carbon nanotube composite anode for lithium-ion batteries

Author’s Accepted Manuscript Simple fabrication of free-standing ZnO/graphene/carbon nanotube composite anode for lithium-ion batteries Yongguang Zhang, Yaqiong Wei, Haipeng Li, Yan Zhao, Fuxing Yin, Xin Wang www.elsevier.com

PII: DOI: Reference:

S0167-577X(16)31291-5 http://dx.doi.org/10.1016/j.matlet.2016.08.017 MLBLUE21301

To appear in: Materials Letters Received date: 29 April 2016 Revised date: 19 July 2016 Accepted date: 4 August 2016 Cite this article as: Yongguang Zhang, Yaqiong Wei, Haipeng Li, Yan Zhao, Fuxing Yin and Xin Wang, Simple fabrication of free-standing ZnO/graphene/carbon nanotube composite anode for lithium-ion batteries, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.08.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Simple fabrication of free-standing ZnO/graphene/carbon nanotube composite anode for lithium-ion batteries 1,2,3

Yongguang Zhang1,2, Yaqiong Wei1,2,3, Haipeng Li

*, Yan Zhao1,2, Fuxing Yin1,2, Xin Wang4*

1

Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300130, China 2

Tianjin Key Laboratory of Laminating Fabrication and Interface Control Technology for Advanced Materials, Hebei University of Technology, Tianjin 300130, China

3

School of Material Science & Engineering, Hebei University of Technology, Tianjin 300130 China

4

Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, China [email protected] [email protected]

*Corresponding author. Tel.: +86-22-60201447.

Abstract

A three-dimensional ZnO/graphene/CNT (carbon nanotube) composite was prepared by a one-step sol-gel synthetic technique followed by vacuum-assisted filtration. High resolution transmission electron microscopy revealed the formation of highly-dispersed ZnO nanoparticles, uniformly laid on the crumpled graphene sheets and CNT. The as-prepared ZnO/graphene/CNT composite exhibited excellent cyclability and rate capability when used as anode for lithium ion batteries, affording 620 mAh g-1 stable reversible discharge capacity after 100 cycles at 100 mA g-1. The unique composite structure provided a large surface area and highly conductive network which maintains good electronic contact between particles, suppresses ZnO aggregation during the charge/discharge processes, and accommodates the large volume changes of ZnO upon cycling.

Keywords: Lithium ion battery, Anode, ZnO/graphene/CNT composite, Free-standing, Nanoparticles,

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Energy storage and conversion

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Introduction In face of the increasing demand for next-generation lithium ion batteries (LIBs) with high energy

density and good cycle stability, ZnO has attracted much attention due to its high theoretical capacity (978 mAh g-1), low cost, abundance and environmentally friendly nature [1]. However, there are still some challenges for the practical applications of ZnO as anode materials, such as low electronic conductivity and large volume change during the lithiation/delithiation process, which result in fast capacity fading and poor rate capability [2]. To solve the above problems, many researchers have been carried out to fabricate the nanosized composite by incorporating ZnO on the carbon matrix, such as carbon nanotube (CNT), graphene and mesoporous carbon, since carbon has high electronic conductivity and negligible volume change [3]. More recently, the free-standing composite electrodes with three-dimensional (3D) structure have attracted much attention, and in the electrode, the binder and the current collector are rendered unnecessary, improving the specific capacity of a “whole” LIB. Herein is reported the vacuum-assisted filtration fabrication of a novel free-standing ZnO/graphene/CNT ternary composite. The graphene-CNT matrix serves as a large surface area, highly conductive network which can provide good contact between ZnO particles, tolerating large volume changes and suppressing aggregation of ZnO particles during charge/discharge processes. Furthermore, both CNT and graphene act as conducting materials to improve overall electronic conductivity of the ZnO based ternary composite, leading to fast reaction kinetics [4]. 2.

Experimental The schematic of the fabrication process for ZnO/graphene/CNT composite is shown in Fig. 1a.

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Firstly, 2.86 g zinc acetate (Zn(CH3COO)2, ≥99%, Tianjin Fuchen), 0.47 g carbon nanotubes dispersion (CNT, 9 wt%, Chengdu Organic Chemicals) and 85 mL graphene aqueous suspension (0.5 mg mL-1, Nanjing Xianfeng) were dispersed in 130 mL of ethanol, followed by the addition of 130 mL solution of 0.754 g of lithium hydroxide (LiOH, ≥ 90%, Tianjin Fuchen) in ethanol. After stirring the mixture for 24 h, a black gel was formed. The gel was filtered using a 0.22 μm pore size polyvinylidene difluoride (PVDF) membrane (Shanghai Xingya, 50 mm diameter) and dried in a vacuum oven at 50 °C for 12 h. After drying, a free-standing film of the ZnO/graphene/CNT ternary composite could be easily peeled off, cut into 1 cm diameter disks, and used directly as electrode. The crystalline nature of the material was probed by X-ray diffractometry (XRD, Smart Lab, Rigaku Co.) in the 2θ range from 20° to 80° using Cu Kα radiation. The samples morphology and structure were examined using scanning electron microscopy (SEM, S-4800, Hitachi Limited) and transmission electron microscopy (TEM, JEM-2100F, JEOL). Thermogravimetric analysis (TGA, SDT Q-600, TA Instruments-Waters LLC) was conducted under air atmosphere with a heating rate of 10 °C min-1. The electrochemical performance of the material was investigated using coin-type cells (CR2025) with lithium disks serving as counter and pseudo-reference electrodes. A 1 mole L-1 solution of LiPF6 in dimethyl carbonate/diethyl carbonate/ethylene carbonate mixture (1:1:1 by volume) was used as electrolyte and a microporous polypropylene film as separator. The coin cells were assembled in an Argon (99.9995%) filled glove box (MBraun) and tested galvanostatically at room temperature on a multichannel battery tester (BTS-5V5mA, Neware), using different current densities and a cut-off potential window of 0.01-3.0 V vs. Li+/Li. 3.

Results and discussion Fig. 1b shows the XRD patterns of the ZnO, CNT and ZnO/graphene/CNT ternary composite. The 3

peak observed at 26° corresponds to graphitic (002) plane while the remainder peaks can all be indexed to hexagonal wurtzite ZnO (JCPDS No. 36-1451) [5]. This indicates that crystalline ZnO particles were successfully formed in a composite with graphene sheets and CNT. The thermogravimetric (TG) spectrum for the ZnO/graphene/CNT ternary composite is shown in Fig. 1c. About 24% weight loss is observed during the temperature sweep to 800 °C. When the temperature is further increased to 1000 °C, the change of weight loss becomes insignificant. The weight loss process can be divided into two regions. The first region (100-200 °C) corresponds to ~9 wt% weight loss due to evaporation of adsorbed water whereas the second region (~15 wt%, 200-800 °C) is likely related to the pyrolysis of C from the composite. Therefore, the amount of ZnO in the composite is approximately 84 wt% [6].

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Fig. 1 Fabrication process, crystallographic and thermogravimetric analysis of the ZnO/graphene/CNT composite. (a) Schematic of the fabrication process for ZnO/graphene/CNT composite; (b) Powder XRD patterns of CNT, pure and composite ZnO; (c) Weight percent loss curves of the composite. Fig. 2a shows the morphology of the ZnO/graphene/CNT composite. One can clearly see that numerous CNTs are well attached to the surface of graphene sheets, forming a 3D architecture, and that highly-dispersed ZnO nanoparticles are uniformly laid on the crumpled graphene sheets and CNTs [7]. This 3D structure can significantly increase the electrode effective surface and facilitate the diffusion of the Li+ ions. In the composite, CNTs not only works as a modifier of graphene, altering their surface 4

properties and preventing possible agglomeration, but also acts as a link between graphene layers. The inset in Fig. 2a shows a photograph of the ZnO/graphene/CNT film held by tweezers, demonstrating its good flexibility. Fig. 2b-2f depict typical TEM micrographs of the samples. As shown in Fig. 2b, ZnO nanoparticles are uniformly distributed on the surface of graphene/CNT. At a higher magnification, Fig. 2c, the CNTs can be seen anchored onto the graphene nanosheets with no obvious agglomeration of the ZnO nanoparticles. The inset in Fig. 2c shows the ZnO nanoparticle size distribution obtained from the analysis of TEM data. ZnO nanoparticles have a uniform size distribution with average size of 8 nm. HRTEM (Fig. 2d) of ZnO/graphene/CNT composite reveals spacing of 0.118 nm and 0.248 nm between the adjacent fringes, corresponding respectively to the (104) and (101) planes of ZnO [5]. Furthermore, the SAED pattern in Fig. 2e displays seven distinct diffraction rings characteristic of nanoscopic polycrystalline material. Fig. 2f shows elemental distribution of C, Zn and O in the hybrid composite. One can see that the ZnO nanoparticles are homogeneous distributed over the 3D graphene/CNT, a desirable feature for good electrochemical performance of the ZnO/graphene/CNT composite anode material.

Fig. 2 Electron microscopy analysis of the ZnO/graphene/CNT composite. (a) SEM micrographs (inset: image of flexible composite); (b)-(c) TEM micrographs at different magnifications (inset: particle size distribution obtained from TEM data); (d) HRTEM image; (e) SAED pattern; (f) TEM micrograph and 5

corresponding EDS elemental maps. Fig. 3a depicts typical initial three cycles galvanostatic discharge-charge curves of the ZnO/graphene/CNT composite at a current density of 100 mA g-1. In the first discharge curve, a slightly sloped plateaus located at 0.5-0.7 V vs. Li+/Li, corresponding to the reduction process of ZnO to Zn metal (ZnO + 2Li →Zn+ Li2O) [8]. One can see the second voltage plateau at 0.2 V appearing upon deep discharge, which is ascribed to the formation of lithium-zinc alloy (xLi + Zn →LixZn) [8]. In the first charge curve, different slopes at 0.3, 0.5 and 0.7 V for ZnO/graphene/CNT ternary composite can be attributed to multi-step dealloying process of Li-Zn alloy, which gives Zn metal through several stages, like LiZn→Li2Zn3→LiZn2→Li2Zn5, respectively [9]. The slope at 1.3 V for ZnO/graphene/CNT ternary composite should be ascribed to the formation of ZnO. The initial discharge capacity for the ZnO/graphene/CNT anode is 1503 mAh g-1. Upon charge, only 887 mAh g-1 capacity is recovered, leading to a low coulombic efficiency of 60%. The long plateau and initial irreversible capacity loss is a common phenomenon for the ZnO anode attributed to the formation of a solid electrolyte interface (SEI) [10]. After the initial activation, the discharge curves are different from the first one, and the potential profiles in the subsequent cycles are similar in shape, indicating that the lithiation process of ZnO is reversible [11]. The signature near 0.1 V seen in the second and third discharge curves is attributed to the contribution from the carbon matrix lithiation. The cycling performance of the ZnO/graphene/CNT composite with 3D carbon network is shown in Fig. 3b. A high reversible discharge capacity of 890 mAh g-1 is obtained on the second cycle, which decreases slightly and stabilizes at 620 mAh g-1 after 100 cycles. This represents an increase of at least 200 mAh g-1 reversible specific capacity compared to what is obtained for nanostructured ZnO anode without graphene/CNT at using similar cycling rate [11, 12]. Furthermore, the coulombic efficiency is almost 100 % from the 7th cycle. The excellent cycling performance of the ZnO/graphene/CNT composite can be attributed to the unique 3D structure of the carbon network, a robust framework in which the ZnO nanoparticles are uniformly dispersed, benefiting the stress relaxation and electron transport. The rate performance of ZnO/graphene/CNT composite is shown in Fig. 3c-3d. An increase in current rate from 200 mA g-1 to 1600 mA g-1 caused a drop in discharge capacity from about 682 mAh g-1 to 261 mAh g-1. 6

Notwithstanding, the ternary composite recovered most of the stable capacity when the current rate was reduced back to 200 mA g-1. This good rate capability is attributed to high conductivity, uniform ZnO distribution, and unique 3D structure of ZnO/graphene/CNT composite.

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Fig. 3 Electrochemical performance of ZnO/graphene/CNT electrodes. (a) Discharge-charge curves of the ZnO/graphene/CNT electrodes at 100 mA g-1 within a voltage window of 0.01-3.0 V vs. Li+/Li; (b) Cycling performance of the ZnO/graphene/CNT anodes at 100 mA g-1; (c)-(d) Rate capability of lithium cells with the ZnO/graphene/CNT electrode. Conclusions In conclusion, we have successfully prepared a free-standing ZnO/graphene/CNT composite electrode with good electrochemical performance. In the hybrid composite, numerous CNTs are well attached to the surface of graphene sheets forming a 3D architecture, and the ZnO nanoparticles are well dispersed and uniformly laid on the crumpled graphene sheets and CNT. These structural characteristics extend the electrode/electrolyte interface, enhance the electrical conductivity, and improve the structural stability of the composite. As a result, the ZnO/graphene/CNT electrodes exhibit notable electrochemical

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performance including high initial discharge capacity and good cycle/rate performance. 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), Ministry of Human Resources and Social Security of China (Grant No. CG2015003002), Natural Science Foundation of Hebei Province of China (Project No. E2015202037) and Science and Technology Correspondent Project of Tianjin (Project No. 14JCTPJC00496). References [1] Y. Zhao, H.P. Li, Y. Zhang, H.X. Xie, F.X. Yin, Int. J. Electrochem. Sci. 11 (2016) 3179-3189. [2] C.T. Hsieh, C.Y. Lin, Y.F. Chen, J.S. Lin, Electrochim. Acta 111 (2013) 359-365. [3] X.H. Huang, X.H. Xia, Y.F. Yuan, F. Zhou, Electrochim. Acta 56 (2011) 4960-4965. [4] Y. Zhang, Z. Luo, Q. Xiao, T. Sun, G. Lei, Z. Li, X. Li, J. Power Sources 297 (2015) 442-448. [5] B. Zeng, X.H. Chen, X.T. Ning, C.S. Chen, A.P. Hu, W.N. Deng, Catal. Commun. 43 (2014) 235-239. [6] H. Bai, Y. Xu, L. Zhao, C. Li, G. Shi, Chem. Commun. (2009) 1667-1669. [7] P. Li, Y. Liu, J.Y. Liu, Z.T. Li, G.L. Wu, M.B. Wu, Chem. Eng. J. 271 (2015) 173-179. [8] C. Zhang, L.L. Ren, X.Y. Wang, T.X. Liu, J. Phys. Chem. C 114 (2010) 11435-11440. [9] Q.S. Xie, X.Q. Zhang, X.B. Wu, H.Y. Wu, X. Liu, G.H. Yue, Y. Yang, D.L. Peng, Electrochim. Acta 125 (2014) 659-665. [10] B. Zhang, Q.B. Zheng, Z.D. Huang, Carbon 49 (2011) 4524-4534. [11] H.P. Li, Y.Q. Wei, Y. Zhao, Y. Zhang, F.X. Yin, C.W. Zhang, Z. Bakenov, J. Nanomater. 2016 (2016) 4675960. [12] H.P. Li, Y.Q. Wei, Y. Zhang, F.X. Yin, C.W. Zhang, G.K. Wang, Z. Bakenov, Ionics 22 (2016)

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1387-1393.

Highlights:

  

A free-standing ZnO/graphene/CNT ternary composite was successfully synthesized. The ZnO/graphene/CNT composite exhibits good cycle performance and rate capability.

The graphene-CNT matrix serves as a large surface area, highly conductive network.

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