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Graphene supported Zn2SnO4 nanoflowers with superior electrochemical performance as lithium-ion battery anode
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 15183–15190 www.elsevier.com/locate/ceramint
Graphene supported Zn2SnO4 nanoflowers with superior electrochemical performance as lithium-ion battery anode Ke Wang, Ying Huangn, Yuanyuan Shen, Lele Xue, Haijian Huang, Haiwei Wu, Yanli Wang Department of Applied Chemistry, The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi'an 710072, PR China Received 30 May 2014; received in revised form 6 June 2014; accepted 30 June 2014 Available online 7 July 2014
Abstract A hydrothermal synthesis approach has been developed to distribute the Zn2SnO4 nanoflowers on the graphene sheets (GNS). The as-prepared Zn2SnO4 nanoflowers/GNS composites were characterized by XRD, BET, FTIR, Raman, TGA, SEM, TEM and electrochemical measurements. The results show that the Zn2SnO4 nanoflowers have particular 3-D structure and homogeneously adhere on graphene sheets. Electrochemical measurements suggest that Zn2SnO4 nanoflowers/GNS composites exhibit better cycling properties and lower initial irreversible capacities as anode materials for lithium-ion batteries. Galvanostatic cycling shows 1967 mAh g 1 of initial discharge capacity and 1087 mAh g 1 of initial charge capacity. A higher reversible capacity of 850 mAh g 1 is obtained after 10 cycles at a current density of 300 mA g 1. The higher reversible capacity and good stability can be ascribed to the presence of graphene. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Zn2SnO4 nanoflowers/GNS composites; Hydrothermal synthesis; Lithium-ion batteries; Electrochemical properties
1. Introduction Lithium-ion batteries have dominated the battery industry for the past several years in portable electronic devices due to their high volumetric and gravimetric energy densities (650 Wh L 1 and 210 Wh kg 1) [1]. However, the theoretical capacity of graphite is not high enough to meet the demands for battery in small-scale applications translates to large-scale applications [2,3]. Increasing efforts have been diverted to the exploration of new anode materials with higher theoretical capacity. In recent years, major research accomplishments are the proposal of Snbased materials [4,5], and which have been deemed as promising negative electrode materials for LIBs. As a kind of tin–zinc composite oxide, Zn2SnO4 attracts researchers' more attention since it has an inverse spinel structure, high electron mobility, high electrical conductivity and low visible absorption. These outstanding properties enable its application in lithium-ion n
Corresponding author. Tel.: þ86 29 88431636. E-mail addresses: [email protected] (K. Wang), [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.ceramint.2014.06.133 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
battery anode [6]. The practical usage of Zn2SnO4 anodes is, however, hindered by their rapid capacity fade due to the large volume changes upon Li-insertion/extraction processes as anode material (about 300% volume change between the fully lithiated state, Li4.4Sn, and the non-lithiated state, Sn) [7,8]. There were considerable approaches proposed to accommodate the large volume changes and alleviate the Sn-based particle aggregation in order to get better cycling performance [9–11]. One strategy is to design the nanostructure of electrode materials [12,13]. As known, Zn2SnO4 micro/nanocrystals have been synthesized in various morphologies such as spherical, cubeshaped, or rod-like nanocrystals [14–16]. Another approach is to load the nanoparticles onto a matrix that is able to accommodate these volume changes during the alloying process [17,18]. Recently, graphene with superior electrical conductivities, high surface area of over 2600 m2 g 1, excellent thermal property and mechanical property, has attracted much attention in the field of materials science [19–22], which is more attractive as a matrix to support Zn2SnO4 nanoparticles. The flexible graphene not only buffers the volume changes during Li-insertion/extraction processes but also prevents the aggregation of the nanoparticles upon
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long-term cycling due to its large specific surface area [23] and high mechanical strength [24]. In this work, graphene framework was combined to improve the cycling stability of Zn2SnO4 nanoflowers electrodes. The graphene framework and the multidimensional nanostructure of Zn2SnO4 create a double buffer structure for Zn2SnO4, and provide additional mechanical strength to prevent the crack and pulverization of the electrode structure during repeated volume expansion and contraction. The Zn2SnO4 nanoflowers/GNS composites show the superior electrochemical performance. 2. Experimental 2.1. Sample preparation All of the reagents were of analytical grade and were used without any further purification. Graphite oxide (GO) was first
synthesized with a modified Hummers' method. Zinc chloride (ZnCl2) and tin chloride dihydrate (SnCl2 2H2O) were used as precursors and NaOH as the mineralizer for the hydrothermal synthesis of Zn2SnO4 nanoflowers. The Zn2SnO4 nanoflowers grew onto the graphene sheets by a hydrothermal process. The process began with dissolving SnCl2 2H2O (1.35 g), ZnCl2 (0.82 g) and GO (0.240 g) into 30 mL ethanol separately to form three well-distributed solutions. Then GO solution and 40 mL NaOH (1 M) solution were successive added little by little facilitated by magnetic stirring into the tin chloride dihydrate transparent solution. Zinc chloride solution was added drop by drop into above mixed solution under vigorous stirring using magnetic stirrer at room temperature until the formation of precipitate of the hybrid complex. Finally, the hybrid complex was transferred into a 200 mL Teflon-lined stainless autoclave with a fill factor of approximately 70%. The autoclave was sealed and maintained in a furnace at 220 1C for
Fig. 1. (a)XRD patterns of Zn2SnO4 nanoflowers and Zn2SnO4 nanoflowers/GNS composites; (b) adsorption/desorption isotherms of the Zn2SnO4 nanoflowers/ GNS composites and Zn2SnO4 nanoflowers; (c) TGA curve of the Zn2SnO4 nanoflowers/GNS composites; and (d) FTIR spectrum of the Zn2SnO4 nanoflowers/ GNS composites.
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24 h. Then, the product was separated and washed for several times with ultrapure water and ethanol after the hydrothermal reaction was terminated. After these, the product was dried under vacuum at 60 1C for 12 h to obtain the precursor. Finally, the precursor was sintered at 800 1C for 4 h under argon atmosphere, leading to the reduction of GO to obtain Zn2SnO4 nanoflowers/GNS composites. The bare Zn2SnO4 nanoflowers were synthesized under the same condition without the addition of graphene oxide for comparison. 2.2. Materials characterization The structures of the prepared samples were characterized by X-ray diffraction analysis (XRD) (Rigaku, model D/max2500 system at 40 kV and 100 mA of Cu Kα). The Raman spectras of the composite samples were obtained by using a Laser-Raman spectrometer (Renishaw Co., England) with a 514 nm radiation. The surface morphology of the composites was performed by a model Tecnai F30 G2 (FEI Co., USA) field emission transmission electron microscope (FETEM) and scanning electron microscope (SEM, SuPRA 55, German ZEISS). The Fourier transform infrared spectroscopy (FTIR) spectras were obtained by using a Model NIcolETiS10 Fourier transform spectrometer (Thermo SCI-ENTIFIC Co., USA) with a 2 cm 1 resolution in the range of 500–4000 cm 1. Electrochemical performance was evaluated by a CR2016type coin cell with a multi-channel current static system Land (LAND CT200IA). The anode electrodes were prepared by coating slurries consisting of Zn2SnO4 nanoflowers/GNS composites (65 wt%) with acetylene black (15 wt%) and PVDF (20 wt%) as a binder dissolved in 1-methyl-2pyrrolidinone (NMP) solution on a copper foil. Li foil was used as a counter electrode, and polypropylene (PP) film (Celgard 2400) as the separator. The electrolyte was a solution of 1 M LiPF6 in a mixture of ethylene (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1, v/v/v).
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which also provide a high conductive medium for electron transfer. To quantify the mass percentage of Zn2SnO4, Fig. 1(c) displays the TGA curves of the Zn2SnO4 nanoflowers/GNS composites that have been carried out in air. The first weight loss in the temperature between 30 1C and 140 1C is attributed to the release of the absorbed water. The rapid mass loss occurs between 200 1C and 650 1C due to the oxidation and decomposition of graphene. Therefore, according to the change of weight, it is estimated that the amount of the Zn2SnO4 in the composites is 83 wt%. Fig. 1(d) shows the Fourier transform infrared spectroscopy (FTIR) spectrum of the Zn2SnO4 nanoflowers/GNS composites. The vibration at about 520 cm 1 is seen in the low wave number region corresponding to the Sn–O vibration mode of Zn2SnO4, the peak at 3414 cm 1 can be attributed to the O–H stretching vibrations of the structural –OH groups [25]. The absorption at 1616 cm 1 can be assigned to the skeletal vibration of graphene sheets [26]. The presence of carboxyl can also be detected at around 1122 cm 1. Here the absorption of the oxygen-containing groups at the edges and on the basal planes of graphene sheets owing to the oxidation process can be partly removed during thermal expansion. Fig. 2 presents the Raman spectra of the Zn2SnO4 nanoflowers/GNS composites and the as-prepared composites before annealing. The peak at about 1592 cm 1 (G band) is
3. Results and discussion Fig. 1(a) shows the XRD patterns of the as-prepared Zn2SnO4 nanoflowers and the Zn2SnO4 nanoflowers/GNS composites. In the XRD curve of Zn2SnO4 nanoflowers (i), all main diffraction peaks of the sample are consistent with the (JCPDS card no. 24-1470) data of the pure reverse-spinel Zn2SnO4. The major diffraction peaks of Zn2SnO4 nanoflowers/GNS composites (ii) can be indexed to those of Zn2SnO4 (JCPDS card no. 24-1470). Furthermore, the characteristic diffraction peak of the GNS (n002) confirms the presence of graphene in the composite. The results indicate that growth of the Zn2SnO4 nanoflowers are inhibited in the hybrid due to the intervention of graphene. Fig. 1(b) displays the N2 adsorption/desorption isotherms of the Zn2SnO4 nanoflowers/GNS composites and Zn2SnO4 nanoflowers. Indeed, the BET surface area of the composites increases from 40.56 to 481.29 m2 g 1after graphene loading. The presence of graphene is the key reason for having large surface area of the Zn2SnO4 nanoflowers/GNS composites, and
Fig. 2. Raman spectras of Zn2SnO4 nanoflowers/GNS composites and the as-prepared composites before annealing.
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Fig. 3. SEM and TEM images of the Zn2SnO4 nanoflowers.
related to the vibration of sp2-bonded carbon atoms in a 2-dimensional hexagonal lattice, while the 1334 cm 1 peak (D band) is ascribed to edge planes and disordered structures [27,28]. In the Raman spectrum of two samples are all contains both D and G bands (at 1580 cm 1 and 1330 cm 1, respectively). However, the D/G intensity ratio (ID/IG=0.97) of the Zn2SnO4 nanoflowers/GNS is larger than that of the asprepared composites before annealing (ID/IG=0.88). The results are in agreement with previous reports on that extensive oxidation and rapid thermal exfoliation have induced a substantial decrease of the size of the in-plane sp2 domains [29]. The morphology of the Zn2SnO4 nanoflowers before and after graphene sheet encapsulation were examined by SEM and TEM. The SEM and TEM images of the Zn2SnO4 nanoflowers are shown in Fig. 3. From the SEM images (Fig. 3a,b), it can be observed that the samples are composed of flower-like structures in high yield, which is stacked by numerous rod-like tips protrude, presenting a specials 3D nanostructure. Further TEM observation (Fig. 3c,d) reveal that
the particles are flower-like in shape and nearly monodisperse in size with an average diameter of 500 nm, and the diameter of the rod-like tips is about 12.5 nm. The special nanostructures of Zn2SnO4 nanoflowers can facilitate the diffusion of electrolyte, meanwhile, the loose nanostructures can alleviate volume change during the alloying and dealloying reactions between Sn and Li to improve the cycle performance. The microstructure of the Zn2SnO4 nanoflowers/GNS composites are shown in Fig. 4. As shown in images, Zn2SnO4 nanoflowers are successfully deposited onto the surfaces of graphene sheets. The SEM images (Fig. 4a,b) reveal that the average particle sizes of Zn2SnO4 nanoflowers in the Zn2SnO4 nanoflowers/GNS composites are smaller than that of the bare Zn2SnO4 nanoflowers. This result indicates that the nanoparticles deposited on graphene sheets can also prevent themselves from stacking into multilayers. The novel structure of the nanocomposites will possess good cycle performance as a potential anode material due to the large amount of void
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Fig. 4. SEM and TEM images of the Zn2SnO4 nanoflowers/GNS composites.
spaces of Zn2SnO4 nanoflowers and the flexible graphene sheets, resulting double buffering system to prevent the aggregation of these nanoparticles to a certain extent [30]. The electrochemical performance of the as-prepared Zn2SnO4 nanoflowers/GNS composites was evaluated by galvanostatic discharge–charge cycling at a current density of 300 mA g 1 for the first two cycles. For comparison, we also present the result of Zn2SnO4 nanoflowers prepared by the same procedure under the same electrochemical conditions which is shown in Fig. 5a. As can be seen, the capacities from the first discharge and charge cycles are 1750 and 880 mAh g 1 for the Zn2SnO4 nanoflowers electrode, while 1967 and 1087 mAh g 1 for the layered Zn2SnO4 nanoflowers/GNS composites electrode, respectively. These initial capacity losses could come from incomplete conversion reaction (Eqs. (1) and (2)) and the irreversible loss of lithium ions due to the formation of a solid electrolyte interphase layer (SEI). The Zn2SnO4 nanoflowers/GNS composites electrode shows high capacity, and delivers a better coulombic efficiency (55.3%)
compare to that of Zn2SnO4 nanoflowers electrode (50.3%). These results demonstrate the synergistic effect between Zn2SnO4 nanoflowers and graphene sheets can improve the electrochemical performance of this hybrid composites electrode. The lithium-ion insertion/extraction reactions of the Zn2SnO4 nanoflowers/GNS composites electrode were investigated by CV experiments. Fig. 5b shows representative CV curves of the composites electrode conducted over voltages between 3.0 and 0.0 V at a scanning rate of 0.5 mV s 1. It can be seen that the cathodic peak in the first cycle is observed at 0.10 V, which can be attributed to the formation of the SEI layer [31], two other cathodic peaks are observed at 0.25 and 0.82 V, corresponding to the delithiation process (Eqs. (1) and (2)). Meanwhile, two main anodic peaks are located near 1.32 and 0.56 V vs Li þ /Li, corresponding to the delithiation process (Eqs. (3) and (4)). Beyond those, 0.35 V can be illustrated with Li Zn alloy formation, according to Wang et al. [32]. In the following cycles, there is no substantial
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Fig. 5. (a) Discharge/charge voltage profiles of the Zn2SnO4 nanoflowers/GNS composites and Zn2SnO4 nanoflowers for the first cycle; (b) cyclic voltammetry of the Zn2SnO4 nanoflowers/GNS composites between 0.01 and 2.5 V at a scan rate of 0.2 mV s 1; (c) comparative cycling performance of the Zn2SnO4 nanoflowers/ GNS composites with Zn2SnO4 nanoflowers, these tests are conducted at a current density of 300 mA g 1 between 0.01 and 2.0 V; (d) rate capability of the Zn2SnO4 nanoflowers/GNS composites which is obtained between 0.01 and 2.0 V at various current densities.
change in the peak potentials and curve shape, the negative electrode peaks are slightly moved to the right, while anodic peaks shift positively, which could be attributed to the alloying and de-alloying process between Li and Sn. The currents and the integrated areas of the two peaks decreased substantially with cycling, indicating capacity loss due to the irreversibility of the electrochemical reaction. 4Li þ þ Zn2SnO4 þ 4e -Sn þ 2Li2O þ 2ZnO þ
(1)
8Li þ Zn2SnO4 þ 8e -2Zn þ Sn þ 4Li2O
(2)
xLi þ þ Sn þ xe 2LixSn (0 rx r 4.4)
(3)
yLi þ þ Zn þ ye 2LiyZn (y r 1)
(4)
In order to evaluate the cycling performance and coulombic efficiency of the Zn2SnO4 nanoflowers/GNS composites electrode, cycling performance and rate capability tests were carried out. As shown in Fig. 5c, the Zn2SnO4 nanoflowers/
GNS composites electrode exhibit enhanced lithium storage capacity with a high charge capacity over 540 mAh g 1 at a current density of 300 mA g 1 after 50 cycles, while the Zn2SnO4 nanoflowers retain only 501 mAh g 1. It is noteworthy that the coulombic efficiency of the Zn2SnO4 nanoflowers/GNS composites electrode reaches up 91.4% in the second cycle and 98% in the tenth cycle and then is maintained as high as 99% afterwards. The significant improvement of battery performance with the Zn2SnO4 nanoflowers/GNS composites electrode should be attributed to the distinctive synergetic effects of Zn2SnO4 nanoflowers and conductive graphene network. Fig. 5d shows the rate capability of Zn2SnO4 nanoflowers/ GNS composites which is obtained between 0.01 and 2.0 V at various current densities of 300, 600, 800, 1200, and the high current densities of 2500 mA g 1. Before 10 cycles at a current density of 300 mA g 1, the reversible capacity fading of the Zn2SnO4 nanoflowers/GNS composites is average 4%
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graphene can improve the electronic contact among the active particles. Therefore, the fact confirms that the incorporation of graphene can not only preserve the high conductivity of the Zn2SnO4 nanoflowers/GNS composites, but also greatly enhance rapid electron transport during electrochemical lithium insertion/extraction process, resulting in significant improvement in the electrochemical performances of the Zn2SnO4 nanoflowers/GNS composites hybrid. 4. Conclusions
Fig. 6. EIS of the Zn2SnO4 nanoflowers/GNS composites and Zn2SnO4 nanoflowers before cycling (0.01–100 kHz), including the equivalent circuit model of the studied system.
per cycle, and after 10 cycles, a discharge capacity over 850 mAh g 1 is still maintained. After 50 cycles at a current density of 800 mA g 1, the Zn2SnO4 nanoflowers/GNS composites keep a reversible discharge capacity of over 500 mAh g 1. Even at high current densities of 2500 mA g 1, a charge capacity over 350 mAh g 1 can be still kept, indicating the high-rate cycling stability of the Zn2SnO4 nanoflowers/GNS composites. The improved electrochemical performance of the Zn2SnO4 nanoflowers/GNS composites can be attributed to the following two points. Firstly, the special nanostructures of the Zn2SnO4 nanoflowers can alleviate the pulverization problem and enhance the cycling performance. In addition, graphene provides a highly conductive medium for electron transfer during the lithiation and de-lithiation process [33]. This class of composites may hold great promise for the development of high-performance lithiumion batteries. To further prove the good electrochemical performance of the prepared the Zn2SnO4 nanoflowers/GNS composites, EIS measurements were conducted on different samples. Fig. 6 shows the EIS profiles of the Zn2SnO4 nanoflowers and the Zn2SnO4 nanoflowers/GNS composites. The impedance curves consist of one semicircle in the medium frequency region and an inclined line in the low-frequency region. Both the impedance plots can be fitted by the equivalent circuit diagram. In the equivalent circuit diagram, Rs is the electrolyte resistance, Rf is the SEI resistance, W is the Warburg impedance related to the diffusion of lithium-ions into the bulk of the electrode materials, CPE1 and CPE2 are two constant phase elements associated with the interfacial resistance and charge-transfer resistance, respectively. Rct is the charge-transfer resistance, which can be calculated by the diameter of the semicircle [34,35]. It can be observed, the charge-transfer resistance of cell with Zn2SnO4 nanoflowers/ GNS composites was 120 Ω which was obviously lower than the cell with Zn2SnO4 nanoflowers (180 Ω). These results indicate a smaller electrochemical reaction resistance because
In summary, the Zn2SnO4 nanoflowers/GNS composites were fabricated by a simple hydrothermal method, and the composites suggest the structural compatibility between the layered structural Zn2SnO4 nanoflowers and graphene. When tested for lithium storage properties, the Zn2SnO4 nanoflowers/ GNS composites show significantly improved cycling performance compared to the Zn2SnO4 nanoflowers, retaining a capacity of 540 mAh g 1 after 50 cycles at the current density of 300 mA g 1. Even at high current densities of 2500 mAh g 1, the Zn2SnO4 nanoflowers/GNS composites show the high-rate cycling stability as well. The impressive electrochemical performance may be attributed to that the double buffering matrix and synergistic interactions formed by graphene and special nanostructures of the Zn2SnO4 nanoflowers. Acknowledgments This work was supported by the Spaceflight Foundation of the People's Republic of China under Grant no. NBXW0001, the Research Fund for the Doctoral Program of Higher Education of China under Grant no. 20136102110046, the Innovation Foundation of Shanghai Aerospace Science and Technology Grant no. SAST201373, the Graduate Starting Seed Fund of Northwestern Polytechnical University No. Z2014623, and the Basic Research Foundation of Northwestern Polytechnical University under Grant no. JC201269. References [1] F. Croce, M.L. Focarete, J. Hassoun, I. Meschini, B. Scrosati, A safe, high-rate and high-energy polymer lithium-ion battery based on gelled membranes prepared by electrospinning, Energy Environ. Sci. 4 (2011) 921. [2] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [3] M.S. Whittingham, Materials challenges facing electrical energy storage, MRS Bull. 33 (2008) 411–419. [4] Q. Wang, Z.H. Wen, J.H. Li, Fast and reversible lithium-induced electrochemical alloying in tin-based composite oxide hierarchical microspheres assembled by nanoplate building blocks, J. Power Sources 182 (2008) 334. [5] Y.F. Li, Y. Wang, C.Y. Chen, A.Y. Pang, M.D. Wei, Incorporating Zn2SnO4 quantum dots and aggregates for enhanced performance in dyesensitized ZnO solar cells, Chem.—A Eur. J. 18 (2012) 11716–11722. [6] X.Z. Zheng, Y.F. Li, Y.X. Xu, Z.S. Hong, M.D. Wei, Metal-organic frameworks: promising materials for enhancing electrochemical properties of nanostructured Zn2SnO4 anode in Li-ion batteries, CrystEngComm 14 (2012) 2112.
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Ceramics International 40 (2014) 15183–15190 www.elsevier.com/locate/ceramint
Graphene supported Zn2SnO4 nanoflowers with superior electrochemical performance as lithium-ion battery anode Ke Wang, Ying Huangn, Yuanyuan Shen, Lele Xue, Haijian Huang, Haiwei Wu, Yanli Wang Department of Applied Chemistry, The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi'an 710072, PR China Received 30 May 2014; received in revised form 6 June 2014; accepted 30 June 2014 Available online 7 July 2014
Abstract A hydrothermal synthesis approach has been developed to distribute the Zn2SnO4 nanoflowers on the graphene sheets (GNS). The as-prepared Zn2SnO4 nanoflowers/GNS composites were characterized by XRD, BET, FTIR, Raman, TGA, SEM, TEM and electrochemical measurements. The results show that the Zn2SnO4 nanoflowers have particular 3-D structure and homogeneously adhere on graphene sheets. Electrochemical measurements suggest that Zn2SnO4 nanoflowers/GNS composites exhibit better cycling properties and lower initial irreversible capacities as anode materials for lithium-ion batteries. Galvanostatic cycling shows 1967 mAh g 1 of initial discharge capacity and 1087 mAh g 1 of initial charge capacity. A higher reversible capacity of 850 mAh g 1 is obtained after 10 cycles at a current density of 300 mA g 1. The higher reversible capacity and good stability can be ascribed to the presence of graphene. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Zn2SnO4 nanoflowers/GNS composites; Hydrothermal synthesis; Lithium-ion batteries; Electrochemical properties
1. Introduction Lithium-ion batteries have dominated the battery industry for the past several years in portable electronic devices due to their high volumetric and gravimetric energy densities (650 Wh L 1 and 210 Wh kg 1) [1]. However, the theoretical capacity of graphite is not high enough to meet the demands for battery in small-scale applications translates to large-scale applications [2,3]. Increasing efforts have been diverted to the exploration of new anode materials with higher theoretical capacity. In recent years, major research accomplishments are the proposal of Snbased materials [4,5], and which have been deemed as promising negative electrode materials for LIBs. As a kind of tin–zinc composite oxide, Zn2SnO4 attracts researchers' more attention since it has an inverse spinel structure, high electron mobility, high electrical conductivity and low visible absorption. These outstanding properties enable its application in lithium-ion n
Corresponding author. Tel.: þ86 29 88431636. E-mail addresses: [email protected] (K. Wang), [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.ceramint.2014.06.133 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
battery anode [6]. The practical usage of Zn2SnO4 anodes is, however, hindered by their rapid capacity fade due to the large volume changes upon Li-insertion/extraction processes as anode material (about 300% volume change between the fully lithiated state, Li4.4Sn, and the non-lithiated state, Sn) [7,8]. There were considerable approaches proposed to accommodate the large volume changes and alleviate the Sn-based particle aggregation in order to get better cycling performance [9–11]. One strategy is to design the nanostructure of electrode materials [12,13]. As known, Zn2SnO4 micro/nanocrystals have been synthesized in various morphologies such as spherical, cubeshaped, or rod-like nanocrystals [14–16]. Another approach is to load the nanoparticles onto a matrix that is able to accommodate these volume changes during the alloying process [17,18]. Recently, graphene with superior electrical conductivities, high surface area of over 2600 m2 g 1, excellent thermal property and mechanical property, has attracted much attention in the field of materials science [19–22], which is more attractive as a matrix to support Zn2SnO4 nanoparticles. The flexible graphene not only buffers the volume changes during Li-insertion/extraction processes but also prevents the aggregation of the nanoparticles upon
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long-term cycling due to its large specific surface area [23] and high mechanical strength [24]. In this work, graphene framework was combined to improve the cycling stability of Zn2SnO4 nanoflowers electrodes. The graphene framework and the multidimensional nanostructure of Zn2SnO4 create a double buffer structure for Zn2SnO4, and provide additional mechanical strength to prevent the crack and pulverization of the electrode structure during repeated volume expansion and contraction. The Zn2SnO4 nanoflowers/GNS composites show the superior electrochemical performance. 2. Experimental 2.1. Sample preparation All of the reagents were of analytical grade and were used without any further purification. Graphite oxide (GO) was first
synthesized with a modified Hummers' method. Zinc chloride (ZnCl2) and tin chloride dihydrate (SnCl2 2H2O) were used as precursors and NaOH as the mineralizer for the hydrothermal synthesis of Zn2SnO4 nanoflowers. The Zn2SnO4 nanoflowers grew onto the graphene sheets by a hydrothermal process. The process began with dissolving SnCl2 2H2O (1.35 g), ZnCl2 (0.82 g) and GO (0.240 g) into 30 mL ethanol separately to form three well-distributed solutions. Then GO solution and 40 mL NaOH (1 M) solution were successive added little by little facilitated by magnetic stirring into the tin chloride dihydrate transparent solution. Zinc chloride solution was added drop by drop into above mixed solution under vigorous stirring using magnetic stirrer at room temperature until the formation of precipitate of the hybrid complex. Finally, the hybrid complex was transferred into a 200 mL Teflon-lined stainless autoclave with a fill factor of approximately 70%. The autoclave was sealed and maintained in a furnace at 220 1C for
Fig. 1. (a)XRD patterns of Zn2SnO4 nanoflowers and Zn2SnO4 nanoflowers/GNS composites; (b) adsorption/desorption isotherms of the Zn2SnO4 nanoflowers/ GNS composites and Zn2SnO4 nanoflowers; (c) TGA curve of the Zn2SnO4 nanoflowers/GNS composites; and (d) FTIR spectrum of the Zn2SnO4 nanoflowers/ GNS composites.
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24 h. Then, the product was separated and washed for several times with ultrapure water and ethanol after the hydrothermal reaction was terminated. After these, the product was dried under vacuum at 60 1C for 12 h to obtain the precursor. Finally, the precursor was sintered at 800 1C for 4 h under argon atmosphere, leading to the reduction of GO to obtain Zn2SnO4 nanoflowers/GNS composites. The bare Zn2SnO4 nanoflowers were synthesized under the same condition without the addition of graphene oxide for comparison. 2.2. Materials characterization The structures of the prepared samples were characterized by X-ray diffraction analysis (XRD) (Rigaku, model D/max2500 system at 40 kV and 100 mA of Cu Kα). The Raman spectras of the composite samples were obtained by using a Laser-Raman spectrometer (Renishaw Co., England) with a 514 nm radiation. The surface morphology of the composites was performed by a model Tecnai F30 G2 (FEI Co., USA) field emission transmission electron microscope (FETEM) and scanning electron microscope (SEM, SuPRA 55, German ZEISS). The Fourier transform infrared spectroscopy (FTIR) spectras were obtained by using a Model NIcolETiS10 Fourier transform spectrometer (Thermo SCI-ENTIFIC Co., USA) with a 2 cm 1 resolution in the range of 500–4000 cm 1. Electrochemical performance was evaluated by a CR2016type coin cell with a multi-channel current static system Land (LAND CT200IA). The anode electrodes were prepared by coating slurries consisting of Zn2SnO4 nanoflowers/GNS composites (65 wt%) with acetylene black (15 wt%) and PVDF (20 wt%) as a binder dissolved in 1-methyl-2pyrrolidinone (NMP) solution on a copper foil. Li foil was used as a counter electrode, and polypropylene (PP) film (Celgard 2400) as the separator. The electrolyte was a solution of 1 M LiPF6 in a mixture of ethylene (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1, v/v/v).
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which also provide a high conductive medium for electron transfer. To quantify the mass percentage of Zn2SnO4, Fig. 1(c) displays the TGA curves of the Zn2SnO4 nanoflowers/GNS composites that have been carried out in air. The first weight loss in the temperature between 30 1C and 140 1C is attributed to the release of the absorbed water. The rapid mass loss occurs between 200 1C and 650 1C due to the oxidation and decomposition of graphene. Therefore, according to the change of weight, it is estimated that the amount of the Zn2SnO4 in the composites is 83 wt%. Fig. 1(d) shows the Fourier transform infrared spectroscopy (FTIR) spectrum of the Zn2SnO4 nanoflowers/GNS composites. The vibration at about 520 cm 1 is seen in the low wave number region corresponding to the Sn–O vibration mode of Zn2SnO4, the peak at 3414 cm 1 can be attributed to the O–H stretching vibrations of the structural –OH groups [25]. The absorption at 1616 cm 1 can be assigned to the skeletal vibration of graphene sheets [26]. The presence of carboxyl can also be detected at around 1122 cm 1. Here the absorption of the oxygen-containing groups at the edges and on the basal planes of graphene sheets owing to the oxidation process can be partly removed during thermal expansion. Fig. 2 presents the Raman spectra of the Zn2SnO4 nanoflowers/GNS composites and the as-prepared composites before annealing. The peak at about 1592 cm 1 (G band) is
3. Results and discussion Fig. 1(a) shows the XRD patterns of the as-prepared Zn2SnO4 nanoflowers and the Zn2SnO4 nanoflowers/GNS composites. In the XRD curve of Zn2SnO4 nanoflowers (i), all main diffraction peaks of the sample are consistent with the (JCPDS card no. 24-1470) data of the pure reverse-spinel Zn2SnO4. The major diffraction peaks of Zn2SnO4 nanoflowers/GNS composites (ii) can be indexed to those of Zn2SnO4 (JCPDS card no. 24-1470). Furthermore, the characteristic diffraction peak of the GNS (n002) confirms the presence of graphene in the composite. The results indicate that growth of the Zn2SnO4 nanoflowers are inhibited in the hybrid due to the intervention of graphene. Fig. 1(b) displays the N2 adsorption/desorption isotherms of the Zn2SnO4 nanoflowers/GNS composites and Zn2SnO4 nanoflowers. Indeed, the BET surface area of the composites increases from 40.56 to 481.29 m2 g 1after graphene loading. The presence of graphene is the key reason for having large surface area of the Zn2SnO4 nanoflowers/GNS composites, and
Fig. 2. Raman spectras of Zn2SnO4 nanoflowers/GNS composites and the as-prepared composites before annealing.
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Fig. 3. SEM and TEM images of the Zn2SnO4 nanoflowers.
related to the vibration of sp2-bonded carbon atoms in a 2-dimensional hexagonal lattice, while the 1334 cm 1 peak (D band) is ascribed to edge planes and disordered structures [27,28]. In the Raman spectrum of two samples are all contains both D and G bands (at 1580 cm 1 and 1330 cm 1, respectively). However, the D/G intensity ratio (ID/IG=0.97) of the Zn2SnO4 nanoflowers/GNS is larger than that of the asprepared composites before annealing (ID/IG=0.88). The results are in agreement with previous reports on that extensive oxidation and rapid thermal exfoliation have induced a substantial decrease of the size of the in-plane sp2 domains [29]. The morphology of the Zn2SnO4 nanoflowers before and after graphene sheet encapsulation were examined by SEM and TEM. The SEM and TEM images of the Zn2SnO4 nanoflowers are shown in Fig. 3. From the SEM images (Fig. 3a,b), it can be observed that the samples are composed of flower-like structures in high yield, which is stacked by numerous rod-like tips protrude, presenting a specials 3D nanostructure. Further TEM observation (Fig. 3c,d) reveal that
the particles are flower-like in shape and nearly monodisperse in size with an average diameter of 500 nm, and the diameter of the rod-like tips is about 12.5 nm. The special nanostructures of Zn2SnO4 nanoflowers can facilitate the diffusion of electrolyte, meanwhile, the loose nanostructures can alleviate volume change during the alloying and dealloying reactions between Sn and Li to improve the cycle performance. The microstructure of the Zn2SnO4 nanoflowers/GNS composites are shown in Fig. 4. As shown in images, Zn2SnO4 nanoflowers are successfully deposited onto the surfaces of graphene sheets. The SEM images (Fig. 4a,b) reveal that the average particle sizes of Zn2SnO4 nanoflowers in the Zn2SnO4 nanoflowers/GNS composites are smaller than that of the bare Zn2SnO4 nanoflowers. This result indicates that the nanoparticles deposited on graphene sheets can also prevent themselves from stacking into multilayers. The novel structure of the nanocomposites will possess good cycle performance as a potential anode material due to the large amount of void
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Fig. 4. SEM and TEM images of the Zn2SnO4 nanoflowers/GNS composites.
spaces of Zn2SnO4 nanoflowers and the flexible graphene sheets, resulting double buffering system to prevent the aggregation of these nanoparticles to a certain extent [30]. The electrochemical performance of the as-prepared Zn2SnO4 nanoflowers/GNS composites was evaluated by galvanostatic discharge–charge cycling at a current density of 300 mA g 1 for the first two cycles. For comparison, we also present the result of Zn2SnO4 nanoflowers prepared by the same procedure under the same electrochemical conditions which is shown in Fig. 5a. As can be seen, the capacities from the first discharge and charge cycles are 1750 and 880 mAh g 1 for the Zn2SnO4 nanoflowers electrode, while 1967 and 1087 mAh g 1 for the layered Zn2SnO4 nanoflowers/GNS composites electrode, respectively. These initial capacity losses could come from incomplete conversion reaction (Eqs. (1) and (2)) and the irreversible loss of lithium ions due to the formation of a solid electrolyte interphase layer (SEI). The Zn2SnO4 nanoflowers/GNS composites electrode shows high capacity, and delivers a better coulombic efficiency (55.3%)
compare to that of Zn2SnO4 nanoflowers electrode (50.3%). These results demonstrate the synergistic effect between Zn2SnO4 nanoflowers and graphene sheets can improve the electrochemical performance of this hybrid composites electrode. The lithium-ion insertion/extraction reactions of the Zn2SnO4 nanoflowers/GNS composites electrode were investigated by CV experiments. Fig. 5b shows representative CV curves of the composites electrode conducted over voltages between 3.0 and 0.0 V at a scanning rate of 0.5 mV s 1. It can be seen that the cathodic peak in the first cycle is observed at 0.10 V, which can be attributed to the formation of the SEI layer [31], two other cathodic peaks are observed at 0.25 and 0.82 V, corresponding to the delithiation process (Eqs. (1) and (2)). Meanwhile, two main anodic peaks are located near 1.32 and 0.56 V vs Li þ /Li, corresponding to the delithiation process (Eqs. (3) and (4)). Beyond those, 0.35 V can be illustrated with Li Zn alloy formation, according to Wang et al. [32]. In the following cycles, there is no substantial
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Fig. 5. (a) Discharge/charge voltage profiles of the Zn2SnO4 nanoflowers/GNS composites and Zn2SnO4 nanoflowers for the first cycle; (b) cyclic voltammetry of the Zn2SnO4 nanoflowers/GNS composites between 0.01 and 2.5 V at a scan rate of 0.2 mV s 1; (c) comparative cycling performance of the Zn2SnO4 nanoflowers/ GNS composites with Zn2SnO4 nanoflowers, these tests are conducted at a current density of 300 mA g 1 between 0.01 and 2.0 V; (d) rate capability of the Zn2SnO4 nanoflowers/GNS composites which is obtained between 0.01 and 2.0 V at various current densities.
change in the peak potentials and curve shape, the negative electrode peaks are slightly moved to the right, while anodic peaks shift positively, which could be attributed to the alloying and de-alloying process between Li and Sn. The currents and the integrated areas of the two peaks decreased substantially with cycling, indicating capacity loss due to the irreversibility of the electrochemical reaction. 4Li þ þ Zn2SnO4 þ 4e -Sn þ 2Li2O þ 2ZnO þ
(1)
8Li þ Zn2SnO4 þ 8e -2Zn þ Sn þ 4Li2O
(2)
xLi þ þ Sn þ xe 2LixSn (0 rx r 4.4)
(3)
yLi þ þ Zn þ ye 2LiyZn (y r 1)
(4)
In order to evaluate the cycling performance and coulombic efficiency of the Zn2SnO4 nanoflowers/GNS composites electrode, cycling performance and rate capability tests were carried out. As shown in Fig. 5c, the Zn2SnO4 nanoflowers/
GNS composites electrode exhibit enhanced lithium storage capacity with a high charge capacity over 540 mAh g 1 at a current density of 300 mA g 1 after 50 cycles, while the Zn2SnO4 nanoflowers retain only 501 mAh g 1. It is noteworthy that the coulombic efficiency of the Zn2SnO4 nanoflowers/GNS composites electrode reaches up 91.4% in the second cycle and 98% in the tenth cycle and then is maintained as high as 99% afterwards. The significant improvement of battery performance with the Zn2SnO4 nanoflowers/GNS composites electrode should be attributed to the distinctive synergetic effects of Zn2SnO4 nanoflowers and conductive graphene network. Fig. 5d shows the rate capability of Zn2SnO4 nanoflowers/ GNS composites which is obtained between 0.01 and 2.0 V at various current densities of 300, 600, 800, 1200, and the high current densities of 2500 mA g 1. Before 10 cycles at a current density of 300 mA g 1, the reversible capacity fading of the Zn2SnO4 nanoflowers/GNS composites is average 4%
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graphene can improve the electronic contact among the active particles. Therefore, the fact confirms that the incorporation of graphene can not only preserve the high conductivity of the Zn2SnO4 nanoflowers/GNS composites, but also greatly enhance rapid electron transport during electrochemical lithium insertion/extraction process, resulting in significant improvement in the electrochemical performances of the Zn2SnO4 nanoflowers/GNS composites hybrid. 4. Conclusions
Fig. 6. EIS of the Zn2SnO4 nanoflowers/GNS composites and Zn2SnO4 nanoflowers before cycling (0.01–100 kHz), including the equivalent circuit model of the studied system.
per cycle, and after 10 cycles, a discharge capacity over 850 mAh g 1 is still maintained. After 50 cycles at a current density of 800 mA g 1, the Zn2SnO4 nanoflowers/GNS composites keep a reversible discharge capacity of over 500 mAh g 1. Even at high current densities of 2500 mA g 1, a charge capacity over 350 mAh g 1 can be still kept, indicating the high-rate cycling stability of the Zn2SnO4 nanoflowers/GNS composites. The improved electrochemical performance of the Zn2SnO4 nanoflowers/GNS composites can be attributed to the following two points. Firstly, the special nanostructures of the Zn2SnO4 nanoflowers can alleviate the pulverization problem and enhance the cycling performance. In addition, graphene provides a highly conductive medium for electron transfer during the lithiation and de-lithiation process [33]. This class of composites may hold great promise for the development of high-performance lithiumion batteries. To further prove the good electrochemical performance of the prepared the Zn2SnO4 nanoflowers/GNS composites, EIS measurements were conducted on different samples. Fig. 6 shows the EIS profiles of the Zn2SnO4 nanoflowers and the Zn2SnO4 nanoflowers/GNS composites. The impedance curves consist of one semicircle in the medium frequency region and an inclined line in the low-frequency region. Both the impedance plots can be fitted by the equivalent circuit diagram. In the equivalent circuit diagram, Rs is the electrolyte resistance, Rf is the SEI resistance, W is the Warburg impedance related to the diffusion of lithium-ions into the bulk of the electrode materials, CPE1 and CPE2 are two constant phase elements associated with the interfacial resistance and charge-transfer resistance, respectively. Rct is the charge-transfer resistance, which can be calculated by the diameter of the semicircle [34,35]. It can be observed, the charge-transfer resistance of cell with Zn2SnO4 nanoflowers/ GNS composites was 120 Ω which was obviously lower than the cell with Zn2SnO4 nanoflowers (180 Ω). These results indicate a smaller electrochemical reaction resistance because
In summary, the Zn2SnO4 nanoflowers/GNS composites were fabricated by a simple hydrothermal method, and the composites suggest the structural compatibility between the layered structural Zn2SnO4 nanoflowers and graphene. When tested for lithium storage properties, the Zn2SnO4 nanoflowers/ GNS composites show significantly improved cycling performance compared to the Zn2SnO4 nanoflowers, retaining a capacity of 540 mAh g 1 after 50 cycles at the current density of 300 mA g 1. Even at high current densities of 2500 mAh g 1, the Zn2SnO4 nanoflowers/GNS composites show the high-rate cycling stability as well. The impressive electrochemical performance may be attributed to that the double buffering matrix and synergistic interactions formed by graphene and special nanostructures of the Zn2SnO4 nanoflowers. Acknowledgments This work was supported by the Spaceflight Foundation of the People's Republic of China under Grant no. NBXW0001, the Research Fund for the Doctoral Program of Higher Education of China under Grant no. 20136102110046, the Innovation Foundation of Shanghai Aerospace Science and Technology Grant no. SAST201373, the Graduate Starting Seed Fund of Northwestern Polytechnical University No. Z2014623, and the Basic Research Foundation of Northwestern Polytechnical University under Grant no. JC201269. References [1] F. Croce, M.L. Focarete, J. Hassoun, I. Meschini, B. Scrosati, A safe, high-rate and high-energy polymer lithium-ion battery based on gelled membranes prepared by electrospinning, Energy Environ. Sci. 4 (2011) 921. [2] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [3] M.S. Whittingham, Materials challenges facing electrical energy storage, MRS Bull. 33 (2008) 411–419. [4] Q. Wang, Z.H. Wen, J.H. Li, Fast and reversible lithium-induced electrochemical alloying in tin-based composite oxide hierarchical microspheres assembled by nanoplate building blocks, J. Power Sources 182 (2008) 334. [5] Y.F. Li, Y. Wang, C.Y. Chen, A.Y. Pang, M.D. Wei, Incorporating Zn2SnO4 quantum dots and aggregates for enhanced performance in dyesensitized ZnO solar cells, Chem.—A Eur. J. 18 (2012) 11716–11722. [6] X.Z. Zheng, Y.F. Li, Y.X. Xu, Z.S. Hong, M.D. Wei, Metal-organic frameworks: promising materials for enhancing electrochemical properties of nanostructured Zn2SnO4 anode in Li-ion batteries, CrystEngComm 14 (2012) 2112.
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