C Hybrids as Anode Materials for High

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Lotus Root-like MnO/C Hybrids as Anode Materials for High Performance Lithium Ion Batteries Zhaoxia Cao, Mengjiao Shi, Yanmin Ding, Jun Zhang, Zhichao Wang, Hongyu Dong, Yanhong Yin, and Shu-Ting Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11144 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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The Journal of Physical Chemistry

Lotus Root-like MnO/C Hybrids as Anode Materials for High Performance Lithium Ion Batteries AUTHOR NAMES Zhaoxia Cao*†,‡,§, MengjiaoShi†,‡,§, YanminDing†,‡,§, JunZhang†,‡,§, Zhichao Wang†,‡,§, Hongyu Dong†,‡,§, Yanhong Yin†,‡,§and Shuting Yang*†,‡,§

AUTHOR ADDRESS †

School of Chemistry and Chemical Engineering, Henan Normal University,

Xinxiang Henan 453007, China. ‡

National and Local Joint Engineering Laboratory of Motive Power and Key

Materials, Xinxiang, Henan 453007, China. §

Collaborative Innovation Center of Henan Province for Green Motive Power and

Key Materials, Henan Normal University, Xinxiang, Henan 453007, China.

ACS Paragon Plus Environment


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ABSTRACT

Lotus root-like MnO/C mesoscale hybrids, featured with nanometer-sized monodisperse metal oxide particles uniformly embedded in porous carbon matrix, are in situ synthesized via a facile and scalable method: polyvinyl alcohol-assisted aqueous precipitation followed by thermal decomposition of precursors. As a result, the hierarchical-structured MnO/C hybrids show a desirable capacity, superior cycling durability and rate capability. These performances may be ascribed to its particular structure: the cavity of lotus root as well as the void between manganese oxides and carbon as sufficient mechanical buffers to avoid agglomeration and pulverization, and conductive pathway of well interconnected carbon "wall" to favor the fast electron and Li ions transport during the electrochemical cycling.


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1. Introduction Due to the advantages of high energy density, fast charge/discharge rate, long cycling life and low cost, lithium ion batteries (LIBs) have been considered as a new generation of power equipments, especially in high energy storage applications such as Electric vehicles (EVs) and hybrid electric vehicles (HEVs).1-5 However, graphite-based carbon, which is the current conventional anode material, cannot meet the demands for large energy and power density, because of its limited theoretical capacity of 372 mA h g-1. Current strategy is based on the modification of existing electrode materials, and searching for low-cost electrode materials with high energy density and cycling stability.6 Transition metal oxides (MOx), including SnO2,7 MoO2,8 NiO,9 Fe2O3,10 Mn3O4,11-13 CuO,14 CoO

15

and MnO2,16-17 have been extensively studied in terms of

high theoretical specific capacity. Among the transition metal oxides, manganese monoxide (MnO), possessing high theoretical capacity (755 mA h g-1), low electromotive force (1.032 V vs Li/Li+), high density (5.43 g cm-3) and abundant natural resources, therefore, has received tremendous research interests.18-25 However, there are still challenges in the application of MnO to practical LIBs: the slow kinetics of Li ions and electron transport in electrodes and at the interface of electrode/electrolyte, low electronic conductivity, large volume expansion and severe collapse of the electrode upon cycling. Generally, these problems can be partly solved by constructing micro/nanostructure or forming metal oxide/carbon composites with favorable morphology and structure (nanorod,24, 26-29 microsphere,30-31 microdisk,32 or



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core-shell33-36), corresponding to various processes, such as use of toxic chemicals or complicated conditions (e.g. hydrothermal process under high temperature and high pressure condition). So far, superior electrochemical performances have been achieved, which are ascribed to particular structures and addition of carbon could effectively relieve large volume expansion, improve the electrode/electrolyte contact area to favor the fast Li ions transport and enhance the reaction kinetics during the electrochemical reaction. Specially, some biomimic structures, for example, pomegranate-like culster structure37 and hierarchical vine-tree-like carbon nanotube architectures,38 are elaborately designed to play a role in high performance electrode materials fabrication for energy storage devices. Inspired by the above reports and the fact that our group successfully applied lotus root-like carbon materials in lithium sulfur batteries,39 we proposed to in situ fabricate lotus root-like MnO/C mesoscale hybrids via a simple thermal decomposition of MnC2O4•xH2O-nPVA complex precursor. In this structure, the wall of "lotus root" consisting of small manganese oxide particles uniformly embedded in porous carbon, can allow Li ions pass through and provide conductive pathway. The cavity of "lotus root" as well as the void between manganese oxides and carbon, can buffer the volume change and severe collapse of the electrode during the electrochemical reaction process. For its large interface between electrode and electrolyte, short Li ions diffusion path, and excellent structure stability during cycling, the lotus root-like MnO/C hybrid exhibits high specific capacity, superior cyclability, and excellent rate capability as anode materials in LIBs.


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2. Experimental 2.1 Synthesis of lotus root-like MnO/C hybrids In a typical synthesis, 3.54 g MnCl2 was dissolved into 20 mL absolute ethyl alcohol, and then dropped into 225 mL of 1.0 wt% PVA aqueous solution under stirring. After equivalent amount of 20 wt% H2C2O4•2H2O aqueous solution was introduced into the above resulting solution, white precipitate appeared. The precipitate was centrifugalized, washed with deionized water and dried in vacuum. Then, the precursor was annealed at 600 °C with a heating rate of 5 °C min-1 for 2 h in nitrogen. Finally, the target product was named as MnO/C-1.0% PVA and defined as the typical MnO/C hybrid. For comparison, pure MnO sample and other MnO/C hybrids were also prepared, named as pure MnO, MnO/C-0.2% PVA and MnO/C-2.0% PVA respectively, only adjusting the PVA concentration (0, 0.2 and 2.0 wt%). 2.2 Material characterization The crystalline structure was measured by X-ray powder diffraction (XRD, Bruker AXS D8) analysis with Cu Kɑ radiation in the 2θ range from 10o to 80o. The thermal decomposition behavior of the precursors and final samples were examined by thermal gravimetric analysis (TGA; NETZSCH STA 449F3) at a heating rate of 10 °C/min from 20 to 800 °C in nitrogen flow and air flow, respectively. A Fourier transform infrared spectrometer (FTIR, Themo Nicolet 670FT-IR) was used for recording the FTIR spectra of the sample from 500 to 4000 cm-1. Raman data were collected by an HR Evolution Raman spectrometer, using an excitation laser of 633



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nm. The surface morphologies and composition were characterized by field-emission scanning electron microscopy (FE-SEM; SU8010) with an energy-dispersive spectroscopy (EDS) attachment. Microstructural properties were obtained using high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). The specific surface area was measured using the Brunauer-Emmet-Teller method (BET, Tristar-II, Micromeritics) with N2 adsorption and desorption isotherms. The pore size distribution was calculated by the Barrett-Joyner Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the surface composition of the sample by using an ESCALAB 250 spectrometer (Perkin-Elmer). 2.3 Electrochemical measurements The electrochemical tests of the samples were performed using CR2032 coin cell, and assembled in a glove box filled with pure argon (99.9%). Lithium metal was used as counter electrode, and micropores polypropylene membrane (Cellgard 2400) as separator film. The working electrodes were comprised of dispersing active material, polyvinglidene difuoride (PVDF) and acetylene black with the weight ratio of 60:10:30 in N-methyl-2-pyrrolidone (NMP) solvent to produce slurry, then the slurry was coated evenly on a copper foil and dried at 60

in vacuum overnight. The active

material mass loading of the electrodes was about 1.0 mg cm-2, which was close to that in literatures. The electrolyte was 1 mol/L LiPF6 dissolved in a mixture of ethylene carbonate/dimethyl carbonate (v/v = 1:1). Galvanostatic measurement was tested in the voltage range from 0.01 to 3.00 V on a LAND Battery test system


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(CT2001A, Wuhan, China). Cyclic voltammetry (CV) test was carried out in the potential window of 0.01 to 3.00 V and at a scanning rate of 0.25 mV s-1 by an electrochemical

workstation

(CHI

660B).

The

electrochemical

impedance

spectroscopy (EIS) was performed on an electrochemical workstation (CHI 660B) over the frequency range of 10-1 Hz to 105 Hz.

3. Results and discussion The formation process of MnO/C hybrids is schematically illustrated in Figure 1, by the assist of PVA as chelating agent, carbon source and structure-director, which involves the precipitation of precursor and the in-situ formation of MnO/C hybrid during thermal decomposition in N2 atmosphere. Here, PVA is an effective structure-director because the OH ligands of the PVA can adsorb metal cations, resulting in the anisotropic growth of a solid material.40-41 The rules also be applied to interpret the formation of MnC2O4•xH2O-PVA hybrids. Firstly, PVA chains chelated Mn2+ and formed of Mn2+-nPVA complex with OH ligands as bridges. After oxalic acid solution was dropped into the above solution, white floccus precipitation immediately appeared due to the Mn2+-nPVA was rapid transformed into MnC2O4•xH2O precursor. Then, the MnC2O4•xH2O seeds grew up to form rod-like structures or further gather to bundles with the confinement of PVA carbon backbone grids. This self-assembled cluster structure is strongly influence by the PVA dosages (See more details in the following section of morphology evolution). Upon annealing, the decomposition of precursor gave MnO/C hybrids. The corresponding equations can be described as:



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Mn2+ +nPVA → Mn2+-nPVA Mn2+-nPVA + H2C2O4•2H2O → MnC2O4•xH2O-nPVA MnC2O4•xH2O-nPVA

→

MnO/C + yCO2 +H2O

Figure 1 Schematic illustration of the synthesis route.

This proposed process was well confirmed by the gradual enhance of PVA signals in FTIR spectrum and different XRD patterns of precursors (Figure S1 and S2). The strong absorption peak at 3390 cm-1 in FTIR spectrum can be assigned to the stretching vibrations of crystal water in oxalate. This band becomes broad, weak and shifts toward lower wavenumber with increasing PVA dosage. This shift owes to the formation of intermolecular hydrogen bonds by the OH of PVA with Mn2+, indicating the enhanced interaction between Mn2+ and PVA. The XRD patterns show that those precursors are made of different proportions of the orthorhombic phase of MnC2O4 and MnC2O4•2H2O, moreover MnC2O4 phase increases with higher PVA concentration. TG analyses of all the precursors in N2 show similar curves (Figure S4). The initial weight loss of 5 %-20 % appears up to 200 °C, which is comparable to the


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loss of crystallized water from each mole of the precursor, respectively. After which, a sharp decline occurs and no weight loss above 600 °C, mainly associated with the complete decomposition of oxalate and the conversion of anhydrous MnC2O4 to MnO products. Those precursors exhibit different weight loss at each platform due to different proportions of MnC2O4, MnC2O4•2H2O and PVA, which is in agreement with the IR and XRD analysis. The residual ratios of all precursors are about ~40 ％ (Table S1).

Figure 2 XRD pattern (a) and Ranman spectrum (b) of the typical MnO/C hybrid.

After heat treatment, the MnC2O4•xH2O-nPVA precursors turn into MnO/C hybrids, analyzed by XRD and Raman spectrum. Figure 2a shows the XRD pattern of the typical MnO/C hybrid. The characteristic peaks at 35.0°, 40.6°, 58.7°, 70.2° and 73.8° can be indexed as (111), (200), (220), (311) and (222) planes of the MnO, respectively. No other diffraction peaks are detected. The typical Raman shifts of MnO (~648 cm-1)42-44 and C (~1324 and ~1594 cm-1)45-47 can also been observed in Figure 2b. The high intensity of a D-band suggests the existence of non-graphitic carbon. The carbon content of the typical MnO/C hybrid, determined from TG



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analysis in air (Figure S5 and Table S2), is 3.3 %, which in accordance with the absence of diffraction peaks in XRD pattern.

Figure 3 FE-SEM images (a, b) and EDS spectrum (c) of the typical MnO/C hybrid. (d) the element mapping images for Mn, O and C, respectively.

The FE-SEM images exhibit the morphology evolution of precursors without and with different PVA dosages, as shown in Figure S6. As comparison, a spindle-shaped appearance is observed with a length about 100 µm for the sample without PVA (Figure S6a). When PVA concentration ranging from 0.2 wt% to 1.0 wt%, the morphology gradually change from thick rod (diameter about 8 µm, Figure S6b) to slim one (diameters of 1-2 µm, Figure S6c) with cracks. Further increasing PVA concentration to 2.0 wt%, the rod just becomes a little thick and nonuniform (Figure


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S6d). According to above results, we surmise that PVA influences the size and orientation of MnC2O4•xH2O in the nucleation and growing process, the morphology of the precursors also depends strongly on the PVA dosages. The morphology of the typical MnO/C hybrid was also investigated in detail by FE-SEM. The MnO/C hybrid consisting of many fine nanocrystals inherits the general morphology of its precursor except visible surface porous structure (Figure 3a). The high-magnification FE-SEM image of a single rod with open tips, reveals a layered and interior hollow lotus root-like structure, based on the distinct contrast between the outer wall and the middle cavity region (Figure 3b). Furthermore, the EDS spectrum and element distribution images (Figure 3c, d) confirm the presence of Mn, O, and C elements. As for samples of pure MnO and MnO/C-0.2 % PVA (Figure S7a-b), just some cracks appear on the surface, compared with their precursors. Interestingly, the sample of MnO/C-2.0 % PVA remains the porous structure, while the MnO particles embedded in the carbon networks become bigger and carbon networks get more identifiable (Figure S7c-d).

Figure 4 TEM images and the corresponding HRTEM images of the typical MnO/C hybrid (a, b, c), MnO/C-2.0% PVA hybrid as a contrast (d, e, f).



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More details of the microstructure for the typical MnO/C hybrid were further studied by TEM (Figure 4a). An individual porous MnO/C tube is consisting of amorphous carbon networks and about 150 nm embedded particles, which agrees well with FE-SEM observations. The magnified HRTEM images (Figure 4b and c) are the different regions marked in Figure 4a, corresponding to amorphous carbon and MnO. The carbon networks with mutual interconnection in all the directions can ensure a continuous protective layer when applied as electrode materials. The lattice spacing of 0.27 nm corresponds to the (111) crystal plane of MnO (JCPDS No. 07-0230), and the interplanar distance of 0.51 nm is consistent with the (020) plane (JCPDS No. 04-0326), indicating good crystallization of MnO particles. As the surface area and pore-size distribution are the two key factors for the electrochemical applications of such electrode materials, N2 adsorption/desorption isothermal measurements were performed. Figure S8a shows the isothermal plot of the typical MnO/C hybrid, which is a type IV with H1 type hysteresis loops. A high specific surface area of 22.52 m2 g-1 is obtained, implying the much accessible electroactive sites for surface Faradaic reactions. The pore size distribution curve (Figure S8b) exhibits a multilevel pore structure, in good agreement with the FE-SEM and TEM observations. The larger pore size at 117.8 nm may be corresponding to the inner cavity of the lotus root like stucture. While the most pores around 26.49 nm, may be ascribed to the particles packing. As a contrast, N2 adsorption/desorption isotherms and pore size distribution (Figure S8) of pure MnO and other MnO/C hybrids were summarized in Table S3. The surface area and pore volume firstly increase with increasing PVA and then


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decrease, ranging from 2.76, 4.71, 22.53 to 12.73 m2 g-1, and 0.017, 0.057, 0.123, 0.084 cm3 g-1, respectively. We may expect the porous and hollow tubular structure could not only buffer the volume change,48 but also facilitate an efficient contact of the internal active materials with electrolyte, leading to a fast electron and Li ions transportation, thus bring about high rate capability and excellent cycle performance when applied to lithium storage.8

Figure 5 XPS spectra for the typical MnO/C hybrid: (a) the survey spectrum and the high resolution spectra for (b) C 1s, (c) Mn 2p, (d) O 1s.

Additionally, X-ray photoelectron spectroscopy (XPS) was further employed to investigate the surface electronic state and the composition. As shown in Figure 5a, the detected peaks of Mn (2s, 2p, 3s and 3p), O (1s, KLL), and C 1s in the survey



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spectrum confirm the presence of Mn, O, and C in the typical MnO/C hybrid. The C 1s spectrum (Figure 5b) can be fitted into three parts centered at 284.6, 286.4 and 288.3 eV, corresponding to sp3C-sp3C, C-O-C and C-O bonds, respectively.20, 49 The Mn 2p spectrum (Figure 5c) exhibits two obvious signals at 641.3 and 652.9 eV, which can be attributed to Mn 2p3/2 and Mn 2p1/2, and the pair of satellite peaks at 643.0 and 654.6 eV are consistent with MnO, agreeing well with those of the previous literatures.18, 50-51 The O1s high-resolution spectrum (Figure 5d) displays two peaks at 530.1 and 531.7 eV, which refer to Mn-O and C-O-C.52 As for sample of MnO/C-2.0 % PVA, the similar information was found (Figure S9). These results demonstrate the successful transformation of MnC2O4•xH2O-nPVA complex into MnO/C hybrid.


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Figure 6 (a) CV curves of the typical MnO/C hybrid for the first five cycles. (b) Discharge and

charge profiles of the typical MnO/C hybrid at 0.75 A g-1 between 0.01 and 3.00 V for different cycles. (c) XRD patterns of the typical MnO/C electrode after cycling. (d) Selected local diffraction patterns of (c). Differential charge vs. voltage plots (e) at region I (discharge) and (f) at region II (charge) derived from (b).

The electrochemical lithium storage properties were firstly studied by the cyclic voltammograms (CV). Figure 6a displays representative CV curves of the first five cycles, which accords with the previous reports.50 In the first cycle, the typical MnO/C electrode shows two reduction peaks at ~1.5 V and ~1.8 V possibly corresponding to the reduction of Mn3+ or Mn4+ to Mn2+. The traced MnxOy impurity resulting from the partial oxidation of Mn2+ in the product can be confirmed by the XPS of Mn 3s (Figure S10).26 Additionally, a broad reduction peak around 0.68 V is assigned to the irreversible reduction of electrolyte and the formation of a solid electrolyte interphase (SEI) layer.19, 22, 53 Furthermore, the main cathodic peak at 0.08 V for typical electrode is corresponding to the complete reduction of Mn2+ to Mn0. From the second cycle onward, this peak shifts to 0.26 V. The drastic change is possibly due to the improved kinetics of the MnO electrode.21,

54-55

The main

oxidation peak located at about 1.32 V can be ascribed to the oxidation of Mn0 to Mn2+. The CV profiles of the typical MnO/C after the first cycle overlap very well, indicating high reversibility and good stability of the MnO/C electrode. Figure 6b shows the discharge-charge curves of the typical MnO/C electrode at a current density of 0.75 A g-1. In the first discharge curve, a long voltage plateau at



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about 0.25 V is the reduction of Mn2+ to Mn0. Whereas for the charge process, the slope from 1.2 V to 1.5 V is related to the oxidation of Mn0 to Mn2+, which is typical characteristic of voltage trends for the MnO electrode.26, 56 From the second cycle, the discharge plateau shifts to about 0.5 V, expressing the irreversible phase transformation of Mn and Li2O.54, 57-58 XRD measurement was used to understand the transformation process of typical MnO/C electrode during the discharge-charge process. As shown in Figure 6c,d, when the cell was discharged to 0.01 V after 1st and 3rd cycles, the electrochemical reaction results in the formation of metal Mn, whereas in the corresponding charged state, the MnO phase can be regenerated.59-61 These processes can be expressed by the following reactions: MnO + 2Li+ + 2e−→ Mn + Li2O Mn + Li2O → MnO + 2Li+ + 2e− The initial discharge capacity of the typical MnO/C is 1410 mA h g-1, which is higher than the theoretical capacity of MnO based on conversion reaction. This may be attributed to the following reasons. 1) The formation of solid electrolyte interface (SEI) film and reversible gel-like polymer film on the electrode surface.60 2) An interfacial reaction of lithium within the Mn/Li2O matrix produces lithium storage in the grain boundary regions between Li2O and Mn grains. In addition, Mn2+ in MnO/C could be reoxidized to a higher oxidation which also reflects in the capacity vs. voltage curves extracted from the galvanostatic profiles. Apart from the aforementioned redox peaks in the CV curves and voltage profiles, two new peaks


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around 0.87 V and 2.17 V appeared (Figure 6e, f). The anodic peak at about 2.17 V was concerned with the re-oxidation of Mn2+ to high valence for many Mn-based materials. The cathodic peak at 0.87 V was corresponded to the high valence state reduced to Mn2+. As shown in Figure 6f, a charge slope at around 2.17 V appears, gradually broadens, and finally transforms into a long plateau upon cycling, indicating an ever-increasing capacity and Li ion reactivity. Both the peaks were visible after 100 cycles and their intensities increased gradually, foreboding the existence of multiple high valence.22 The typical MnO/C electrode shows the excellent cycling performance, with a reversible capacity over the theoretical capacity, compared to the other MnO/C and pure MnO samples, as shown by Figure 7a. The cycling performance was evaluated at a current density of 0.75 A g-1. It can be clearly observed that the reversible capacity of the typical MnO/C electrode is as high as 1084 mA h g-1 even after 250 cycles, and with much higher capacity retention of 77 %. It should be pointed out that the capacity keeps a relatively gradual increase after 50 cycles on the whole, with the appearance of a unique fluctuation upon cycling. On the basis of the related literatures,23, 56, 62-63 this phenomenon may be reasonably attributed to the formation of polymeric gel-like film due to the kinetically active electrolyte degradation during conversion process, metal valence transform into higher states oxided by Li2O, the formation and dissolution of the electrolyte catalyzed by as-formed metal nanograins and lithium storage via interfacial charging within metal/Li2O matrix which both leading to more ''extra capacities''. In addition, the conversion-reaction kinetics can be



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improved by the formation of defects in the electrodes during cycling, leading to the oxidation of Mn2+ to a higher oxidation state and the appearance of a unique fluctuation, which has also been observed in other nanostructured Mn-based

Figure 7 (a) Cycling performance of pure MnO and MnO/C electrodes at 0.75 A g-1. (b)

Comparison of rate performance: pure MnO and MnO/C electrodes at various current densities. (c) Cycling performance of the typical MnO/C hybrid at 1.51 A g-1 for 250 cycles. (d,e) TEM images along with the corresponding HRTEM images (f,g) of the typical MnO/C electrode after 250 cycles at 3.0 V.


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oxides.20, 50 The capacities of other MnO/C electrodes only show a slightly gradual increase. In contrast, the capacity of pure MnO just goes down upon cycling. This may be related to the different structure, pore size, particle size and carbon content of those four samples. The small size of MnO particles, high surface area and pore volume, suitable microstructure favor the excellent electrochemical performance. Here the carbon network with mutual interconnection in all the directions can prevent the aggregation of active materials, alleviate the volumetric variation on cycling, and ensure a continuous conductive network. What’s more, the typical MnO/C electrode possesses wonderful rate capability. Figure 7b shows the rate capability of four electrodes by increasing the current densities stepwise from 0.08 to 3.77 A g-1. The discharge capacities at 0.08, 0.15, 0.38, 0.75, 1.51 and 3.77 A g-1 are 1801, 1432, 1239, 947, 745 and 462 mA h g-1 for typical MnO/C electrode. Whereas other electrodes deliver decreased average discharge capacities, respectively. It is noted that a satisfactory discharge capacity of 1841 mA h g-1 can be recovered when the current density is reset to 0.08 A g-1. On the contrary, the pure MnO shows the smallest discharging capacity, worst cycling performance and rate performance. In addition, a reversible capacity of 695 mA h g-1 for typical MnO/C electrode at a high current density of 1.51 A g-1 after 250 cycles can be maintained (Figure 7c). The excellent electrochemical performance should be closely associated to the unique structure, which was verified by the electrochemical impedance spectrum (EIS) measurements and corresponding fitting results of Nyquist plots using the equivalent



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circuit after 250 cycles at 0.75 A g-1. As shown in Figure S11, the Nyquist plots all have similar features, the semicircle in the high frequency region is associated with the charge transfer impedance at the electrolyte/electrode interface. The inclined line in the low frequency region is related to the diffusion of Li ions in the bulk of the electrode. Among the fitting results (Table S4), the typical MnO/C electrode shows the smallest Rs, Rct, especially Zw values. It is believed that small MnO particles lead to short Li ions diffusion distances, meanwhile the open porous structure provides rapid ion transport pathway. However, as reported by Liu64 that effective conducting and buffering matrix cannot be formed for MnO nanoparticles when the carbon content is reduced to a certain degree such as 5.3 wt%, this work shows the MnO/C electrode still exhibits a superior cycling performance even the carbon content is as low as 3.3 %. We may believe that main contribution is the unique lotus root-like structure with small MnO particles embedded in carbon matrix, which provides large enough room for the expansion upon lithiation rather than the carbon network can offer effective conducting. That is why we chose a high conductive content as 30 % to obtain the best electrochemical performance based on corresponding literatures (Table S5). To further understand and confirm the electrochemical mechanism of the MnO/C hybrid, we investigated the electrode after 250 cycles by TEM measurements. The lotus root-like structure is maintained when the MnO/C hybrid is recharged to 3.0 V (Figure 7d, e) or redischarged to 0.01 V (Figure S12a-b), suggesting the excellent structural stability of the current restructured electrode. It needs to point out that the


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empty internal void is well retained during cycles. However, the stripe widths (d(200)) of MnO (Figure 7f, g) and (d(211)) of Mn (Figure S12c-d) can still be recognized by HRTEM respectively when the MnO/C hybrid is recharged/redischarged to 3.0 V/0.01V.

Figure 8 Energy storage charateristics of crossing-sectional view and axial side cutaway view in

MnO/C hybrid.

Lotus root-like MnO/C mesoscale hybrids do not show similar superior electrochemical performances as some nanostructure hybrids like MnO/graphene composites. But the electrode still delivers a higher specific capacity than most of the reported MnOx/C materials with special structure design, such as Peanut-like MnO/C, Core-shell MnO@Carbon wires and MnO/C microtubes (Table S5). It can be attributed to the following reasons, as illustrated in Figure 8. Fistly, the architecture with well interconnected pores and open space between layered arrays further provide electrolyte permeability and a shorter Li ions diffusion during the electrochemical reaction. Secondly, the incorporation of carbon would alleviate the mechanical stress induced by the Li ions insertion/extraction, rather than enhance the electronic conductivity due to the low carbon content of the MnO/C hybrid in this work.



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4. Conclusions In summary, we demonstrated a facile synthesis method to fabricate lotus root-like MnO/C hybrid with a uniform size distribution and good dispersity. The resultant deliver a better lithium storage performance used as anode material in LIBs. A reversible capacity of 1084 mA h g-1 could be maintained at 0.75 A g-1 after 250 cycles for the typical MnO/C hybrid, and it displays a capacity of 695 mA h g-1 after 250 cycles at 1.51 A g-1. Moreover, it also shows high rate performance and recovery capability. The better electrochemical performance is connected with its unique layered and interior hollow structure coupled with carbon incorporation. In addition, the synthesis strategy is easy operation, low cost, and may open up new avenues to prepare special structure transition oxide@carbon composites, which could potentially be used in future energy conversion technologies.

Supporting Information. Figure S1 FT-IR spectra of oxalate precursors synthesized under various PVA concentrations.

Figure S2 XRD patterns of oxalate precursors synthesized under various PVA concentrations.

Figure S3 XRD patterns of MnO/C and pure MnO. Figure S4 TG curves of the precursors with different usages of PVA. Figure S5 TG curves of the as-formed MnO/C hybrids with different usages of PVA. Figure S6 FE-SEM images of precursors (a) without PVA, (b) 0.2% PVA, (c) 1.0% PVA, (d) 2.0% PVA.


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Figure S7 FE-SEM images of (a) pure MnO, (b)MnO/C-0.2% PVA, (c,d) MnO/C-2.0% PVA.

Figure S8 N2 adsorption/desorption isotherms and pore size distribution of MnO/C hybrids, and pure MnO as a contrast.

Figure S9 XPS spectra of MnO/C-2.0% PVA. Figure S10 (a) XPS spectra of Mn 3s for the typical MnO/C hybrid. (b) XPS spectra of Mn 2p for the typical MnO/C electrode after 10 cycles.

Figure S11 (a) Nyquist plots of the MnO/C hybrids after 250 cycles at 0.75 A g-1. (b) The magnification of red area marked in (a).

Figure S12 (a, b) TEM images along with the corresponding HRTEM images (d, e) of the typical MnO/C electrode after 250 cycles at 0.01 V.

Table S1 The residual ratios of the precursors in N2 atmosphere. Table S2 The carbon centent of samples with different PVA usage calculated by TG. Table S3 Physical properties of pure MnO and MnO/C hybrids. Table S4 Fitting results of Nyquist plots using the equivalent circuit. Table S5 The comparison of the capacity of present work with reported MnOx-based anodes for Li-ion batteries.

Author Information Corresponding Author

*E-mail:[email protected];Fax:(+86)-373-3326439; Tel: (+86)-373-3326439 *E-mail: [email protected]; Fax:(+86)-373-3326439; Tel: (+86)-373-3326439



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Notes

The authors declare no competing financial interest.

Acknowledgement This work was financially supported by National High Technology Research and Development Program of China (863 Program) (2013AA110104), National Nature Science Foundation of China under award (21471049) and Research Fund for the Doctoral Program of Henan Normal University (No. 11116).

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TOC Graphic


C Hybrids as Anode Materials for High

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