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modified graphite composite material as anode of lithium ion battery
Materials Chemistry and Physics 167 (2015) 303e308
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electrochemical performance of SnO2/modified graphite composite material as anode of lithium ion battery Hong-Qiang Wang a, b, Guan-Hua Yang a, You-Guo Huang a, Xiao-Hui Zhang a, Zhi-Xiong Yan a, Qing-Yu Li a, * a
Guangxi Key Laboratory of Low Carbon Energy Materials, School of Chemical and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, China Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang 438000, China
b
h i g h l i g h t s A simple synthetic method of SnO2/modified graphite composite as anode. The as-prepared composite with layered structure alleviates the huge reunion of SnO2. The composite exhibits a good capacity retention rate of 85.8% after 25 cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 September 2014 Received in revised form 1 September 2015 Accepted 18 October 2015 Available online 26 October 2015
In this report, we synthesized SnO2/modified graphite anode composite material by a simple reflux method using SnCl4$5H2O as tin source and modified graphite as carbon source. The as-obtained composite was investigated with the help of X-ray diffraction (XRD), scanning electron microscopy (SEM) and galvanostatic cycling tests. The results show that the composite has a wave-shaped fold structure and the SnO2 nanoparticles on it have an average size of about 50 nm. Compared to pure modified graphite, the SnO2/modified graphite exhibits a better electrochemical performance with a reversible specific capacity of 581.7 mAh g1 after 80 cycles, owing to high mechanical stress and elasticity of modified graphite could hinder the volume effect of SnO2 nanoparticles during the Liþ insertion/extraction process. All these favourable characters reveal that the composite is a great potential anode material in highperformance lithium ion batteries. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Nanostructures Electron microscopy Electrochemical properties
1. Introduction Lithium-ion batteries (LIBs) as energy storage device of power sources, have been widely used in portable electronic devices, electric bicycles, electric vehicles and hybrid vehicles [1e4]. With the development of the society and the progress of technology, the applications of LIBs are increasingly extended. Anode materials as part of the battery, have a pivotal role on the performance of the batteries and have been put forward further requirement for them in LIBs [5e7]. Graphite, as an important anode material, has been extensively used in commercial lithium-ion batteries. However, the theoretical capacity of it is only 372 mAh g1, which can hardly
* Corresponding author. E-mail address: [email protected] (Q.-Y. Li). http://dx.doi.org/10.1016/j.matchemphys.2015.10.048 0254-0584/© 2015 Elsevier B.V. All rights reserved.
meet the needs of high-performance LIBs. In order to obtain highperformance LIBs, numerous efforts are trying to find alternative anode materials. Among the alternative anode materials, SnO2 has attracted great attention from researchers due to its high storage capacity and low cost. The theoretical specific capacity of SnO2 is 782 mAh g1, which is more than twice of the graphite [8,9]. However, SnO2 has large volume expansion effect in the process of insertion and extraction of lithium. It is easy to cause the electrode material pulverization and exfoliation from the current collector, which lead to the rapid decrease of battery capacity and poor cycle performance, eventually hinder its application in actual [10e13]. In order to overcome these shortcomings, an effective strategy is the introduction of an active or inert element that acts as a buffer to accommodate the large volume change of SnO2 during cycling. It is an effective approach to alleviate the volume change of SnO2 by forming a composite material of SnO2 and carbon, which can
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improve the capacity and cycle performance of anode material [14e17]. Introduction of carbon not only buffers the volume change of SnO2, but also strengthens the SnO2 electrical contact and improves the utilization of the active material. In addition, carbon can also provide some specific capacity for the electrode. Graphene, with excellent electrical conductivity and mechanical properties, is a new type of carbon material, which is an excellent choice for SnO2 modification [18e24]. And some studies of SnO2/graphene composite as anode materials have been reported. Fabrice M. Courtel et al. prepared nano-SnO2/carbon composites with small particle sizes (5 nm) in situ via the polyol method using microwave-assisted heating method [25]. Zhifeng Du et al. utilized a in situ chemical synthesis method to deposit nano-SnO2 on the surface of graphene [26]. Chaofeng Zhang et al. used hydrothermal method to synthesize carbon-coated graphene/tin oxide composites [27]. However, these anode materials preparation process is complicated, timeconsuming, or using toxic solvents, which limit the large-scale application of them. In this paper, SnO2/modified graphite composite is prepared by a simple reflux in an aqueous solution. SnO2 particles in the composite material are approximately 50 nm and distribute evenly in the modified graphite layers. The electrode material deterioration due to the huge volume change of SnO2 has been gained a certain inhibition and reversible of the battery is also improved. The specific capacity of 581.7 mAh g1 is obtained after 80 cycles. Thus, the as-prepared SnO2/modified graphite composite material is a promising anode material for lithium ion battery.
graphite composite and modified graphite were analysed by X-ray diffraction (XRD: Rigaku, D/max 2500v/pc), field emission scanning electron microscopy (FESEM, Philips, FEI Quanta 200 FEG), and transmission electron microscope (TEM, Tokyo, Japan, JEOL 2100F). The component of the sample was analysed by an energy dispersive spectroscope (EDS, INCA) attached to the SEM. Thermogravimetric analysis TG-DSC (SETARAM, LABSYS evo) was carried out in the temperature range from 30 to 1000 C at a scanning rate of 10 C min1 in dry air. 2.3. Electrochemical evaluation The working electrodes were prepared by mixing the asobtained composite, carbon black and polyvinylidene fluoride (PVDF) (85:10:5 in weight ratio) in N-methyl pyrrolidone (NMP). In this research, the loading amount of anode active materials in the electrodes SnO2/modified graphite composite is 0.8e1.0 mg/cm2. The as-obtained homogeneous slurry was coated on copper foil and dried at 80 C under vacuum for 12 h. The lithium foil was used as counter electrode and the electrolyte used was 1 M LiPF6 in a 1:1 (v/ v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Electrochemical measurements were carried out using CR2032 coin-type cells, which were assembled in an argon-filled glove-box. The galvanostatic chargeedischarge in the voltage ranging from 5 mV to 3.0 V was conducted with a Battery Tester (Land, Wuhan, China) at 25 C. Cyclic voltammetry (CV) (0.1 mVe3.0 V, 0.1 mV s1) was performed using an IM6 electrochemical workstation (Zahner, Germany).
2. Experiments 2.1. Preparation of SnO2/modified graphite composite Graphite oxide (GO) with layer structure was synthesized from flake graphite by a modified Hummer's method [28]. 1 g GO was dispersed in 100 ml of deionized water by sonication for 30 min, forming a stable GO suspension. Then, the graphite oxide was added to the three-neck round bottom flask, followed by this 2 g NaBH4 and 50 ml aqueous solution of SnCl4$5H2O (3.75 g) were slowly added into the above solution, and heated at 100 C under reflux for 8 h. Subsequently, the products were collected by filtration and washed with deionized water, dried at 80 C. In order to improve the crystallinity of SnO2 in composite materials, the products were annealed at 550 C for 3 h in N2 atmosphere. 2.2. Characterization The crystal structure and morphology of the SnO2/modified
3. Results and discussion Fig. 1a shows the X-ray diffraction patterns of the SnO2/modified graphite, bare SnO2, and modified graphite. The diffraction peaks of crystalline SnO2 nanoparticles in SnO2/modified graphite are clearly distinguishable. Peaks at 2q of 26.6 , 33.9 , 37.9 , 51.8 , 54.7, 61.9 , 65.9 , 71.3 , 78.7, 83.7 exhibit the major diffraction peaks at (110), (101), (200), (211), (220), (310), (301), (202), (321) and (222) of tetragonal crystalline SnO2 (JCPDS No.41-1445) [29,30]. In the XRD pattern of SnO2/modified graphite, no obvious carbon peaks can be found, which is due to the overlap of the main carbon (002) facet with SnO2 (110) facet. In addition, the diffraction peak of metal Sn cannot be observed either. It demonstrates that Sn is mainly in the form of SnO2 in the composite material at the low heat treatment temperature. The pure modified graphite displays a strong (002) diffraction peak and two weak (100), (110) diffraction peaks, suggesting the amorphous nature of modified graphite. XRD confirms the coexisting phases of SnO2 and modified graphite and the
Fig. 1. XRD pattern of SnO2/modified graphite, bare SnO2, and modified graphite (a), and TG/DTA behaviour of SnO2/modified graphite (b).
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Fig. 2. SEM image of modified graphite layers (a), and SEM image of SnO2/modified graphite composite, inset: partial enlargement of the composite (b), and TEM image of the SnO2/ modified graphite composite (c), and EDS surface scanning analysis image of SnO2/modified graphite composite (d), and low-magnification SEM image of SnO2/modified graphite (e) and corresponding EDS maps for C, O and Sn (feh).
formation of SnO2/modified graphite composite. In order to quantify the composition of modified graphite and SnO2, the as-prepared composite is analysed by TGA. As shown in
the TG/DTA curves (Fig. 1b), the weight loss between 30 and 250 C could be attributed mainly to the release of adsorbed water. The dramatic weight loss from 550 C to 650 C may be correlated to the
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Fig. 3. The chargeedischarge curve of SnO2/modified graphite composite (a), and the chargeedischarge curve of modified graphite for the 1st, 2nd, 20th cycle (b).
removal of modified graphite [13,31]. Therefore, according to the change of weight, the composition of the SnO2 and modified graphite in the as-prepared composite is determined to be about 31% and 47% by weight respectively. The morphologies of modified graphite layers and SnO2/modified graphite composite are characterized by field emission scanning electron microscopy, as shown in Fig. 2a and Fig. 2b, respectively. From Fig. 2a, it can be seen that the sheet structure of graphite has been opened after the oxidation process. The modified graphite has a layered sandwich structure, which can be used as carriers of SnO2 to prevent aggregation of SnO2. In addition, the modified graphite may work as a buffer, which relieves the volume expansion and contraction of SnO2 during the Liþ insertion/ extraction process. Fig. 2b and the inset show that SnO2 nanoparticles uniformly distribute in the matrix of the modified graphite and the modified graphite layers. The SnO2 particles firmly bond to the modified graphite layers, which is contributed to improve the conductivity and specific capacity of the material. In order to investigate the details of the microstructure in the composite, TEM observation is carried out, as shown in Fig. 2c. It shows that a lot of SnO2 particles, about 50 nm in size, homogeneously disperse in the matrix of the modified graphite. The results of EDS, as shown in Fig. 2d, indicate that the composite material contains O, Sn, C, and the content of each element is 40.82%, 36.66%, 22.52%, respectively. Based on the atomic number ratio of SnO2, the atomic ratio of Sn:O equals to 1:8.26, which is far lower than the ratio (1:2) of SnO2. This is attributed to the modified graphite layers could not be completely recovered by thermal reduction at 550 C. Further, the low-magnification SEM image and presence of individual elements of SnO2/modified graphite are shown in Fig. 2eeh. The elemental mapping of SnO2/modified graphite indicates the presence of C, O and Sn in the sample.
Fig. 5. Schematic illustration of synthesis of SnO2/modified graphite: (1) and (2) were the Hummer's method for preparation of modified graphite, (3) the dispersion of SnO2 on the modified graphite, (4) the carbonization of SnO2/modified graphite.
Substantial amounts of oxygen are also detected, further indicating that the oxygen in the composite has not been completely removed after the heat treatment. These maps reveal that SnO2 nanoparticles homogeneously distribute on the modified graphite [6]. To investigate the electrochemical performances of modified graphite and SnO2/modified graphite, chargeedischarge tests are implemented. Fig. 3 shows the 1st, 2nd, 20th chargeedischarge experimental results at the voltage range of 5 mVe3.0 V (vs. Li/Liþ) under room temperature. In Fig. 3a, SnO2/modified graphite
Fig. 4. The cycle performance of SnO2/modified graphite composite material and modified graphite with current density of 0.1 A g1 (a), and rate performance of SnO2/modified graphite composite at different chargeedischarge rates (b).
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Fig. 6. Cyclic voltammetry of the SnO2/modified graphite at a scanning rate of 0.1 mV s1 (a), and Nyquist plots of SnO2/modified graphite before and after 10 cycles (b).
electrode displays a initial discharge capacity of 1373.6 mAh g1 and a charge capacity of 784.1 mAh g1. The voltage drops rapidly from 2.5 V to 1.5 V during the discharge process, which is due to the reaction between Li and SnO2 and the generation of non-reactive Li2O. A platform appears gradually around 0.8 V and disappears in the subsequent cycle, which is mainly caused by the decomposition of electrolyte on the electrode surface to form a solid electrolyte interface (SEI). Owing to the irreversible formation of inactive Li2O and SEI, the coulomb efficiency is low (around 57.1%) in the first cycle. As the voltage tardily decreased, the second platform appears at approximately 0.1 V, which is corresponded to the formation of intercalation compounds causing by the intercalation of lithium ions into the modified graphite interlayer [19,32]. In addition, it can be seen that the chargeedischarge capacity of the battery is relatively consistent during cycles, revealing that performance of the electrode material tends to be stable. As a comparison, the chargeedischarge performances of modified graphite are investigated (as display in Fig. 3b). It shows that an intense voltage drop appear at about 0.75 V in the first discharge process and disappears in the subsequent cycle, which is considered to be correspond to the formation of SEI. The profile of 10 mVe0.5 V corresponds to the process of the lithium ions embedding and extraction out from the modified graphite layers. According to Fig. 3b, modified graphite displays a discharge capacity of 707.3 mAh g1 and a charge capacity of 324.5 mAh g1 in the first cycle. Thus, it demonstrates that the specific capacity of the composite due to the load of SnO2 nanoparticles is higher than the capacity of modified graphite. Fig. 4a shows the cyclic performances of SnO2/modified graphite composite material and modified graphite with current density of 0.1 A g1. Owing to the irreversible formation of amorphous Li2O matrix and decomposition of electrolyte on the electrode surface to formation of SEI, the first discharge capacity of SnO2/modified graphite decreases promptly. In the succedent chargeedischarge cycles, the SnO2/modified graphite exhibits good reversible capacity, which is mainly because the combination of the high mechanical stress and elasticity of modified graphite and Liþ reversibly insertion into and extraction out from Sn. Meanwhile, the modified graphite has a layered sandwich structure with undulating folds (as shown in Fig. 2a), which contributes to establish the reaction of solid/liquid interface and improves the electrochemical activity of lithium ions in the solid/liquid interface. Compared with modified graphite, the SnO2/modified graphite exhibits a higher specific capacity, which may be caused by SnO2 nanoparticles embedding into the layered sandwich structure of modified graphite or the gaps during preparing the composite material (as the schematic illustration shown in Fig. 5). After 80 cycles, the dischargeecharge capacity of composite is 589.6 mAh g1 and 581.7 mAh g1,
respectively, which is higher than the dischargeecharge capacity of modified graphite. It indicates that both SnO2 and modified graphite have good synergy, in which modified graphite acts as the buffer of SnO2 to mitigate volume effect of SnO2 in the process of dischargeecharge and the SnO2 increases the specific capacity of the composite. Fig. 4b shows the rate performance of the SnO2/modified graphite. The cell is first cycled at current density of 0.1 A g1, and then the current density is increased to 0.2 and 2 A g1, finally returned to 0.1 A g1, successively. As can be seen from it, the SnO2/ modified graphite displays an excellent rate capability. The SnO2/ modified graphite is still capable of delivering a substantial capacity of 901.5, 782.0, 639.7, 541.4, and 420.5 mAh g1 at various current densities of 0.1, 0.2, 0.5, 1, and 2 A g1, respectively. It should be noted that, when the current density is reversed back to 0.1 A g1 after 50 cycles, the specific charge capacity of 814.3 mAh g1 could be obtained, demonstrating that the SnO2/modified graphite could tolerate varied dischargeecharge rates. This good rate capability is attribute to the layered modified graphite work as a highly conductive matrix to anchor SnO2 nanoparticles and the SnO2 nanoparticles can shorten the transport path for both lithium ions and electrons [9,32]. In order to understand the electrochemical reactivity of the SnO2/modified graphite better, cyclic voltammetry is performed ranging from 1 mV to 3.0 V at a sweep rate of 0.1 mV s1 (Fig. 6a). In the first cycle, two peaks appear in the cathodic process, seating around 1.26 V and 0.01 V, which is associated with the alloying reaction between lithium and Sn and the formation of the LixC compounds, respectively. The peak at around 0.81 V is generally considered to be attributed to the reduction of SnO2 to Sn, the production of Li2O and the formation of SEI film, which result in the generation of the irreversible capacity. In the anodic process, the oxidation peak around 0.1 V is corresponded to the lithium extraction out from the modified graphite sheets. Peaks at about 0.60 V and 1.20 V, are related to the reaction of lithium extraction out from LixSn alloy [32,33]. Obviously, the anodic/cathodic peak almost remains unchanged after another two cyclic voltammetry, exhibiting that the composite has good redox reversibility, excellent electrical conductivity and good cycle performance. Fig. 6b shows the Nyquist plots of SnO2/modified graphite before and after 10 cycles in the 100 kHz to 10 mHz frequency range. It demonstrates the similar Nyquist plots with a semicircle in high frequency range and a sloping straight line in low frequency range. According to previous studies [22,34], the semicircle in the high-frequency is attributed to the formation of SEI film and contact resistance, the semicircle in medium-frequency region is associated to the charge-transfer impedance on and the inclined line at lower frequency represents the Warburg impedance (W),
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which corresponds to the Liþ diffusion in the electrode materials. It can be clearly seen that the charge transfer resistance of SnO2/ modified graphite electrode after 10 cycles (185 U) is smaller than the composite electrode before cycle (693 U). The result indicates that the presence of modified graphite with layered nanostructure can accommodate the volume change of SnO2 during cycles effectively to reduce the charge transfer impedance and increases the electronic conductivity of the composite. This phenomenon further confirms the result from CVs that the SnO2/modified graphite electrode exhibits good cycling performance. 4. Conclusions In summary, SnO2/modified graphite composite is prepared by a simple reflux in an aqueous solution. The experimental results exhibit that the SnO2/modified graphite composite is composed of uniformly distributed nanosized particles and can deliver a reversible capacity of 581.7 mAh g1, which is higher than the dischargeecharge capacity of modified graphite. Therefore, the results presented in this paper prove that SnO2/modified graphite composite is a good candidate for use as anode material in lithium ion battery. Acknowledgements This research was supported by NSF of China (51474110, 51364004 and 51064004), Guangxi Natural Science Foundation (2011GXNSFA018016). References [1] F.M. Hassan, Z. Chen, A. Yu, et al., Sn/SnO2 embedded in mesoporous carbon nanocomposites as negative electrode for lithium ion batteries, Electrochimica Acta 87 (2013) 844e852. [2] Fei Wang, Xiaoping Song, Gang Yao, et al., Carbon-coated mesoporous SnO2 nanospheres as anode material for lithium ion batteries, Scr. Mater. 66 (2012) 562e565. [3] Wenbo Yue, Sheng Yang, Yunling Liu, et al., A facile synthesis of mesoporous graphene-tin composites as high-performance anodes for lithium-ion batteries, Mater. Res. Bull. 48 (2013) 1575e1580. [4] Seung-Min Paek, EunJoo Yoo, Itaru Honma, Enhanced cyclic performance and lithium storage capacity of SnO2/graphene nanoporous electrodes with threedimensionally delaminated flexible structure, Nano Lett. 9 (1) (2009) 72e75. [5] Hui Liu, Renzong Hu, Wei Sun, et al., nanocomposites prepared by oxygen plasma-assisted milling as cyclic durable anodes for lithium ion batteries, J. Power Sources 242 (2013) 114e121. [6] Yuezeng Su, Shuang Li, Dongqing Wu, et al., Two-dimensional carbon-coated graphene/metal oxide hybrids for enhanced lithium storage, Acs Nano 6 (9) (2012) 8349e8356. [7] Jian Xie, Shuang-Yu Liu, Xue-Fei Chen, et al., Nanocrystal-SnO2-loaded graphene with improved Li-storage properties prepared by a facile one-pot hydrothermal route, Int. J. Electrochem. Sci. 6 (11) (2011) 5539e5549. [8] Yong Wang, Fabing Su, Jim Yang Lee, et al., Crystalline carbon hollow spheres, crystalline carbon-SnO2 hollow spheres, and crystalline SnO2 hollow spheres: synthesis and performance in reversible Li-ion storage, Chem. Mater. 18 (2006) 1347e1353. [9] Dongniu Wang, Xifei Li, Jiajun Wang, et al., Defect-rich crystalline SnO2 immobilized on graphene nanosheets with enhanced cycle performance for Li ion batteries, J. Phys. Chem. C 116 (42) (2012) 22149e22156. [10] Xifei Li, Xiangbo Meng, Jian Liu, et al., Tin oxide with controlled morphology and crystallinity by atomic layer deposition onto graphene nanosheets for enhanced lithium storage, Adv. Funct. Mater. 22 (2012) 1647e1654. [11] Hui Qiao, Jing Li, Jiapeng Fu, et al., Sonochemical synthesis of ordered SnO2/ CMK-3 nanocomposites and their lithium storage properties, ACS Appl. Mater.
Interfaces 3 (2011) 3704e3708. [12] Jun Song Chen, Yan Ling Cheah, Yuan Ting Chen, et al., SnO2 nanoparticles with controlled carbon nanocoating as high-capacity anode materials for lithium-ion batteries, J. Phys. Chem. C 113 (2009) 20504e20508. [13] Xiong Wen Lou, Jun Song Chen, Peng Chen, et al., One-pot synthesis of carboncoated SnO2 nanocolloids with improved reversible lithium storage properties, Chem. Mater. 21 (2009) 2868e02874. [14] Xiong Wen Lou, Da Deng, Jim Yang Lee, et al., Preparation of SnO2/carbon composite hollow spheres and their lithium storage properties, J. Mater. Chem. 20 (20) (2008) 6562e6566. [15] Shu-Lei Chou, Jia-Zhao Wang, Chao Zhonga, et al., A facile route to carboncoated SnO2 nanoparticles combined with a new binder for enhanced cyclability of Li-ion rechargeable batteries, Electrochimica Acta 54 (28) (2009) 7519e7524. [16] Guodong Du, Chao Zhong, Peng Zhang, et al., Tin dioxide/carbon nanotube composites with high uniform SnO2 loading as anode materials for lithium ion batteries, Carbon 49 (1) (2011) 133e139. [17] Bin Liu, Zai Ping Guo, Guodong Du, et al., In situ synthesis of ultra-fine, porous, tin oxide-carbon nanocomposites via a molten salt method for lithium-ion batteries, J. Power Sources 195 (16) (2010) 5382e5386. [18] Seung-Keun Park, Seung-Ho Yu, Nicola Pinna, et al., A facile hydrazineassisted hydrothermal method for the deposition of monodisperse SnO2 nanoparticles onto graphene for lithium ion batteries, J. Mater. Chem. 22 (2012) 2520e2525. [19] Hongdong Liu, Jiamu Huang, Xinlu Li, et al., Flower-like SnO2/graphene composite for high-capacity lithium storage, Appl. Surf. Sci. 258 (2012) 4917e4921. [20] Peichao Lian, Xuefeng Zhu, Shuzhao Liang, et al., High reversible capacity of SnO2/graphene nanocomposite as an anode material for lithium-ion batteries, Electrochimica Acta 56 (2011) 4532e4539. [21] Jia Liu, Jiamu Huang, Longlong Hao, et al., SnO2 nano-spheres/graphene hybrid for high-performance lithium ion battery anodes, Ceram. Int. 39 (2013) 8623e8627. [22] Junsheng Zhu, Dianlong Wang, Lin Wang, et al., Facile synthesis of sulfur coated SnO2-graphene nanocomposites for enhanced lithium ion storage, Electrochimica Acta 91 (2013) 323e329. [23] Wei Sun, Xiuzheng Wang, Yuhua Wang, et al., Application of graphene-SnO2 nanocomposite modified electrode for the sensitive electrochemical detection of dopamine, Electrochimica Acta 87 (2013) 317e322. [24] Zhiyong Wang, Hao Zhang, Nan Li, et al., Laterally confined graphene nanosheets and graphene/SnO2 composites as high-rate anode materials for lithium-ion batteries, Nano Res. 3 (10) (2010) 748e756. [25] Fabrice M. Courtel, Elena A. Baranova, et al., In situ polyol-assisted synthesis of nano-SnO2/carbon composite materials as anodes for lithium-ion batteries, J. Power Sources 195 (8) (2010) 2355e2361. [26] Zhifeng Du, Xiaoming Yin, Ming Zhang, et al., In situ synthesis of SnO2/graphene nanocomposite and their application as anode material for lithium ion battery, Mater. Lett. 64 (2010) 2076e2079. [27] Chaofeng Zhang, Xing Peng, Zaiping Guo, et al., Carbon-coated SnO2/graphene nanosheets as highly reversible anode materials for lithium ion batteries, Carbon 50 (2012) 1897e1903. [28] Hongdong Liu, Jiamu Huang, Xinlu Li, et al., Hydrothermal synthesis and characterization of graphene/self-assembled SnO2 hybrid, Phys. E 44 (2012) 1931e1935. [29] Jane Yao, Xiaoping Shen, Bei Wang, et al., In situ chemical synthesis of SnO2graphene nanocomposite as anode materials for lithium-ion batteries, Electrochem. Commun. 11 (2009) 1849e1852. [30] Junfei Liang, Wei Wei, Da Zhong, et al., One-step in situ synthesis of SnO2/ graphene nanocomposites and its application as an anode material for Li-ion batteries, Appl. Mater. Interfaces 4 (2012) 454e459. [31] Ping Wu, Ning Du, Hui Zhang, et al., Self-templating synthesis of SnO2 carbon hybrid hollow spheres for superior reversible lithium ion storage, Appl. Mater. Interfaces 3 (2011) 1946e1952. [32] Bei Wang, Dawei Su, Jinsoo Park, et al., Graphene-supported SnO2 nanoparticles prepared by a solvothermal approach for an enhanced electrochemical performance in lithium-ion batteries, Nanoscale Res. Lett. 7 (2012) 215e222. [33] Jiaxin Li, Yi Zhao, Ning Wang, et al., A high performance carrier for SnO2 nanoparticles used in lithium ion battery, Chem. Commun. 47 (2011) 5238e5240. [34] Jeng-Yu Lin, Ming-Hung Chou, Y.-C. Kuo, Rapid synthesis of tin oxide decorated carbon nanotube nanocomposites as anode materials for lithium-ion batteries, J. Alloys Compd. 589 (2014) 472e478.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electrochemical performance of SnO2/modified graphite composite material as anode of lithium ion battery Hong-Qiang Wang a, b, Guan-Hua Yang a, You-Guo Huang a, Xiao-Hui Zhang a, Zhi-Xiong Yan a, Qing-Yu Li a, * a
Guangxi Key Laboratory of Low Carbon Energy Materials, School of Chemical and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, China Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang 438000, China
b
h i g h l i g h t s A simple synthetic method of SnO2/modified graphite composite as anode. The as-prepared composite with layered structure alleviates the huge reunion of SnO2. The composite exhibits a good capacity retention rate of 85.8% after 25 cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 September 2014 Received in revised form 1 September 2015 Accepted 18 October 2015 Available online 26 October 2015
In this report, we synthesized SnO2/modified graphite anode composite material by a simple reflux method using SnCl4$5H2O as tin source and modified graphite as carbon source. The as-obtained composite was investigated with the help of X-ray diffraction (XRD), scanning electron microscopy (SEM) and galvanostatic cycling tests. The results show that the composite has a wave-shaped fold structure and the SnO2 nanoparticles on it have an average size of about 50 nm. Compared to pure modified graphite, the SnO2/modified graphite exhibits a better electrochemical performance with a reversible specific capacity of 581.7 mAh g1 after 80 cycles, owing to high mechanical stress and elasticity of modified graphite could hinder the volume effect of SnO2 nanoparticles during the Liþ insertion/extraction process. All these favourable characters reveal that the composite is a great potential anode material in highperformance lithium ion batteries. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Nanostructures Electron microscopy Electrochemical properties
1. Introduction Lithium-ion batteries (LIBs) as energy storage device of power sources, have been widely used in portable electronic devices, electric bicycles, electric vehicles and hybrid vehicles [1e4]. With the development of the society and the progress of technology, the applications of LIBs are increasingly extended. Anode materials as part of the battery, have a pivotal role on the performance of the batteries and have been put forward further requirement for them in LIBs [5e7]. Graphite, as an important anode material, has been extensively used in commercial lithium-ion batteries. However, the theoretical capacity of it is only 372 mAh g1, which can hardly
* Corresponding author. E-mail address: [email protected] (Q.-Y. Li). http://dx.doi.org/10.1016/j.matchemphys.2015.10.048 0254-0584/© 2015 Elsevier B.V. All rights reserved.
meet the needs of high-performance LIBs. In order to obtain highperformance LIBs, numerous efforts are trying to find alternative anode materials. Among the alternative anode materials, SnO2 has attracted great attention from researchers due to its high storage capacity and low cost. The theoretical specific capacity of SnO2 is 782 mAh g1, which is more than twice of the graphite [8,9]. However, SnO2 has large volume expansion effect in the process of insertion and extraction of lithium. It is easy to cause the electrode material pulverization and exfoliation from the current collector, which lead to the rapid decrease of battery capacity and poor cycle performance, eventually hinder its application in actual [10e13]. In order to overcome these shortcomings, an effective strategy is the introduction of an active or inert element that acts as a buffer to accommodate the large volume change of SnO2 during cycling. It is an effective approach to alleviate the volume change of SnO2 by forming a composite material of SnO2 and carbon, which can
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improve the capacity and cycle performance of anode material [14e17]. Introduction of carbon not only buffers the volume change of SnO2, but also strengthens the SnO2 electrical contact and improves the utilization of the active material. In addition, carbon can also provide some specific capacity for the electrode. Graphene, with excellent electrical conductivity and mechanical properties, is a new type of carbon material, which is an excellent choice for SnO2 modification [18e24]. And some studies of SnO2/graphene composite as anode materials have been reported. Fabrice M. Courtel et al. prepared nano-SnO2/carbon composites with small particle sizes (5 nm) in situ via the polyol method using microwave-assisted heating method [25]. Zhifeng Du et al. utilized a in situ chemical synthesis method to deposit nano-SnO2 on the surface of graphene [26]. Chaofeng Zhang et al. used hydrothermal method to synthesize carbon-coated graphene/tin oxide composites [27]. However, these anode materials preparation process is complicated, timeconsuming, or using toxic solvents, which limit the large-scale application of them. In this paper, SnO2/modified graphite composite is prepared by a simple reflux in an aqueous solution. SnO2 particles in the composite material are approximately 50 nm and distribute evenly in the modified graphite layers. The electrode material deterioration due to the huge volume change of SnO2 has been gained a certain inhibition and reversible of the battery is also improved. The specific capacity of 581.7 mAh g1 is obtained after 80 cycles. Thus, the as-prepared SnO2/modified graphite composite material is a promising anode material for lithium ion battery.
graphite composite and modified graphite were analysed by X-ray diffraction (XRD: Rigaku, D/max 2500v/pc), field emission scanning electron microscopy (FESEM, Philips, FEI Quanta 200 FEG), and transmission electron microscope (TEM, Tokyo, Japan, JEOL 2100F). The component of the sample was analysed by an energy dispersive spectroscope (EDS, INCA) attached to the SEM. Thermogravimetric analysis TG-DSC (SETARAM, LABSYS evo) was carried out in the temperature range from 30 to 1000 C at a scanning rate of 10 C min1 in dry air. 2.3. Electrochemical evaluation The working electrodes were prepared by mixing the asobtained composite, carbon black and polyvinylidene fluoride (PVDF) (85:10:5 in weight ratio) in N-methyl pyrrolidone (NMP). In this research, the loading amount of anode active materials in the electrodes SnO2/modified graphite composite is 0.8e1.0 mg/cm2. The as-obtained homogeneous slurry was coated on copper foil and dried at 80 C under vacuum for 12 h. The lithium foil was used as counter electrode and the electrolyte used was 1 M LiPF6 in a 1:1 (v/ v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Electrochemical measurements were carried out using CR2032 coin-type cells, which were assembled in an argon-filled glove-box. The galvanostatic chargeedischarge in the voltage ranging from 5 mV to 3.0 V was conducted with a Battery Tester (Land, Wuhan, China) at 25 C. Cyclic voltammetry (CV) (0.1 mVe3.0 V, 0.1 mV s1) was performed using an IM6 electrochemical workstation (Zahner, Germany).
2. Experiments 2.1. Preparation of SnO2/modified graphite composite Graphite oxide (GO) with layer structure was synthesized from flake graphite by a modified Hummer's method [28]. 1 g GO was dispersed in 100 ml of deionized water by sonication for 30 min, forming a stable GO suspension. Then, the graphite oxide was added to the three-neck round bottom flask, followed by this 2 g NaBH4 and 50 ml aqueous solution of SnCl4$5H2O (3.75 g) were slowly added into the above solution, and heated at 100 C under reflux for 8 h. Subsequently, the products were collected by filtration and washed with deionized water, dried at 80 C. In order to improve the crystallinity of SnO2 in composite materials, the products were annealed at 550 C for 3 h in N2 atmosphere. 2.2. Characterization The crystal structure and morphology of the SnO2/modified
3. Results and discussion Fig. 1a shows the X-ray diffraction patterns of the SnO2/modified graphite, bare SnO2, and modified graphite. The diffraction peaks of crystalline SnO2 nanoparticles in SnO2/modified graphite are clearly distinguishable. Peaks at 2q of 26.6 , 33.9 , 37.9 , 51.8 , 54.7, 61.9 , 65.9 , 71.3 , 78.7, 83.7 exhibit the major diffraction peaks at (110), (101), (200), (211), (220), (310), (301), (202), (321) and (222) of tetragonal crystalline SnO2 (JCPDS No.41-1445) [29,30]. In the XRD pattern of SnO2/modified graphite, no obvious carbon peaks can be found, which is due to the overlap of the main carbon (002) facet with SnO2 (110) facet. In addition, the diffraction peak of metal Sn cannot be observed either. It demonstrates that Sn is mainly in the form of SnO2 in the composite material at the low heat treatment temperature. The pure modified graphite displays a strong (002) diffraction peak and two weak (100), (110) diffraction peaks, suggesting the amorphous nature of modified graphite. XRD confirms the coexisting phases of SnO2 and modified graphite and the
Fig. 1. XRD pattern of SnO2/modified graphite, bare SnO2, and modified graphite (a), and TG/DTA behaviour of SnO2/modified graphite (b).
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Fig. 2. SEM image of modified graphite layers (a), and SEM image of SnO2/modified graphite composite, inset: partial enlargement of the composite (b), and TEM image of the SnO2/ modified graphite composite (c), and EDS surface scanning analysis image of SnO2/modified graphite composite (d), and low-magnification SEM image of SnO2/modified graphite (e) and corresponding EDS maps for C, O and Sn (feh).
formation of SnO2/modified graphite composite. In order to quantify the composition of modified graphite and SnO2, the as-prepared composite is analysed by TGA. As shown in
the TG/DTA curves (Fig. 1b), the weight loss between 30 and 250 C could be attributed mainly to the release of adsorbed water. The dramatic weight loss from 550 C to 650 C may be correlated to the
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Fig. 3. The chargeedischarge curve of SnO2/modified graphite composite (a), and the chargeedischarge curve of modified graphite for the 1st, 2nd, 20th cycle (b).
removal of modified graphite [13,31]. Therefore, according to the change of weight, the composition of the SnO2 and modified graphite in the as-prepared composite is determined to be about 31% and 47% by weight respectively. The morphologies of modified graphite layers and SnO2/modified graphite composite are characterized by field emission scanning electron microscopy, as shown in Fig. 2a and Fig. 2b, respectively. From Fig. 2a, it can be seen that the sheet structure of graphite has been opened after the oxidation process. The modified graphite has a layered sandwich structure, which can be used as carriers of SnO2 to prevent aggregation of SnO2. In addition, the modified graphite may work as a buffer, which relieves the volume expansion and contraction of SnO2 during the Liþ insertion/ extraction process. Fig. 2b and the inset show that SnO2 nanoparticles uniformly distribute in the matrix of the modified graphite and the modified graphite layers. The SnO2 particles firmly bond to the modified graphite layers, which is contributed to improve the conductivity and specific capacity of the material. In order to investigate the details of the microstructure in the composite, TEM observation is carried out, as shown in Fig. 2c. It shows that a lot of SnO2 particles, about 50 nm in size, homogeneously disperse in the matrix of the modified graphite. The results of EDS, as shown in Fig. 2d, indicate that the composite material contains O, Sn, C, and the content of each element is 40.82%, 36.66%, 22.52%, respectively. Based on the atomic number ratio of SnO2, the atomic ratio of Sn:O equals to 1:8.26, which is far lower than the ratio (1:2) of SnO2. This is attributed to the modified graphite layers could not be completely recovered by thermal reduction at 550 C. Further, the low-magnification SEM image and presence of individual elements of SnO2/modified graphite are shown in Fig. 2eeh. The elemental mapping of SnO2/modified graphite indicates the presence of C, O and Sn in the sample.
Fig. 5. Schematic illustration of synthesis of SnO2/modified graphite: (1) and (2) were the Hummer's method for preparation of modified graphite, (3) the dispersion of SnO2 on the modified graphite, (4) the carbonization of SnO2/modified graphite.
Substantial amounts of oxygen are also detected, further indicating that the oxygen in the composite has not been completely removed after the heat treatment. These maps reveal that SnO2 nanoparticles homogeneously distribute on the modified graphite [6]. To investigate the electrochemical performances of modified graphite and SnO2/modified graphite, chargeedischarge tests are implemented. Fig. 3 shows the 1st, 2nd, 20th chargeedischarge experimental results at the voltage range of 5 mVe3.0 V (vs. Li/Liþ) under room temperature. In Fig. 3a, SnO2/modified graphite
Fig. 4. The cycle performance of SnO2/modified graphite composite material and modified graphite with current density of 0.1 A g1 (a), and rate performance of SnO2/modified graphite composite at different chargeedischarge rates (b).
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Fig. 6. Cyclic voltammetry of the SnO2/modified graphite at a scanning rate of 0.1 mV s1 (a), and Nyquist plots of SnO2/modified graphite before and after 10 cycles (b).
electrode displays a initial discharge capacity of 1373.6 mAh g1 and a charge capacity of 784.1 mAh g1. The voltage drops rapidly from 2.5 V to 1.5 V during the discharge process, which is due to the reaction between Li and SnO2 and the generation of non-reactive Li2O. A platform appears gradually around 0.8 V and disappears in the subsequent cycle, which is mainly caused by the decomposition of electrolyte on the electrode surface to form a solid electrolyte interface (SEI). Owing to the irreversible formation of inactive Li2O and SEI, the coulomb efficiency is low (around 57.1%) in the first cycle. As the voltage tardily decreased, the second platform appears at approximately 0.1 V, which is corresponded to the formation of intercalation compounds causing by the intercalation of lithium ions into the modified graphite interlayer [19,32]. In addition, it can be seen that the chargeedischarge capacity of the battery is relatively consistent during cycles, revealing that performance of the electrode material tends to be stable. As a comparison, the chargeedischarge performances of modified graphite are investigated (as display in Fig. 3b). It shows that an intense voltage drop appear at about 0.75 V in the first discharge process and disappears in the subsequent cycle, which is considered to be correspond to the formation of SEI. The profile of 10 mVe0.5 V corresponds to the process of the lithium ions embedding and extraction out from the modified graphite layers. According to Fig. 3b, modified graphite displays a discharge capacity of 707.3 mAh g1 and a charge capacity of 324.5 mAh g1 in the first cycle. Thus, it demonstrates that the specific capacity of the composite due to the load of SnO2 nanoparticles is higher than the capacity of modified graphite. Fig. 4a shows the cyclic performances of SnO2/modified graphite composite material and modified graphite with current density of 0.1 A g1. Owing to the irreversible formation of amorphous Li2O matrix and decomposition of electrolyte on the electrode surface to formation of SEI, the first discharge capacity of SnO2/modified graphite decreases promptly. In the succedent chargeedischarge cycles, the SnO2/modified graphite exhibits good reversible capacity, which is mainly because the combination of the high mechanical stress and elasticity of modified graphite and Liþ reversibly insertion into and extraction out from Sn. Meanwhile, the modified graphite has a layered sandwich structure with undulating folds (as shown in Fig. 2a), which contributes to establish the reaction of solid/liquid interface and improves the electrochemical activity of lithium ions in the solid/liquid interface. Compared with modified graphite, the SnO2/modified graphite exhibits a higher specific capacity, which may be caused by SnO2 nanoparticles embedding into the layered sandwich structure of modified graphite or the gaps during preparing the composite material (as the schematic illustration shown in Fig. 5). After 80 cycles, the dischargeecharge capacity of composite is 589.6 mAh g1 and 581.7 mAh g1,
respectively, which is higher than the dischargeecharge capacity of modified graphite. It indicates that both SnO2 and modified graphite have good synergy, in which modified graphite acts as the buffer of SnO2 to mitigate volume effect of SnO2 in the process of dischargeecharge and the SnO2 increases the specific capacity of the composite. Fig. 4b shows the rate performance of the SnO2/modified graphite. The cell is first cycled at current density of 0.1 A g1, and then the current density is increased to 0.2 and 2 A g1, finally returned to 0.1 A g1, successively. As can be seen from it, the SnO2/ modified graphite displays an excellent rate capability. The SnO2/ modified graphite is still capable of delivering a substantial capacity of 901.5, 782.0, 639.7, 541.4, and 420.5 mAh g1 at various current densities of 0.1, 0.2, 0.5, 1, and 2 A g1, respectively. It should be noted that, when the current density is reversed back to 0.1 A g1 after 50 cycles, the specific charge capacity of 814.3 mAh g1 could be obtained, demonstrating that the SnO2/modified graphite could tolerate varied dischargeecharge rates. This good rate capability is attribute to the layered modified graphite work as a highly conductive matrix to anchor SnO2 nanoparticles and the SnO2 nanoparticles can shorten the transport path for both lithium ions and electrons [9,32]. In order to understand the electrochemical reactivity of the SnO2/modified graphite better, cyclic voltammetry is performed ranging from 1 mV to 3.0 V at a sweep rate of 0.1 mV s1 (Fig. 6a). In the first cycle, two peaks appear in the cathodic process, seating around 1.26 V and 0.01 V, which is associated with the alloying reaction between lithium and Sn and the formation of the LixC compounds, respectively. The peak at around 0.81 V is generally considered to be attributed to the reduction of SnO2 to Sn, the production of Li2O and the formation of SEI film, which result in the generation of the irreversible capacity. In the anodic process, the oxidation peak around 0.1 V is corresponded to the lithium extraction out from the modified graphite sheets. Peaks at about 0.60 V and 1.20 V, are related to the reaction of lithium extraction out from LixSn alloy [32,33]. Obviously, the anodic/cathodic peak almost remains unchanged after another two cyclic voltammetry, exhibiting that the composite has good redox reversibility, excellent electrical conductivity and good cycle performance. Fig. 6b shows the Nyquist plots of SnO2/modified graphite before and after 10 cycles in the 100 kHz to 10 mHz frequency range. It demonstrates the similar Nyquist plots with a semicircle in high frequency range and a sloping straight line in low frequency range. According to previous studies [22,34], the semicircle in the high-frequency is attributed to the formation of SEI film and contact resistance, the semicircle in medium-frequency region is associated to the charge-transfer impedance on and the inclined line at lower frequency represents the Warburg impedance (W),
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which corresponds to the Liþ diffusion in the electrode materials. It can be clearly seen that the charge transfer resistance of SnO2/ modified graphite electrode after 10 cycles (185 U) is smaller than the composite electrode before cycle (693 U). The result indicates that the presence of modified graphite with layered nanostructure can accommodate the volume change of SnO2 during cycles effectively to reduce the charge transfer impedance and increases the electronic conductivity of the composite. This phenomenon further confirms the result from CVs that the SnO2/modified graphite electrode exhibits good cycling performance. 4. Conclusions In summary, SnO2/modified graphite composite is prepared by a simple reflux in an aqueous solution. The experimental results exhibit that the SnO2/modified graphite composite is composed of uniformly distributed nanosized particles and can deliver a reversible capacity of 581.7 mAh g1, which is higher than the dischargeecharge capacity of modified graphite. Therefore, the results presented in this paper prove that SnO2/modified graphite composite is a good candidate for use as anode material in lithium ion battery. Acknowledgements This research was supported by NSF of China (51474110, 51364004 and 51064004), Guangxi Natural Science Foundation (2011GXNSFA018016). References [1] F.M. Hassan, Z. Chen, A. Yu, et al., Sn/SnO2 embedded in mesoporous carbon nanocomposites as negative electrode for lithium ion batteries, Electrochimica Acta 87 (2013) 844e852. [2] Fei Wang, Xiaoping Song, Gang Yao, et al., Carbon-coated mesoporous SnO2 nanospheres as anode material for lithium ion batteries, Scr. Mater. 66 (2012) 562e565. [3] Wenbo Yue, Sheng Yang, Yunling Liu, et al., A facile synthesis of mesoporous graphene-tin composites as high-performance anodes for lithium-ion batteries, Mater. Res. Bull. 48 (2013) 1575e1580. [4] Seung-Min Paek, EunJoo Yoo, Itaru Honma, Enhanced cyclic performance and lithium storage capacity of SnO2/graphene nanoporous electrodes with threedimensionally delaminated flexible structure, Nano Lett. 9 (1) (2009) 72e75. [5] Hui Liu, Renzong Hu, Wei Sun, et al., nanocomposites prepared by oxygen plasma-assisted milling as cyclic durable anodes for lithium ion batteries, J. Power Sources 242 (2013) 114e121. [6] Yuezeng Su, Shuang Li, Dongqing Wu, et al., Two-dimensional carbon-coated graphene/metal oxide hybrids for enhanced lithium storage, Acs Nano 6 (9) (2012) 8349e8356. [7] Jian Xie, Shuang-Yu Liu, Xue-Fei Chen, et al., Nanocrystal-SnO2-loaded graphene with improved Li-storage properties prepared by a facile one-pot hydrothermal route, Int. J. Electrochem. Sci. 6 (11) (2011) 5539e5549. [8] Yong Wang, Fabing Su, Jim Yang Lee, et al., Crystalline carbon hollow spheres, crystalline carbon-SnO2 hollow spheres, and crystalline SnO2 hollow spheres: synthesis and performance in reversible Li-ion storage, Chem. Mater. 18 (2006) 1347e1353. [9] Dongniu Wang, Xifei Li, Jiajun Wang, et al., Defect-rich crystalline SnO2 immobilized on graphene nanosheets with enhanced cycle performance for Li ion batteries, J. Phys. Chem. C 116 (42) (2012) 22149e22156. [10] Xifei Li, Xiangbo Meng, Jian Liu, et al., Tin oxide with controlled morphology and crystallinity by atomic layer deposition onto graphene nanosheets for enhanced lithium storage, Adv. Funct. Mater. 22 (2012) 1647e1654. [11] Hui Qiao, Jing Li, Jiapeng Fu, et al., Sonochemical synthesis of ordered SnO2/ CMK-3 nanocomposites and their lithium storage properties, ACS Appl. Mater.
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