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Porous reduced graphene oxide sheet wrapped silicon composite fabricated by steam etching for lithium-ion battery application
Journal of Power Sources 286 (2015) 431e437
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Porous reduced graphene oxide sheet wrapped silicon composite fabricated by steam etching for lithium-ion battery application H. Tang a, J. Zhang b, Y.J. Zhang a, Q.Q. Xiong a, Y.Y. Tong a, Y. Li a, X.L. Wang a, C.D. Gu a, J.P. Tu a, * a
State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Provence, and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China b
h i g h l i g h t s A novel preparation method is fabricated by steam etching of Si/rGO aerogel. This method is easy to handle and exhibited excellent performances. The in-situ TEM verifies the well-retained integrity of electrode in the substrate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2015 Received in revised form 28 March 2015 Accepted 30 March 2015 Available online 31 March 2015
A novel of Si/porous reduced graphene oxide (rGO) composite is fabricated by steam etching of Si/rGO aerogel. The rGO sheets with nano-holes build a unique three-dimensional porous network and can encapsulate the Si nanoparticles. The porous structure of Si/rGO composite can reduce the transfer distance of Li ions and restrain the aggregation and destruction of Si particles. The in-situ transmission electron microscopy (TEM) observation demonstrates that the porous rGO sheets help the entire electrode to maintain highly conductive and facilitate the lithiation of Si nanoparticles. The composite electrode presents high specific capacity and good cycling stability (1004 mAh g1 at 50 mA g1 up to 100 cycles). © 2015 Elsevier B.V. All rights reserved.
Keywords: Silicon Reduced graphene oxide Steam etching In-situ transmission electron microscopy Lithium-ion battery
1. Introduction Lithium ion batteries (LiBs) are the important power source in different fields, including electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) [1,2]. High energy and power density, and excellent cycling stability are factors for the applications of LIBs. However, to meet these demands, the high capacity anode materials, such as transition metal oxides and phosphides [3e6], tin alloys and oxides [7,8], and silicon [9e11], have been studied to substitute commercial graphite with low theoretical specific capacity (372 mAh g1) [12,13]. Among these anode materials, Si is the most promising alternative because of
* Corresponding author. E-mail addresses: [email protected], [email protected] (J.P. Tu). http://dx.doi.org/10.1016/j.jpowsour.2015.03.185 0378-7753/© 2015 Elsevier B.V. All rights reserved.
its low cost, natural abundance and especially its highest theoretical capacity (4200 mAh g1, corresponding to the Li22Si5 alloy at room temperature) with a low potential window [14,15]. However, due to its low intrinsic electrical conductivity (1.56 103 S m1) and large volume change (>300%), Si suffers from limited cycling stability during the charge/discharge process [16,17]. In order to overcome these problems, the most usual approach is to prepare composites with Si nanoparticles dispersed uniformly in a carbonaceous matrix [18,19]. Graphene, a two-dimensional carbon nano-material, has a potential application for energy storage due to its large surface area, excellent conductivity, flexibility and chemical stability [20e22]. The reduced graphene oxide (rGO) as an active matrix can improve the electrochemical performance of Si-based materials [1,23,24]. The improved electrochemical performance can be
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Scheme 1. Schematic of the synthesis procedure of Si/rGP.
attributed to the fact that rGO can not only enhance the electrical conductivity of Si and provide a support for dispersing Si nanostructures, but also effectively release the volume change and aggregation of Si nanoparticles during the charge/discharge process. In this present work, we fabricated a Si/rGO nano-porous network composite (Si/rGP) anode material by simple steam etching Si/rGO aerogel. The stable porous network structure with high specific surface area is capable to reduce the transfer resistance of Li ions, and the Si/rGP composite electrode exhibits the improved reversible capacity and cycling performance. The facile method provides a new approach for the Si-based anode materials for LIBs.
Fig. 1. TG curves of Si/rGP, rGO and pure Si in air at a heating rate of 5 C min1.
Fig. 2. XRD patterns of Si/GO and Si/rGP powders.
Fig. 3. XPS spectra of Si/rGP: (a) survey scan, (b) Si2p region of Si/rGP, (c) C 1s curve of Si/rGP.
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Fig. 4. SEM images of (a) the Si/rGO after hydrothermal treatment and (b) the Si/rGP after steam etching at low magnification, the inset of (a) is a digital photograph of Si/rGO hydrogel; (c) the Si/rGP at high magnification; TEM images of the Si/rGP ((d) at low magnification and (e) at high magnification); (f) Raman spectra of Si/GO and Si/rGP.
2. Experimental section 2.1. Materials Si were received from CWNANO (<100 nm in size, 99.9% purity) without further purification. Graphite powder (99.95% metals basis) and polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) were obtained from Aladdin. H2SO4, H3PO4, NaNO3, KMnO4, H2O2, HCl and thiourea were purchased from Sinopharm Chemical Reagent Co., Ltd without further purification. 2.2. Preparation of Si/r-GP composite material Graphene oxide (GO) was prepared from graphite powder according to the modified Hummers' method [25]. Si nanoparticles (1.4 g) and 10 ml of 20 wt.% PDDA aqueous solution were dispersed in water (1000 mL) by sonication in a water bath (KQ2200DE, 99 kHz). The excess PDDA was removed by four repeated centrifugation (14000 rpm 10 min)/wash/redispersion cycles. After that, GO was exfoliated in distilled water with a concentration of 2 mg mL1. Then the Si-PDDA aqueous solution was diluted to the GO aqueous dispersion. After that, 0.5 g of thiourea was added into the homogeneous Si-PDDA/GO aqueous dispersion, and then the mixture was sealed in a 100 ml Teflon-lined stainless steel autoclave and maintained at 180 C for 4.5 h. Finally, the Si/rGO assembly was dipped into distilled water for 24 h to remove the residual thiourea [26]. And vacuum freeze-drying was used to remove water and obtain dry Si/rGO aerogel. In order to obtaining the Si/rGP, the aerogel was put into the beaker and sealed in a
100 ml Teflon-lined stainless steel autoclave with 10 ml DI water and heated to 200 C for 10 h. Finally, the Si/rGP was prepared by freeze-drying under vacuum (<20 Pa) for 48 h. The detailed synthesis is schematically demonstrated in Scheme 1. For comparison, the same method was used to prepare the pure nano-porous rGO (rGP), and the Si/rGO without nano-holes on the rGO sheets was prepared by hydrothermal treatment. 2.3. In-situ electrochemical experiments The in-situ electrochemical experiments were carried out using a Nanofactory TEM-scanning tunneling microscopy (STM) holder in a JEOLJEM-2100F TEM. Briefly, a 0.25 mm-thick gold rod was cut to produce a clean and fresh cross section; then the Si/rGP powder was glued to the pretreated gold rod with conductive epoxy as the working electrode. A sharp tungsten STM tip was used to scratch Li metal surface to fetch some fresh Li inside a glovebox filled with argon. The Li layer on the tip of the W rod was served as the counter electrode and lithium source. Both the Si/rGP and lithium electrodes were mounted onto a Nanofactory STM-TEM holder inside the same glovebox. The holder was quickly transferred into the TEM column; a native Li2O layer formed on the surface of Li metal due to the exposure to air, which was served as the solid-state electrolyte to allow the transport of Liþ. The Li2O/Li electrode was mounted on the mobile STM probe, which could be driven to contact the Si/rGP electrode inside the TEM. Lithiation took place after a negative bias was applied on the Si/rGP with respect to the lithium metal to drive Liþ transport through the solid-state Li2O layer, and the bias was then reversed to positive to facilitate delithiation.
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2.4. Materials characterization The microstructure and morphology of the products were characterized by Xray diffraction (XRD, Rigaku D/max 2550 PC, Cu Ka), Xray photoelectron spectroscopy (XPS, AXIS UTLTRADLD), scanning electron microscopy (SEM, Hitachi S4700) and transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). The Raman spectra were measured on a JobinYvon Labor Raman HR800 using Arion laser of 514.5 nm. Zetasizer 3000HSA was used to test the Zeta potential for characterization of charged silicon, rGO and Si-PDDA/rGO composite. The thermogravimetric (TG) analysis of Si/rGP during annealing was obtained using a TA Q600 instrument in a temperature range from 25 to 800 C with a heating rate of 5 C min1 in air. The specific surface area and pore diameter were determined by BET(BrunauerEmmett-Teller) measurements using a Autosorb-1-C surface area analyzer. 2.5. Electrochemical measurements To prepare working electrodes, the active material, Super-P carbon black, and polyvinylidene fluoride (PVDF) with mass ratio 80: 10: 10 were mixed into a homogeneous slurry with mortar and pestle, and then the slurry was pasted onto pure Cu foil. The electrochemical tests were performed using a cointype half cell (CR 2025). Test cells were assembled in an argonfilled glove box with the metallic lithium foil as both the reference and counter electrodes, 1 M LiPF6 in ethylene carbonate (EC)dimethyl carbonate (DME) (1: 1 in volume) as the electrolyte, and a polypropylene (PP) microporous film (Cellgard 2300) as the separator. The galvanostatic chargedischarge tests were conducted on a LAND battery program control test system at different current densities between 0.01 and 1.5 V at room temperature (25 ± 1 C). Cyclic voltammetry (CV) tests were performed on a CHI660C electrochemical workstation in the potential range of 0e1.5 V (vs. Li/Liþ) at a scanning rate of 0.1 mV s1. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 100 kHz to 0.01 Hz under AC stimulus with 5 mV of amplitude.
Obviously, the diffraction peaks of the Si/rGP and Si/GO are matched well with the silicon crystal planes of (111), (220), (311). The diffraction peak (001) at 10.9 of Si/GO is a typical peak of GO and the additional broadened peak (002) confirms the conversion of GO to rGO after the hydrothermal treatment as described in the previous literature [17,31]. Fig. 3a shows the XPS spectrum of the Si/rGP composite. In the survey region, it is clear that C, O, and Si elements coexist in the sample. Fig. 3b presents the highresolution spectrum of Si 2p with three components of SieC at 99.9 eV, monatomic silicon at 99.0 eV and Si/SiOx (x < 2) at 103.5 eV. There are no peaks (110 and 105 eV, i.e.) from SiO2 [32]. The formation of SiOx shells is possibly by the oxidation of Si nano-crystals with a high surface activity and the surface charge of Si is obtained negatively (zeta potential: 19.5 mV). And then, the Si/SiOx can adsorb PDDA through electrostatic attraction to change the surface charge from negative to positive (Si-PDDA zeta potential: þ77.5 mV). Finally, the obtained positively charged SiPDDA nanoparticles assemble with negatively charged GO by electrostatic attraction (Si-PDDA/GO zeta potential: 38 mV). Therefore, a good coating effect of GO is obtained and the rGO sheets can encapsulate the Si nanoparticles. The C 1s core level peak can be split into three components (Fig. 3c). The peaks of CeC, SieC and CeOH are detected at 284.6, 284.3, and 285.6 eV, respectively, which indicates that most of the oxygen-containing groups are removed after hydrothermal treatment [31]. Fig. 4a and b shows the SEM image of Si/rGO after just hydrothermal treatment and Si/rGP after steam etching treatment, respectively. It is obviously that Si/rGO exhibits a 3D network
3. Results and discussion The formation of the nano-porous network structure is based on the steam etching Si/rGO aerogel. In the previous works [27,28], the carbon-based materials can be converted to gaseous fuels such as CO and H2 by the so-called water gas reaction with hot steam. Therefore, with the steam etching process (Scheme 1), carbon atoms can react with H2O vapor to produce CO and H2, leaving behind carbon vacancies that eventually grow into large pores. To determine the content of Si in the Si/rGP composite, TGA measurements were performed in a temperature range from 25 to 800 C in air. As shown in Fig. 1, a weight loss occurs about 600 C for the Si/rGP hybrid, indicating the oxidation and decomposition of graphene in air as described in the previous literature [29,30]. The oxidation and decomposition temperature of the composite is about 600 C, higher than that (about 500 C) of pure rGO from GO, which strongly suggests the occurrence of interactions between the rGO sheets and Si nanoparticles. Due to the unoxidized Si powder at 600 C in air, it is reasonable to determine the weight percentage of Si in the composite from the largest weight loss in the TGA curve. The mass fraction of silicon in the Si/rGP composite is about 50%. With increasing the temperature, the Si and negligible SiOx are left alone. The XRD patterns of the Si/rGP and Si/GO are presented in Fig. 2.
Fig. 5. (a) Nitrogen adsorption and desorption isotherms of Si/rGP and Si/rGO, (b) the corresponding pore size distribution plots of Si/rGP and Si/rGO.
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structure with interconnected pores. The inset in Fig. 4a is a digital photograph of Si/rGO after hydrothermal treatment. The Si/rGP composite exhibits the same structure, indicating that the 3D porous structure is still retention after steam etching treatment (Fig. 4b). As shown in Fig. 4c and d, the Si nanoparticles are well-encapsulated by the rGO sheets with some wrinkles (black line in Fig. 4d), which is typical for rGO sheets [33]. In the high resolution TEM image of Si/rGP (Fig. 4e), the nano-holes in the rGO sheets can be found. Meanwhile, a free of SiOx shell on Si surface is well-encapsulated with a thickness of about 10 nm. These nano-holes on the rGO sheets not only supply plenty of channels for efficient Liþ diffusion/transport access to the active material, but also provide the space to release the volume change of Si. Besides, the good coating effect plays an important role in enhancing the conductivity of overall electrode and the availability of active materials. Fig. 4f displays the Raman spectra of the Si/GO and Si/rGP. Two characteristic peaks are observed corresponding to the D and G bands of graphene based materials in the Raman spectra. In general, the ID/IG ratio is an important parameter to evaluate the graphitization degree of carbonaceous materials and the density of defects in graphene based materials [20,23]. After steam etching treatment, the ID/IG ratio increases from 0.88 to 1.05 and this result may be attributed to the presence of localized sp3 defects within the sp2 carbon network upon reduction of the exfoliated GO. The particular specific surface area and pore size distribution performance of Si/rGP and Si/rGO by nitrogen adsorptionedesorption isotherms experiments. As shown in Fig. 5, based
Fig. 6. (a) CV curves of Si/rGP in the initial four cycles at a scan rate of 0.1 mV s1 in the potential range of 0e1.5 V (vs. Li/Liþ), (b) galvanostatic dischargeecharge curves for different cycles.
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on the BarretteJoynereHalenda (BJH) equation, both materials possess a similar average pore diameter of 5 nm (Fig. 5b), but they have very different specific surface areas (Si/rGP: 241.187 m2 g1 and Si/rGO: 71.679 m2 g1) and pore volumes (2.602 m3 g1 for the
Fig. 7. (a) Cycling performance of Si/rGP at a current density of 50 mA g1, (b) cycling performance of Si/rGP, Si/rGO, rGO and pure silicon at a current density of 50 mA g1, (c) rate capability of Si/rGP at current densities ranging from 50 to 200 mA g1 and then 200 mA g1 for 100 cycles, (d) Nyquist plots of Si/rGP and Si particle electrodes after 50 cycles from 100 kHz to 0.01 Hz in the fully charged state.
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Fig. 8. (a) Schematic illustration of the in-situ experimental setup, the first lithiation of Si/rGP with lithiation time of (b) 0 s, (c)10 s, (d) 60 s, (e) schematic of lithiation behavior of crystalline Si nanoparticle.
Si/rGP and 1.071 m3 g1 for the Si/rGO). In principle, the Si/rGP composite with higher specific surface can have steady cycling performance, but on the other hand, it may cause lower initial coulombic efficiency than some others [16,34,35]. The CV curves of Si/rGP at a scan rate of 0.1 mV s1 is shown in Fig. 6a. In the first cycle, the reaction of crystalline silicon with lithium to form amorphous LixSi appears at below 0.1 V. There is broad peak at around 0.7 V, which can be ascribed to the formation of the solid-electrolyte interphase (SEI) on porous composite and the reaction of lithium ions with the residual oxygen-containing groups of rGO [1,36]. During the subsequent charging process, two anodic branches located at 0.3 and 0.5 V and one cathodic peak located at 0.2 V are observed, which corresponds to the delithiation and lithiation of amorphous Si, respectively, consistent with previous works for Si/rGO composite anodes [37,38]. Fig. 6b shows the 1st, 2nd, 10th, 30th and 50th dischargeecharge voltage profiles of the Si/rGP electrode. The initial discharge capacity is 3168.2 mAh g1 with a reversible charge capacity up to 1536.2 mAh g1. The reasons of irreversible capacity loss are ascribed to the reaction of crystalline silicon with lithium to form amorphous LixSi at the initial discharge voltage plateau of 0.1 V and the formation of the SEI film [1,36]. After the first cycle, the voltage plateaus between 0.2 and 0.6 V corresponds to the alloy-dealloy of LixSi phase, and the coulombic efficiency increases and stabilizes at about 98% in subsequent cycles. Fig. 7a shows the cycling performance of the Si/rGP electrode at a current density of 50 mA g1 between 0.01 and 1.5 V. Apparently, the Si/rGP electrode exhibits good cycling performance (1004 mAh g1 after 100 cycles). For comparison, the electrochemical performance of the Si/rGO and rGO was also investigated under the same electrochemical condition. After 50 cycles, they delivers a reversible capacity of 801 mAh g1 and 294 mAh g1 (Fig. 7b), respectively, much lower than the Si/rGP electrode. Moreover, the rate capability of the Si/rGP was also evaluated from 50 mA g1 to 200 mA g1. As shown in Fig. 7c, the Si/rGP electrode can deliver reversible capacities of about 1763, 1824, 1358 and 1507 mAh g1 at a current density of 50, 100, 200 and 50 mA g1, respectively, and then the capacity remains 849 mAh g1 at a current density of 200 mA g1 after 100 cycles. The improved electrochemical performance of the Si/rGP composite could be attributed to the unique porous structure and good conductivity. The more nano-channels for Li ions can easier get to the active material and shorten the distance of Li ions. The porous structure of the rGO sheets also supplies more spaces to alleviating the volume change of the silicon particles during the
charge/discharge process. The coating effect of rGO sheets avoids the aggregation phenomenon of Si and improves the overall electrode conductivity [39,40]. EIS measurements were employed to deeply understand the electrochemical performance of Si/rGP and Si particle electrodes. Fig. 7d shows the Nyquist plots of both the electrodes after 50 cycles from 100 kHz to 0.01 Hz in the fully charged state. The first semicircle in high frequency region is related to the ionic conduction through the electrolyte and electronic conduction between the substrate and the active material [41,42]. With decreasing the frequency, the spectrum is dominated by the formation of SEI film. Meanwhile, the sloping straight line corresponds to the Li-ion diffusion in the bulk materials [43e45]. It clearly shows the diameter of the semicircle for the Si/ rGP composite is much smaller than that of the pure Si electrode. It indicates that the porous rGO sheets can reduce the charge transfer at the electrode/electrolyte interface, and consequently decrease the internal resistance. To further check the electrochemical reactions of Si nanoparticles encapsulated by porous rGO sheets, we used in-situ TEM to understand the effect of rGO sheets on phase transformation and concomitant volume change of embedded Si nanoparticles. The nanoscale electrochemical LIB device to enable direct observation is schematically illustrated in Fig. 8a. As shown in Fig. 8bed (the corresponding video is in the Supplementary Movie) and the illustration in Fig. 8e, it can be seen that the interface is gradually formed between amorphous LixSi and crystalline Si. These mean that the lithiation behavior of crystalline Si nanoparticles encapsulated by the rGO sheets shows many similarities to the pure crystalline Si nanoparticles. And the results are also consistent with the CV tests. The lithiation kinetics cannot be dramatically promoted by higher mobility of Li ions since the interface reaction plays a more significant role. The lithiation is limited by the reaction rate at the interface (e.g. the break of SieSi bond) of LixSi and crystalline Si, rather than by the diffusion of lithium through the amorphous phase. This explains the lithiation behavior of the composite is similar to that of the pure Si nanoparticles [46e48]. But from the supplementary video, it indicates that the rGO sheets still attach to the interface of the silicon particles and help multiple Si nanoparticles to maintain the integrity, meanwhile, supply diffusion pathways of electron and Liþ during charge/discharge cycles. Due to the elasticity of rGO, large volume expansion during lithiation can be released and suppress the pulverization of Si and disintegration of Si electrode by relaxing the stress during lithiation, while it would not release the significant Si volume expansion itself [40,49e53].
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Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.03.185. 4. Conclusions In summary, a novel Si/rGO nano-porous network composite electrode has been successfully fabricated by steam etching Si/rGO aerogel. The in-situ TEM demonstrates the lithiation behavior of crystalline Si nanoparticles, i.e, the lithiation is limited by the reaction rate at the interface of LixSi and crystalline Si. The stable porous network structure with high specific surface area is capable to reduce the transfer resistance of Li ions in the electrode and improve the electrochemical performance of silicon anode material. Meanwhile, the synthetic process can offer a new progress for the next generation of rechargeable lithium ion batteries. Acknowledgments This work is supported by the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037) and Key Science and Technology Innovation Team of Zhejiang Province (2010R50013). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.03.185. References [1] R.Z. Hu, W. Sun, Y.L. Chen, M.Q. Zeng, M. Zhu, Nanoscale 5 (2013) 1470e1474. [2] R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Nature 414 (2001) 359e367. [3] Q.Q. Xiong, J.P. Tu, X.H. Xia, X.Y. Zhao, C.D. Gu, X.L. Wang, Nanoscale 5 (2013) 7906e7912. [4] Z.Y. Wang, D.Y. Luan, S. Madhavi, Y. Hu, X.W. Lou, Energy Environ. Sci. 5 (2012) 5252e5256. [5] Q.Q. Xiong, J.P. Tu, Y. Lu, J. Chen, Y.X. Yu, X.L. Wang, C.D. Gu, J. Mater. Chem. 22 (2012) 18639e18645. [6] Y. Lu, J.P. Tu, Q.Q. Xiong, J.Y. Xiang, Y.J. Mai, J. Zhang, Y.Q. Qiao, X.L. Wang, C.D. Gu, S.X. Mao, Adv. Funct. Mater. 22 (2012) 3927e3935. [7] Y.H. Xu, J.C. Guo, C.S. Wang, J. Mater. Chem. 22 (2012) 9562e9567. [8] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395e1397. [9] I.A. Profatilova, C. Stock, A. Schmitz, S. Passerini, M. Winter, J. Power Sources 222 (2013) 140e149. [10] H. Liu, L.B. Hu, Y.S. Meng, Q. Li, Nanoscale 5 (2013) 10376e10383. [11] Y.Y. Tang, X.H. Xia, Y.X. Yu, S.J. Shi, J. Chen, Y.Q. Zhang, J.P. Tu, Electrochim. Acta 88 (2013) 664e670. [12] S. Iwamura, H. Nishihara, T. Kyotani, J. Power Sources 222 (2013) 400e409. [13] X.S. Zhou, Y.X. Yin, L.J. Wan, Y.G. Guo, Chem. Commun. 48 (2012) 2198e2200. [14] R.Z. Hu, W. Sun, Y.L. Chen, M.Q. Zeng, M. Zhu, J. Mater. Chem. A 2 (2014) 9118e9125. [15] S.L. Jing, H. Jiang, Y.J. Hu, C.Z. Li, J. Mater. Chem. A 2 (2014) 16360e16364. [16] M. Ko, S. Chae, S. Jeong, P. Oh, J. Cho, ACS Nano 8 (2014) 8591e8599.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Porous reduced graphene oxide sheet wrapped silicon composite fabricated by steam etching for lithium-ion battery application H. Tang a, J. Zhang b, Y.J. Zhang a, Q.Q. Xiong a, Y.Y. Tong a, Y. Li a, X.L. Wang a, C.D. Gu a, J.P. Tu a, * a
State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Provence, and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China b
h i g h l i g h t s A novel preparation method is fabricated by steam etching of Si/rGO aerogel. This method is easy to handle and exhibited excellent performances. The in-situ TEM verifies the well-retained integrity of electrode in the substrate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2015 Received in revised form 28 March 2015 Accepted 30 March 2015 Available online 31 March 2015
A novel of Si/porous reduced graphene oxide (rGO) composite is fabricated by steam etching of Si/rGO aerogel. The rGO sheets with nano-holes build a unique three-dimensional porous network and can encapsulate the Si nanoparticles. The porous structure of Si/rGO composite can reduce the transfer distance of Li ions and restrain the aggregation and destruction of Si particles. The in-situ transmission electron microscopy (TEM) observation demonstrates that the porous rGO sheets help the entire electrode to maintain highly conductive and facilitate the lithiation of Si nanoparticles. The composite electrode presents high specific capacity and good cycling stability (1004 mAh g1 at 50 mA g1 up to 100 cycles). © 2015 Elsevier B.V. All rights reserved.
Keywords: Silicon Reduced graphene oxide Steam etching In-situ transmission electron microscopy Lithium-ion battery
1. Introduction Lithium ion batteries (LiBs) are the important power source in different fields, including electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) [1,2]. High energy and power density, and excellent cycling stability are factors for the applications of LIBs. However, to meet these demands, the high capacity anode materials, such as transition metal oxides and phosphides [3e6], tin alloys and oxides [7,8], and silicon [9e11], have been studied to substitute commercial graphite with low theoretical specific capacity (372 mAh g1) [12,13]. Among these anode materials, Si is the most promising alternative because of
* Corresponding author. E-mail addresses: [email protected], [email protected] (J.P. Tu). http://dx.doi.org/10.1016/j.jpowsour.2015.03.185 0378-7753/© 2015 Elsevier B.V. All rights reserved.
its low cost, natural abundance and especially its highest theoretical capacity (4200 mAh g1, corresponding to the Li22Si5 alloy at room temperature) with a low potential window [14,15]. However, due to its low intrinsic electrical conductivity (1.56 103 S m1) and large volume change (>300%), Si suffers from limited cycling stability during the charge/discharge process [16,17]. In order to overcome these problems, the most usual approach is to prepare composites with Si nanoparticles dispersed uniformly in a carbonaceous matrix [18,19]. Graphene, a two-dimensional carbon nano-material, has a potential application for energy storage due to its large surface area, excellent conductivity, flexibility and chemical stability [20e22]. The reduced graphene oxide (rGO) as an active matrix can improve the electrochemical performance of Si-based materials [1,23,24]. The improved electrochemical performance can be
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Scheme 1. Schematic of the synthesis procedure of Si/rGP.
attributed to the fact that rGO can not only enhance the electrical conductivity of Si and provide a support for dispersing Si nanostructures, but also effectively release the volume change and aggregation of Si nanoparticles during the charge/discharge process. In this present work, we fabricated a Si/rGO nano-porous network composite (Si/rGP) anode material by simple steam etching Si/rGO aerogel. The stable porous network structure with high specific surface area is capable to reduce the transfer resistance of Li ions, and the Si/rGP composite electrode exhibits the improved reversible capacity and cycling performance. The facile method provides a new approach for the Si-based anode materials for LIBs.
Fig. 1. TG curves of Si/rGP, rGO and pure Si in air at a heating rate of 5 C min1.
Fig. 2. XRD patterns of Si/GO and Si/rGP powders.
Fig. 3. XPS spectra of Si/rGP: (a) survey scan, (b) Si2p region of Si/rGP, (c) C 1s curve of Si/rGP.
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Fig. 4. SEM images of (a) the Si/rGO after hydrothermal treatment and (b) the Si/rGP after steam etching at low magnification, the inset of (a) is a digital photograph of Si/rGO hydrogel; (c) the Si/rGP at high magnification; TEM images of the Si/rGP ((d) at low magnification and (e) at high magnification); (f) Raman spectra of Si/GO and Si/rGP.
2. Experimental section 2.1. Materials Si were received from CWNANO (<100 nm in size, 99.9% purity) without further purification. Graphite powder (99.95% metals basis) and polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) were obtained from Aladdin. H2SO4, H3PO4, NaNO3, KMnO4, H2O2, HCl and thiourea were purchased from Sinopharm Chemical Reagent Co., Ltd without further purification. 2.2. Preparation of Si/r-GP composite material Graphene oxide (GO) was prepared from graphite powder according to the modified Hummers' method [25]. Si nanoparticles (1.4 g) and 10 ml of 20 wt.% PDDA aqueous solution were dispersed in water (1000 mL) by sonication in a water bath (KQ2200DE, 99 kHz). The excess PDDA was removed by four repeated centrifugation (14000 rpm 10 min)/wash/redispersion cycles. After that, GO was exfoliated in distilled water with a concentration of 2 mg mL1. Then the Si-PDDA aqueous solution was diluted to the GO aqueous dispersion. After that, 0.5 g of thiourea was added into the homogeneous Si-PDDA/GO aqueous dispersion, and then the mixture was sealed in a 100 ml Teflon-lined stainless steel autoclave and maintained at 180 C for 4.5 h. Finally, the Si/rGO assembly was dipped into distilled water for 24 h to remove the residual thiourea [26]. And vacuum freeze-drying was used to remove water and obtain dry Si/rGO aerogel. In order to obtaining the Si/rGP, the aerogel was put into the beaker and sealed in a
100 ml Teflon-lined stainless steel autoclave with 10 ml DI water and heated to 200 C for 10 h. Finally, the Si/rGP was prepared by freeze-drying under vacuum (<20 Pa) for 48 h. The detailed synthesis is schematically demonstrated in Scheme 1. For comparison, the same method was used to prepare the pure nano-porous rGO (rGP), and the Si/rGO without nano-holes on the rGO sheets was prepared by hydrothermal treatment. 2.3. In-situ electrochemical experiments The in-situ electrochemical experiments were carried out using a Nanofactory TEM-scanning tunneling microscopy (STM) holder in a JEOLJEM-2100F TEM. Briefly, a 0.25 mm-thick gold rod was cut to produce a clean and fresh cross section; then the Si/rGP powder was glued to the pretreated gold rod with conductive epoxy as the working electrode. A sharp tungsten STM tip was used to scratch Li metal surface to fetch some fresh Li inside a glovebox filled with argon. The Li layer on the tip of the W rod was served as the counter electrode and lithium source. Both the Si/rGP and lithium electrodes were mounted onto a Nanofactory STM-TEM holder inside the same glovebox. The holder was quickly transferred into the TEM column; a native Li2O layer formed on the surface of Li metal due to the exposure to air, which was served as the solid-state electrolyte to allow the transport of Liþ. The Li2O/Li electrode was mounted on the mobile STM probe, which could be driven to contact the Si/rGP electrode inside the TEM. Lithiation took place after a negative bias was applied on the Si/rGP with respect to the lithium metal to drive Liþ transport through the solid-state Li2O layer, and the bias was then reversed to positive to facilitate delithiation.
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2.4. Materials characterization The microstructure and morphology of the products were characterized by Xray diffraction (XRD, Rigaku D/max 2550 PC, Cu Ka), Xray photoelectron spectroscopy (XPS, AXIS UTLTRADLD), scanning electron microscopy (SEM, Hitachi S4700) and transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). The Raman spectra were measured on a JobinYvon Labor Raman HR800 using Arion laser of 514.5 nm. Zetasizer 3000HSA was used to test the Zeta potential for characterization of charged silicon, rGO and Si-PDDA/rGO composite. The thermogravimetric (TG) analysis of Si/rGP during annealing was obtained using a TA Q600 instrument in a temperature range from 25 to 800 C with a heating rate of 5 C min1 in air. The specific surface area and pore diameter were determined by BET(BrunauerEmmett-Teller) measurements using a Autosorb-1-C surface area analyzer. 2.5. Electrochemical measurements To prepare working electrodes, the active material, Super-P carbon black, and polyvinylidene fluoride (PVDF) with mass ratio 80: 10: 10 were mixed into a homogeneous slurry with mortar and pestle, and then the slurry was pasted onto pure Cu foil. The electrochemical tests were performed using a cointype half cell (CR 2025). Test cells were assembled in an argonfilled glove box with the metallic lithium foil as both the reference and counter electrodes, 1 M LiPF6 in ethylene carbonate (EC)dimethyl carbonate (DME) (1: 1 in volume) as the electrolyte, and a polypropylene (PP) microporous film (Cellgard 2300) as the separator. The galvanostatic chargedischarge tests were conducted on a LAND battery program control test system at different current densities between 0.01 and 1.5 V at room temperature (25 ± 1 C). Cyclic voltammetry (CV) tests were performed on a CHI660C electrochemical workstation in the potential range of 0e1.5 V (vs. Li/Liþ) at a scanning rate of 0.1 mV s1. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 100 kHz to 0.01 Hz under AC stimulus with 5 mV of amplitude.
Obviously, the diffraction peaks of the Si/rGP and Si/GO are matched well with the silicon crystal planes of (111), (220), (311). The diffraction peak (001) at 10.9 of Si/GO is a typical peak of GO and the additional broadened peak (002) confirms the conversion of GO to rGO after the hydrothermal treatment as described in the previous literature [17,31]. Fig. 3a shows the XPS spectrum of the Si/rGP composite. In the survey region, it is clear that C, O, and Si elements coexist in the sample. Fig. 3b presents the highresolution spectrum of Si 2p with three components of SieC at 99.9 eV, monatomic silicon at 99.0 eV and Si/SiOx (x < 2) at 103.5 eV. There are no peaks (110 and 105 eV, i.e.) from SiO2 [32]. The formation of SiOx shells is possibly by the oxidation of Si nano-crystals with a high surface activity and the surface charge of Si is obtained negatively (zeta potential: 19.5 mV). And then, the Si/SiOx can adsorb PDDA through electrostatic attraction to change the surface charge from negative to positive (Si-PDDA zeta potential: þ77.5 mV). Finally, the obtained positively charged SiPDDA nanoparticles assemble with negatively charged GO by electrostatic attraction (Si-PDDA/GO zeta potential: 38 mV). Therefore, a good coating effect of GO is obtained and the rGO sheets can encapsulate the Si nanoparticles. The C 1s core level peak can be split into three components (Fig. 3c). The peaks of CeC, SieC and CeOH are detected at 284.6, 284.3, and 285.6 eV, respectively, which indicates that most of the oxygen-containing groups are removed after hydrothermal treatment [31]. Fig. 4a and b shows the SEM image of Si/rGO after just hydrothermal treatment and Si/rGP after steam etching treatment, respectively. It is obviously that Si/rGO exhibits a 3D network
3. Results and discussion The formation of the nano-porous network structure is based on the steam etching Si/rGO aerogel. In the previous works [27,28], the carbon-based materials can be converted to gaseous fuels such as CO and H2 by the so-called water gas reaction with hot steam. Therefore, with the steam etching process (Scheme 1), carbon atoms can react with H2O vapor to produce CO and H2, leaving behind carbon vacancies that eventually grow into large pores. To determine the content of Si in the Si/rGP composite, TGA measurements were performed in a temperature range from 25 to 800 C in air. As shown in Fig. 1, a weight loss occurs about 600 C for the Si/rGP hybrid, indicating the oxidation and decomposition of graphene in air as described in the previous literature [29,30]. The oxidation and decomposition temperature of the composite is about 600 C, higher than that (about 500 C) of pure rGO from GO, which strongly suggests the occurrence of interactions between the rGO sheets and Si nanoparticles. Due to the unoxidized Si powder at 600 C in air, it is reasonable to determine the weight percentage of Si in the composite from the largest weight loss in the TGA curve. The mass fraction of silicon in the Si/rGP composite is about 50%. With increasing the temperature, the Si and negligible SiOx are left alone. The XRD patterns of the Si/rGP and Si/GO are presented in Fig. 2.
Fig. 5. (a) Nitrogen adsorption and desorption isotherms of Si/rGP and Si/rGO, (b) the corresponding pore size distribution plots of Si/rGP and Si/rGO.
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structure with interconnected pores. The inset in Fig. 4a is a digital photograph of Si/rGO after hydrothermal treatment. The Si/rGP composite exhibits the same structure, indicating that the 3D porous structure is still retention after steam etching treatment (Fig. 4b). As shown in Fig. 4c and d, the Si nanoparticles are well-encapsulated by the rGO sheets with some wrinkles (black line in Fig. 4d), which is typical for rGO sheets [33]. In the high resolution TEM image of Si/rGP (Fig. 4e), the nano-holes in the rGO sheets can be found. Meanwhile, a free of SiOx shell on Si surface is well-encapsulated with a thickness of about 10 nm. These nano-holes on the rGO sheets not only supply plenty of channels for efficient Liþ diffusion/transport access to the active material, but also provide the space to release the volume change of Si. Besides, the good coating effect plays an important role in enhancing the conductivity of overall electrode and the availability of active materials. Fig. 4f displays the Raman spectra of the Si/GO and Si/rGP. Two characteristic peaks are observed corresponding to the D and G bands of graphene based materials in the Raman spectra. In general, the ID/IG ratio is an important parameter to evaluate the graphitization degree of carbonaceous materials and the density of defects in graphene based materials [20,23]. After steam etching treatment, the ID/IG ratio increases from 0.88 to 1.05 and this result may be attributed to the presence of localized sp3 defects within the sp2 carbon network upon reduction of the exfoliated GO. The particular specific surface area and pore size distribution performance of Si/rGP and Si/rGO by nitrogen adsorptionedesorption isotherms experiments. As shown in Fig. 5, based
Fig. 6. (a) CV curves of Si/rGP in the initial four cycles at a scan rate of 0.1 mV s1 in the potential range of 0e1.5 V (vs. Li/Liþ), (b) galvanostatic dischargeecharge curves for different cycles.
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on the BarretteJoynereHalenda (BJH) equation, both materials possess a similar average pore diameter of 5 nm (Fig. 5b), but they have very different specific surface areas (Si/rGP: 241.187 m2 g1 and Si/rGO: 71.679 m2 g1) and pore volumes (2.602 m3 g1 for the
Fig. 7. (a) Cycling performance of Si/rGP at a current density of 50 mA g1, (b) cycling performance of Si/rGP, Si/rGO, rGO and pure silicon at a current density of 50 mA g1, (c) rate capability of Si/rGP at current densities ranging from 50 to 200 mA g1 and then 200 mA g1 for 100 cycles, (d) Nyquist plots of Si/rGP and Si particle electrodes after 50 cycles from 100 kHz to 0.01 Hz in the fully charged state.
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Fig. 8. (a) Schematic illustration of the in-situ experimental setup, the first lithiation of Si/rGP with lithiation time of (b) 0 s, (c)10 s, (d) 60 s, (e) schematic of lithiation behavior of crystalline Si nanoparticle.
Si/rGP and 1.071 m3 g1 for the Si/rGO). In principle, the Si/rGP composite with higher specific surface can have steady cycling performance, but on the other hand, it may cause lower initial coulombic efficiency than some others [16,34,35]. The CV curves of Si/rGP at a scan rate of 0.1 mV s1 is shown in Fig. 6a. In the first cycle, the reaction of crystalline silicon with lithium to form amorphous LixSi appears at below 0.1 V. There is broad peak at around 0.7 V, which can be ascribed to the formation of the solid-electrolyte interphase (SEI) on porous composite and the reaction of lithium ions with the residual oxygen-containing groups of rGO [1,36]. During the subsequent charging process, two anodic branches located at 0.3 and 0.5 V and one cathodic peak located at 0.2 V are observed, which corresponds to the delithiation and lithiation of amorphous Si, respectively, consistent with previous works for Si/rGO composite anodes [37,38]. Fig. 6b shows the 1st, 2nd, 10th, 30th and 50th dischargeecharge voltage profiles of the Si/rGP electrode. The initial discharge capacity is 3168.2 mAh g1 with a reversible charge capacity up to 1536.2 mAh g1. The reasons of irreversible capacity loss are ascribed to the reaction of crystalline silicon with lithium to form amorphous LixSi at the initial discharge voltage plateau of 0.1 V and the formation of the SEI film [1,36]. After the first cycle, the voltage plateaus between 0.2 and 0.6 V corresponds to the alloy-dealloy of LixSi phase, and the coulombic efficiency increases and stabilizes at about 98% in subsequent cycles. Fig. 7a shows the cycling performance of the Si/rGP electrode at a current density of 50 mA g1 between 0.01 and 1.5 V. Apparently, the Si/rGP electrode exhibits good cycling performance (1004 mAh g1 after 100 cycles). For comparison, the electrochemical performance of the Si/rGO and rGO was also investigated under the same electrochemical condition. After 50 cycles, they delivers a reversible capacity of 801 mAh g1 and 294 mAh g1 (Fig. 7b), respectively, much lower than the Si/rGP electrode. Moreover, the rate capability of the Si/rGP was also evaluated from 50 mA g1 to 200 mA g1. As shown in Fig. 7c, the Si/rGP electrode can deliver reversible capacities of about 1763, 1824, 1358 and 1507 mAh g1 at a current density of 50, 100, 200 and 50 mA g1, respectively, and then the capacity remains 849 mAh g1 at a current density of 200 mA g1 after 100 cycles. The improved electrochemical performance of the Si/rGP composite could be attributed to the unique porous structure and good conductivity. The more nano-channels for Li ions can easier get to the active material and shorten the distance of Li ions. The porous structure of the rGO sheets also supplies more spaces to alleviating the volume change of the silicon particles during the
charge/discharge process. The coating effect of rGO sheets avoids the aggregation phenomenon of Si and improves the overall electrode conductivity [39,40]. EIS measurements were employed to deeply understand the electrochemical performance of Si/rGP and Si particle electrodes. Fig. 7d shows the Nyquist plots of both the electrodes after 50 cycles from 100 kHz to 0.01 Hz in the fully charged state. The first semicircle in high frequency region is related to the ionic conduction through the electrolyte and electronic conduction between the substrate and the active material [41,42]. With decreasing the frequency, the spectrum is dominated by the formation of SEI film. Meanwhile, the sloping straight line corresponds to the Li-ion diffusion in the bulk materials [43e45]. It clearly shows the diameter of the semicircle for the Si/ rGP composite is much smaller than that of the pure Si electrode. It indicates that the porous rGO sheets can reduce the charge transfer at the electrode/electrolyte interface, and consequently decrease the internal resistance. To further check the electrochemical reactions of Si nanoparticles encapsulated by porous rGO sheets, we used in-situ TEM to understand the effect of rGO sheets on phase transformation and concomitant volume change of embedded Si nanoparticles. The nanoscale electrochemical LIB device to enable direct observation is schematically illustrated in Fig. 8a. As shown in Fig. 8bed (the corresponding video is in the Supplementary Movie) and the illustration in Fig. 8e, it can be seen that the interface is gradually formed between amorphous LixSi and crystalline Si. These mean that the lithiation behavior of crystalline Si nanoparticles encapsulated by the rGO sheets shows many similarities to the pure crystalline Si nanoparticles. And the results are also consistent with the CV tests. The lithiation kinetics cannot be dramatically promoted by higher mobility of Li ions since the interface reaction plays a more significant role. The lithiation is limited by the reaction rate at the interface (e.g. the break of SieSi bond) of LixSi and crystalline Si, rather than by the diffusion of lithium through the amorphous phase. This explains the lithiation behavior of the composite is similar to that of the pure Si nanoparticles [46e48]. But from the supplementary video, it indicates that the rGO sheets still attach to the interface of the silicon particles and help multiple Si nanoparticles to maintain the integrity, meanwhile, supply diffusion pathways of electron and Liþ during charge/discharge cycles. Due to the elasticity of rGO, large volume expansion during lithiation can be released and suppress the pulverization of Si and disintegration of Si electrode by relaxing the stress during lithiation, while it would not release the significant Si volume expansion itself [40,49e53].
H. Tang et al. / Journal of Power Sources 286 (2015) 431e437
Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.03.185. 4. Conclusions In summary, a novel Si/rGO nano-porous network composite electrode has been successfully fabricated by steam etching Si/rGO aerogel. The in-situ TEM demonstrates the lithiation behavior of crystalline Si nanoparticles, i.e, the lithiation is limited by the reaction rate at the interface of LixSi and crystalline Si. The stable porous network structure with high specific surface area is capable to reduce the transfer resistance of Li ions in the electrode and improve the electrochemical performance of silicon anode material. Meanwhile, the synthetic process can offer a new progress for the next generation of rechargeable lithium ion batteries. Acknowledgments This work is supported by the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037) and Key Science and Technology Innovation Team of Zhejiang Province (2010R50013). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.03.185. References [1] R.Z. Hu, W. Sun, Y.L. Chen, M.Q. Zeng, M. Zhu, Nanoscale 5 (2013) 1470e1474. [2] R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Nature 414 (2001) 359e367. [3] Q.Q. Xiong, J.P. Tu, X.H. Xia, X.Y. Zhao, C.D. Gu, X.L. Wang, Nanoscale 5 (2013) 7906e7912. [4] Z.Y. Wang, D.Y. Luan, S. Madhavi, Y. Hu, X.W. Lou, Energy Environ. Sci. 5 (2012) 5252e5256. [5] Q.Q. Xiong, J.P. Tu, Y. Lu, J. Chen, Y.X. Yu, X.L. Wang, C.D. Gu, J. Mater. Chem. 22 (2012) 18639e18645. [6] Y. Lu, J.P. Tu, Q.Q. Xiong, J.Y. Xiang, Y.J. Mai, J. Zhang, Y.Q. Qiao, X.L. Wang, C.D. Gu, S.X. Mao, Adv. Funct. Mater. 22 (2012) 3927e3935. [7] Y.H. Xu, J.C. Guo, C.S. Wang, J. Mater. Chem. 22 (2012) 9562e9567. [8] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395e1397. [9] I.A. Profatilova, C. Stock, A. Schmitz, S. Passerini, M. Winter, J. Power Sources 222 (2013) 140e149. [10] H. Liu, L.B. Hu, Y.S. Meng, Q. Li, Nanoscale 5 (2013) 10376e10383. [11] Y.Y. Tang, X.H. Xia, Y.X. Yu, S.J. Shi, J. Chen, Y.Q. Zhang, J.P. Tu, Electrochim. Acta 88 (2013) 664e670. [12] S. Iwamura, H. Nishihara, T. Kyotani, J. Power Sources 222 (2013) 400e409. [13] X.S. Zhou, Y.X. Yin, L.J. Wan, Y.G. Guo, Chem. Commun. 48 (2012) 2198e2200. [14] R.Z. Hu, W. Sun, Y.L. Chen, M.Q. Zeng, M. Zhu, J. Mater. Chem. A 2 (2014) 9118e9125. [15] S.L. Jing, H. Jiang, Y.J. Hu, C.Z. Li, J. Mater. Chem. A 2 (2014) 16360e16364. [16] M. Ko, S. Chae, S. Jeong, P. Oh, J. Cho, ACS Nano 8 (2014) 8591e8599.
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