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Fabrication of graphene nanoplatelets-supported SiOx-disordered carbon composite and its application in lithium-ion batteries
Journal of Power Sources 293 (2015) 976e982
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Fabrication of graphene nanoplatelets-supported SiOx-disordered carbon composite and its application in lithium-ion batteries Mingqi Li a, b, *, Yan Yu a, Jing Li a, Baoling Chen a, Aishuak Konarov a, P. Chen a a b
Department of Chemical Engineering, University of Waterloo, Waterloo, N2L3G1, Canada College of Chemistry and Chemical Engineering, China West Normal University, Nanchong, 637009, China
h i g h l i g h t s SiOx-C/GNPs is fabricated by self-assembly and heat treatment. SiOx-C/GNPs exhibits a high reversible capacity with excellent capacity retention. This work provides a facile route to improve the electrochemical performance of SiOx.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2015 Received in revised form 23 May 2015 Accepted 4 June 2015 Available online 14 June 2015
Developing high-capacity electrode materials is critical to enhance the energy-density of lithium-ion batteries. Here, a new graphene nanoplatelets-supported SiOx-disordered carbon composite (SiOx-C/ €ber-type process and high-temperature treatment. When used as an anode GNPs) is fabricated by a Sto for lithium-ion batteries, SiOx-C/GNPs exhibits excellent cycle stability and rate capability even under deep chargeedischarge cycling. A stable reversible capacity of about 630 mAh g1 (calculated on the total mass of the composite) can be achieved at a current density of 100 mA g1 and the capacity retention is nearly 100% after 250 cycles. Additionally, the plateau voltages of lithium extraction for the composite are only slightly higher than those for the commercial graphite, which ensures the high energy density of the assembled batteries. The superior electrochemical performance of this novel material is due to its unique features: excellent electronic conductivity, short transportation length for both lithium ions and electrons, elastomeric space to accommodate volume changes upon Li insertion/extraction and robust connection between SiOx and GNPs. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.
Keywords: Lithium-ion batteries Graphene nanoplatelets Anode Nonstoichiometric silicon oxide Electrochemical performance
1. Introduction As one of the most important energy-storage devices, lithiumion batteries (LIBs) have attracted much attention in the scientific and industrial fields [1,2]. There is an increasing interest in developing high-capacity anode materials for the next generation of rechargeable LIBs. In this respect, silicon-based materials possess the largest potential due to high specific capacity, satisfying lithium lithiation/delithiation potentials and high abundance. The main handicap for the practical application of silicon-based materials is the huge volume change during alloying/dealloying process, which causes poor cycle stability [3,4]. Many studies have been devoted to
* Corresponding author. College of Chemistry and Chemical Engineering, China West Normal University, Nanchong, 637009, China. E-mail address: [email protected] (M. Li). http://dx.doi.org/10.1016/j.jpowsour.2015.06.019 0378-7753/Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.
the resolution of the problem and demonstrated that the reduction of silicon particles size and the fabrication of silicon-based composite materials with special structure were the most effective means to achieve the improved cycle performance [4e10]. However, it is practically difficult to attempt to synthesize nanosilicons by a facile chemical method. Currently, the preparation of nanosilicons usually experiences a complicated process or needs strict operation conditions [6,7,11,12], which lead to a high cost and make their scale-up production difficult. In comparison with elemental silicon with the same particle size, silicon oxides show less volume change and Li2O and lithium silicate generated during the initial lithiation buffer the volume expansion of silicon, thus silicon oxide electrodes usually show better cyclic stability than silicon electrodes [13e19]. Of course, it should be noted that silicon oxides have a poor electronic conductivity, and the inner stress caused by the volume changes upon Li insertion/extraction can still destroy
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the microstructure of the electrodes during prolonged cycling although the stress has been reduced as compared with that in elemental silicon. Silicon oxides can be divided into stoichiometric SiO and SiO2, and nonstoichiometric SiOx (0 < x < 2, xs1). In the past, most studies about silicon oxides as electrode materials were focused on SiO and SiO2 [17,18,20e23]. Until recently, nonstoichiometric SiOx (1 < x < 2) has attracted researchers' attention because it is found that SiOx (1 < x < 2) can be prepared by hydrolysis reaction of siloxanes or silicon halides under mild reaction conditions [24e28], which not only lowers the preparation cost, but also offers much more choices for their improvement in electrochemical performance. Herein, we report a new way to improve the electrochemical properties of SiOx by engineering graphene nanoplatelets (GNPs)-supported SiOx-disordered carbon composite. GNPs, which are stacked by graphene sheets, have “platelet” morphology with a diameter ranging from 0.5 to 25 mm and an overall thickness of about 5e25 nm [29,30]. Although GNPs present a relatively smaller specific surface area compared with conventional graphene, their facile preparation and dispersion which is very important for scale-up production in a practical application, have attracted many researchers' interest. To date, GNPs have thus been applied in many fields like sensing applications [31], loading catalysts [32] and constructing supercapacitors [33]. However, to the best of our knowledge, there is no report about GNPs-supported SiOx composite as an anode for lithium-ion batteries. In this work, GNPs-supported SiOx-C composite is fabricated by self-assembly using GNPs and C2H5Si (OC2H5)3 (for short EtSi(OEt)3) as starting materials with the help of cetyltrimethylammonium bromide (CTAB) and aqueous ammonia, followed by heat treatment. Here, EtSi(OEt)3 is chosen as starting materials because we have found that siloxane with alkyl can produce a small amount of disordered carbon (see below), which can improve the conductivity of SiOx and solidify the connection between SiOx and GNPs. The as-prepared composite presents excellent cycle stability with high capacity and good rate capability. 2. Experimental 2.1. Synthesis of SiOx-C/GNPs Graphene nanoplatelets (0.25 g, ACS Material) were dispersed in a mixture containing cetyltrimethylammonium bromide (CTAB, 0.24%wt), ethanol and deionized water by pulse sonication. Subsequently, 2.5 mL of ammonium hydroxide (25 wt%) and 2.0 mL of EtSi(OEt)3 were added dropwise to the above solution. The mixed solution was stirred at room temperature for 12 h to finish the reaction. The precipitate was collected, washed three times with deionized water and dried at 100 C in vacuum overnight. The dried sample was thermally treated in a tube furnace at 1000 C in a flow of argon (100 mL min1) for 3 h, and then the furnace was naturally cooled to room temperature. The final product was labeled as SiOxC/GNPs. For comparison, SiOx-C excluding GNPs was also synthesized by similar method. 2.2. Materials characterization X-ray diffraction (XRD) patterns were recorded using D8 Discover (Bruker) equipped with Cu Ka (l ¼ 0.15406 nm) radiation at a scan rate of 3.0 min1 from 10 to 90 . Element analysis (Elementar analyzer, Elementar Americas INC) was performed to determine the carbon mass fraction in the samples. Scanning electron microscopy (SEM, Leo-1530, Zeiss), Transmission electron microscopy (TEM, Philips CM12) and energy dispersive X-ray image maps (EDX-map) were conducted to investigate the morphology,
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microstructure and element distribution of the as-prepared composites. Fourier-transform infrared reflection (FTIR) spectra were recorded on a vertex 70 FTIR spectrometer (Bruker). The specific surface area was determined by the BrunauereEmmeteTeller method (BET, ASAP 2020, Micromeritics). XPS analysis was performed by a multi-technique ultra-high vacuum Imaging XPS Microprobe system (ESCALab 250, Thermo VG Scientific). 2.3. Electrochemical measurement The electrochemical performance of the SiOx-C/GNPs composite was evaluated using 2026 coin cells assembled in an argon-filled glove box. Working electrodes were fabricated by painting slurry with 75 wt% of SiOx-C/GNPs composite as active material, 10 wt% of acetylene black (AB) as conductor and 15 wt% of sodium alginate as binder onto Cu foil. The resulting film was dried at 105 C in vacuum for 12 h. The loaded total mass of active material was 1.8e2.5 mg cm2. A lithium disc was used as the counter electrode and reference electrode. The electrolyte was 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in weight) with 2% vinylene carbonate (VC) and 5% fluoroethylene carbonate (FEC) as additives. Cells were cycled at different current densities over a voltage range of 0.0~3.0 V. Cyclic voltammetry (CV) was carried out in the potential range of 0.0~3.0 V at a scan rate of 0.1 mV s1 on EC-Lab (Biologic Science Instruments). After cell charging ended and the cell was kept at open circuit state for 4 h, EIS measurements were performed at open circuit potential by applying a sine wave with an amplitude of 10 mV over a frequency range from 100 k to 0.01 Hz. 3. Results and discussion €ber-type The SiOx-C/GNPs composite is synthesized by a Sto process and high-temperature treatment. Firstly, the surfactant cations (CTAþ) are adsorbed on a negatively charged GNPs surface by electrostatic interaction and form CTAB micellar assemblies on the surface. With the help of aqueous ammonia, the resulting negatively charged oligomeric silicate species approach the micelle surface through electrostatic interaction. Additionally, the presence of ammonia also can facilitate the formation hydrogen bonds between the CTAB micelles and silicate oligomers [34]. These absorbed oligomeric silicate species polymerize each other to form polymer. After high-temperature treatment, organic components were decomposed or carbonized and the formed SiOx-C film was anchored on the surface of GNPs or embedded into GNPs. XPS, FTIR, Element Analysis and XRD are firstly carried out to determine the composition of the as-prepared composite. Peaks attributable to Si2p, O1s, and C1s are detected in the XPS of the surface layer for both SiOx-C and SiOx-C/GNPs etched by Arþ for 10 min, but the relative intensity of the C peak for the SiOx-C is much weaker (Fig. 1). Here, the disordered carbon comes from the carbonization of CTAB and polysilicate with alkyl. Element analysis shows that the SiOx-C contains about 9.1% disordered carbon, while the total mass fraction of disordered carbon and GNPs in the SiOx-C/ GNPs is about 44.5%. The mean x value for the SiOx-C/GNPs composite is about 1.21 according to its EDS analysis. Compared with the FTIR spectrum of the annealed GNPs at 1000 C for 3h (Fig. 2(a)), three new peaks which are characteristic absorption bands for silicon oxide, appear in the FTIR spectrum of the SiOx/GNPs [35]. 1097 cm1 corresponds to asymmetry SieOeSi stretching vibration, 470 cm1 to SieO rocking vibration and 789 cm1 to SieO bending vibration. This result further confirms that SiOx is successfully anchored on the surface of GNPs. From the XRD patterns of the samples (Fig. 2(b)), the hump peak at 2threta ¼ 21 in SiOx-C/ GNPs corresponds to SiOx and the synthesized SiOx is amorphous or
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(b)
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Fig. 1. XPS spectra of the SiOx-C (a) and SiOx-C/GNPs (b).
Fig. 2. FT-IR spectra of the annealed GNPs and SiOx-C/GNPs (a) and XRD patterns of the SiOx-C, annealed GNPs and SiOx-C/GNPs (b).
low crystalline. Herein, it should be mentioned that since the crystalline structure of the GNPs is still kept, their diffraction peaks are similar to graphite sheets rather than single layer graphene. As shown by Fig. 3, GNPs are composed of thin sheets. According to the data from ACS Material (US), the GNPs have a flake thickness of 2e10 nm with an average lateral particle size of about 5.0 mm.
The SiOx-C/GNPs composite fabricated by self-assembly and heat treatment is some irregular lumps, in which each layer contains many thin flakes loaded with SiOx-C. Magnified SEM and TEM images confirm that SiOx-C is anchored on the surfaces of GNPs in film form instead of individual particle. Elemental mapping images (Fig. 3) show that the hybrids contain large amount of C, Si and O elements with uniform distribution. A high-magnification TEM image demonstrates that SiOx is actually composed of two distinct parts, well-ordered and disordered parts. In the ordered part, lattice fringe is observed and the lattice spacing is about 0.206 nm, which corresponds to (222) lattice plane of SiO2. At a high temperature, nano-SiO2 is readily sintered and transformed into crystalline SiO2 [35]. BET also is used to characterize the as-prepared SiOx-C/GNPs. After self-assembly and heat treatment, a specific surface area of about 28.2 m2 g1 in GNPs is substantially decreased to 10.5 m2 g1 in the SiOx-C/GNPs with the deposition of SiOx-C. Additionally, an obvious hysteresis loop is present in the isotherm absorptionedesorption curves of the SiOx-C/GNPs and the absorbed quantity rises sharply after the relative pressure is increased to 0.8 (Fig. 4), indicating the composite material contains mesopores (2e50 nm) and macropores (>50 nm) [36]. The pore size distribution curve of the SiOx-C/GNPs also supports the above mentioned results, in which the pore size ranges from 2.6 nm to 180 nm with three peaks located at about 2.6 nm, 7 nm and 14 nm. The existence of pores or gaps is expected to provide an extra room for the volume expansion of SiOx upon cycling. The electrochemical performance of the SiOx-C/GNPs was evaluated by using 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) with 2% vinylene carbonate (VC) and 5% fluoroethylene carbonate (FEC) as the electrolyte. Here, the additives VC and FEC are used to form stable SEI film on the surface of the electrodes [37]. From Fig. 5(a) and (b), the chargeedischarge curves of the SiOx-C/GNPs present significant difference from those of both GNPs and SiOx-C. In the initial discharge curve, because of a huge specific surface area, the GNPs exhibit an obvious slope for electrolyte decomposition between 1.0 and 0.25 V, which leads to a low coulombic efficiency (69.8%). The voltage plateau of the lithium insertion/extraction for GNPs is relatively flat and close to 0 V, which are very similar to characteristics of commercial graphite. In the first cycle, the SiOx-C excluding GNPs delivers a discharge capacity of 1044 mAh g1, but its coulombic efficiency is only 37.7% and there is a large polarization between the chargeedischarge curves, which may arise from its poor conductivity. In contrast with the GNPs and SiOx-C, the SiOx-C/GNPs composite exhibits excellent electrochemical behavior. In the first cycle, the SiOx-C/GNPs presents a discharge capacity of 896 mAh g1 and a charge capacity of
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Fig. 3. SEM images of the GNPs (a), and SiOx-C/GNPs: (b) low and (c) high magnification; TEM images of the SiOx-C/GNPs: (d) low and (e) high magnification; Elemental mapping images of C, Si and O for the SiOx-C/GNPs.
Fig. 4. BET isotherms (a) and pore size distribution (b) of the SiOx-C/GNPs.
607 mAh g1 with a coulombic efficiency of 67.7%. With the activation of the electrode, the reversible capacity increases to 626 mAh g1 in the second cycle. Evidently, SiOx is main contributor to the capacity of the SiOx-C/GNPs. Based on the composition of the SiOx-C/GNPs (55.5 wt% SiOx and 44.5 wt% (carbon and GNPs)) and the stable reversible capacity of the GNPs and the disordered carbon (300 mAh g1), it can be calculated that the SiOx in the SiOxC/GNPs contributes a stable reversible capacity of about 887 mAh g1. Additionally, the chargeedischarge curves' characteristics of the SiOx-C/GNPs show its qualification for an anode
material in high-energy lithium-ion batteries. As one knows, although many metal oxides like Fe3O4 [38], SnO2 [39] and GeOx [40], exhibit high specific capacity, they usually have high dealloying potentials with short voltage plateau, which compromises the advantage of their high capacity. In our study, the SiOx-C/GNPs composite possesses a low and long voltage plateau for lithium extraction. Therefore, it is expected to enhance the energy density of full batteries when the composite is used as anode. Finally, the voltage platform of lithium insertion for the SiOx-C/GNPs is higher than those of graphite, which contributes to avoiding the formation
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Fig. 5. Discharge-charge profiles of the SiOx-C and SiOx-C/GNPs in the first cycle (a) and the second cycle (b), and CV curves of the SiOx-C (c) and SiOx-C/GNPs (d) in the initial five cycles.
of dendrite during a long-time cycling and thus enhances the safety of batteries. Fig. 5(c) and (d) show the cyclic voltammetry (CV) behavior of the SiOx-C and SiOx-C/GNPs at a scan rate of 0.1 mV s1. For the SiOx-C/GNPs, two broad reduction peaks at about 1.35 V and 0.70 V in the first cycle, which disappear from the second cycle, are associated with the electrolyte decomposition and the formation of the solid electrolyte interface (SEI) layer. The former corresponds to the decomposition of FEC, while the latter is ascribed to the decomposition of other solvents in the electrolyte [41]. Both contribute to the irreversible capacity of the first discharge process. The reductive reaction potential of SiOx is closely related with the composition of the electrodes. The reduction of the SiOx in the SiOxC electrode happens at about 0.2 V due to a strong polarization, while the reduction of SiOx in the SiOx-C/GNPs electrode shifts to about 0.33 V. Previous studies have demonstrated that elemental silicon and a series of silicate salts are formed during the process [14,26e28,42]. Subsequently, lithium begins to insert the reduced amorphous silicon. The anodic broad and intense peak at 0.26 V in the curve of the SiOx-C/GNPs is overlapped partly by two peaks corresponding to lithium extraction from LixC and LiySi, respectively. The lithium extraction peak of LiySi for the SiOx-C/GNPs decreases to about 0.27 V from 0.56 V for the SiOx-C excluding GNPs and the peak becomes sharper, indicating GNPs substantially improves the electrochemical properties of SiOx. From the second cycle, the characteristics are similar to those of the mixture composed of graphite and amorphous silicon. Fig. 6(a) shows the cycle performance profiles of the SiOx-C, GNPs and SiOx-C/GNPs at a current density of 100 mA g1. Although the SiOx-C delivers a high initial discharge capacity, its reversible capacity is less than 400 mAh g1 and the capacity retention is poor. After 60 cycles, its discharge capacity decreases to 250 mAh g1. The GNPs exhibit an excellent cyclic stability with high coulombic
efficiency except the first cycle, but it only delivers a stable capacity of about 300 mAh g1, which is far from the requirements of electrode materials for high-energy lithium-ion batteries. Compared with the former two, the SiOx-C/GNPs composite possesses prominent advantages in being used as anode for lithiumion batteries. It exhibits a stable cycling performance comparable to GNPs, while it is much higher than GNPs in reversible capacity. After the initial several cycles, the SiOx-C/GNPs composite exhibits a stable reversible capacity of about 630 mAh g1, which is more than twice as much as the GNPs, and the coulombic efficiency is nearly 100%. After 250 cycles, no obvious capacity fading is observed and the coulombic efficiency has kept stable during the whole cycling. As shown in Fig. 6(b), the SiOx-C/GNPs composite also exhibits good cyclic performance at different current densities. The specific capacity is maintained at values of about 585 mAh g1 and 483 mAh g1 after the current density is increased to 200 mA g1 and 400 mA g1, respectively. Even at a current density of 600 mA g1, the composite still delivered a stable capacity of 408 mAh g1, which is higher than the theoretical capacity of graphite (372 mAh g1). After the current density is returned to 100 mA g1, the specific capacity is completely restored. Additionally, the coulombic efficiency is also stable at different current densities. All of these results show that the SiOx-C/GNPs is indeed a promising strategy to improve the electrochemical performance of SiOx. Such superior electrochemical performances are believed to result from the unique features of the SiOx-C/GNPs. Firstly, after SiOx-C is anchored on the surfaces of GNPs in the film form, its conductivity is dramatically enhanced. As shown in Fig. 7(a), the resistance value of the SiOx-C electrode (the diameter of the suppressed semicircle in high-medium frequency range) is approximately twice as much as that of the SiOx-C/GNPs electrode. In such
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Fig. 6. Cycle performance profiles of the SiOx-C/GNPs, GNPs and SiOx-C (a), and rate capability profiles of the SiOx-C/GNPs (b).
Fig. 7. EIS of the SiOx-C and SiOx-C/GNPs before cycling (a) and the SiOx-C/GNPs after different cycles (b).
structure, the electronic transport length in SiOx is effectively shortened. Moreover, the graphene sheets provide a continuous conductive path between SiOx nanoparticles, thus, also reducing the particleeparticle interface resistance. Secondly, flexible nanoplatelets can avoid the crack or pulverization of the electrode material in a repeated chargeedischarge process. Even though volume expansion still happens, the electrode will not pulverize as the sheets can deform resiliently to accommodate such volume changes [40,43]. Thirdly, the SiOx-C/GNPs fabricated by selfassembly and heat treatment has a robust connection between SiOx and GNPs, in which partly nano-SiOx-C has penetrated into graphene nanoplatelets, preventing the peeling-off of SiOx film from the surface of graphene sheets upon cycling. Finally, the nanoscale characteristics of SiOx film ensure the fast Li-ion diffusion in the electrode, which also favors a high rate capability. The EIS and SEM image for the SiOx-C/GNPs electrode after cycling further confirm the above analysis. From Fig. 7(b), no obvious impedance
increase for the SiOx-C/GNPs is observed from 50th to 250th cycle, suggesting that the structural integrity of the electrode has been maintained and the formed SEI film is also stable upon cycling [44]. The cracking of the electrode caused by the large volume change is not observed on the SEM image of the cycled SiOx-C/GNPs electrode and a uniform SEI film is formed on the surface of the electrode (Fig. 8). These results further confirm that our strategy used to resolve the conductivity and volume change of SiOx is feasible and effective. 4. Conclusions In summary, we have developed a facile and scalable solution process to fabricate high-performance Li-ion battery negative electrodes by anchoring SiOx films on the surfaces of GNPs, using relatively cheap starting materials. When used in LIBs, the composite material exhibits a high reversible capacity, good rate
Fig. 8. SEM images of the SiOx-C/GNPs before cycling (a) and after 100 cycles (b).
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capability, excellent cycling stability and low voltage platform of lithium extraction. The notably enhanced electrochemical performances of the SiOx-C/GNPs composite can be attributed to the structural features that provide excellent electronic conductivity, short transportation length for both lithium ions and electrons, enough elastomeric space to accommodate volume changes upon Li insertion/extraction, and the robust connection between GNPs and SiOx. In addition, our described materials design in this work may be extended to other battery electrode material systems that experience large volume changes during cycling and have a poor conductivity. Acknowledgments This research was financially supported by Positec, the Natural Sciences and Engineering Research Council of Canada (NSERC), Canadian Foundation for Innovation (CFI), the Canada Research Chairs (CRC) program and Natural Science Foundation of China (51374175). References [1] G. Jeong, Y. Kim, H. Kim, Y. Kim, H. Sohn, Energ. Environ. Sci. 4 (2011) 1986e2002. [2] B. Scrosati, J. Garche, J. Power Sources 195 (2010) 2419e2430. [3] N. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Angew. Chem. Int. Ed. 51 (2012) 9994e10024. [4] X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B.W. Sheldon, J. Wu, Adv. Energy Mater. (2014), http://dx.doi.org/10.1002/aenm.201300882. [5] M.R. Zamfir, H.T. Nguyen, E. Moyen, Y.H. Lee, D. Pribat, J. Mater. Chem. A 1 (2013) 9566e9586. [6] H. Kim, M. Seo, M. Park, J. Cho, Angew. Chem. 122 (2010) 2192e2195. [7] K. Hyunjung, H. Byunghee, C. Jaebum, C. Jaephil, Angew. Chem. 120 (2008) 10305e10308. [8] R. Yi, F. Dai, M.L. Gordin, H. Sohn, D. Wang, Adv. Energy Mater. 11 (2013) 1507e1515. [9] Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A. van Aken, J. Maier, Adv. Mater. 22 (2010) 2247e2250. [10] Y. Park, N. Choi, S. Park, S.H. Woo, S. Sim, B.Y. Jang, S.M. Oh, S. Park, J. Cho, K.T. Lee, Adv. Energy Mater. 3 (2013) 206e212. [11] Y.S. Hu, R.D. Cakan, M.M. Titirici, J.O. M, R. Schl, M. Antonietti, J. Maier, Angew. Chem. Int. Ed. 47 (2008) 1645e1649. [12] H. Wu, Y. Cui, Nano Today 7 (2012) 414e429. [13] J. Lee, N. Choi, S. Park, Energ. Environ. Sci. 5 (2012) 7878e7882. [14] W. Chang, C. Park, J. Kim, Y. Kim, G. Jeong, H. Sohn, Energ. Environ. Sci. 5 (2012) 6895e6899.
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Fabrication of graphene nanoplatelets-supported SiOx-disordered carbon composite and its application in lithium-ion batteries Mingqi Li a, b, *, Yan Yu a, Jing Li a, Baoling Chen a, Aishuak Konarov a, P. Chen a a b
Department of Chemical Engineering, University of Waterloo, Waterloo, N2L3G1, Canada College of Chemistry and Chemical Engineering, China West Normal University, Nanchong, 637009, China
h i g h l i g h t s SiOx-C/GNPs is fabricated by self-assembly and heat treatment. SiOx-C/GNPs exhibits a high reversible capacity with excellent capacity retention. This work provides a facile route to improve the electrochemical performance of SiOx.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2015 Received in revised form 23 May 2015 Accepted 4 June 2015 Available online 14 June 2015
Developing high-capacity electrode materials is critical to enhance the energy-density of lithium-ion batteries. Here, a new graphene nanoplatelets-supported SiOx-disordered carbon composite (SiOx-C/ €ber-type process and high-temperature treatment. When used as an anode GNPs) is fabricated by a Sto for lithium-ion batteries, SiOx-C/GNPs exhibits excellent cycle stability and rate capability even under deep chargeedischarge cycling. A stable reversible capacity of about 630 mAh g1 (calculated on the total mass of the composite) can be achieved at a current density of 100 mA g1 and the capacity retention is nearly 100% after 250 cycles. Additionally, the plateau voltages of lithium extraction for the composite are only slightly higher than those for the commercial graphite, which ensures the high energy density of the assembled batteries. The superior electrochemical performance of this novel material is due to its unique features: excellent electronic conductivity, short transportation length for both lithium ions and electrons, elastomeric space to accommodate volume changes upon Li insertion/extraction and robust connection between SiOx and GNPs. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.
Keywords: Lithium-ion batteries Graphene nanoplatelets Anode Nonstoichiometric silicon oxide Electrochemical performance
1. Introduction As one of the most important energy-storage devices, lithiumion batteries (LIBs) have attracted much attention in the scientific and industrial fields [1,2]. There is an increasing interest in developing high-capacity anode materials for the next generation of rechargeable LIBs. In this respect, silicon-based materials possess the largest potential due to high specific capacity, satisfying lithium lithiation/delithiation potentials and high abundance. The main handicap for the practical application of silicon-based materials is the huge volume change during alloying/dealloying process, which causes poor cycle stability [3,4]. Many studies have been devoted to
* Corresponding author. College of Chemistry and Chemical Engineering, China West Normal University, Nanchong, 637009, China. E-mail address: [email protected] (M. Li). http://dx.doi.org/10.1016/j.jpowsour.2015.06.019 0378-7753/Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.
the resolution of the problem and demonstrated that the reduction of silicon particles size and the fabrication of silicon-based composite materials with special structure were the most effective means to achieve the improved cycle performance [4e10]. However, it is practically difficult to attempt to synthesize nanosilicons by a facile chemical method. Currently, the preparation of nanosilicons usually experiences a complicated process or needs strict operation conditions [6,7,11,12], which lead to a high cost and make their scale-up production difficult. In comparison with elemental silicon with the same particle size, silicon oxides show less volume change and Li2O and lithium silicate generated during the initial lithiation buffer the volume expansion of silicon, thus silicon oxide electrodes usually show better cyclic stability than silicon electrodes [13e19]. Of course, it should be noted that silicon oxides have a poor electronic conductivity, and the inner stress caused by the volume changes upon Li insertion/extraction can still destroy
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the microstructure of the electrodes during prolonged cycling although the stress has been reduced as compared with that in elemental silicon. Silicon oxides can be divided into stoichiometric SiO and SiO2, and nonstoichiometric SiOx (0 < x < 2, xs1). In the past, most studies about silicon oxides as electrode materials were focused on SiO and SiO2 [17,18,20e23]. Until recently, nonstoichiometric SiOx (1 < x < 2) has attracted researchers' attention because it is found that SiOx (1 < x < 2) can be prepared by hydrolysis reaction of siloxanes or silicon halides under mild reaction conditions [24e28], which not only lowers the preparation cost, but also offers much more choices for their improvement in electrochemical performance. Herein, we report a new way to improve the electrochemical properties of SiOx by engineering graphene nanoplatelets (GNPs)-supported SiOx-disordered carbon composite. GNPs, which are stacked by graphene sheets, have “platelet” morphology with a diameter ranging from 0.5 to 25 mm and an overall thickness of about 5e25 nm [29,30]. Although GNPs present a relatively smaller specific surface area compared with conventional graphene, their facile preparation and dispersion which is very important for scale-up production in a practical application, have attracted many researchers' interest. To date, GNPs have thus been applied in many fields like sensing applications [31], loading catalysts [32] and constructing supercapacitors [33]. However, to the best of our knowledge, there is no report about GNPs-supported SiOx composite as an anode for lithium-ion batteries. In this work, GNPs-supported SiOx-C composite is fabricated by self-assembly using GNPs and C2H5Si (OC2H5)3 (for short EtSi(OEt)3) as starting materials with the help of cetyltrimethylammonium bromide (CTAB) and aqueous ammonia, followed by heat treatment. Here, EtSi(OEt)3 is chosen as starting materials because we have found that siloxane with alkyl can produce a small amount of disordered carbon (see below), which can improve the conductivity of SiOx and solidify the connection between SiOx and GNPs. The as-prepared composite presents excellent cycle stability with high capacity and good rate capability. 2. Experimental 2.1. Synthesis of SiOx-C/GNPs Graphene nanoplatelets (0.25 g, ACS Material) were dispersed in a mixture containing cetyltrimethylammonium bromide (CTAB, 0.24%wt), ethanol and deionized water by pulse sonication. Subsequently, 2.5 mL of ammonium hydroxide (25 wt%) and 2.0 mL of EtSi(OEt)3 were added dropwise to the above solution. The mixed solution was stirred at room temperature for 12 h to finish the reaction. The precipitate was collected, washed three times with deionized water and dried at 100 C in vacuum overnight. The dried sample was thermally treated in a tube furnace at 1000 C in a flow of argon (100 mL min1) for 3 h, and then the furnace was naturally cooled to room temperature. The final product was labeled as SiOxC/GNPs. For comparison, SiOx-C excluding GNPs was also synthesized by similar method. 2.2. Materials characterization X-ray diffraction (XRD) patterns were recorded using D8 Discover (Bruker) equipped with Cu Ka (l ¼ 0.15406 nm) radiation at a scan rate of 3.0 min1 from 10 to 90 . Element analysis (Elementar analyzer, Elementar Americas INC) was performed to determine the carbon mass fraction in the samples. Scanning electron microscopy (SEM, Leo-1530, Zeiss), Transmission electron microscopy (TEM, Philips CM12) and energy dispersive X-ray image maps (EDX-map) were conducted to investigate the morphology,
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microstructure and element distribution of the as-prepared composites. Fourier-transform infrared reflection (FTIR) spectra were recorded on a vertex 70 FTIR spectrometer (Bruker). The specific surface area was determined by the BrunauereEmmeteTeller method (BET, ASAP 2020, Micromeritics). XPS analysis was performed by a multi-technique ultra-high vacuum Imaging XPS Microprobe system (ESCALab 250, Thermo VG Scientific). 2.3. Electrochemical measurement The electrochemical performance of the SiOx-C/GNPs composite was evaluated using 2026 coin cells assembled in an argon-filled glove box. Working electrodes were fabricated by painting slurry with 75 wt% of SiOx-C/GNPs composite as active material, 10 wt% of acetylene black (AB) as conductor and 15 wt% of sodium alginate as binder onto Cu foil. The resulting film was dried at 105 C in vacuum for 12 h. The loaded total mass of active material was 1.8e2.5 mg cm2. A lithium disc was used as the counter electrode and reference electrode. The electrolyte was 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in weight) with 2% vinylene carbonate (VC) and 5% fluoroethylene carbonate (FEC) as additives. Cells were cycled at different current densities over a voltage range of 0.0~3.0 V. Cyclic voltammetry (CV) was carried out in the potential range of 0.0~3.0 V at a scan rate of 0.1 mV s1 on EC-Lab (Biologic Science Instruments). After cell charging ended and the cell was kept at open circuit state for 4 h, EIS measurements were performed at open circuit potential by applying a sine wave with an amplitude of 10 mV over a frequency range from 100 k to 0.01 Hz. 3. Results and discussion €ber-type The SiOx-C/GNPs composite is synthesized by a Sto process and high-temperature treatment. Firstly, the surfactant cations (CTAþ) are adsorbed on a negatively charged GNPs surface by electrostatic interaction and form CTAB micellar assemblies on the surface. With the help of aqueous ammonia, the resulting negatively charged oligomeric silicate species approach the micelle surface through electrostatic interaction. Additionally, the presence of ammonia also can facilitate the formation hydrogen bonds between the CTAB micelles and silicate oligomers [34]. These absorbed oligomeric silicate species polymerize each other to form polymer. After high-temperature treatment, organic components were decomposed or carbonized and the formed SiOx-C film was anchored on the surface of GNPs or embedded into GNPs. XPS, FTIR, Element Analysis and XRD are firstly carried out to determine the composition of the as-prepared composite. Peaks attributable to Si2p, O1s, and C1s are detected in the XPS of the surface layer for both SiOx-C and SiOx-C/GNPs etched by Arþ for 10 min, but the relative intensity of the C peak for the SiOx-C is much weaker (Fig. 1). Here, the disordered carbon comes from the carbonization of CTAB and polysilicate with alkyl. Element analysis shows that the SiOx-C contains about 9.1% disordered carbon, while the total mass fraction of disordered carbon and GNPs in the SiOx-C/ GNPs is about 44.5%. The mean x value for the SiOx-C/GNPs composite is about 1.21 according to its EDS analysis. Compared with the FTIR spectrum of the annealed GNPs at 1000 C for 3h (Fig. 2(a)), three new peaks which are characteristic absorption bands for silicon oxide, appear in the FTIR spectrum of the SiOx/GNPs [35]. 1097 cm1 corresponds to asymmetry SieOeSi stretching vibration, 470 cm1 to SieO rocking vibration and 789 cm1 to SieO bending vibration. This result further confirms that SiOx is successfully anchored on the surface of GNPs. From the XRD patterns of the samples (Fig. 2(b)), the hump peak at 2threta ¼ 21 in SiOx-C/ GNPs corresponds to SiOx and the synthesized SiOx is amorphous or
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(b)
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Fig. 1. XPS spectra of the SiOx-C (a) and SiOx-C/GNPs (b).
Fig. 2. FT-IR spectra of the annealed GNPs and SiOx-C/GNPs (a) and XRD patterns of the SiOx-C, annealed GNPs and SiOx-C/GNPs (b).
low crystalline. Herein, it should be mentioned that since the crystalline structure of the GNPs is still kept, their diffraction peaks are similar to graphite sheets rather than single layer graphene. As shown by Fig. 3, GNPs are composed of thin sheets. According to the data from ACS Material (US), the GNPs have a flake thickness of 2e10 nm with an average lateral particle size of about 5.0 mm.
The SiOx-C/GNPs composite fabricated by self-assembly and heat treatment is some irregular lumps, in which each layer contains many thin flakes loaded with SiOx-C. Magnified SEM and TEM images confirm that SiOx-C is anchored on the surfaces of GNPs in film form instead of individual particle. Elemental mapping images (Fig. 3) show that the hybrids contain large amount of C, Si and O elements with uniform distribution. A high-magnification TEM image demonstrates that SiOx is actually composed of two distinct parts, well-ordered and disordered parts. In the ordered part, lattice fringe is observed and the lattice spacing is about 0.206 nm, which corresponds to (222) lattice plane of SiO2. At a high temperature, nano-SiO2 is readily sintered and transformed into crystalline SiO2 [35]. BET also is used to characterize the as-prepared SiOx-C/GNPs. After self-assembly and heat treatment, a specific surface area of about 28.2 m2 g1 in GNPs is substantially decreased to 10.5 m2 g1 in the SiOx-C/GNPs with the deposition of SiOx-C. Additionally, an obvious hysteresis loop is present in the isotherm absorptionedesorption curves of the SiOx-C/GNPs and the absorbed quantity rises sharply after the relative pressure is increased to 0.8 (Fig. 4), indicating the composite material contains mesopores (2e50 nm) and macropores (>50 nm) [36]. The pore size distribution curve of the SiOx-C/GNPs also supports the above mentioned results, in which the pore size ranges from 2.6 nm to 180 nm with three peaks located at about 2.6 nm, 7 nm and 14 nm. The existence of pores or gaps is expected to provide an extra room for the volume expansion of SiOx upon cycling. The electrochemical performance of the SiOx-C/GNPs was evaluated by using 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) with 2% vinylene carbonate (VC) and 5% fluoroethylene carbonate (FEC) as the electrolyte. Here, the additives VC and FEC are used to form stable SEI film on the surface of the electrodes [37]. From Fig. 5(a) and (b), the chargeedischarge curves of the SiOx-C/GNPs present significant difference from those of both GNPs and SiOx-C. In the initial discharge curve, because of a huge specific surface area, the GNPs exhibit an obvious slope for electrolyte decomposition between 1.0 and 0.25 V, which leads to a low coulombic efficiency (69.8%). The voltage plateau of the lithium insertion/extraction for GNPs is relatively flat and close to 0 V, which are very similar to characteristics of commercial graphite. In the first cycle, the SiOx-C excluding GNPs delivers a discharge capacity of 1044 mAh g1, but its coulombic efficiency is only 37.7% and there is a large polarization between the chargeedischarge curves, which may arise from its poor conductivity. In contrast with the GNPs and SiOx-C, the SiOx-C/GNPs composite exhibits excellent electrochemical behavior. In the first cycle, the SiOx-C/GNPs presents a discharge capacity of 896 mAh g1 and a charge capacity of
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Fig. 3. SEM images of the GNPs (a), and SiOx-C/GNPs: (b) low and (c) high magnification; TEM images of the SiOx-C/GNPs: (d) low and (e) high magnification; Elemental mapping images of C, Si and O for the SiOx-C/GNPs.
Fig. 4. BET isotherms (a) and pore size distribution (b) of the SiOx-C/GNPs.
607 mAh g1 with a coulombic efficiency of 67.7%. With the activation of the electrode, the reversible capacity increases to 626 mAh g1 in the second cycle. Evidently, SiOx is main contributor to the capacity of the SiOx-C/GNPs. Based on the composition of the SiOx-C/GNPs (55.5 wt% SiOx and 44.5 wt% (carbon and GNPs)) and the stable reversible capacity of the GNPs and the disordered carbon (300 mAh g1), it can be calculated that the SiOx in the SiOxC/GNPs contributes a stable reversible capacity of about 887 mAh g1. Additionally, the chargeedischarge curves' characteristics of the SiOx-C/GNPs show its qualification for an anode
material in high-energy lithium-ion batteries. As one knows, although many metal oxides like Fe3O4 [38], SnO2 [39] and GeOx [40], exhibit high specific capacity, they usually have high dealloying potentials with short voltage plateau, which compromises the advantage of their high capacity. In our study, the SiOx-C/GNPs composite possesses a low and long voltage plateau for lithium extraction. Therefore, it is expected to enhance the energy density of full batteries when the composite is used as anode. Finally, the voltage platform of lithium insertion for the SiOx-C/GNPs is higher than those of graphite, which contributes to avoiding the formation
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Fig. 5. Discharge-charge profiles of the SiOx-C and SiOx-C/GNPs in the first cycle (a) and the second cycle (b), and CV curves of the SiOx-C (c) and SiOx-C/GNPs (d) in the initial five cycles.
of dendrite during a long-time cycling and thus enhances the safety of batteries. Fig. 5(c) and (d) show the cyclic voltammetry (CV) behavior of the SiOx-C and SiOx-C/GNPs at a scan rate of 0.1 mV s1. For the SiOx-C/GNPs, two broad reduction peaks at about 1.35 V and 0.70 V in the first cycle, which disappear from the second cycle, are associated with the electrolyte decomposition and the formation of the solid electrolyte interface (SEI) layer. The former corresponds to the decomposition of FEC, while the latter is ascribed to the decomposition of other solvents in the electrolyte [41]. Both contribute to the irreversible capacity of the first discharge process. The reductive reaction potential of SiOx is closely related with the composition of the electrodes. The reduction of the SiOx in the SiOxC electrode happens at about 0.2 V due to a strong polarization, while the reduction of SiOx in the SiOx-C/GNPs electrode shifts to about 0.33 V. Previous studies have demonstrated that elemental silicon and a series of silicate salts are formed during the process [14,26e28,42]. Subsequently, lithium begins to insert the reduced amorphous silicon. The anodic broad and intense peak at 0.26 V in the curve of the SiOx-C/GNPs is overlapped partly by two peaks corresponding to lithium extraction from LixC and LiySi, respectively. The lithium extraction peak of LiySi for the SiOx-C/GNPs decreases to about 0.27 V from 0.56 V for the SiOx-C excluding GNPs and the peak becomes sharper, indicating GNPs substantially improves the electrochemical properties of SiOx. From the second cycle, the characteristics are similar to those of the mixture composed of graphite and amorphous silicon. Fig. 6(a) shows the cycle performance profiles of the SiOx-C, GNPs and SiOx-C/GNPs at a current density of 100 mA g1. Although the SiOx-C delivers a high initial discharge capacity, its reversible capacity is less than 400 mAh g1 and the capacity retention is poor. After 60 cycles, its discharge capacity decreases to 250 mAh g1. The GNPs exhibit an excellent cyclic stability with high coulombic
efficiency except the first cycle, but it only delivers a stable capacity of about 300 mAh g1, which is far from the requirements of electrode materials for high-energy lithium-ion batteries. Compared with the former two, the SiOx-C/GNPs composite possesses prominent advantages in being used as anode for lithiumion batteries. It exhibits a stable cycling performance comparable to GNPs, while it is much higher than GNPs in reversible capacity. After the initial several cycles, the SiOx-C/GNPs composite exhibits a stable reversible capacity of about 630 mAh g1, which is more than twice as much as the GNPs, and the coulombic efficiency is nearly 100%. After 250 cycles, no obvious capacity fading is observed and the coulombic efficiency has kept stable during the whole cycling. As shown in Fig. 6(b), the SiOx-C/GNPs composite also exhibits good cyclic performance at different current densities. The specific capacity is maintained at values of about 585 mAh g1 and 483 mAh g1 after the current density is increased to 200 mA g1 and 400 mA g1, respectively. Even at a current density of 600 mA g1, the composite still delivered a stable capacity of 408 mAh g1, which is higher than the theoretical capacity of graphite (372 mAh g1). After the current density is returned to 100 mA g1, the specific capacity is completely restored. Additionally, the coulombic efficiency is also stable at different current densities. All of these results show that the SiOx-C/GNPs is indeed a promising strategy to improve the electrochemical performance of SiOx. Such superior electrochemical performances are believed to result from the unique features of the SiOx-C/GNPs. Firstly, after SiOx-C is anchored on the surfaces of GNPs in the film form, its conductivity is dramatically enhanced. As shown in Fig. 7(a), the resistance value of the SiOx-C electrode (the diameter of the suppressed semicircle in high-medium frequency range) is approximately twice as much as that of the SiOx-C/GNPs electrode. In such
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Fig. 6. Cycle performance profiles of the SiOx-C/GNPs, GNPs and SiOx-C (a), and rate capability profiles of the SiOx-C/GNPs (b).
Fig. 7. EIS of the SiOx-C and SiOx-C/GNPs before cycling (a) and the SiOx-C/GNPs after different cycles (b).
structure, the electronic transport length in SiOx is effectively shortened. Moreover, the graphene sheets provide a continuous conductive path between SiOx nanoparticles, thus, also reducing the particleeparticle interface resistance. Secondly, flexible nanoplatelets can avoid the crack or pulverization of the electrode material in a repeated chargeedischarge process. Even though volume expansion still happens, the electrode will not pulverize as the sheets can deform resiliently to accommodate such volume changes [40,43]. Thirdly, the SiOx-C/GNPs fabricated by selfassembly and heat treatment has a robust connection between SiOx and GNPs, in which partly nano-SiOx-C has penetrated into graphene nanoplatelets, preventing the peeling-off of SiOx film from the surface of graphene sheets upon cycling. Finally, the nanoscale characteristics of SiOx film ensure the fast Li-ion diffusion in the electrode, which also favors a high rate capability. The EIS and SEM image for the SiOx-C/GNPs electrode after cycling further confirm the above analysis. From Fig. 7(b), no obvious impedance
increase for the SiOx-C/GNPs is observed from 50th to 250th cycle, suggesting that the structural integrity of the electrode has been maintained and the formed SEI film is also stable upon cycling [44]. The cracking of the electrode caused by the large volume change is not observed on the SEM image of the cycled SiOx-C/GNPs electrode and a uniform SEI film is formed on the surface of the electrode (Fig. 8). These results further confirm that our strategy used to resolve the conductivity and volume change of SiOx is feasible and effective. 4. Conclusions In summary, we have developed a facile and scalable solution process to fabricate high-performance Li-ion battery negative electrodes by anchoring SiOx films on the surfaces of GNPs, using relatively cheap starting materials. When used in LIBs, the composite material exhibits a high reversible capacity, good rate
Fig. 8. SEM images of the SiOx-C/GNPs before cycling (a) and after 100 cycles (b).
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capability, excellent cycling stability and low voltage platform of lithium extraction. The notably enhanced electrochemical performances of the SiOx-C/GNPs composite can be attributed to the structural features that provide excellent electronic conductivity, short transportation length for both lithium ions and electrons, enough elastomeric space to accommodate volume changes upon Li insertion/extraction, and the robust connection between GNPs and SiOx. In addition, our described materials design in this work may be extended to other battery electrode material systems that experience large volume changes during cycling and have a poor conductivity. Acknowledgments This research was financially supported by Positec, the Natural Sciences and Engineering Research Council of Canada (NSERC), Canadian Foundation for Innovation (CFI), the Canada Research Chairs (CRC) program and Natural Science Foundation of China (51374175). References [1] G. Jeong, Y. Kim, H. Kim, Y. Kim, H. Sohn, Energ. Environ. Sci. 4 (2011) 1986e2002. [2] B. Scrosati, J. Garche, J. Power Sources 195 (2010) 2419e2430. [3] N. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Angew. Chem. Int. Ed. 51 (2012) 9994e10024. [4] X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B.W. Sheldon, J. Wu, Adv. Energy Mater. (2014), http://dx.doi.org/10.1002/aenm.201300882. [5] M.R. Zamfir, H.T. Nguyen, E. Moyen, Y.H. Lee, D. Pribat, J. Mater. Chem. A 1 (2013) 9566e9586. [6] H. Kim, M. Seo, M. Park, J. Cho, Angew. Chem. 122 (2010) 2192e2195. [7] K. Hyunjung, H. Byunghee, C. Jaebum, C. Jaephil, Angew. Chem. 120 (2008) 10305e10308. [8] R. Yi, F. Dai, M.L. Gordin, H. Sohn, D. Wang, Adv. Energy Mater. 11 (2013) 1507e1515. [9] Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A. van Aken, J. Maier, Adv. Mater. 22 (2010) 2247e2250. [10] Y. Park, N. Choi, S. Park, S.H. Woo, S. Sim, B.Y. Jang, S.M. Oh, S. Park, J. Cho, K.T. Lee, Adv. Energy Mater. 3 (2013) 206e212. [11] Y.S. Hu, R.D. Cakan, M.M. Titirici, J.O. M, R. Schl, M. Antonietti, J. Maier, Angew. Chem. Int. Ed. 47 (2008) 1645e1649. [12] H. Wu, Y. Cui, Nano Today 7 (2012) 414e429. [13] J. Lee, N. Choi, S. Park, Energ. Environ. Sci. 5 (2012) 7878e7882. [14] W. Chang, C. Park, J. Kim, Y. Kim, G. Jeong, H. Sohn, Energ. Environ. Sci. 5 (2012) 6895e6899.
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