Synthesis of Porous NiO-Wrapped Graphene Nanosheets and Their

Oct 25, 2013 - Meili Qi , Dong Xie , Yu Zhong , Minghua Chen , Xinhui Xia ..... Yeonsun Sohn , Byoung-Gyu Kim , Kee Suk Nahm , Kang-Sup Chung , Pil Ki...
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Synthesis of Porous NiO-Wrapped Graphene Nanosheets and Their Improved Lithium Storage Properties Dong Xie, Qingmei Su, Weiwei Yuan, Zimin Dong, Jun Zhang, and Gaohui Du* Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China S Supporting Information *

ABSTRACT: This article reports a facile preparation of NiO−graphene composite by the combination of a solutionbased method and subsequent annealing. X-ray diffraction and electron microscopy reveals that the graphene nanosheets are uniformly wrapped by porous NiO nanosheets in the product. The composite shows highly improved electrochemical performance as anode for Li−ion batteries (LIBs). The NiO−graphene nanosheets deliver a first discharge capacity of 2169.6 mAh g−1 and remain a reversible capacity up to 704.8 mAh g−1 after 50 cycles at a current of 200 mA g−1 in half cells. Contrarily, the pristine NiO nanosheets show only a reversible capacity of 134 mA g−1 after 50 cycles. The NiO− graphene composite also exhibits ameliorative rate capacity of 402.6 mAh g−1 at the current of 1600 mA g−1. In particular, these novel nanostructured composites show exceptional capacity retention in the assembled NiO−graphene/LiNi1/3Mn1/3Co1/3O2 full cell at different current density. The enhanced electrochemical performances are ascribed to the stable sheet-on-sheet architectures and the synergistic effects between the conductive graphene and thin porous NiO nanosheets.

1. INTRODUCTION The ever-growing demand for large-scale energy storage applications has triggered significant research efforts on high energy-density lithium ion batteries (LIBs).1−3 As a commercial anode material, graphite is usually utilized in LIBs with a theoretic capacity of 374 mAh g−1. To further improve the capacity and energy density of the battery system, various electrode materials have be developed for next-generation LIBs. In particular, in situ transmission electron microscopy (TEM) has been employed to understand the dynamic lithiation− delithiation mechanism of electrode materials.4,5 Recently, transition metal oxides have been widely investigated as anode materials for LIBs because they have higher specific capacity and volumetric energy density than graphite.6−11 However, these anode materials always suffer from rapid capacity fading because of poor conductivity and large volume expansion occurring in the cycling process. One approach to avoid these limitations of transition metal oxides is to hybridize with carbonaceous materials for improved conductivity and accommodation of the strain during volume change.12−16 Graphene, as one of the special structures of carbon consisting of monolayers of hybridized carbon atoms arranged in a honeycombed network with six-membered rings, has been widely used as a conducting additive for nanostructured composite materials because of its excellent electronic conductivity, high theoretical surface area of 2630 m2/g, and good mechanical properties.3,17−20 Recently, graphene has been used to prepare composite materials with transition metal oxides to improve the energy storage performance.12,21−26 © 2013 American Chemical Society

Among transition metal oxides, NiO is one of the most promising anode materials for LIBs with high safety, environmental benignity, low cost, and outstanding theoretical capacity of 718 mAh g−1. NiO−graphene composites with various structures have been prepared via different methods, such as chemical precipitation,27 self-assembly,28,29 and hydrothermal method.30 Herein, we report a new method for the fabrication of NiO−graphene composite in which the graphene sheets are uniformly wrapped by porous NiO nanosheets. The novel porous NiO-wrapped graphene nanosheets are found to be a promising anode material for LIBs.

2. EXPERIMENTAL DETAILS 2.1. Synthesis of Graphite Oxide. Graphite oxide (GO) was synthesized from natural graphite powers by a modified hummers method.31 In a typical procedure, 1.0 g of graphite powder was added to the mixture of concentrated H2SO4/ HNO3 (92:23 mL); 3.0 g of KMnO4 was added gradually to the above mixture under constant stirring, and the mixture was kept in an ice bath for 2 h. After removing the ice bath, the mixture was stirred at 35 °C for 2 h. Subsequently, 46 mL of deionized water was added to the mixture and kept at 98 °C for 1 h. When cooled to the room temperature naturally, 10 mL of 30 wt % H2O2 was added, and the color of the mixture changed to brilliant yellow, followed by washing with 15 mL of HCl (10 wt Received: June 3, 2013 Revised: October 2, 2013 Published: October 25, 2013 24121

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process. The reaction process is illustrated and shown in Figure 1. First, GO was sonicated in water to form a suspension of GO

%) aqueous solution. GO was obtained after centrifuging (10 000 rpm), washing with deionized water, and freeze-drying. 2.2. Preparation of NiO−Graphene Composite. GO (0.09 g) was ultrasonically dispersed in 150 mL of deionized water and then a mixture of 0.003 mol of Ni(NO3)2·6H2O and 0.03 mol of hexamethylenetetramine (HMT) was added. The aqueous solution was heated at 98 °C under reflux condition with continuous magnetic stirring for 3 h. After reaction, the solution was cooled naturally to room temperature. NiOH− graphite oxide composite (the precursor) was formed and collected after centrifuging, washing with deionized water and ethanol several times, and dried overnight at 60 °C. The NiO− graphene was obtained by calcining the precursor in N2 at 300 °C for 2 h and then in air at 350 °C for 2 h. For comparison, the pristine NiO nanosheets were prepared by a similar solution-based method and subsequent calcination with the absence of GO. 2.3. Materials Characterization. The products were characterized using X-ray powder diffraction (XRD) on a Philips PW3040/60 X-ray diffractometer with Cu Kα (λ = 1.5418 Å) radiation for phase identification. The morphologies were examined using scanning electron microscopy (SEM) on a Hitachi S4800 microscope, and the microstructures were investigated using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) performed on a JEOL 2100F instrument. Raman spectroscopy was recorded on Renishaw RM1000 spectrometer with a 514.5 nm incident laser light. Thermal gravimetric analysis (TGA) was performed using Netzsch STA 449C thermal analyzer with a heating rate of 5 °C min−1 in flowing air to determine the amount of graphene in sample. 2.4. Electrochemical Measurement. 2032 Type half cells were assembled with the prepared NiO-wrapped graphene composite as the working electrode materials. The working electrodes were prepared by dispersing the as-prepared products (75 wt %), acetylene carbon black (15 wt %), and poly(vinylidene fluoride) binder (10 wt %) in N-methyl-2pyrrolidone solvent to form a slurry. The slurry was pasted onto a Ni foam current collector and was dried at 80 °C for 6 h in a vacuum oven. Thin Li foil was employed as the counter electrode, and a polypropylene membrane (Celgard 2400) was used the separator. The electrolyte solution was 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1. 2032 Coin-type full cells with commercially available LiNi1/3Mn1/3Co1/3O2 as the cathode and NiO-wrapped graphene as the anode were also assembled in the high-purity argon-filled glovebox. The cathode was prepared using the similar method as the preparation of NiO-wrapped graphene slurry. The resultant LiNi1/3Mn1/3Co1/3O2 slurry was uniformly pasted on pure alumina foil and was dried at 80 °C for 6 h in a vacuum oven. Cyclic voltammetry (CV) was carried out on a CHI 660C electrochemistry workstation from 0.01 to 3.0 V at a scan rate of 0.1 mV s−1. The galvanostatic charge-discharge measurements were conducted on LAND battery test system at different current densities between cut-off potentials of 0.01−3 V for half cells and 1.2−3.2 V for full cells. The discharge and charge capacities of NiO-wrapped graphene were calculated based on the total mass of NiO and graphene in the composite.

Figure 1. Schematic illustration of formation process of NiO-wrapped graphene nanosheets (a) and pristine porous NiO nanosheets (b).

nanosheets. Ni2+ ions was then absorbed on the GO nanosheets and reacted with OH− at the reflux condition. This step yielded uniform Ni(OH)2 nanosheets grown on the GO nanosheets. HMT has been extensively used in the fabrication of metal oxides nanosheet structure,32−34 and its role is to release hydroxyl ions slowly and uniformly in the suspension, resulting in the growth of Ni(OH)2 as suggested by the following reactions: (CH 2)6 N4 + 6H 2O → 6HCHO + 4NH3 NH3 + H 2O → NH4 + + OH−

Ni 2 + + 2OH− → Ni(OH)2

The resultant Ni(OH)2 nanosheets anchored tightly on the surface of the GO nanosheets through oxygen-containing functional groups, such as hydroxyl, epoxy, and carboxyl.16,35,36 During the calcination, GO converted to graphene by losing oxygen-containing surface groups, and Ni(OH)2 decomposed to yield porous NiO nanosheets following Ni(OH)2 → NiO + H2O. So the Ni(OH)2−GO precursor converted to NiO− graphene composite finally after annealing at 350 °C. The pristine porous NiO nanosheets would be obtained with the similar reaction process when GO was not added. To explore the optimum experimental condition, control experiments were made by varying the molar ratio of Ni(NO3)2 to HTM and the reflux time (see Figure 1S and 2S, Supporting Information). We found the porous NiO-wrapped graphene composite could be synthesized easily when the molar ratio of Ni(NO3)2 to HTM was around 1:10 and the reflux time was about 3 h, otherwise NiO nanoparticles would be formed on graphene sheets. The XRD patterns of pristine NiO and NiO−graphene composite obtained at the optimized condition are shown in Figure 2a. All of the diffraction peaks of pure NiO sample can be ascribed to cubic NiO (JCPDS No. 071-1179). The characteristic (111), (200), (220), (311), and (222) peaks of cubic NiO are also observed for NiO−graphene composites. The (002) diffraction peak of graphene nanosheets is typically located at about 24° in the XRD pattern, but it is not obvious in Figure 1a because the content of graphene is low and the diffraction of disorderedly stacked graphene nanosheets is quite weaker as compared to the well-crystalline NiO. To confirm the presence of graphene, Raman spectra were recorded. The

3. RESULTS AND DISCUSSION NiO−graphene nanosheets were synthesized by a simple solution route at reflux conditions followed by a heat treatment 24122

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Figure 2. (a) XRD patterns of pristine NiO (red) and NiO−graphene composite (black). (b) Raman spectra of NiO−graphene composite, pristine NiO, and graphene. (c) TGA curve of NiO−graphene composite.

Figure 3. SEM images of graphene (a), pristine NiO nanosheet (b), and NiO−graphene composite (c). TEM images of pristine NiO nanosheet (inset of b) and NiO−graphene nanosheet (d,e). HRTEM image (f) and corresponding FFT pattern (inset) of NiO nanosheet on graphene.

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individual nanosheets. The result suggests that NiO nanosheets exist and are distributed uniformly on the graphene nanosheets. This novel NiO−graphene sheet-on-sheet architecture should be favorable for preventing aggregation or restacking of graphene nanosheets or porous NiO nanosheets during charge−discharge cycles.12,39 Moreover, as an ideal electron conductor, graphene can play an important role as a conductive network within the electrode, which benefits the electrochemical performance.30,38 Lithium storage properties of the as-prepared NiO−graphene composite were first investigated by CV and galvanostatic discharge/charge cycling in half-cell configurations. For comparison, electrodes made of pristine porous NiO nanosheets were also tested under the same electrochemical conditions. The CV curves of NiO−graphene composite and pristine NiO are shown in Figure 5a,b. In the first cycle, there is a characteristic cathodic peak around 0.33 V for pristine NiO, which is shifted to 0.40 V for the NiO−graphene composite. It is a typical indication of the initial reduction of NiO to Ni, the electrochemical formation of Li2O, and a partially irreversible solid electrolyte interphase (SEI) layer.30 The main reduction peak shifts to 0.90 V and becomes smaller in the second cycle for NiO−graphene composites, as well as for the pristine NiO nanosheets, which reveals an irreversible reaction present in the first electrochemical cycle. Two anodic peaks are observed around 1.50 and 2.25 V during the charge cycle, which correspond to the decomposition of electrolyte and Li2O accompanying with the oxidation of metallic Ni.40,41 The first and second charge−discharge curves of pristine NiO and NiO− graphene nanosheets at a current density of 200 mA g−1 are presented in Figure 5c,d. A long voltage plateau is located at about 0.65 V for pristine NiO nanosheets, which is due to the reduction of NiO to Ni and the formation of SEI film.42 This plateau becomes short for NiO−graphene composite. The initial discharge and charge capacities of pristine NiO nanosheets are 2271.3 and 1373.6 mAh g−1, respectively. The NiO−graphene nanosheets deliver specific capacities of 2169.6 and 1467.2 mAh g−1 for the first discharge and charge. The initial capacities are much higher than the theoretical value (718 mAh g−1), which is mainly attributed to the SEI film formation42,43 and additional storage of Li+ in the defects or pores of nanostructured materials.30,44 Indeed, we can see the presence of crystal defects in NiO nanosheets from the HRTEM image in Figure 3f. Moreover, the porous NiO nanosheets are very thin so that more crystal facets are exposed to provide additional sites (e.g., more surface defects) for Listorage. Although the initial discharge capacity (2271.3 mAh g−1) of pristine NiO is slightly higher than that of NiO− graphene (2169.6 mAh g−1), its Coulombic efficiency (60.5%) is lower than NiO−graphene (67.6%). The fact that the capacity of pristine NiO is higher than that of NiO−graphene for the first cycle is caused by the lower capacity of graphene as compared to NiO.38 As shown in Figure 5e, the reversible capacity of pristine NiO decreases to 134 mAh g−1 after 50 cycles, which is almost five times lower than that of NiO− graphene composite electrode (704.8 mAh g−1). The lithiumstorage performance of NiO-wrapped graphene has been compared with the NiO−graphene composites prepared with different methods in the literature (see Table 1S in Supporting Information). Although the performance of porous NiOwrapped graphene is not as good as ultrathin porous NiO nanosheets/graphene hierarchical structure21 and 3D-hierarchical NiO−graphene nanosheets,30 it is improved considerably

Raman spectrum of NiO−graphene composite in Figure 2b shows the characteristic Raman peaks of NiO as well as the D and G characteristic peaks of graphene. The content of graphene in the composite was determined by TGA measurement, as show in Figure 2c. The as-prepared composite consists of 26.9 wt % graphene and 73.1 wt % NiO. The morphology and structure of graphene, pristine NiO, and NiO−graphene composite are compared by SEM and TEM. The graphene shows a typical nanosheet structure with rippled and crumpled feature as shown in Figure 3a. Pure NiO is composed of ultrathin nanosheets (less than 10 nm) with porous structure (Figure 3b). The inset of Figure 3b is a TEM image of a NiO nanosheet showing some favorable hexagonal pores with typical diameters of 20−40 nm. The formation of the pores should be credited to the decomposition of Ni(OH)2 precursors and the high reaction/etching rate at the defect sites in the precursor nanosheets during calcination.37,38 In comparison to graphene and pristine NiO, NiO−graphene composite has different morphology. As shown in Figure 3c, many nanosheets with a lateral dimension in micrometer size are present in the products. Figure 3d shows a TEM image of the NiO−graphene composite, clearly showing flexible and uniform sheet-like morphology. A high magnification TEM image is shown in Figure 3e; it is clearly revealed that the graphene sheet is enclosed with a layer of porous NiO, leading to a NiO−graphene sheet-on-sheet architecture. Numerous nanosized pores with diameters less than 5 nm are clearly seen in the NiO nanosheet, which were also resulted from the decomposition of nickel hydroxide precursor on GO. Thin NiO porous nanosheets are homogeneously anchored on the surface of graphene without aggregation. A HRTEM image of a single nanosheet and its corresponding FFT pattern are shown in Figure 3f. The lattice spacing of 0.24 and 0.21 nm is observed, corresponding to the interspaces of the (111) and (200) planes of the cubic NiO. The FFT pattern also confirms the nanosheet is well-crystalline and that it can be indexed as cubic NiO along the zone of [011̅]. In order to further confirm the NiO−graphene sheet-onsheet structure, the compositional and elemental distributions in the NiO−graphene composite were analyzed. Figure 4 displays the EDS elemental mapping of nickle (Figure 4b), carbon (Figure 4c), and oxygen (Figure 4d), which demonstrate the uniform distribution of these elements in

Figure 4. TEM image (a) and EDS elemental mapping of a NiO− graphene nanosheet: (b) nickel, (c) carbon, and (d) oxygen mapping. 24124

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Figure 5. Cyclic voltammograms of (a) pristine NiO and (b) NiO−graphene nanosheets at a scan rate of 0.1 mV s−1. Voltage profiles showing the first and second charge−discharge cycles of pristine NiO (c) and NiO−graphene nanosheets (d) at a current density of 200 mA g−1. (e) Capacity retention and Coulombic efficiency at a current density of 200 mA g−1 and (f) rate performance at various current densities between 200 and 1600 mA g−1.

Figure 6. (a) TEM image and (b) ED pattern of NiO−graphene composite electrode after 50 cycles at a current density of 200 mA g−1.

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Figure 7. (a) Voltage profiles showing the first (black), second (green), and third (red) charge−discharge cycle of LiNi1/3Mn1/3Co1/3O2/NiO− graphene full cell at a current density of 100 mA g−1 between 1.2 and 3.2 V. (b) Capacity retention at a current density of 100 mA g−1 and (c) rate performance at current densities between 100 and 800 mA g−1.

in Figure 6, the ultrafine porous NiO nanosheets anchored on graphene nanosheets almost retain its original morphology. It demonstrates the long-term stability of the as-prepared NiO− graphene composite as anode in LIBs. There are numerous nanosized pores in the NiO nanosheets, which can provide space for volume changes upon cycling. In particular, the aggregation of the ultrafine NiO is avoided under the protection of graphene. Vise versa, the NiO porous nanosheets can also be helpful for preventing aggregating or restacking of graphene nanosheets. The corresponding electron diffraction (ED) pattern in Figure 6b reveals that the electrode material is still NiO, indicating a highly reversible conversion reaction for the prepared material. For transition metal oxides as anode materials, large volume change and stresses accompanying the phase transition upon lithiation−delithiation usually cause pulverization of the electrode, which leads to weak contact of the materials and fast decay in capacity upon cycling.5 In this article, the as-prepared porous NiO-wrapped graphene nanosheets can avoid the widespread problem of cracking or pulverization of transition metal oxides as anode materials and exhibit long-term stability and integrity upon cycling. To prove the feasibility of using porous NiO-wrapped graphene nanosheets in practical energy storage application, the full cells were assembled by using commercially available LiNi1/3Mn1/3Co1/3O2 as the cathode and porous NiO-wrapped graphene as the anode. To the best of our knowledge, there is still no published study using NiO−graphene composite as anodes in full cells. The voltage profile and cycle properties of the NiO−graphene/LiNi1/3Mn1/3Co1/3O2 full cells with a cutoff voltage range of 1.2−3.2 V are shown in Figure 7, in which the specific capacities are calculated according to the mass of NiO−graphene anode. As shown in Figure 7a,b, the full cell exhibits a charge/discharge capacity of about 1311.4/617.2 mAh g−1 in the first cycle at a current density of 100 mA g−1.

compared with mesoporous NiO nanoplate/graphene composite45 and NiO nanoparticle/graphene hybrid.46 The Coulombic efficiencies of NiO−graphene composite are stable around 96−98%, while the pure NiO electrode shows obviously lower Coulombic efficiencies in the 5th−20th cycles. The inferior cycling performance of pristine NiO should be caused by large volume change, poor electric conductivity, and aggregation of NiO nanosheets during the charge−discharge cycles. In contrast, the NiO−graphene sheet-on-sheet architectures show improved cycling performance because the excellent flexibility and conductivity of graphene can not only buffer the volume change but also prevent the aggregation of the active materials upon cycling. Figure 5f demonstrates the rate capability of pristine NiO and NiO−graphene nanosheet electrodes from current densities of 200 to 1600 mA g−1 for ten cycles at each current density. In the initial 10 cycles with a small current density (200 mA g−1), the capacities of pristine NiO and NiO−graphene composite are comparable, and the pristine NiO is even a little advantageous. However, when the current density increases, the situation reverses largely. For example, the NiO−graphene electrode is still able to deliver a discharge capacity of 403.3 mAh g−1 at a current density of 1600 mA g−1, which is much higher than the pristine NiO electrode (22.1 mAh g−1). The improved rate performance of the as-prepared NiO−graphene composite could be reasonably attributed to advantageous combination of the highly conductive graphene and porous NiO nanosheets, which can provide sufficient electrode/ electrolyte contact areas and facilitate continuous and fast conducting pathways for electrons through the electrodes during the lithiation−delithiation process. To reveal the stability of porous NiO-wrapped graphene electrode, the morphology and structure of the composite after 50 charge−discharge cycles was examined by TEM. As shown 24126

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This large irreversible capacity loss in the first cycle is probably due to the high surface area of NiO-wrapped graphene nanostructures.47 The reversible capacity tends to become stable in the subsequent cycles. The full cell delivers a discharge capacity of 592.4 mAh g−1 in the second cycle with a Coulombic efficiency of 82.3%. The reversible capacity stabilizes at 430 mAh g−1 in the 25th cycle with a Coulombic efficiency of 93%. To investigate the rate capability, the full cell was cycled at different current densities as shown in Figure 7c. Each step comprises three charge−discharge cycles at different current from 100 to 800 mA g−1. The discharge capacity of the full cell maintains about 456 mAh g−1 at a high current density of 800 mA g−1 with a Coulombic efficiency of 97.7% and recovers to 570 mAh g−1 when the current is reset to 100 mAh g−1. The good capacity retention and rate performance are vital to realize their potential application in LIBs.

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4. CONCLUSIONS In summary, we have fabricated NiO−graphene composite by solution-based method and subsequent annealing. The graphene nanosheets are found to be uniformly wrapped by porous NiO nanosheets, forming a sheet-on-sheet composite. When evaluated as anode for LIBs, the as-prepared NiO− graphene nanosheets show improved cycling performance with a discharge capacity of 704.8 mAh g−1 after 50 cycles at a current of 200 mA g−1 and deliver ameliorative rate capacity with 402.6 mAh g−1 at a current of 1600 mA g−1. Furthermore, the coin-type full cell consisting of LiNi1/3Mn1/3Co1/3O2 cathode and NiO−graphene anode demonstrates good cycle performance and rate capability. These improved and desirable electrochemical properties of NiO−graphene composite are primarily attributed to the advantageous combination of conducting graphene and porous NiO nanosheets and the stable NiO−graphene sheet-on-sheet architecture.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of the NiO−graphene composite prepared with different molar ratio of Ni(NO3)2 to HTM and different reflux time. Table of the preparation and lithium-storage performance of NiO−graphene composites in the literature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(G.D.) Fax: 86-579-82282595. Tel: 86-579-82283897. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-11-1081) and the National Science Foundation of China (No. 21203168).

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REFERENCES

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dx.doi.org/10.1021/jp4054814 | J. Phys. Chem. C 2013, 117, 24121−24128