In situ sol-gel synthesis of ultrafine ZnO nanocrystals anchored on graphene as anode material for lithium-ion batteries

Ceramics International 42 (2016) 12371–12377

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In situ sol-gel synthesis of ultrafine ZnO nanocrystals anchored on graphene as anode material for lithium-ion batteries Haipeng Li a,b,c, Yaqiong Wei a,b,c, Yongguang Zhang a,b,n, Chengwei Zhang a,b, Gongkai Wang a,b, Yan Zhao a,b, Fuxing Yin a,b, Zhumabay Bakenov d a

Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300130, China Tianjin Key Laboratory of Laminating Fabrication and Interface Control Technology for Advanced Materials, Hebei University of Technology, Tianjin 300130, China c School of Material Science & Engineering, Hebei University of Technology, Tianjin 300130, China d Institute of Batteries LLC, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan b

art ic l e i nf o

a b s t r a c t

Article history: Received 1 January 2016 Received in revised form 2 May 2016 Accepted 3 May 2016 Available online 4 May 2016

Ultrafine ZnO nanocrystals anchored on graphene were synthesized by a facile and highly efficient in situ sol-gel method. Uniform ZnO nanocrystals with an average size of 9.3 nm were well dispersed on graphene nanosheets forming two-dimensional nanostructured ZnO/Graphene hybrids. Due to the intimate integration and strong synergistic effects between the ZnO nanocrystals and graphene nanosheets these hybrids exhibited a stable electrochemical performance. Along with this the graphene anchoring provides to the system high conductivity and large surface area and buffers the ZnO volume change during cycling. Furthermore, ultrafine ZnO nanocrystals provide a short diffusion path for Li þ upon insertion/ deinsertion. These structure and property advantages allow the as-prepared ZnO/graphene composite to exhibit a high reversible operation as an anode for lithium batteries with a stable specific discharge capacity of 516 mAh g  1 after 100 cycles at a current density of 200 mA g  1 and a good rate capability with a discharge capacity of 304 mAhg  1 even at a cycling rate of 1500 mA g  1. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sol-gel processes B. Nanocomposites D. ZnO E. Batteries

1. Introduction Due to their high capacity and long cycle life lithium-ion batteries (LIBs) have been widely applied as power sources in portable electronic devices, such as notebook computers, mobile phones, and digital cameras [1]. In this high performance advantages, the anode materials play a key role. Currently, graphite dominates the market of anode materials for lithium-ion batteries due to its low cost, high yield, and long cycle life [2]. However, low electrochemical capacity of graphite (372 mAh g  1) limits its consideration for application in new generation batteries, especially considering the development of new high capacity cathodes which require coupling with the enhanced energy storage capacity negative electrodes [3]. Therefore, tremendous efforts have been made to develop new anode materials for the next-generation LIBs. In the worldwide research for better lithium-ion anode materials, metal oxides (MO, M ¼Sn, Mn, Fe, Cu, Ni, Co, etc.) [4–12] have n Corresponding author at: Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300130, China. E-mail address: [email protected] (Y. Zhang).

http://dx.doi.org/10.1016/j.ceramint.2016.05.010 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

shown excellent promises due to their theoretical capacity higher than that of graphite, making them prospective anode materials with superior performance for next generation LIBs [13]. Among various metal oxides, ZnO with a high theoretical capacity of 978 mAh g  1 has attracted a special attention due to its low cost, non-toxicity and chemically stability compared with other candidate materials [14]. However, increased ZnO particle size and aggregation of ZnO particles during its charge-discharge negatively affect its reversible capacity and promote a capacity fading upon prolonged cycling. In order to circumvent these intractable problems, a variety of nanostructured carbonaceous materials such as mesoporous carbon, graphene, and carbon nanotubes have been used to composite with ZnO [13,15,16]. Introduction of carbon materials not only functions as a structural buffering layer to cushion the mechanical stress due to large volume changes of ZnO upon cycling, but also acts as a robust network to preserve good electronic conductivity of the electrode [17]. Among various promising carbonaceous materials, graphene has been suggested as an ideal one, due to its high surface area over 2600 m2 g  1, high electronic conductivity of 200 S m  1 and good mechanical properties [18]. Therefore, graphene has been used to incorporate with ZnO to synthesize hybrid materials to tackle the conductivity issues [19]. Hsieh et al. [19] prepared a

H. Li et al. / Ceramics International 42 (2016) 12371–12377

Fig. 1 shows XRD patterns of the as-prepared ZnO, ZnO/graphene composite and graphene nanosheets. The XRD patterns of ZnO and ZnO/graphene composite could be indexed as hexagonal phase of ZnO (JCPDS Card, no. 36-1451), which confirms that in situ sol-gel synthesis route is an efficiency way to prepare the ZnO based materials with a high degree of crystallinity. It should be noted that a broadened peak at ca. 25° can be observed in ZnO/ graphene composite, which is corresponding to the graphene (002). The average ZnO crystalline size in the ZnO/graphene composite calculated by the Debye Scherrer equation based on the most obvious peak (101) is 9.3 nm. The results are in good agreement with the following TEM/SEM data. Fig. 2 shows the FTIR patterns of graphene, ZnO/graphene composite and ZnO recorded in the range of wave lengths of 4000–360 cm  1. The characteristic features of graphene are the absorption bands corresponding to O–H stretching at 3400 cm  1, the peaks at 2976 and 2903 cm  1 are designated as the asymmetric stretching and symmetric vibrations of C–H. C-C skeleton vibration of carbon ring in graphene was at 1579 cm  1. The absorption peak at around 1072 cm  1 and 1225 cm  1 are attributed to the characteristic stretching vibration of C–O and C ¼O of COOH group, the peak observed at 894 cm  1 may be assigned to the ¼C– H out of plane vibration. In the spectrum of ZnO/graphene, the peak intensity of C–O and ¼C–H, belonged to graphene, obviously disappeared which indicated that the graphene hybridized with ZnO to form ZnO/graphene composite. In the spectrum of ZnO, there is a broad band at 3429 cm  1 with a very low intensity corresponding to the vibration mode of the water O–H group

20

40

50

(103) (200) (112) (201) (004)

(110)

(102)

30

60

70

(202)

ZnO/graphene ZnO Graphene

(101)

Zinc acetate (Zn(CH3COO)2, Z99%), lithium hydroxide (LiOH, Z90%), and ethanol (CH3CH2OH, Z 99.7%) were obtained from Beijing Chemical Reagents Company. Graphene aqueous suspension (3 wt%) was purchased from Nanjing Xianfeng Nanomaterials Technology Company (reduced graphene oxide with a few COOH functional groups). Firstly, 0.754 g of LiOH was added into 130 mL ethanol, followed by the addition of 130 mL of ethanol solution containing 2.86 g of Zn(CH3COO)2 and 0.085 g of graphene. After stirring the mixture for 24 h at room temperature, a black gel was formed. Further, this black gel was centrifuged and the precipitate was washed several times with double-distilled water and ethanol. Finally, after drying at 80 °C in an oven and grounding in an agate mortar, the ZnO/graphene composite was obtained. A sample without addition of graphene was prepared as well for comparison of the resulting product structure. Powder X-ray diffraction (XRD, Smart Lab, Rigaku Corporation), using Cu Kα radiation was employed to identify the crystalline phase of the synthesized materials. The data was recorded in the 2θ range of 20–80° with a 10° step size. Infrared absorption spectra were measured at room temperature on a FTIR spectrometer (FTIR, V80, Bruker Corporation) using the KBr pelleting technique. Samples were dried, gently mixed with 300 mg of KBr powder and compressed into discs at a force of 17 kN for 5 min using a manual tablet presser. X-ray photoelectron spectroscopy (XPS) was conducted with a Shimadzu Axis Ultra spectrometer with a Mg-Kα radiation source. The morphologies of the samples were investigated by scanning electron microscopy (SEM, S-4800, Hitachi Limited) and transmission electron microscopy (TEM, JEM-2100F, JEOL). Raman spectroscopy of the ZnO/graphene powders were performed by using the 532 nm line of Ar þ laser as the excitation source on ThermoFisher DXR Raman Microscope to validate the quality of ZnO/graphene composites. Thermo gravimetric analysis (TGA, SDT Q-600, TA Instruments-Waters LLC) was conducted from room temperature to 1000 °C in air with a heating rate of 10 °C min  1. The electrochemical performance of sample was investigated using coin-type cells (CR2025). Lithium foil was used as both counter and reference electrodes. The electrolyte was 1 mol dm  3 LiPF6 dissolved in a mixture of dimethyl carbonate/diethyl carbonate/ethylene carbonate (1:1:1 by volume), and the separator was microporous polypropylene film. The ZnO/graphene electrode was prepared by mixing 80 wt% as-prepared dispersed ZnO/graphene

3. Results and discussion

(100) (002)

2. Experimental

nanoparticle powders, 10 wt% polyvinylidene fluoride (PVDF) (Kynar, HSV900) as a binder and 10 wt% acetylene black (MTI, 99.5% purity) conducting agent in 1-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich, Z99.5% purity). The resultant slurry was uniformly spread onto Al foil using a doctor blade and dried at 50 °C for 12 h. The resulting ZnO/graphene film was used to prepare the electrodes by punching circular disks with 1 cm in diameter. The active material loading in each electrode was about 2 mg cm  2. The coin cells were assembled in an Ar (99.9995%) filled glove box (MBraun) and tested galvanostatically on a multichannel battery tester (BTS-5V5mA, Neware) in a cut-off potential window of 0.005–3.0 V vs. Li þ /Li electrode at different current densities. All electrochemical measurements were performed at room temperature.

Graphene (002)

novel ZnO/graphene composite with highly crystallized ZnO nanocrystals (the size of 80–100 nm) via a microwave-assisted deposition followed by a heat treatment in H2-containing atmosphere. This hybrid material exhibited enhanced performance in terms of cyclability and rate capability (460 mAh g  1 at 1 C for 50 cycles) compared with the graphene counterpart. However, the preparation method used in this work is complicated, demanding a prolonged processing and cannot be adopted for mass production. Thus it is noteworthy to develop a facile, low-cost and scalable synthesis route for ZnO/graphene composite materials [20]. To the best of our knowledge, a few studies on a simplified preparation of ZnO/graphene composite have been reported so far. In this work, we adopt a one-pot sol-gel synthesis route to design a novel ZnO/graphene composite; this in situ synthesis method promotes homogenous distribution of nanosized ZnO particles attached to the surface of graphene nanosheets. A synergistic effect of properties of the ZnO nanocrystals and their anchoring on the graphene nanosheet support on the composite material performance as an anode for LIBs are systemically investigated as well.

Intensity (a.u.)

12372

80

2 Theta (degree) Fig. 1. XRD patterns of the ZnO, graphene, and ZnO/graphene composite.

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Zn 2p3/2 (a)

b ZnO/graphene c ZnO

Zn 2p1/2 C-H

a

O-H C-O

=C-H

C-C b

C=O

Intensity (a.u.)

Transmittance(%)

a Graphene

O 1S C 1S

c

500

1000

1500

2000

2500

Wave number

3000

cm

3500

4000

0

200

400

600

800

1000

1200

-1

Binding energy (eV)

Fig. 2. FTIR spectra of graphene, ZnO and ZnO/graphene composite.

Intensity (arb.unit)

D

G

ZnO/graphene Graphene

(b)

Intensity (a.u.)

Zn 2p3/2

Zn 2p1/2

1020

1030

1040

1050

Binding energy (ev)

(c) C-O

Intensity (a.u.)

indicating the presence of small amount of water adsorbed on the ZnO surface. The characteristic Zn–O stretching vibration of ZnO shifted to a lower wave number of 447 cm  1 from the reported values of 457 cm  1 due to the presence of carboxylic functional groups in the functionalized hybrid composites involved in the formation of Zn–O–C carbonaceous bonds [21–25]. Raman spectroscopy can assess ordered and disordered crystal structures of carbonaceous materials and the single-, bi- and multilayer characteristics of graphene and/or graphene oxide layers. Fig. 3 presents the Raman spectra of ZnO/graphene and graphene, which showed three major peaks of D, G and 2D bands. However, the intensity ratio of the D and G bands (ID/IG) of the composite is much larger than that of graphene, attributed to interactions between the ZnO nanoparticles and the graphene sheets. According to the literature, the 2D peak for the single-layer graphene sheet should be located at about 2679 cm  1 [26–28]. In our case, the 2D band is located at about 2679 cm  1, which indicates that our graphene is single-layered. In addition, the ZnO/ graphene composite peaks at 448 cm  1 is attributed to crystalline ZnO, which confirms that the as-prepared ZnO/graphene composite is composed of pure graphene and crystalline ZnO nanoparticles [29]. Chemical composition and bonds configuration of ZnO/graphene was further investigated by XPS. Fig. 4a presents the survey spectrum of ZnO/graphene composite, and the only Zn, O, and C elements were detected suggesting the purity of the samples. The high-resolution scan of Zn 2p, shown in Fig. 4b, identifies the exact peak location of Zn 2p3/2 at 1022.7 eV and of Zn 2p1/2 at

280

C-C

C=O Zn-O-C

285 290 Binding energy (eV)

295

Fig. 4. Survey XPS spectrum of ZnO/graphene composite; XPS spectra of Zn 2p (b) and C 1s (c).

2D

ZnO

500

1000

1500

2000

Raman shift cm

2500

3000

-1

Fig. 3. Raman spectra of graphene and ZnO/graphene composite.

1045.7 eV. The C1s (Fig. 4c) spectra exhibits characteristic features of graphene with three main components that corresponds to the carbon atoms in different functional groups: the C–C (284.8 eV), C– O (286.1 eV) and C ¼O (288.9 eV) bonds. In the C 1s spectrum of ZnO/graphene, an additional peak at 283.6 eV can be detected. From the discussion above, it is safe to say that the peak could be assigned to Zn–O–C bond [30–32]. The SEM images of the as-prepared ZnO nanoparticles and ZnO/graphene composite are shown in Fig. 5. One can see from Fig. 5a that the individual ZnO particles exhibit obvious agglomeration and aggregated into large clusters by the interactions such

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Fig. 5. SEM images of the (a) ZnO and (b)–(d) ZnO/graphene samples at different magnifications.

as Van der Waals forces [33]. In contrast, the ZnO/graphene composite particles are well dispersed and have a smaller particle size from 6 nm to 13 nm as it can be observed in Fig. 5d. One can clearly see that ZnO nanocrystals with a much lower degree of aggregation uniformly laid on the surface of crumpled graphene sheets (Fig. 5b and c). The Fig. 5d inset shows the ZnO nanoparticles diameter distribution obtained from Fig. 5d; the average diameter of the ZnO nanoparticles was found to be around 9 nm. This phenomenon suggests that graphene nanosheets could not only act as effective heterogeneous nucleation and growth sites for formation of ZnO but also could limit the ZnO nanoparticles aggregation during one-pot sol-gel synthesis. It should be noted that nanosized ZnO particles can provide shorten transport path for Li þ ions diffusion and the additional space to accommodate the material volume expansion upon cycling. The ultrafine ZnO nanocrystals anchored on graphene, obtained in this work, could be favorable for enhanced electrochemical performance of this anode. The TEM measurement results are shown in Fig. 6; it can be seen that the ZnO nanocrystals are densely grown on the surface of graphene. As depicted in Fig. 6a and b, the ZnO nanoparticles are uniformly anchored on the surface of the graphene without any serious aggregation. One can see that in the corresponding

HRTEM at a higher magnification (Fig. 6c), the lattice fringes could be clearly identified and have a d-spacing of 0.281 nm, corresponding to the (100) planes of crystalline ZnO. The SAED pattern (Fig. 6d) exhibits properties of polycrystalline ZnO with homogeneous diffraction rings, indicating a high crystallinity of ZnO. Thermo gravimetric (TG) measurement was carried out in air flux to determine the mass percentage of ZnO in the prepared hybrids at a temperature rise rate of 10 °C min  1. As it is shown in Fig. 7, there is a slight weight loss below 120 °C which can be ascribed to evaporation of adsorbed water molecules. A following major weight loss from approximately 300 °C to 800 °C is attributed to the graphene combustion. Upon a further temperature is a sample keeps a constant weight above 800 °C, which is suggested to be a weight of ZnO. Hence, the content of ZnO in the nanocomposites is calculated to be about 74.5 wt%. Fig. 8 displays the potential curves of the ZnO/graphene composite anode upon galvanostatic cycling at a current density of 200 mA g  1. One can see that at the first cycle the ZnO/graphene composite possesses a large discharge capacity of 1583 mAh g  1, which has similar trends to those reported in the literature for the ZnO-based anodes [34]: a long slope region can be observed in the initial discharge but it disappears in the subsequent cycles, which is

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Fig. 6. (a)–(b) Different magnifications TEM image; (c) HRTEM images of the ZnO/graphene samples; (d) SAED pattern images of the ZnO/graphene sample.

Voltage/V

3

2 1st cycle 2nd cycle 3rd cycle

1

0 Fig. 7. TG curves of the ZnO/graphene composite at a heating rate of 10 °C min  1 under air.

mainly related to the reduction of ZnO into Zn, the formation of LiZn alloy, and the formation of a solid electrolyte interface (SEI). The following charge-discharge cycles include two reversible steps [35]:

0

500

1000

Capacity/ mAh g

1500 -1

Fig. 8. Charge-discharge curves of the ZnO/graphene electrodes at 200 mA g  1 within a voltage range of 0.005–3.0 V vs. Li þ /Li.

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Capacity/ mAh g

-1

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1500

200 mA g-1

1000

ZnO/Gn charge ZnO/Gn discharge ZnO charge ZnO discharge

500 0

0

20

40 60 Cycle number

80

100

Fig. 9. Cycling performance of the ZnO and ZnO/graphene negative electrodes at 200 mA g  1.

Capacity/ mAh g-1

1500 ZnO charge ZnO discharge ZnO/Gn charge ZnO/Gn discharge

1000

-1

200 mA g

-1

-1

500 mA g

500 0

1500 mA g

-1

200 mA g

-1

1000 mA g

0

10

20 30 40 Cycle number

50

Fig. 10. Rate capability of lithium cells with the ZnO and ZnO/graphene electrode.

ZnOþ2Li2Zn þLi2O Zn þLi2LiZn Total reaction: ZnOþ3Li2LiZn þLi2O Furthermore, it should be noted that, in this composite, not only ZnO but the graphene as well is electrochemically active to react with lithium and can provide some additional Li þ storage, which greatly favor the total reversible capacity [36]. It can be seen from Fig. 8 that during a subsequent anodic scan, the discharge curves features differ from the initial one, and upon further cycling they maintain similar shape and size, which indicates a reversible character of the lithiation process of ZnO/graphene composite [37]. Cyclability data of ZnO/graphene composite and its bare ZnO counterpart at a current density of 200 mA g  1 are presented in Fig. 9. It can be seen that a bare ZnO anode shows gradually fading cycling ability with the first discharge capacity of 1504 mAh g  1 and only 251 mAh g  1 retained after 100 cycles. In contrast, the ZnO/graphene composite exhibits remarkably enhanced cycling

stability and higher specific capacities than ZnO: a high discharge capacity of 516 mAh g  1 was maintained by this composite anode over 100 cycles. This significant performance improvement is due to the fact that the graphene nanosheets only prevent aggregation of ZnO nanoparticle, as it was confirmed by SEM measurement, but the graphene nanosheets could both successful accommodate the mechanical strain resulted from the lithiation and delithiation processes and provide channels for electron rapid transfer, resulting in a higher conductivity and favorable conditions for the enhanced electrochemical performance [38–39]. Fig. 10 shows the rate capabilities of ZnO and ZnO/graphene composite anodes in lithium half-cell at various current densities from 200 to 1500 mA g  1. It can be seen that the capacities of both anode materials gradually decrease with the increase in current density and the ZnO/graphene composite exhibits enhanced rate capability compared with a bare ZnO. At a current density of 1500 mA g  1 the capacity of ZnO/graphene composite is 304 mAh g  1, while the corresponding value of ZnO is only 190 mAh g  1. It should be noted that the ZnO/graphene composite presents a steady reversible capacity at a current of 1500 mA g  1, and along with this, this anode recovers about 90% of its original reversible capacity (611 mAh g  1) when the current density is turned back to 200 mA g  1 after cycling at the higher cycling rates [36,40–41]. This enhanced reversibility and cycling stability indicates that graphene can provide an effective electron conduction path and their sheet-like structure forms a stable carrier for ultrafine ZnO nanocrystals anchored on it [19,41–43]. Furthermore, highly dispersed and evenly distributed over the highly crosslinked thin graphene nanosheets ZnO nanoparticles shorten the Li þ diffusion distances and provide a large contact interfaces between electrode and electrolyte as well. Based on the results obtained on the ZnO/graphene electrode, it can be concluded that the composite prepared in this work exhibits superior electrochemical performance to those reported in the literature, as it can be seen from the comparative data in Table 1. These results indicate that a graphene additive could remarkably improve the electrochemical performance of ZnO nanoparticles.

4. Conclusions In summary, the ZnO/graphene composite was successfully synthesized by a versatile and scalable sol-gel method. The prepared ZnO/graphene composite contains ZnO nanoparticles with an average size of 9.3 nm that are uniformly dispersed and anchored on the graphene sheets. The ZnO/graphene displays superb cycling stability and rate capability when used as anodes in LIBs. The excellent performance of the ZnO/graphene composite is contributed to the graphene, which provides high surface areas for highly dispersed of ZnO nanoparticles and alleviate the volume expansion during the charge-discharge cycling. The present work provides insights for a better design of the ZnO composite anode with high electrochemical performance for lithium-ion batteries. For example, by inducing the highly-dispersed ZnO nanoparticles

Table 1 Comparison of performance of ZnO/graphene nanocomposite anodes for LIBs. Materials

Reversible capacity (mAh g-1)

Cycle number

Current density

Applied potential range (V)

Ref.

ZnO-CoO-C ZnO/graphene composites ZnO@GN framework graphene/ZnO composites Alumina-ZnO-graphene composites ZnO/graphene nanocomposite ZnO/graphene composite

438 404 460 401 487 300 516

50th 100th 50th 30th 100th 25th 100th

0.5 C 0.5 C 1C 100 mA g  1 100 mA g  1 50 mA g  1 200 mA g  1

0.0–2.5 0.001–3.0 0.01–3.0 0.01–3.0 0.01–3.0 0.05–2.0 0.005–3.0

[34] [13] [19] [44] [45] [46] Our work

H. Li et al. / Ceramics International 42 (2016) 12371–12377

into carbonaceous host frameworks to improve the battery performance via enhanced conductivity of the system and stabilizing its mechanical strength against periodical volume expansion and shrinkage upon battery operation.

Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant no. 21406052), the Program for the Outstanding Young Talents of Hebei Province (Grant no. BJ2014010), Natural Science Foundation of Hebei Province of China (Project no. E2015202037) and Science and Technology Correspondent Project of Tianjin (Project no. 14JCTPJC00496). ZB acknowledges the financial support from the research Grant 5156/GF from the Ministry of Education and Science of Kazakhstan.

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