Unusually Huge Charge Storage Capacity of Mn3O4âGraphene

Article

Subscriber access provided by SUNY DOWNSTATE 3

4

Unusually Huge Charge Storage Capacity of MnO–Graphene Nanocomposite Achieved by the Incorporation of Inorganic Nanosheets Kanyaporn Adpakpang, Xiaoyan Jin, Seul Lee, Seung Mi Oh, Nam-Suk Lee, and Seong-Ju Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00208 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on April 28, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39

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

ACS Applied Materials & Interfaces

Unusually Huge Charge Storage Capacity of Mn3O4−Graphene Nanocomposite Achieved by the Incorporation of Inorganic Nanosheets Kanyaporn Adpakpang,a Xiaoyan Jin,a Seul Lee,a Seung Mi Oh,a Nam-Suk Lee,b and SeongJu Hwang*,a a

Department of Chemistry and Nanoscience, College of Natural Sciences, Ewha Womans

University, Seoul 03760, Korea b

National Institute for Nanomaterials Technology (NINT), Pohang University of Science and

Technology (POSTECH), Pohang 37666, Korea

* To whom all correspondances are addressed. Tel: +82-2-3277-4370 Fax: +82-2-3277-3419 E-mail: [email protected]

ACS Paragon Plus Environment

1


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

Page 2 of 39

ABSTRACT The remarkable improvement of the electrode performance of Mn3O4−graphene nanocomposite for lithium ion batteries can be obtained by the incorporation of only a small amount of exfoliated layered MnO2 or RuO2 nanosheets.

The metal oxide nanosheet-

incorporated Mn3O4−reduced graphene oxide (rG-O) nanocomposites are synthesized via the crystal growth of Mn3O4 nanocrystals in the mesoporous networks of rG-O and MnO2/RuO2 2D nanosheets. The incorporation of metal oxide nanosheets is highly effective in optimizing porous composite structure and charge transport property resulting in remarkably increasing the discharge capacity of Mn3O4−rG-O nanocomposite with significant improvement of cyclability and rate performance.

The observed enormous discharge capacity of the

synthesized Mn3O4−rG-O−MnO2 nanocomposite (~1600 mA h g−1 for the 100th cycle) is the greatest one among the reported data of Mn3O4−rG-O nanocomposite. Despite much lower electrical conductivity of MnO2 than RuO2, the MnO2-incorporated nanocomposite at optimal composition (2.5wt%) shows even larger discharge capacities with comparable rate characteristics compared with the RuO2-incorporated homologue. This finding underscores that the electrode performance of the resulting nanosheet-incorporated nanocomposite is strongly dependent on its pore and composite structures rather than on the intrinsic electrical conductivity of the additive nanosheet.

The present study clearly demonstrates that,

regardless of electrical conductivity, the incorporation of metal oxide 2D nanosheet is an effective way to efficiently optimize the electrode functionality of graphene-based nanocomposites. KEYWORDS: Graphene, Layered compounds, Nanosheets, Nanocomposites, Energy storage


2

Page 3 of 39

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


1. Introduction Recently exfoliated 2D nanosheets of layered metal oxides attract intense research interest in regard to the unique physicochemical properties and valuable functionalities initiating from extremely high structural and morphological anisotropy.1−3

In comparison with

graphene nanosheet, layered metal oxide nanosheets possess much wider spectrum of chemical compositions, crystal structures, and physicochemical properties.4,5 Concerning to crystal morphology and colloidal nature, there are close similarities between layered metal oxide nanosheet and reduced graphene oxide (rG-O) nanosheet, which makes possible the homogeneous mixing of these two types of nanosheets.6 Due to the lack of free electron clouds in layered metal oxide, the intervention of the metal oxide nanosheet between the rGO nanosheets is supposed to minimize the π−π interaction of rG-O and also to improve the porous structure and charge transfer kinetics of restacked rG-O assembly.7

Thus the

incorporation of metal oxide nanosheets provides useful method to optimize diverse functionalities of rG-O-based nanocomposites.8

In this hybrid system, the electrical

conductivity of the metal oxide nanosheet incorporated is supposed to have profound influence on the electrode performance of the resulting nanocomposite; if the electrical conductivity of the additive nanosheet dominantly determines the electrode performance of the resulting nanocomposite, the addition of highly conductive metal oxide nanosheet is fairly crucial in synthesizing high performance nanocomposite electrode material. Otherwise, there is no significant dependence of the electrode activity of the nanocomposite on the electrical conductivity of the incorporated nanosheet. Since the electrical conductivity of metal oxide is closely related to its chemical composition and thus to its preparation cost, it is crucial to elucidate the relationship between the electrical conductivity of additive nanosheet and electrode functionality of the resulting nanocomposite for establishing an economical route to high performance electrode materials. Among many layered metal oxides, metallic layered


3


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

Page 4 of 39

RuO2 is one of the most conductive materials and its nanosheet can be easily prepared by soft-chemical exfoliation reaction of alkali metal ruthenate.9 However, this material has a drawback of the relatively high price of Ru element. In contrast to the RuO2 nanosheet, the semiconductive MnO2 nanosheet can be obtained by a simple single-step solution-based synthesis at room temperature.10 In addition to cheap price and environmental benignity of Mn element, a facile preparation of the colloidal MnO2 nanosheet can promote the use of this material as an additive for graphene-based nanocomposites. The effect of the incorporation of the RuO2 and MnO2 nanosheets on the electrode functionality can provide valuable insight for understanding the relationship between the electrical conductivity of the additive nanosheet and the electrode performance of the resulting nanocomposite. In fact, there are numerous efforts on the graphene and its nanocomposites as for the structural reinforcement and electrical conductivity enhancement of the nanocomposites.11,12 The aforesaid materials have been widely used in electrochemical applications such as sensor, photocatalyst, fuel cell, as well as lithium ion batteries (LIBs).13−27 As for LIBs anodes, graphene-metal oxide nanocomposites have been extensively studied. Most of studies about metal oxide−graphene nanocomposites can be classified as three categories; the first one is a simple crystal growth of metal oxide nanoparticles in the graphene colloids forming various metal oxide-graphene configurations such as anchoring, wrapping, sandwich structures, and so on.18−22 In this case, a serious self-stacking of graphene nanosheets usually occurs, which can cause the inhomogeneous hybridization between metal oxide and graphene, and is difficult to optimize the porous structure of the resulting nanocomposites. Thus it is hard to achieve the large discharge capacity comparable to the theoretical number of the nanocomposites with this conventional approach. The second category is the morphology-controlled synthesis of metal oxide−graphene nanocomposite such as encapsulated structure.23−25 Even though the control of morphology is somewhat beneficial in enhancing the electrode functionality of the


4

Page 5 of 39

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


nanocomposite, the beneficial effect of morphology control is somewhat limited since the inner composite and pore structure cannot be optimized by the tailoring of morphology. The third category is a simple physical blending between graphene and metal oxide via a conventional mechanical mixing.26,27

Despite its facile synthesis, the resulting

nanocomposites rarely show promising electrochemical performance, which is ascribed to an insufficient chemical interaction between metal oxide and graphene, leading to the nonoptimized charge transport properties of the nanocomposites. In comparison with these previous approaches, the present synthetic strategy of nanosheet incorporation shows much higher efficiency in improving the pore structure, charge transport, and electrode activity of the resulting metal oxide−graphene nanocomposites. Also this method is fairly simple and scalable in that only additional process required is to add a small amount of MnO2 nanosheet into the colloidal suspension of rG-O. More importantly, this strategy is easily applicable for most of ever-reported synthesis methods of graphene-based nanocomposite by utilizing the mixed colloidal suspension of rG-O and MnO2 nanosheets instead of the pure colloidal suspension of rG-O. Judging from these advantages of the present synthetic strategy over the previously reported methods of graphene-based nanocomposites, the incorporation of metal oxide nanosheet can provide fairly novel and useful way to improve various functionalities of graphene-based nanocomposites. Yet at the time of publication of this study, we are aware of no comparative study on the optimization of the electrode functionality of graphene-based nanocomposites via the incorporation of different metal oxide nanosheets with dissimilar electrical conductivities.


5


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

Page 6 of 39

Figure 1. Schematic diagram for the synthesis of the MGM/MGR nanocomposites.

In this work, a series of Mn3O4−graphene−MO2 (M = Mn and Ru) nanocomposites are synthesized by the immobilization of Mn3O4 nanocrystals in the mesoporous stacked assembly of rG-O and MnO2/RuO2 nanosheets, as illustrated in Figure 1. The influences of the electrical conductivity and concentration of incorporated metal oxide nanosheets on the physicochemical properties and electrochemical activity of the resulting nanocomposite are investigated. For this purpose, several metal oxide/rG-O ratios are applied for the synthesis of Mn3O4−graphene−MO2 (M = Mn and Ru) nanocomposites.

Hereafter the resulting

nanocomposites with different MnO2/rG-O ratios (0, 1, 2.5, and 5wt%) are denoted as MG0, MGM1, MGM2.5, and MGM5, respectively. The homologues with RuO2 nanosheets are denoted as MGR1, MGR2.5, and MGR5, respectively.

2. Experimental 2.1. Synthesis. The precursor of layered RuO2 nanosheet was obtained in the form of aqueous colloidal suspension via the exfoliation of the pristine layered Na0.2RuO2, which was achieved by the reaction of the protonated Na0.2RuO2 material with tetrabutylammonium


6

Page 7 of 39

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


(TBA+) ions.9

The colloidal suspension of MnO2 2D nanosheet was obtained by the

oxidation reaction of MnCl2 with H2O2 in the presence of tetramethylammonium ions (TMA+).10 The other precursor of graphene oxide (G-O) nanosheet was prepared by the oxidation of graphite with a modified Hummers' method.28 The colloidal mixtures of layered MnO2/RuO2 and G-O nanosheets were obtained by mixing of the aqueous suspensions of the component nanosheets. As illustrated in Figure 1, the MGM/MGR nanocomposites were synthesized by the reaction of the colloidal mixtures of G-O (48 mL) and MnO2/RuO2 (0−5wt% to G-O) nanosheets with 2.4 mL of 0.2 M Mn(Ac)2 and 1 mL of 30% NH4OH aqueous solution, and 1 mL of H2O at 80 °C for 10 h. Then the obtained suspension was transferred into the hydrothermal vessel and reaction continued at 150 °C for 3 h. After the reaction was complete, precipitates were segregated from the solution by centrifugation, leaving only transparent supernatant solution.

This observation showed the complete

incorporation of Mn2+, MnO2/RuO2, and G-O into the final products. The resulting materials were thoroughly washed with ethanol and distilled water, and finally freeze-dried. The weight ratio of Mn3O4:MnO2/RuO2:rG-O components was estimated to 2.3:0:1 for MG0, 2.3:0.01:1 for MGM1/MGR1, 2.3:0.025:1 for MGM2.5/MGR2.5, and 2.3:0.05:1 for MGM5/MGR5, respectively. 2.2. Characterization. The zeta potentials of the colloidal suspensions of MnO2, RuO2, and G-O nanosheets and their mixtures were examined using Malvern Zetasizer Nano ZS (Malvern, UK). Powder X-ray diffraction (XRD) analysis was carried out to examine the crystal structures of the MGM and MGR nanocomposites using Rigaku D/Max-2000/PC diffractometer (Cu Kα radiation, 298 K). The crystal shapes and local crystal structures of the resulting nanocomposites were characterized with field emission-scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and high resolution-transmission electron microscopy/selected area electron diffraction (HR-TEM/SAED, Jeol JEM-2100F, an


7


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

Page 8 of 39

accelerating voltage of 200 kV). Energy dispersive spectrometry (EDS)−elemental mapping analysis was utilized to probe the spatial distribution of component elements of the present nanocomposites.

The chemical bonding natures of the present nanocomposites were

examined with micro-Raman spectroscopic analysis (JY LabRam HR spectrometer), in which an excitation wavelength of 514.5 nm was utilized. The oxidation states of component ions in the nanocomposites were determined by X-ray photoelectron spectroscopy (XPS) using XPS spectrometer (Thermo VG, UK, Al Kα).

The energies of the measured spectra were

referenced to the adventitious C 1s peak at 284.8 eV to remove the possible spectral modification caused by the charging effect. Mn K-edge and Ru K-edge X-ray absorption near-edge structure (XANES) spectroscopic analyses were performed at the beam line 10C of the Pohang Accelerator Laboratory (PAL) in Korea. The surface areas and porous structures of the nanocomposites were determined with N2 adsorption−desorption analysis (Micromeritics ASAP 2020, 77 K). For the activation of pore structure, degassing process at 150 °C for 3 h under vacuum was employed prior to the measurement. 2.3. Electrochemical measurements. The present nanocomposites were applied as anode materials

for

lithium

ion

batteries

using

Maccor

(Series

4000)

multichannel

galvanostat/potentiostat in the voltage range of 0.01−3.0 V (vs. Li/Li+) at current density of 200−2000 mA g−1. The working electrode was prepared by mixing 80wt% active material, 10wt% Super P, and 10wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2pyrrolidinone (NMP). The composite electrodes were prepared by the deposition of electrode slurry onto a copper foil and vacuum-dried at 120 °C for 12 h. 1 M LiPF6 in the ethylene carbonate/diethyl carbonate (EC/DEC = 50:50) mixture with 3vol% fluoroethylene carbonate (FEC) was used as an electrolyte. The composite electrode, the electrolyte, and the Li metal anode were assembled into 2016 coin-type cell in an argon-filled glove box. The charge transport behaviors of the present nanocomposites were examined by measuring


8

Page 9 of 39

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


electrochemical impedance spectroscopic (EIS) data with the frequency range of 100 KHz−0.01 Hz.

3. Results and Discussion 3.1. Characterization of precursor colloidal suspensions. The stable colloidal mixtures can be obtained by the direct mixing the colloidal suspensions of exfoliated MnO2 and RuO2 nanosheets with the G-O suspensions, which is ascribable to their similar negative surface charges and crystal dimensions to those of G-O nanosheets (see Supporting Information of Figure S1). The crystal growth of Mn3O4 in the existence of colloidal mixtures of the layered MnO2/RuO2

and

G-O

nanosheets

yields

metal

oxide

nanosheet-incorporated

Mn3O4−graphene−MO2 (M = Mn and Ru) nanocomposites. Since the exfoliated MnO2/RuO2 and G-O nanosheets commonly possess negative surface charge, the precursor Mn2+ ions can electrostatically interact on the surface of both the nanosheets of metal oxide and graphene oxide, leading to the immobilization of Mn3O4 crystals on these nanosheets.

Figure 2. Powder XRD patterns of the Mn3O4−graphene−MO2 (M = Mn and Ru) nanocomposites of (a) MG0, (b) MGM1/MGR1, (c) MGM2.5/MGR2.5, and (d)


9


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

Page 10 of 39

MGM5/MGR5. The A and B panels represent the data of the MnO2- and RuO2incorporated nanocomposites, respectively.

3.2. Powder XRD analysis.

Figure 2 presents powder XRD patterns of the

Mn3O4−graphene−MO2 (M = Mn and Ru) nanocomposites.

Notwithstanding the

incorporation of metal oxide nanosheets, all the nanocomposites under investigation display typical XRD peaks of spinel-structured Mn3O4 phase, indicating the formation of Mn3O4 phase during the hydrothermal reaction. Conversely no Bragg reflections of graphene and layered MnO2/RuO2 phase are observable in the XRD patterns, ascribing to the homogeneous dispersion of these nanosheets in the present nanocomposites without any phase segregation. According to the least-square fitting analysis, there is no systematic variation in the lattice parameters of Mn3O4 phase upon the incorporation of exfoliated MnO2 and/or RuO2 nanosheets, as listed in Table S1 of Supporting Information. The present result clearly demonstrates that the incorporation of exfoliated metal oxide nanosheets into Mn3O4−rG-O nanocomposite has negligible influence on the crystal structure of Mn3O4 nanocrystals. The particle sizes of Mn3O4 crystals in the present nanocomposites are calculated using Scherrer equation. All the present nanocomposites possess similar particle size of Mn3O4 component of 14−19 nm, indicating negligible influence of layered MnO2/RuO2 nanosheets on the particle size of Mn3O4 component. 3.3. FE-SEM and TEM analyses. The FE-SEM images of the Mn3O4−graphene−MO2 (M = Mn and Ru) nanocomposites are depicted in Figure 3. All the present nanocomposites exhibit the immobilization of Mn3O4 nanoparticles on the surface of sheet-like crystallites, indicating the formation of Mn3O4−graphene−MO2 nanocomposites. Mesoporous stacking structure of sheet-like crystallites is distinctly discernible for all the present nanocomposites, reflecting the formation of mesopores during the composite formation.


10

Page 11 of 39

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


Figure 3. FE-SEM images of the Mn3O4−graphene−MO2 (M = Mn and Ru) nanocomposites of (a) MG0, (b) MGM1, (c) MGM2.5, (d) MGM5, (e) MGR1, (f) MGR2.5, and (g) MGR5. Figure 4 represents the TEM and HR-TEM images of the present nanocomposites of MG0, MGM2.5, and MGR2.5. All of the present materials display the immobilization of Mn3O4 nanocrystals on the surface of the metal oxide and rG-O nanosheets. In the enlarged HRTEM images of all the present nanocomposites, the interline distances between two neighboring fringes are estimated as ~0.49 nm, which corresponds to the interplanar distance of {101} planes of Mn3O4 phase. The present findings provide strong evidence for the anchoring of the cubic Mn3O4 nanocrystals on the surface of the nanosheets. As shown in Figures 4B and 4C, both the MGM2.5 and MGR2.5 nanocomposites also demonstrate distinct lattice fringes corresponding to layered MnO2/RuO2 nanosheets and rG-O nanosheets, underscoring the incorporation of metal oxide and graphene nanosheets. The co-existence of these components of Mn3O4, layered MnO2/RuO2, and rG-O in the present nanocomposites is further confirmed by the corresponding SAED patterns of these species. The EDS−elemental mapping analysis of the MGR2.5 nanocomposite clearly demonstrates the homogeneous distribution of Mn, Ru, O, and C troughout its structure, confirming the uniform composite formation among Mn3O4 nanoparticles, RuO2 nanosheets, and graphene nanosheets.


11


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

Page 12 of 39

Figure 4. (a,b,c) TEM/HR-TEM images of (A) MG0, (B) MGM2.5, and (C) MGR2.5 nanocomposites. The insets of (c) represent the FT-SAED data. The bottom of right panel depicts EDS−elemental map and TEM image of MGR2.5 nanocomposite. 3.4. Micro-Raman spectroscopic analysis. The micro-Raman spectra of the present MGM and MGR nanocomposites are plotted in Figure 5. Two intense Raman peaks D and G, characteristic of graphene species, are observed at >1000 cm−1 for all the present nanocomposites, indicating the existence of graphene nanosheets in these materials.29 The observation of additional peak D' for the present nanocomposites indicates N-doping for graphene (Figure 5B), since this spectral feature becomes discernible with a significant modification of the sp2 carbon of graphene by the nitrogen doping. In the lower wavenumber region of 0.8, demonstrating the limited concentration of mesopores. The isotherm behavior of the present materials can be classified as Brunauer−Deming−Deming−Teller (BDDT)-type-IV shape and H3-type weak hysteresis loop in the IUPAC classification, which is typically attributed to the aggregates of plate-like particles with slit-shaped mesopores.38,39 No significant change in the overall isotherm shape upon the incorporation of MnO2 and RuO2 nanosheets is observed, suggesting the maintenance of the type of the pore structure of Mn3O4−rG-O nanocomposite upon the incorporation of metal oxide nanosheet.

Figure 8. N2 adsorption−desorption isotherms of the nanocomposites of (a) MG0, (b) MGM1, (c) MGM2.5, (d) MGM5, (e) MGR1, (f) MGR2.5, and (g) MGR5.

The surface area calculation on the basis of the Brunauer−Emmett−Teller (BET) equation exhibits a slight surface expansion of the Mn3O4−rG-O nanocomposite upon the incorporation of MnO2 nanosheets; the surface areas of the present nanocomposites are determined as 68 m2g−1 for MG0, 72 m2g−1 for MGM1, 78 m2g−1 for MGM2.5, 58 m2g−1 for MGM5, 68 m2g−1 for MGR1, 69 m2g−1 for MGR2.5, and 59 m2g−1 for MGR5, respectively. The calculated pore volumes of the present materials are in the range of 0.267−0.375 cm3 g−1.


17


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

Page 18 of 39

Although the incorporation of small amount of MnO2 nanosheets induces a slight increase of the surface area up to the optimal content of 2.5%, a further increase of MnO2 content depresses the surface area of Mn3O4−graphene nanocomposite. In comparison with the incorporation of MnO2 nanosheet, that of RuO2 nanosheet is less effective in expanding the surface of the Mn3O4−rG-O nanocomposite. According to the rigid nature and the lack of free electron clouds of the metal oxide nanosheets, the initial surface expansion caused by the incorporation of metal oxide nanosheet originates from the minimization of π−π interaction between rG-O nanosheets by the intervention of metal oxide nanosheets. This gives rise to the prevention of high self-stacking tendency of graphene nanosheets.

The observed

effective role of metal oxide nanosheet in optimizing the porous structure of restacked graphene-based nanocomposite is already reported for Pt−layered titanate−graphene nanocomposite, suggesting the universal merit of the nanosheet addition.8 It is worthwhile to mention that the observed surface expansion upon the incorporation of metal oxide nanosheets into the Mn3O4-grown rG-O network is much less prominent than the reported surface expansion of the pure rG-O network upon the addition of MnO2 nanosheets.40 This is attributable to the blocking of the pore structure of the MnO2/RuO2−rG-O network by the crystal growth of Mn3O4 nanoparticles. However, according to the previous EIS study,40 even with relatively weak expansion of surface area, the incorporation of metal oxide nanosheet is fairly effective in enhancing the charge transfer properties of rG-O-based nanocomposite, strongly suggesting the formation of efficient internal charge transfer pathway by intervened metal oxide nanosheets via the improved mixing of Mn3O4 and rG-O. 3.7. Electrochemical cycling tests. The evolution of the electrochemical activity of Mn3O4−rG-O nanocomposite upon the incorporation of metal oxide nanosheets is investigated by applying the present nanocomposites as anode materials for lithium ion batteries. The galvanostatic discharge−charge potential plots of the present MGM2.5 and


18

Page 19 of 39

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


MGR2.5 nanocomposites are presented in Figures 9A and 9B. In the first discharge cycle, all the present nanocomposites show a sloping plateau at ~0.3−1.6 V, which is ascribed to the formation of solid−electrolyte interface (SEI) film and the initial reduction of Mn3O4.41 The well-defined plateau at ~0.3 V corresponds to the reductive transformation of Mn3O4 into amorphous Mn and Li2O (Mn3O4 + 8Li+ +8e− → 3Mn + 4Li2O).30,41,42 The interfacial storage of Li+ ions makes additional contribution to the longer plateau below 0.3 V.43 During the charging process, all the nanocomposites under investigation demonstrate two anodic plateaus at ~1.3 and ~2.2 V, which originate from the oxidation of Mn element to MnO and MnO to Mn3O4 (or Mn4+), respectively.41,44 In the following cycle, the electrochemical lithiation and delithiation processes of Mn3O4 occur at higher and lower potentials, respectively, as compared to those observed in the first cycling. This alteration of working potentials strongly suggests the improvement of reaction kinetics.45 As shown in Figures 9A, 9B, and S2 of Supporting Information, the incorporation of metal oxide nanosheets has negligible influence on the potential profiles of the Mn3O4−rG-O nanocomposite, which are typical features of Mn3O4 phase.41

In fact, a very small concentration of incorporated

MnO2/RuO2 nanosheets makes it difficult to experimentally observe their electrochemical redox process. Figures 9C and 9D present the discharge−charge capacity plots of the present nanocomposites measured at current density of 200 mA g−1. The huge initial discharge capacities of 1148, 1196, 1319, 1328, 1286, 1384, and 1343 mA h g−1 are obtained for MG0, MGM1, MGM2.5, MGM5, MGR1, MGR2.5, and MGR5, respectively. At the first cycle, all the present nanocomposites exhibit significant capacity losses, which are mainly ascribable to the formation of SEI layer.41,42 After the first cycle, high coulombic efficiency of >95% occurs for all the present nanocomposites, indicating the highly reversible insertion/extraction of Li+ ions.

The capacities of the present nanocomposites become


19


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

Page 20 of 39

increasing with proceeding the cycle. The observed gradual increase of discharge capacities originates from the improvement of the electrode kinetics and the development of stable Li+ diffusion paths during the electrochemical cycling, as frequently reported for porous materials.44,46−48

Figure 9. Potential profiles for the nanocomposite of (A) MGM2.5 and (B) MGR2.5. Capacity plots and rate capability profiles of (C/E) MGM and (D/F) MGR nanocomposites for (a) MG0, (b) MGM1/MGR1, (c) MGM2.5/MGR2.5, and (d) MGM5/MGR5.

Since a gradual increase of the discharge capacity of the present nanocomposite might originate from the decomposition of the electrolyte, the variation of the differential capacity plots of the MGM2.5 electrode with proceeding the cycle is investigated to check out the


20

Page 21 of 39

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


possible electrolyte decomposition.

As shown in Figure S3 of Supporting Information,

intense peaks denoted as a and b are clearly observable at ~1.4 and ~0.3 V in the discharge process, which are assigned as the reductive reaction of Mn3O4 with Li+ ions to yield Li2O and MnO/Mn, respectively.30,41,42 Additionally a small reduction peak b’ corresponding to the decomposition of electrolyte and the resulting formation of solid-electrolyte-interface film appears at ~0.4 V for the first cycle and then disappears in the following cycles, suggesting the negligible occurrence of the decomposition of electrolyte after the first cycle. At the lower potential of ~0.01−0.02 V, another reduction peak c is discernible, which is attributable to the insertion of Li+ ions into the graphene lattice.49,50 As the cycle proceeds, a small reduction peak a’ appears at ~1 V, which can be assigned as the reduction of higher-valent manganese oxide into MnO phase.41,44 The advent of this additional peak strongly suggests that the increased capacity upon repeated cycling is attributable to the enhanced kinetics of the electrode, leading to the formation of higher-valent Mn species. In the case of charging process, the MGM2.5 nanocomposite displays a small peak denoted as d at ~0.13 V corresponding to the extraction of Li+ ions from the graphene lattice as well as two intense peaks denoted as e and f at ~1.3 and ~2.2 V related to the oxidation of Mn into MnO and higher-valent manganese oxide, respectively.41,44

No observation of electrolyte-related

features allows us to conclude negligible contribution of the decomposition of electrolyte to the observed increase of discharge capacity with proceeding the cycle. This conclusion is further confirmed by the coulombic efficiency plots of the MGM and MGR nanocomposites in Figure S4 of Supporting Information. After the several initial cycles, all the present nanocomposites commonly display a high coulombic efficiency of >95%, highlighting negligible contribution of side reaction caused by the degradation of electrolyte. On the basis of the experimental findings presented here, we are able to conclude that the increased capacity of the present nanocomposite is attributed to the enhanced kinetics of the electrode


21


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

Page 22 of 39

with the promoted formation of higher-valent Mn species which can react with larger number of Li+ ions not to the decomposition of electrolyte. In fact, similar notable increase of the discharge capacity with proceeding the cycle has been already observed for other nanocomposites of metal oxide−graphene, which was also interpreted as a result of the improved kinetics rather than the decomposition of the electrolyte.44,51 As can be seen clearly from Figures 9C and 9D, the incorporation of a small amount of the layered MnO2 and RuO2 nanosheets commonly results in the remarkable increase of the discharge capacity of the Mn3O4−rG-O nanocomposite. The discharge capacities of the present nanocomposites are quite sensitive to a minute variation of the content of metal oxide nanosheet incorporated. Among the present nanocomposites, the MGM2.5 nanocomposite delivers the largest discharge capacity at current density of 200 mA g−1. The discharge capacities of the present nanocomposites for the 100th cycle are estimated to be ~1200 mA h g−1 for MGM1, ~1600 mA h g−1 for MGM2.5, ~1300 mA h g−1 for MGM5, ~1000 mA h g−1 for MGR1, ~1200 mA h g−1 for MGR2.5, and ~1400 mA h g−1 for MGR5, which are much greater than that of the metal oxide nanosheet-free MG0 nanocomposite (~700 mA h g−1). Of noteworthy is that, even with a small content of metal oxide nanosheet incorporated, the discharge capacities of the MGM2.5 and MGR2.5 nanocomposites are more than twice greater than that of the MG0 one, highlighting the remarkably high efficiency of the metal oxide addition in improving the electrode performance of Mn3O4−graphene nanocomposite. To the best of our knowledge, the discharge capacity of the present MGM2.5 material is the largest reversible discharge capacity of Mn3O4−graphene-based materials ever-reported (see Supporting Information of Table S2). Judging from the fact that all the components in the present nanocomposites are electrochemically active, the theoretical discharge capacity of these materials can be calculated to be ~1023 mA h g−1 for MGM and ~1022 mA h g−1 for MGR on the basis of the theoretical capacities of the component materials (i.e. 937 mA h g−1


22

Page 23 of 39

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


for Mn3O4, 1216 mA h g−1 for N-doped rG-O, 1230 mA h g−1 for MnO2, and 1100 mA h g−1 for RuO2).7,30,52,53

The obtained theoretical capacities of the MGM and MGR

nanocomposites are much smaller than the experimental data of the present MGM2.5 and MGR2.5 nanocomposites.

Such unusually huge discharge capacities of the present

nanocomposites with MnO2/RuO2 nanosheets can be ascribed to several factors as follows; (1) the interfacial storage of Li+ ions of the composite structure created by the surface expansion upon composite formation,54 (2) the provision of more Li insertion sites and the increase of Li adsorption energies at the vacancy sites created by the N-doping for the rG-O component,55−57 (3) the creation of more active sites for Li-ion insertion caused by the amorphization of Mn3O4 nanoparticles upon the cycling process,45,58 and (4) the generation of more open stacking composite structure of graphene−metal oxide network increasing additional active sites for Li+ insertion.8 Between the two kinds of metal oxide nanosheets employed here, the layered MnO2 nanosheet is a more effective additive for improving the electrode performance of Mn3O4−rG-O nanocomposite at the current density of 200 mA g−1 (see Figures 9C and 9D). Taking into account the greater surface area of MGM than MGR and the lower electrical conductivity of MnO2 than RuO2, the superior electrode performance of the MGM nanocomposites can be interpreted as evidence for a more crucial role of the composite structure in optimizing the electrochemical activity of graphene-based nanocomposite, compared with the intrinsic electrical conductivity of the incorporated metal oxide nanosheet. Additionally, the long-term electrochemical stability of the present MGM2.5 nanocomposite showing the best electrode performance is investigated by extended electrochemical cyclings at a high current density of 1000 mA h g−1 (see Supporting Information of Figure S5). At the 300th cycle, this MnO2-incorporated nanocomposite still exhibits a large discharge capacity of ~1100 mA h g−1, clearly demonstrating an effective role


23


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

Page 24 of 39

of MnO2 nanosheet in optimizing the long-term activity of graphene-based nanocomposite electrode. Also the effect of layered metal oxide nanosheet on the rate characteristics of Mn3O4−rG-O nanocomposite is examined at various current densities, see Figures 9E and 9F. All of the metal oxide nanosheet-incorporated MGM and MGR nanocomposites show better electrode performance under high current density compared with the metal oxide nanosheetfree MG0 nanocomposite, highlighting the beneficial role of additive metal oxide nanosheet in improving the rate characteristics of graphene-based nanocomposite. In comparison with layered MnO2 nanosheet, layered RuO2 nanosheet is slightly more effective in improving the rate performance of Mn3O4−rG-O nanocomposite (see Supporting Information of Figure S6), which is attributable to the higher electrical conductivity of RuO2 than MnO2. Of noteworthy is that, at the optimal content of 2.5wt%, the rate performance of MGM2.5 nanocomposite is only slightly inferior or comparable to that of MGR2.5 nanocomposite; both the MGM2.5 and MGR2.5 nanocomposites show larger capacities at 200 mA g−1 by 228 and 164% than does the MG0 material. At a higher current density of 2000 mA g−1, the discharge capacities of the MGM2.5 and MGR2.5 nanocomposites are much larger by 140 and 160% than that of the MG0 material, strongly suggesting the usefulness of the MnO2 nanosheet as an economic and efficient additive. The present experimental findings clearly demonstrate that, regardless of the electrical conductivity, the incorporation of layered metal oxide nanosheet is fairly effective in improving the anode performance of the graphene-based nanocomposite. Additionally the electrode activity of the MGM2.5 nanocomposite is also examined in the high potential range of 2.0−4.0 V to probe the effect of high-valent MnO2 nanosheets on the cathode functionality of the present nanocomposite. As depicted in Figure S7 of Supporting Information, the present nanocomposite can deliver only a very small discharge capacity of ~1 mA h g−1, indicating its poor electrode performance as cathode material for lithium ion


24

Page 25 of 39

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


batteries. This finding clearly demonstrates that the present nanocomposite material is useful as anode materials for lithium ion batteries not as cathode materials. 3.8. EIS analysis. The evolution of the charge transport of the present nanocomposites upon the incorporation of MnO2 and RuO2 nanosheets is investigated with EIS analysis. Figure 10A illustrates the EIS spectra of the present MGM2.5, MGR2.5 and MG0 nanocomposites. All of the materials under investigation display similar EIS data composed of partially overlapping semicircles at high-to-medium frequencies and straight-lines at low frequencies. The equivalent circuit model and the fitting parameters are shown in the inset of Figure 10A and Table 1, respectively.

The equivalent circuit is composed of the cell-

components and electrolyte resistance (Rs), polarization resistance (Rf), surface film capacitance (Cf), charge-transfer resistance (Rct), double-layer capacitance (Cdl) and Warburg impedance (Wo).59

Figure 10. (A) EIS spectra and (B) Zre Vs ω−1/2 plot for the fresh cells of MG0 (black), MGM2.5 (blue), and MGR2.5 (red). The circles and solid lines represent the experimental and fitted spectra, respectively. Insets of (A) depict the equivalent circuit and EIS spectra at high-medium frequency.

As listed in Table 1, the Rs values appear nearly identical for all the present nanocomposites. The Rct values of the MG0, MGM2.5, and MGR2.5 nanocomposites are calculated to be 911, 229, and 312 Ω, respectively, indicating the superior charge-transport kinetics of the MnO2/RuO2 nanosheet-incorporated nanocomposites over the nanosheet-free


25


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

Page 26 of 39

one. This result strongly demonstrates that the optimization of composite and pore structures upon the incorporation of metal oxide nanosheets can effectively enhance the charge transport property of the nanocomposites. This is ascribable to the improved nanoscale mixing of Mn3O4 nanoparticles and rG-O nanosheets caused by the depression of selfagglomeration of graphene nanosheets by the intervened metal oxide nanosheets.

Table 1. Parameters obtained from the fitting analysis to experimental EIS spectra. Cdl (µF)

σw (Ω s−1/2)

DLi+ (cm s−1)

911

19.04

481.3

3.79×10−18

8.77

229

11.73

361.0

6.73×10−18

20.77

312

8.39

346.6

7.30×10−18

Material

Rs (Ω)

Rf (Ω)

Cf (µF) Rct (Ω)

MG0

6.46

98.55

7.63

MGM2.5

5.59

57.51

MGR2.5

4.49

30.85

Figure 10B depicts the Zre vs. ω−1/2 plot in the Warburg region. The slope of this plot corresponds to the Warburg coefficient (σw), which is inversely proportional to the measure of diffusion coefficient of the ions taking part in the solid-state diffusion in the electrode material.

Both the MGR2.5 and MGM2.5 nanocomposites possess higher Li+ ion

diffusivities with smaller σw of 346.6 and 361.0 Ω·s−1/2 , respectively, compared with that of MG0 nanocomposite (σw: 481.3 Ω·s1/2).

According to the obtained values, diffusion

coefficient of Li+ can be calculated based on the equation (1),60 DLi+ = R2T2/2A2n4F4C2σ2

(1)

where D is the diffusion coefficient of Li+, n is the number of electron per molecule, A is the surface area of the active material, R is the gas constant, T is the absolute temperature, F is the Faraday constant, C is the concentration of Li+ ions, and σ is the Warburg coefficient. The calculated diffusion coefficients of the MGR2.5, MGM2.5, and MGR0 nanocomposites


26

Page 27 of 39

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


are 7.30×10−18, 6.73×10−18, and 3.79×10−18 cm s−1, respectively. This result indicates the improved polarizability of the metal oxide nanosheet-incorporated nanocomposites and the efficient roles of the metal oxide nanosheets over the rG-O in optimizing the Li+-transfer kinetics of the electrodes. 3.9. Powder XRD and FE-SEM analyses for electrochemically cycled derivatives. The structural and morphological variations of the present nanocomposites upon the electrochemical cycling are studied with powder XRD and FE-SEM analyses. As plotted in Figure 11A, all the present nanocomposites commonly experience the transformation of crystalline Mn3O4 into disordered amorphous structure with the formation of Li2O during the electrochemical cycling, which leads to a marked increase of Li+ diffusion paths.45 The incorporated metal oxide nanosheets do not affect notably the crystal structures of the cycled derivatives.

Figure 11. (A) Powder XRD patterns and (B) FE-SEM images for the electrochemically-cycled nanocomposites of (a) MG0, (b) MGM2.5, and (c) MGR2.5.

As can be seen from Figure 11B, the incorporation of metal oxide nanosheet gives rise to the decrease of the particle sizes of the MGM2.5 and MGR2.5 nanocomposites upon electrochemical cycling without significant agglomeration. Conversely, the nanosheet-free


27


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

Page 28 of 39

MG0 nanocomposite experiences the notable aggregation of electrode particles after the electrochemical cycling. This observation clearly demonstrates the improved morphological stability of the present nanocomposites achieved by the addition of metal oxide nanosheets. According to the N2 adsorption−desorption isotherm analysis for the restacked rG-O and MnO2 nanosheets without the Mn3O4 nanocrystals, the incorporation of MnO2 nanosheets leads to the significant increase of the surface area of the restacked rG-O nanosheets with the remarkable enhancement of porosity; the hybrid networks of 2D MnO2 and rG-O nanosheets with the MnO2/rG-O ratios of 2.7, 7.1, and 14.9wt% exhibit greater surface area of 49, 75, and 162 m2 g−1, respectively, compared with that of pure rG-O network (35 m2 g−1).40 The present result clearly demonstrates that the incorporation of exfoliated metal oxide nanosheets is highly effective in enhancing the porosity of the restacked rG-O network via the diminishment of the π−π interaction between the rG-O nanosheets. The optimized pore structure of restacked nanosheets provides efficient pathways of Li+ ion diffusion during the electrochemical cycling, leading to the negligible perturbation of the composite structure and morphology of the nanocomposite. Additionally the improved pore structure of the restacked metal oxide/rG-O hybrid networks makes possible more homogeneous distribution of Mn3O4 nanocrystals in the nanocomposite matrix, which in turn hinders the strong interaction between rG-O nanosheets and depresses the agglomeration of rG-O nanosheets upon the synthesis process and also during the cycling. Also such highly porous structure of hybrid metal oxide/rG-O network can provide greater number of anchoring sites for the reduction products of Mn and Li2O formed by the electrochemical discharging process. This facilitates the reversible conversion reaction of Mn3O4 nanocrystals without significant frustration of the matrix network of restacked metal oxide/rG-O nanosheets, leading to the improvement of the morphological stability of the present nanocomposites upon the incorporation of metal oxide nanosheets. The resulting retention of the open stacking structure of the present


28

Page 29 of 39

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


nanocomposites is beneficial in accommodating the severe volume change of Mn3O4 during the electrochemical cycling, leading to the enhanced electrode performance of the nanocomposite. The present finding strongly suggests that the efficient role of metal oxide nanosheets incorporated is mainly attributable to the enhanced morphological stability of composite structure rather than to the alteration of structural stability.

The initial

homogeneous mixing between Mn3O4 and rG-O in the present nanocomposites leads to the improvement of Li+ ion diffusion paths and to the enhanced electronic coupling between Mn3O4 and rG-O nanosheet.

4. Conclusion In this study, very efficient Mn3O4-based nanocomposite electrode materials with huge discharge capacity can be synthesized by the incorporation of small amount of exfoliated metal oxide nanosheets into the Mn3O4−graphene nanocomposite. The charge transport and pore structure of the Mn3O4−graphene nanocomposites can be remarkably improved by the addition of metal oxide nanosheets. The MnO2 nanosheet-incorporated Mn3O4−graphene nanocomposite delivers huge discharge capacity of ~1600 mA h g−1 for the 100th cycle with excellent cyclability and rate performance, which is much superior to that of Mn3O4−graphene nanocomposite (~700 mA h g−1). The electrode performance of the present nanocomposites are superior to those of all the previously reported Mn3O4-based electrode materials (see Supporting Information of Table S2). The outstanding electrode performance of the present metal oxide nanosheet-incorporated nanocomposite is ascribable to the provision of more Li+ storage sites and the enhancement of charge transport behavior upon the incorporation of metal oxide nanosheet via the optimization of composite and pore structures. It is worthwhile to mention that, even though the MnO2 nanosheet possesses a lower electrical conductivity than does the RuO2 one, the MnO2-incorporated MGM


29


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

Page 30 of 39

nanocomposites show superior electrode performance at low current density over the RuO2incorporated MGR homologue.

Even at high current density condition, the MGM

nanocomposite with optimal MnO2 content displays comparable electrode performance to the MGR nanocomposite. This result underscores that the optimization of composite and pore structures upon the incorporation of additive metal oxide nanosheet plays a more important role in improving the electrode activity of graphene-based nanocomposite than does the intrinsic electrical conductivity of the additive nanosheet. Taking into account valuable merits of the MnO2 nanosheet over the RuO2 nanosheet in terms of production cost and environmental benignity, the better electrode activities of the MGM nanocomposites than the MGR ones are promising results for the practical application of the present synthetic strategy. The optimization of porous composite structure and electrochemical activity caused by the incorporation of metal oxide nanosheet is expected to be highly efficient in improving other functionalities of graphene-based nanocomposites as electrocatalysts, photocatalysts, redox catalysts, nanobio materials, etc. Our current research project is the exploration of novel graphene-based functional materials with versatile applications such as solar cells, photocatalysts, and so on.

Supporting Information.

The photoimages and zeta potential data of the colloidal

suspensions of MnO2, RuO2, and G-O nanosheets, and their colloidal mixtures, the lattice parameters, charge−discharge profiles, differential capacity plots, coulombic efficiency plots, long-term discharge capacity plot, and of the present nanocomposites, and the reported electrode performance data of Mn3O4−graphene nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS


30

Page 31 of 39

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


This research is supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2010-C1AAA001-2010-0029065) and the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning. The experiments at PAL were supported by MOST & POSTECH.

REFERENCES 1. Wang, L.; Sasaki, T. Titanium Oxide Nanosheets: Graphene Analogues with Versatile Functionalities. Chem. Rev. 2014, 114, 9455–9486. 2. Oh, E. -J.; Kim, T. W.; Lee, K. M.; Song, M. -S.; Jee, A. -Y.; Lim, S. T.; Ha, H. -W.; Lee, M.; Choy, J. -H.; Hwang, S. -J. Unilamellar Nanosheet of Layered Manganese Cobalt Nickel Oxide and Its Heterolayered Film with Polycations. ACS Nano 2010, 4, 4437−4444. 3. Kim, T. W.; Oh, E. J.; Lim, S. T.; Park, D. H.; Jee, A. Y.; Lee, M.; Hyun, S. H.; Choy, J. -H.; Hwang, S. -J. Exfoliated Nanosheets of Layered Cobalt Oxide and Their Application for Film Deposition and Nanoparticle Synthesis. Chem. Eur. J. 2009, 15, 10752−10761. 4. Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutierrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F. Progress, Challenges, and Opportunities in TwoDimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898–2926. 5.

Xu, M.; Liang, T.; Shi, M.; Chen, H. Graphene-Like Two-Dimensional Materials. Chem. Rev. 2013, 113, 3766–3798.

6. Lee, Y. R.; Kim, I. Y.; Kim, T. W.; Lee, K. M.; Hwang, S. -J. Mixed Colloidal Suspensions of Reduced Graphene Oxide and Layered Metal Oxide Nanosheets: Useful Precursors for the Porous Nanocomposites and Hybrid Films of Graphene/Metal Oxide. Chem. Eur. J. 2012, 18, 2263–2271.


31


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

Page 32 of 39

7. Jin, X.; Adpakpang, K.; Kim, I. Y.; Oh, S. M.; Lee, N. -S.; Hwang, S. -J. An Effective Way to Optimize the Functionality of Graphene-Based Nanocomposites: Use of the Colloidal Mixture of Graphene and Inorganic Nanosheets. Sci. Rep. 2015, 5, 11057– 11069. 8. Shin, S. I.; Go, A.; Kim, I. Y.; Lee, J. M.; Lee, Y.; Hwang, S. -J. A Beneficial Role of Exfoliated Layered Metal Oxide Nanosheets in Optimizing the Electrocatalytic Activity and Pore Structure of Pt−Reduced Graphene Oxide Nanocomposites. Energy Environ. Sci. 2013, 6, 608–617. 9. Fukuda, K.; Saida, T.; Sato, J.; Yonezawa, M.; Takasu, Y.; Sugimoto, W. Synthesis of Nanosheet Crystallites of Ruthenate with an α-NaFeO2-Related Structure and Its Electrochemical Supercapacitor Property. Inorg. Chem. 2010, 49, 4391–4393. 10. Kai, K.; Yoshida, Y.; Kageyama, H.; Saito, G.; Ishigaki, T.; Furukawa, Y.; Kawamata, J. Room-Temperature Synthesis of Manganese Oxide Monosheets. J. Am. Chem. Soc. 2008, 130, 15938–15943. 11. Vlassiouk, I; Polizos, G.; Cooper, R.; Ivanov, I.; Keum, J. K.; Paulauskas, F.; Datskos, P.; Smirnov, S. Strong and Electrically Conductive Graphene-Based Composite Fibers and Laminates. ACS Appl. Mater. Interfaces 2015, 7, 10702–10709. 12. Matsumoto, H; Imaizumi, S; Konosu, Y; Ashizawa, M; Minagawa, M; Tanioka, A; Lu, W; Tour, J. M. Electrospun Composite Nanofiber Yarns Containing Oriented Graphene Nanoribbons. ACS Appl. Mater. Interfaces 2013, 5, 6225–6231. 13. Salamon, J.; Sathishkumar, Y.; Ramachandran, K.; Lee, Y. S.; Yoo, D. J.; Kim, A. R.; Gnana Kumar, G. One-Pot Synthesis of Magnetite Nanorods/Graphene Composites and Its Catalytic Activity Toward Electrochemical Detection of Dopamine. Biosens. Bioelectron. 2015, 64, 269–276.


32

Page 33 of 39

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


14. Gnana Kumar, G.; Babu, K. J.; Nahm, K. S.; Hwang, Y. J. A Facile One-Pot Green Synthesis of Reduced Graphene Oxide and Its Composites for Non-Enzymatic Hydrogen Peroxide Sensor Applications. RSC Adv. 2014, 4, 7944–7951. 15. Senthilkumar, N.; Babu, K. J.; Gnana Kumar, G.; Kim, A. R.; Yoo, D. J. Flexible Electrospun PVdF-HFP/Ni/Co Membranes for Efficient and Highly Selective Enzyme Free Glucose Detection. Ind. Eng. Chem. Res. 2014, 53, 10347−10357. 16. Gnana Kumar, G.; Awan, Z.; Nahm, K. S.; Xavier, J. S. Nanotubular MnO2/Graphene Oxide Composites for the Application of Open Air-Breathing Cathode Microbial Fuel Cells. Biosens. Bioelectron. 2014, 53, 528–534. 17. Pan, X.; Zhao, Y.; Liu, S.; Korzeniewski, C. L.; Wang, S.; Fan, Z. Comparing Graphene−TiO2 Nanowire and Graphene-TiO2 Nanoparticle Composite Photocatalysts. ACS Appl. Mater. Interfaces 2012, 4, 3944–3950. 18. Zhang, M.; Lei, D.; Du, Z. F.; Yin, X. M.; Chen, L. B.; Li, Q. H.; Wang, Y. G.; Wang, T. H. Fast Synthesis of SnO2/Graphene Composites by Reducing Graphene Oxide with Stannous Ions. J. Mater. Chem. 2011, 21, 1673–1676. 19. Xu, C.; Wang, X.; Yang, L. C.; Wu, Y.P. Fabrication of a Graphene–Cuprous Oxide Composite. J. Solid State Chem. 2009, 182, 2486–2490. 20. Yang, S. B.; Feng, X. L.; Wang, L.; Tang, K.; Maier, J.; Müllen, K. Graphene-Based Nanosheets with a Sandwich Structure. Angew. Chem. Int. Ed. 2010, 49, 4795–4799. 21. Zhou, G. M.; Wang, D. W.; Li, F.; Zhang, L. L.; Li, N.; Wu, Z. S.; Wen, L.; Lu, G. Q.; Cheng, H. M. Graphene-Wrapped Fe3O4 Anode Material with Improved Reversible Capacity and Cyclic Stability for Lithium Ion Batteries. Chem. Mater. 2010, 22, 5306– 5313. 22. Kim, H.; Seo, D. H.; Kim, S. W.; Kim, J.; Kang, K. Highly Reversible Co3O4/graphene Hybrid Anode for Lithium Rechargeable Batteries. Carbon 2011, 49, 326–332.


33


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

23. Yang, S. B.; Feng, X. L.; Ivanovici, S.; Müllen, K.

Page 34 of 39

Fabrication of Graphene-

Encapsulated Oxide Nanoparticles: Towards High-Performance Anode Materials for Lithium Storage. Angew. Chem. Int. Ed. 2010, 49, 8408–8411. 24. Wei, W.; Yang, S.; Zhou, H.; Lieberwirth, I.; Feng, X.; Müllen, K. 3D Graphene Foams Cross-linked with Pre-encapsulated Fe3O4 Nanospheres for Enhanced Lithium Storage. Adv. Mater. 2013, 25, 2909–2914. 25. Yang, X.; Fan, K.; Zhu, Y.; Shen, J.; Jiang, X. Zhao, P.; Lia, C. Tailored GrapheneEncapsulated Mesoporous Co3O4 Composite Microspheres for High-Performance Lithium Ion Batteries. J. Mater. Chem. 2012, 22, 17278-17283. 26. Ye, F.; Zhao, B.; Ran, R.; Shao, Z. Facile Mechanochemical Synthesis of Nano SnO2/Graphene Composite from Coarse Metallic Sn and Graphite Oxide: An Outstanding Anode Material for Lithium-Ion Batteries. Chem. Eur. J. 2014, 20, 4055– 4063. 27. Wang, Y.; Yang, L.; Hu, R.; Sun, W.; Liu, J.; Ouyang, L.; Yuan, B.; Wang, H.; Zhu, M. A stable and High-Capacity Anode for Lithium-Ion Battery: Fe2O3 Wrapped by Few Layered Graphene. J. Power Sources 2015, 288, 314–319. 28. Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780–786. 29. Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558–1565. 30. Li, L.; Guo, Z.; Du, A.; Liu, H. Rapid Microwave-Assisted Synthesis of Mn3O4– Graphene Nanocomposite and Its Lithium Storage Properties. J. Mater. Chem. 2012, 22, 3600–3605.


34

Page 35 of 39

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


31. Li, Q.; Guo, B.; Yu, J.; Ran, J.; Zhang, B.; Yan, H.; Gong, J. R. Highly Efficient VisibleLight-Driven Photocatalytic Hydrogen Production of CdS-Cluster-Decorated Graphene Nanosheets. J. Am. Chem. Soc. 2011, 133, 10878–10884. 32. Shao, Y.; Zhang, S.; Engelhard, M. H.; Li, G.; Shao, G.; Wang, Y.; Liu, J.; Aksay, I. A.; Lin, Y. J. Nitrogen-Doped Graphene and Its Electrochemical Applications. J. Mater. Chem. 2010, 20, 7491–7496. 33. Fan, Q.; Zhang, W.; Duan, J.; Hong, K.; Xue, L.; Huang, Y. Effects of Binders on Electrochemical Performance of Nitrogen-Doped Carbon Nanotube Anode in SodiumIon Battery. Electrochim. Acta 2015, 174, 970–977. 34. Nesbitt, H. W.; Banerjee, D. Interpretation of XPS Mn(2p) Spectra of Mn Oxyhydroxides and Constraints on the Mechanism of MnO2 Precipitation. Am. Mineral. 1998, 83, 305– 315. 35. Manceau, A.; Gorshkov, A. I.; Drits, V. A. Structural Chemistry of Manganese, Iron, Cobalt, and Nickel in Manganese Hydrous Oxides: Part I. Information from XANES Spectroscopy. Am. Mineral. 1992, 77, 1133–1143. 36. Hwang, S. -J.; Kwon, C. W.; Portier, J.; Campet, G.; Park, H. S.; Choy, J. H.; Huong, P. V.; Yoshimura, M.; Kakihana, M. Local Crystal Structure around Manganese in New Potassium-Based Nanocrystalline Manganese Oxyiodide. J. Phys. Chem. B 2002, 106, 4053–4060. 37. Hwang, S. -J.; Choy, J. -H. Local Atomic Arrangement and Electronic Structure of Nanocrystalline Transition Metal Oxides Determined by X-ray Absorption Spectroscopy. J. Phys. Chem. B 2003, 107, 5791–5796. 38. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, Academic Press, London, 1982. 39. Allen, T. Particle Size Measurement, Chapman and Hall, London, 1980.


35


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

Page 36 of 39

40. Gu, T. -H.; Gunjakar, J. L.; Kim, I. Y.; Patil, S. B.; Lee, J. M.; Jin, X.; Lee, N. -S.; Hwang, S. -J. Porous Hybrid Network of Graphene and Metal Oxide Nanosheets as Useful Matrix for Improving the Electrode Performance of Layered Double Hydroxide. Small 2015, 11, 3921−3931. 41. Gao, J.; Lowe, M. A.; Abruña, H. D. Spongelike Nanosized Mn3O4 as a High-Capacity Anode Material for Rechargeable Lithium Batteries. Chem. Mater. 2011, 23, 3223–3227. 42. Wang, H.; Cui, L. -F.; Yang, Y.; Casalongue, H. S.; Robinson, J. T.; Liang, Y.; Cui, Y.; Dai, H. Mn3O4−Graphene Hybrid as a High-Capacity Anode Material for Lithium Ion Batteries. J. Am. Chem. Soc. 2010, 132, 13978–13980. 43. Hu, Y. -Y.; Liu, Z.; Nam, K. -W.; Borkiewicz, O. J.; Cheng, J.; Hua, X.; Dunstan, M. T.; Yu, X.; Wiaderek, K. M.; Du, L. -S.; Chapman, K. W.; Chupas, P. J.; Yang, X. -Q.; Grey, C. P. Origin of Additional Capacities in Metal Oxide Lithium-Ion Battery Electrodes. Nat. Mater. 2013, 12, 1130–1136. 44. Sun, Y.; Hu, X.; Luo, W.; Xia, F; Huang, Y. Reconstruction of Conformal Nanoscale MnO on Graphene as a High-Capacity and Long-Life Anode Material for Lithium Ion Batteries. Adv. Funct. Mater. 2013, 23, 2436–2444. 45. Ko, Y. N.; Park, S. B.; Choi, S. H.; Kang, Y. C. One-Pot Synthesis of Manganese OxideCarbon Composite Microspheres with Three Dimensional Channels for Li-Ion Batteries. Sci. Rep. 2014, 4, 5751. 46. Li, X.; Xiong, S.; Li, J.; Liang, X.; Wang, J.; Bai, J.; Qian, Y. MnO@Carbon Core–Shell Nanowires as Stable High-Performance Anodes for Lithium-Ion Batteries. Chem. Eur. J. 2013, 19, 11310–11319. 47. Lee, S. M.; Choi, S. H.; Lee, J. -K.; Kang, Y. C. Electrochemical Properties of GrapheneMnO Composite and Hollow-Structured MnO Powders Prepared by a Simple One-Pot Spray Pyrolysis Process. Electrochim. Acta 2014, 132, 441–447.


36

Page 37 of 39

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


48. Zhou, X.; Wan, L. -J.; Guo, Y. -G. Binding SnO2 Nanocrystals in Nitrogen-Doped Graphene Sheets as Anode Materials for Lithium-Ion Batteries. Adv. Mater. 2013, 25, 2152–2157. 49. Lian, P.; Zhu, X.; Liang, S.; Li, Z.; Yang, W.; Wang, H. Large Reversible Capacity of High Quality Graphene Sheets as an Anode Material for Lithium-Ion Batteries. Electrochim. Acta 2010, 55, 3909–3914. 50. Yao, J.; Shen, X.; Wang, B.; Liu, H.; Wang, G. In Situ Chemical Synthesis of SnO2– Graphene Nanocomposite as Anode Materials for Lithium-Ion Batteries. Electrochem. Commun. 2009, 11, 1849–1852. 51. Peng, C.; Chen, B.; Qin, Y.; Yang, S.; Li, C.; Zuo, Y.; Liu, S.; Yang, J. Facile Ultrasonic Synthesis of CoO Quantum Dot/Graphene Nanosheet Composites with High Lithium Storage Capacity. ACS Nano 2012, 6, 1074–1081. 52. Li, L.; Raji, A. -R. O.; Tour, J. M. Graphene-Wrapped MnO2–Graphene Nanoribbons as Anode Materials for High-Performance Lithium Ion Batteries. Adv. Mater. 2013, 25, 6298–6302. 53. Balaya, P.; Li, H.; Kienle, L.; Maier, J. Fully Reversible Homogeneous and Heterogeneous Li Storage in RuO2 with High Capacity. Adv. Funct. Mater. 2003, 13, 621–625. 54. Zhou, G.; Wang, D. -W.; Yin, L. -C.; Li, N.; Li, F.; Cheng, H. -M. Oxygen Bridges Between NiO Nanosheets and Graphene for Improvement of Lithium Storage. ACS Nano 2012, 6, 3214–3223. 55. Su, F.; Poh, C.K.; Chen, J. S.; Xu, G.; Wang, D.; Li, Q.; Lin, J.; Lou, X.W. NitrogenContaining Microporous Carbon Nanospheres with Improved Capacitive Properties. Energy Environ. Sci. 2011, 4, 717–724


37


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

Page 38 of 39

56. Wu, Z. -S.; Ren, W; Xu, L.; Li, F.; Cheng, H. -M. Doped Graphene Sheets as Anode Materials with Super high Rate and Large Capacity for Lithium Ion Batteries. ACS Nano 2011, 5, 5463–5471. 57. Wang, Z. H.; Qie, L.; Yuan, L. X.; Zhang, W. X.; Hu, X. L.; Huang, Y. H. Functionalized N-doped Interconnected Carbon Nanofibers as an Anode Material for Sodium-Ion Storage with Excellent Performance. Carbon 2013, 55, 328–334. 58. Zhong, K.; Xia, X.; Zhang, B.; Li, H.; Wang, Z.; Chen, L. MnO Powder as Anode Active Materials for Lithium Ion Batteries. J. Power Sources 2010, 195, 3300–3308. 59. Sahu, T. S.; Mitra, S. Exfoliated MoS2 Sheets and Reduced Graphene Oxide-An Excellent and Fast Anode for Sodium-ion Battery. Sci. Rep. 2015, 5, 12571. 60. Yuvaraj, S.; Amaresh, S.; Lee, Y. S.; Selvan, R. K. Effect of Carbon Coating on the Electrochemical Properties of Co2SnO4 for Negative Electrodes in Li-Ion Batteries. RSC Adv. 2014, 4, 6407–6416.


38

Page 39 of 39

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


Table of Contents


39

Unusually Huge Charge Storage Capacity of Mn3O4âGraphene

Recommend Documents

Unusually Huge Charge Storage Capacity of Mn3O4âGraphene