and MnO/N-C/rGO mixtures. XPS spectra of MnO@N-C/rGO. Raman spectra of MnO@N-C/rGO and MnO/rGO TGA of MnO@N-C/rGO. Rate performance, charge capacity and CE of MnO@N-C.

AUTHOR INFORMATION Corresponding author* Tel: +86 27 87651837 E-mail: [email protected]; [email protected]. Author contribution Experimental procedures and content of the manuscript were accomplished through the contributions of all authors. Every author participated in the discussion of the results, and fully approved the final version.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors wish to gratefully acknowledge the financial support of the National. This work was financially supported by the National Key Research and Development Program of

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China (2016YFA0202603), and the National Natural Science Foundation of China (51477125, 51372186). We are grateful to Professor Junlin Xie and Dr Xiaoqing Liu of the Center for Materials Research and Analysis of Wuhan University of Technology for materials characterization.

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List of figure captions Figure 1. (a) Schematic illustration of the synthesis route for mesopore MnO@N-C/rGO hybrids consisting of N-doped carbon encapsulated MnO nanoparticles supported on reduced graphene oxide (rGO). SEM images of MnO2 nanorod precursor (b), MnO2@PANi/GO intermediate (c) and MnO@N-C/rGO (d). Figure 2. TEM (a) and high-resolution TEM (b) images of MnO@N-C/rGO (the inset is the corresponding FFT image). A TEM image and corresponding EDS mappings of MnO@N-C/rGO (c). Figure 3. XRD pattern (a), High-resolution XPS spectra of Mn 2p (b), O 1s (c), C 1s (d) and N 1s (e), and N2 adsorption-desorption isotherms (f) (the inset presents the pore size distribution curve) for MnO@N-C/rGO. Figure 4. (a) Representative CV curves of the as-prepared MnO@N-C/rGO with a 0.2 mV s-1 scan rate over a potential window of 0.01-3V versus Li/Li+. (b) Discharge and charge voltage profiles of MnO@N-C/rGO at a current density of 100 mA g-1. (c) Charge and discharge capacities and coulombic efficiency (CE) of MnO@N-C/rGO at 500 mA g-1 for 50 cycles. The unit of current density denoted for each period is mA g-1. (d) Rate performance of electrodes with different samples cycled at various current densities. (e) The CE and various charge capacities of electrodes with different samples cycled at high current densities. The current density was 100 mA g-1 for the first ten cycles and 2000 mA g-1 for later cycles. All specific capacities of MnO@N-C/rGO anodes are based on the total mass of active materials (MnO, N-doped carbon and rGO).

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Graphical Abstract (TOC)

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Top-Down Strategy to Synthesize Mesoporous Dual Carbon Armored

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