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Rational Design of Graphene Reinforced MnO Nanowires with Enhanced Electrochemical Performance for Li-Ion Batteries Qi Sun, Zhijie Wang, Zijiao Zhang, Qian Yu, Yan Qu, Jingyu Zhang, Yan Yu, and Bin Xiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00122 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016

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ACS Applied Materials & Interfaces

Rational Design of Graphene Reinforced MnO Nanowireswith Enhanced Electrochemical Performance for Li-Ion Batteries Qi Sun,1Zhijie Wang,1Zijiao Zhang,2 Qian Yu,2* Yan Qu,3 Jingyu Zhang,4 Yan Yu,1 Bin Xiang1*

1

Department of Materials Science & Engineering, CAS Key Lab of Materials for Energy Conversion, Synergetic Innovation Center of Quantum Information Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China. 2 Center of Electron Microscopy and State Key Laboratory of Silicon Materials, Department of Materials Science & Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China. 3 The Sixth Element Materials Technology Co. Ltd, Changzhou, Jiangsu, 213145, China 4 Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720, USA. *Corresponding author: [email protected]; [email protected]

ACS Paragon Plus Environment


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ABSTRACT Recently, transition metal oxides (TMOs) mixed with carbon materials have attracted attention as lithium-ion battery (LIB) anode materials. However, the aggregation issue in TMOs hinders the development of an ideal encapsulation structure with carbon materials. In this paper, we report graphene reinforced MnO nanowires with enhanced electrochemical performance as an anode in LIB. The graphene nanosheets (GNs)/MnO feature was confirmed by transmission electron microscopy, X-ray diffraction, Raman scattering, and X-ray photoelectron spectroscopy. The GNs/MnO nanowires delivered a highly stable discharge capacity of ~815 mAh g-1 at a current density of 100 mA g-1 after 200 cycles, which is 1.5 times higher than that of pure MnO nanowires. This GNs/MnO structure with a specific capacity of ~995 mAh g-1 at a current density of 50 mA g-1 also exhibited excellent Li storage properties. The superior cycling and high rate capability were attributed to the intimate incorporation between the MnO and GNs. The structure of the GNs/MnO nanowires effectively accommodated the volume change of the MnO nanowires and prevented structure collapse during cycling.

Keywords: Lithium-ion battery; MnO; Graphene nanosheets; Anode；Nanowire morphology


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To meet the requirements for long cycle life, high capacity, and safety, the development of rechargeable lithium-ion batteries (LIBs) has stimulated a significant research effort on high-performance electrode materials for LIBs.1,2,3 For anode materials, transitional metal oxides (TMOs), such as Co3O4 ,4,5 Fe2O3,6 and Mn3O4 ,7 have received increasing attention because of their high theoretical capacity. With its very low electrochemical motivation force (1.032 Vvs. Li/Li+), low conversion potential, and low voltage hysteresis,8,9,10 MnO has become one of the most popular anode materials for LIBs. However, the low electrical conductivity of MnO leads to a poor rate capability. Specifically, the volume-change-induced structure collapse that occurs during the Li-ion insertion/extractionprocesses, resulting in a capacity decline.1,8-11 MnO is typically coated by a buffer layer of carbon materials to enhance its mechanical strength and electrical conductivity.9,10,12 The carbon materials are cycling-compatible materials with novel electrical conductivity, mechanical flexibility, and chemical stability.10,13 The graphene-wrapped structure and MnO anchored in graphene have been widely reported.11,12,14 However, the aggregation issue between the MnO nanostructures results in a non-uniform carbon material coating on the MnO nanostructures. Therefore, achieving adequate stability and high rate capability of MnO in LIB applications remains challenging. In this paper, we report graphene reinforcedMnO nanowiressynthesized from a Mn(oAc)2·4H2O precursor mixed with graphene oxide (GO) in polyvinylpyrrolidone (PVP). The nanowire configuration was investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD),



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Raman scattering and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the graphene nanosheets (GNs)/MnO structure as an anode material in LIBs was examined. Chemicals and materials：All the reagents used in the experiments were analytical grade and were used without further purification. PVP (Mw =1 300 000) was purchased from Alfa Aesar (China) Chemical Co. Ltd. Manganese acetate and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. GO was obtained from the Sixth Element (Changzhou) Materials Technology Co. Ltd. Synthesis of the GNs/MnO: First, the GO (5 mg/ml) was diluted to 1 mg/ml by adding ethanol and sonicated for 2 h. Then, the solution was centrifuged at 5000 rpm for 2 min to remove any unstripped GO flakes. In a typical synthesis, PVP (1 g) was dissolved in of ethanol solution of GO (10 mL, 1 mg/mL with stirring for 12 h. After heating to 55 °C, the GO solution was added to Mn(oAc)2·4H2O (0.5g). The heating and stirring were maintained for 15 min; the Mn(oAc)2•4H2O was rapidly dissolved, and a homogeneously distributed solution was obtained. The solution was injected into a syringe, which was fixed in a syringe pump. The size of the needle for the electrospinning experiment was 19G, and the needle was connected to a high-voltage power supply. The flow rate of the syringe pump was fixed at 0.5 ml/h, and the distance between the needle and sample collector was 16 cm with an applied voltage of 16 kV. The as-electrospun Mn(oAc)2/PVP/GO nanowires were dried at 80 °C for 12 h in air. Then, the nanowires were annealed at 700 °C in Ar/H2(95:5 by volume) for 1 h to remove the polymer matrix and to reduce the GO to GNs. Finally, the


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GNs/MnO nanowires were obtained. The pure MnO nanowires were obtained by using the same process without GO in the precursor. Characterization:The morphologies of the as-prepared GNs/MnO and pure MnO nanowires were investigated using field-emission scanning electron microscopy (EFSEM, JSM-6700F, JEOL). X-Ray powder diffraction (XRD,MXPAHF, Mac Science Co. Ltd., Japan) was used to characterize the structure of the samples using Cu Kα radiation. These nanowires were also examinedusing X-ray photoelectron spectroscopy (XPS; ESCALAB250Xi) with Al Kα radiation (1486.6 eV). Electrochemical characterization:A coin-type (CR2032) cell with pure lithium slice (99.9%) as the counter electrode and reference electrode was used to assemble the samples in an argon-filled glove box. The ratio of active material, carbon black, and polyvinylidene ﬂuoride (PVDF) is 8:1:1.The separator was a polypropylene membrane (Celgard 2400), and the electrolyte was 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC) (1:1 by volume). Cyclic voltammetry (CV) was performed on an electrochemical workstation (CHI600E). The discharge and charge measurements of the batteries were performed on a Land system (CT2001A) in the voltage window between 0.005 and 3 V at room temperature. All the current densities were determined based on themass of the active materials. The typical synthesis process for the nanowires is illustrated in the schematics in Figure 1a. The Mn(oAc)2/PVP nanowires (Figure S1, Supporting Information) were prepared using Mn(oAc)2 as a precursor and the electrospinning method (details in Methods). The MnO nanowires were formed through thermal decomposition that



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occurred in the Mn(oAc)2/PVP nanowires after the annealing process. Because of the gas release and mass loss in the calcination, the MnO nanowire surface became rough (Figure 1b) compared with that of the Mn(oAc)2/PVP nanowires.16 To obtain GNs/MnO composite nanowires, we proposed the addition of GO solution into the Mn(oAc)2 precursor solution instead of working with the after-growth MnO samples.17 After thermal treatments of the Mn(oAc)2/PVP/GO nanowires (details in Methods), GNs/MnO nanowires with a smooth surface were obtained, as demonstrated in Figure 1c. The transmission electron microscopy (TEM) images in Figures 1d and 1e reveal that the GNs were not only uniformly wrapped on the surface of the MnO nanowires but also embedded in the MnO nanowires. Therefore, the compactness between the GNs and MnO nanowires was extensively enhanced.


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Figure 1. (a) Schematic illustration for the synthesis of GNs/MnO nanowires. The different color represents chemical contents in the nanowire: Mn(oAc)2/PVP/GO (green) and GNs/MnO (blue). (b) FESEM images of the pure MnO nanowires and (c) GNs/MnO nanowires after thermal treatment at 700℃in Ar/H2 for 1h. Insert images are the high magnification of the nanowire morphologies. (d) Low magnification TEM image of the GNs/MnO nanowire. (e) HRTEM image of the GNs/MnO nanowire.



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We further performed XRD, Raman scattering, and XPS characterization to confirm the structure and chemical composition of the GNs/MnO nanowires. The XRD pattern revealed a cubic crystal phase in the as-grown GNs/MnO with the representative peaks as indexed in Figure 2a. The observed G band (~1598 cm-1) and D band (~1354 cm-1) peaks in the Raman spectrum (Figure 2b) reveal the presence of reduced GO in the as-grown GNs/MnO. The G band is a characteristic feature of the E2g mode of graphite carbon species (sp2), whereas the D band reflects thequantity of defects. In general, the band peak intensity ratio of ID/IG is utilized to qualitatively evaluate the density of defects in the GO.13 A larger defect density in the GO represents more reduced GO obtained during a reduction process.18,19 From the measured Raman spectrum (Figure 2b), we calculated the ratio of ID/IG of the as-grown GNs/MnO to be 1.2865, which was larger than that of GO (0.873). Thus, the defect density in the as-grown GNs/MnO was larger than that of GO, and we verified that the reduced GO was successfully achieved in our as-grown GNs/MnO during the thermal processes. To probe the chemical composition and binding energy, we performed XPS measurements on the as-grown GNs/MnO. The binding energies of Mn (2s, 2p, 3s, and 3p), C (1s), and O (1s) are shown in Figure 2c. The high-resolution analysis (Figure 2d) elucidated the Mn (II) oxidationstate by the prominent binding energy of Mn (II) 2p 3/2 and 2p 1/2 with the peak locations of 641.2 and 652.8 eV, respectively. The binding energy of C (1s) at a peak of 284.4 eV (Figure 2e) corresponds to graphitic carbon in graphene. The other oxygenated carbons (C–O, C=O, and —COOH) were also identified from the fitting curves, as


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shown in Figure 2e. Compared with the spectrum before the thermal process (Figure S2, Supporting Information), the peak-covered-area percentage of the graphitic carbon increased from 30.6% to 71.7%, and the ratio of oxygenated carbons was extensively decreased (Table S1).This results indicate that the GO was well reduced to GNs during the thermal processes.18,20,21 However, the binding energy peaks of C– O and H–O (Figure 2f) reveal a small amount of remnant O2- species still bondedwith C atoms in the GNs. To explore the content of the graphene in the GNs/MnO, the thermogravimetric analysis (TG) was carried out (Figure S3, Supporting Information). below the 150℃, water absorbed onthe surface of the GNs/MnO was evaporated and thus about 0.5% weight loss could be observed. During 350℃ to 450℃, ~ 6.1 % weight loss was observed corresponding to the oxidation of GNs. Because the oxidation temperature of MnO is higher than 500℃, a slight weight increase observed in TG curve was attributed to the oxidation of the MnO into Mn2O3. In addition, we have also conduct the elemental analysis toverify the TG results. The GNs/MnO with a weight of 2.914 mg was burned in a reaction tube at 950℃, then was held at 550℃. The graphene in the GNs/MnO was reacted and the content of the graphene was measured to be ~ 6.61%, which is consistent with the obtained TG data.



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Figure 2. (a) XRD pattern showing a cubic phase of GNs/MnO nanowires. (b) Raman spectrum for GNs/MnO nanowires. XPS spectra of GNs/MnO nanowires: (c)Suvey spectrum and high-resolution spectra of (d) MnO_2p, (e) C_1s, (f) O_1s.


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To evaluate the electrochemical performance of the GNs/MnO and MnO nanowires, we characterized their lithium-storage properties by using the nanowires as anode materials for LIBs. The charge/discharge curves of the GNs/MnO and MnO nanowires are presented in Figures 3a and 3b, respectively. After the first cycle, the capacity decreased in both samples because of the formation of the solid electrolyte interphase (SEI) film.22The voltage of the GNs/MnO nanowires at the discharge platform increased from 0.3 to 0.5 V after 2 cycles and then dropped to 0.4 V, which is smaller than that in the MnO nanowires (0.5 V). We attributed this behavior to the GNs, which greatly prevent the pulverization of the GNs/MnO nanowires.23A reduced polarization effect was thus observed compared with that in the MnO nanowires. The charge/discharge curves of the 100th cycle mostly coincided with the 150th cycle in the GNs/MnO nanowires (Figure 1a), indicating a highly reversible lithium storage process with a nearly 100% coulombic efficiency. The cycle performance of the GNs/MnO and pure MnO nanowires (Figure 3c) indicated that the initialreversible capacity of the GNs/MnO nanowires was 1347.5 mAh g-1 at a current density of 100 mA g-1, whereas that of the pure MnO nanowires was 999.7 mAh g-1. A capacity drop was observed in the first tens of cycles, and then, the capacity started to increase. This behavior could be due to a process of electrochemical activation and the gel-like polymeric layer formed reversibly in the electrode.2,24,25,26 After ~120 cycles, the capacity of the GNs/MnO nanowires reached a stable value of 815.3 mAh g-1, whereas that of the pure MnO remained at 489.4 mAh g1. To probe the rate capability, the specific capacities of the GNs/MnO and pure MnO nanowires were examined at



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various current densities (Figure 3d). The GNs/MnO nanowires delivered average capacities of 994.6, 832.7, 667.93, 541.0, 439.6, and 285.1 mAh g-1 at current densities of 50, 100, 250, 500, 1000, and 2500 mA g-1, respectively. After deep charge/discharge for 10 cycles at 2500 mA g-1, an average capacity of 1000.19 mA h g-1 was recovered by scanning again at 50 mA g-1. The GNs with high electrical conductivity facilitated the carrier transport in the GNs/MnO nanowires, leading to this high rate capability. The pure MnO exhibited a lower capacity of 539 mA h g-1 at 100

mA

g-1

and

161

mA

h

g-1

at

2500

mA

g-1.

Figure 3. Lithium storage performance of GNs/MnO nanowires. Galvanostatic discharge and charge curves of (a) GNs/MnO nanowires and (b) pure MnO nanowires at a current density of 100 mA g-1 cycled in the voltage range of 3–0.005V vs. Li/Li+ . (c) Capacity - cycle number curves of GNs/MnO and pure MnO nanowires at a


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current density of 100 mA g-1. (d) Rate performances of the GNs/MnO and pure MnO nanowires. The morphologies of the GNs/MnO and pure MnO nanowires were examined after lithium ion insertion and de-insertion at the rate of 100 mA g-1. The GNs/MnO maintained their nanowire structure, whereas the pure MnO nanowires collapsed into nanoparticles (Figure S4, Supporting Information). This finding suggested that the mechanical properties of the GNs/MnO nanowires were improved by the GNs, which were cycling-compatible materials with promising mechanical properties.The GNs cushion the volume changes from the surrounding MnO during cycling.Thus, the mechanical strength of the GNs/MnO nanowires was enhanced compared with that of the pure MnO nanowires. To further understand the enhanced performance in the GNs/MnO electrode, the electrochemical impedance spectroscopy(EIS) was conducted in the GNs/MnO and MnO electrodes (Figure S5, Supporting Information). Compared to the pure MnO electrode after 10 cycles, the charge-transfer resistance of the GNs/MnO electrode after 10 cycles exhibited a lower charge-transfer resistance. As we know, MnO is a wide band gap semiconductor and not well-conductive. The EIS measurements indicated that the graphene improved the conductivity for the cycling processes.As a result, the GNs/MnO nanowires exhibited better rate capabilitywith the enhanced conversion reaction kinetics compared with that of the pure MnO nanowires. To study the chemical reaction and physics change during the discharge/charge cycling, representative CV curves of the GNs/MnOwere obtained for the initial three



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cycles (scan rate: 0.1 mV s-1; voltage range: 0.005–3 V vs. Li), and the results are presented in the Figure 4a. In the cathodic scan, two peaks at 0.12 and 0.67 V were observed, which are attributed to the initial reduction of Mn2+ to Mn0 and the formation of a SEI, respectively. During the anodic scan, only one peak at ~1.23 V was observed, which resulted from the formation of MnO and decomposition of lithium oxide. Then, the voltages of the corresponding peaks were shifted to 0.36 and 1.27 V in the following cathode/anode scans, which could be due to microstructural changes of the active material after the first lithiation.11 In addition, excellent reversibility behavior of the GNs/MnO nanowires was also observed from the overlap curves of the 2nd to3rd cycle. Using various scanning rates ranging from 0.1 to 1 mV s-1, a slight separation between the anodic and cathodic peak was observed, as shown in Figure 4b, which was attributed to the polarization effect resulting from the change of crystallographic phase.27-28 Figure 4c presents a plot of log(peak current) vs. log(scan rate) from 0.1 to 1 mVs-1 for the anodic peak.These data can be fitted by the relationship between the measuredcurrent (i) and scan rate (v):27,28,29 i=avb The parameter b determined by the slope of the log (v)-log (i)plot is utilized to describe the carrier transport type. Generally, a b-value of 0.5 represents a total diffusion-controlledbehavior, whereas a value of 1.0 indicates a capacitive process.27,28,29The calculated slope of 0.507 obtained from the plot (Figure 4c) indicates that total diffusion-controlled carrier transport behavior occurred in the GNs/MnO nanowires during the delithiation process.


(1)

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Figure 4. Kinetics analysis of the electrochemical behavior towards Li+ for the GNs/MnO nanowire electrode. (a) CV curves of the GNs/MnO nanowires at a scan rate of 0.1 mV s-1 in the voltage range of 3–0.005 V vs. Li/Li+. (b) CV curves at



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various scan rates, from 0.1 to 1 mV s-1. (c) Determination of the b-value using the relationship between the peak current and scan rate. We developed a structure of graphene reinforced MnO nanowires by adding GO solution into the precursor solution for nanowire synthesis. The GNs improved the nanowire structure electrical conductivity and prevented MnO nanowires from structure collapse during cycling. Therefore, the GNs/MnO nanowires exhibited excellent Li storage properties witha specific capacity of ~995 mAh g-1 at a current density of 50 mA g-1. This synthesis method opens up a new pathto improve the electrochemical performance of other anodematerials such as Co3O4, CuO, and SnO2.


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ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21373196, 11434009), the National Program for Thousand Young Talents of China and the Fundamental Research Funds for the Central Universities (WK2340000050, WK2060140014). Q Yu was supported by the 973 Program of China no. 2015CB65930.

ASSOCIATED CONTENT Supporting Information SEM images of as-electrospun nanowires of (a) Mn(oAc)2/PVP and (b) Mn(oAc)2/PVP/GO;XPS high-resolution spectrum of C (1s) in Mn(oAc)2/PVP/GO nanowires; TGA curve of GNs/MnO nanowires at a heating rate of 10℃min-1 under air ﬂux;SEM images of the anode electrodes after lithium ion insertion and de-insertion processes: (a) GNs/MnO and (b) pure MnO;Electrochemical impedance spectroscopy of the GNs/MnO and pure MnO after the cycling processes; A table of the

position

and

percentage

of

C=C,

C=O,

COOH,

Mn(oAc)2/PVP/GO and GNs/MnO nanowires.


C-N

bonding

in

the


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