ARTICLE pubs.acs.org/JPCC
Electrospray Synthesis of Silicon/Carbon Nanoporous Microspheres as Improved Anode Materials for Lithium-Ion Batteries Ya-Xia Yin,†,‡ Sen Xin,† Li-Jun Wan,† Cong-Ju Li,‡ and Yu-Guo Guo*,† †
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100090, People's Republic of China ‡ Faculty of Material Engineering, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
bS Supporting Information ABSTRACT:
An optimized nanostructure design of Si-based anode material for high-performance lithium-ion batteries is realized in the form of Si/C nanoporous microspheres. Self-assembled Si/C nanoporous microspheres are synthesized by a programmed method and are investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, N2 adsorption desorption isotherms, and electrochemical experiments. The programmed synthesis steps involve electrojetting Si nanoparticle-containing sodium alginate aqueous solution followed by calcination, carbon coating, and final etching. The electrospray step is the key step toward the formation of the microspheres in which sodium alginate acts as a dispersant and a carbon precursor for nano-Si particles as well as a coagulant together with Cu2+. The Si/C nanoporous microspheres exhibit remarkably enhanced cycling performance and rate performance compared with nano-Si particles when used as anode materials in lithium-ion batteries. The improved electrochemical performances benefit from the advanced nano/microstructure with proper size, carbon coating, and porosity as well as from the as-formed Cu3Si with good electronic conductivity and surface stability.
1. INTRODUCTION High-capacity lithium-insertion materials for high-energydensity Li-ion cells have been of rapidly increasing interest for the emerging fields of new generation portable electronic devices and electric vehicles.1 5 Compared with the commonly used carbonaceous anode materials, silicon has attracted increasing attention as a potential high-capacity anode material because of numerous appealing features such as high theoretical specific capacity of 4212 mA h g 1 5,6 and higher safety and higher stability than graphite (lithiated silicon is more stable in typical electrolytes than lithiated graphite).7 Si anode materials, however, suffer from some drawbacks involving the drastic volume change (larger than 300%) during the alloying dealloying reactions with Li, the intrinsic low electrical conductivity, and the unstable solid electrolyte interphase (SEI) in the common electrolyte of LiPF6. These limitations still challenge the investigation and development on identification of higher capacity for the next generation batteries. r 2011 American Chemical Society
To alleviate the volume change during cycling, one approach is to utilize Si nanostructures to decrease the absolute volume variation of Si-based materials. Li et al. reported the first investigation of nano-Si particles as high-capacity anode materials because Si nanostructures could shorten the Li diffusion distance and enhance the electroactivity toward Li uptake.8 Subsequent studies have shown that Si nanowires, nanotubes, and nanonests are able to facilitate rapid transport of Li ions and exhibit improved performance.9 11 In the context of Si nanostructures, nano/microspherical Si material might be attractive for industrial production and application because of the fairly facile preparation of nano/microspheres and the electrode films accompanied by the eliminatory agglomeration of tiny Si particles therein.4 A recent study has demonstrated that highly enhanced Received: May 19, 2011 Revised: June 20, 2011 Published: June 23, 2011 14148
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The Journal of Physical Chemistry C cycle stability could be achieved by Si/C nano/microspheres with an average diameter of 26 μm fabricated by chemical vapor deposition (CVD).12 From the viewpoint of practical applications, it is necessary to develop an alternative, easy-handle, and large-scale approach to assemble Si/C nano/microspheres. An alternative way to solve the huge volume change during cycling is to introduce nanopores into Si-based anode materials. Pre-existing nanopores could provide the volume needed for Si expansion during alloying with Li and allow for fast Li-ion transport when filled with liquid electrolyte. Generally, nanopores are created either by using a porous carbon scaffold12 or by a sacrificing SiO2 template.13,14 The improved cycle stability of nanoporous Si has demonstrated the importance of controllable nanopores to Si-based anode materials. To form stable SEI on the surface of Si, carbon coating is an effective way. For nanostructured Si, high surface area may lead to the risk of secondary reaction in which LiPF6 electrolyte decomposes gradually and produces HF which etches Si resulting in a high irreversible capacity loss (i.e., low Coulombic efficiency) and poor cycle life.15 In addition to stabilizing the SEI layer on the surface of Si, carbon coating can also enhance the electronic conductivity of the Si anode, which thereby promotes lithium storage in Si particles.16,17 Metal coating was also utilized for Si-based anode materials instead of carbon coating. It has been reported that after coating with Ag nanoparticles, three-dimensional porous Si exhibits improved initial Coulombic efficiency and cycling stability.14 So far, many coating materials with good electronic conductivities, including carbon, metal, metal nitride, and metal silicides,18 20 have been used and have demonstrated to be effective for Si-based anode materials with improved elecrochemical performance. Electrospray has been demonstrated to be an effective technique to produce nano/microspheres without the use of hard templates.21,22 Electrospray microspheres, capable of size modulation in the micrometer region, are easily prepared on the basis of using an electrostatic field between a capillary tip such as a nozzle and a flat counter electrode. Microspheres prepared by electrospray have been widely used in drug delivery of which matrix of microspheres typically involves chitosan and sodium alginate (SA). SA is a linear polysaccharide copolymer that consists of two sterically different repeating units and carboxyl groups in varying proportion.23 Previous studies have demonstrated that upon carbonization of SA in inert atmosphere, porous carbon with micro/mesopores and high electronic conductivity can be obtained.24 In addition, alginate aqueous solution is readily hydrogelled in the presence of divalent metal cations,25,26 which is an effective way of introducing necessary metal cations and thereby immobilizing the as-formed nano/ microstructures. In this study, we have designed and assembled the Si/C nanoporous microspheres via a programmed procedure starting from the step of electrojetting Si nanoparticle-containing SA aqueous solution (Figure 1). Copper ions are initially introduced into the electrospray microspheres to obtain the insoluble alginate copper and thereby to form the stable structure of microspheres. Subsequent calcination is performed to allow copper ions to react with Si to form Cu3Si on the surface of Si nanoparticles. The in situ interfacial formation of conductive Cu3Si provides low interfacial resistance, stabilizes the structure of the microspheres, and also acts as a buffering layer against volume change of Si during cycles because of the chemical bonding of Si and copper.27 Further carbon coating is carried
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Figure 1. Schematical illustration of programmed preparation of Si/C nanoporous microspheres.
out on the Si/C microspheres to improve SEI formation, structural integrity, and electrical conductivity. Finally, HF etching is used to enlarge the nanopores in the microspheres and to remove the possible inactive layer of SiO2 on the surface of Si nanoparticles. The superiority of such design of the Si/C nanoporous microspheres has been demonstrated by their improved electrochemical performance when used as the anode material for lithium-ion batteries.
2. EXPERIMENTAL SECTION 2.1. Preparation of Si/C Nanoporous Microspheres. A mixture was prepared for electrospraying by dispersing Si nanoparticles (2.0 wt %) (Beijing Top Vendor Science & Technology Co. Ltd. Chemicals) into aqueous solution of sodium alginate (0.5 wt %) and by mechanical stirring for 24 h. Electrospray was performed using a commercial machine equipped with a variable high-voltage power supply to provide a high voltage (∼25 kV, Spellman High Voltage Electronics Corporation). The flow rate was 0.75 mL h 1, and the needleto-collector distance was 15 cm. Electrospray microspheres were collected in 1 M aqueous solution of CuCl2 (Beijing Chemicals Co.). The as-collected microspheres were first heated at 800 °C for 6 h under Ar-containing 5% H2 and then were dispersed in tetrahydrofuran (THF, Beijing Chemicals Co.) solution of polyvinyl chloride (PVC, Mw ∼ 62 000, Mn ∼35 000, Aldrich). The carbonizing of the microspheres coated with PVC was carried out at 900 °C for 2 h under Ar-containing 5% H2. Si/C nanoporous microspheres were obtained after etching the Si/C microspheres with 49% hydrofluoric acid for 2 h. 2.2. Structural Characterization. X-ray diffraction (XRD) measurements were carried out using a Philips PW3710 using filtered Cu KR radiation (λ = 1.5405 Å). Transmission electron microscopy (TEM, JEM JEOL 2010), high-resolution TEM (HRTEM, Tecnai F20), and scanning electron microscope (SEM, JEOL 6701F, operating at 10 kV) were used to visualize the morphologies, sizes, and structures of the products. Raman spectra were obtained using a Digilab FTS3500 from Bio-Rad with a laser wavelength of 514.5 nm. The nitrogen absorption and desorption isotherms at 77.3 K were obtained with a Nova 2000e surface area pore size analyzer. 2.3. Electrochemical Characterization. Electrochemical measurements were performed with Swagelok-type cells assembled in an argon-filled glovebox. For preparing working electrodes, 14149
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Figure 2. SEM images of the microspheres prepared by a series of procedures. (a c) The eletrosprayed Si/alginate microspheres collected in CuCl2 solution; (d f) the eletrosprayed microspheres obtained after calcination; (g i) the final Si/C microspheres obtained after carbon coating, calcination, and etching.
a mixture of active material, super-P acetylene black, and poly(vinyl difluoride) (PVDF) at a weight ratio of 60:20:20 was pasted on a Cu foil. The loading mass of active materials is about 10 mg cm 2. Lithium foil was used as the counter electrode. A glass fiber (GF/D) from Whatman was used as a separator. The electrolyte consisted of a solution of 1 M LiPF6 salt in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1 in wt %) plus 2 wt % vinylene carbonate (VC) obtained from Tianjing Jinniu Power Sources Material Co. Ltd. Galvanostatic cycling of the assembled cells was carried out using an Arbin BT2000 system in the voltage range of 0 1.1 V (vs Li+/Li).
3. RESULTS AND DISCUSSION A typical SEM image shows that the as-prepared silicon/ copper alginate microspheres exhibit a pearlike morphology and a diameter ranging from 30 to 70 μm (Figure 2a). Close observation depicts a morphological feature with either a sunken pit or a tail (Figure 2b), which can be ascribed to the evolution of the droplets at the end of the needle under the high-voltage electric field.28 High-resolution SEM images show the hierarchical feature of the as-prepared Si/copper alginate microspheres containing silicon nanoparticles (Figure 2c). After calcination at 800 °C under H2 Ar atmosphere for 6 h, the resulting product retains the pearlike morphology of the precursor (Figure 2d, e). Close observation confirms the retention of the hierarchical structure upon the calcination, which consists of Si nanoparticles and carbon matrix (Figure 2f). Because nanopores and carbon coating are two key elements for reducing the mechanical stress caused by the large volume variation and for stabilizing the structure of the microspheres for Li-ion cell anodes,16,29 31 these features were incorporated into the microsphere structures. The carbon-coating step was realized
by mixing the microspheres with PVC solution followed by calcination at 900 °C for 2 h under H2 Ar atmosphere. Subsequently, HF treatment was utilized to enlarge pores in the Si/C spheres by removing the possibly existing SiO2 layer surrounding Si nanoparticles and by dissolving part of the Si particles (Figure 2, g i). The success of the step is confirmed by the N2 absorption desorption isotherms of the Si/C microspheres before and after HF etching in which the diameters of pores are found to greatly increase from 3.8 to 30 nm after HF etching, and the surface areas also increase from 37 m2 g 1 to 57 m2 g 1, although both isotherms show a similar adsorbed/desorbed behavior (Figure 3). SEM image further confirms the existence of visible nanopores in the Si/C microspheres obtained after coating of carbon, calcination, and etching with HF (Figure 2i). The spherical morphology of the final Si/C product is well maintained after etching (Figure 2g), which promises a robust structure for subsequent battery assembling. Figure 4a shows the XRD pattern of the as-prepared Si/C nanoporous microspheres in which the main diffraction peaks at 2θ = 28.4, 47.4, 56.2, 69.2, and 79.5° can be indexed as the (111), (220), (311), (400), and (331) planes of Si crystallites (JCPDS No. 27-1402), respectively. The diffraction peaks centered at 2θ = 44.6°, 45.1°, and 65.2° are also clearly observed, which can be assigned to the (012), (300), and (003) planes of Cu3Si crystallites (JCPDS No. 51-0916), respectively.18,20,32 The result reveals the existence of crystalline Cu3Si in the final Si/C nanoporous microspheres which is formed during the calcination under Ar H2 atmosphere. In addition, no specific peaks of crystalline C are observed except for a broad peak at 2θ = ∼24°, which might be ascribed to the pyrolyzed carbon with low graphitization degree. Raman spectral analysis was also employed to confirm the crystalline phase of Si and the existence of carbon in the Si/C nanoporous microspheres (Figure 4b). The three characteristic 14150
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Figure 3. (a, c) N2 adsorption/desorption isotherms of Si/C nanoporous microspheres before and after HF etching and (b, d) the pore-size distribution plot calculated by the Barrett-Joyner-Halenda (BJH) formula in the desorption branch isotherm of Si/C microspheres before and after HF etching. 9 = adsorbed, red circle = desorbed.
Figure 4. (a) XRD pattern and (b) Raman spectrum of the as-prepared Si/C nanoporous microspheres.
peaks at about 516, 1346, and 1600 cm 1 are in good agreement with the typical Raman mode of Si, the D-band (disorderedinduced phonon mode), and the G-band (graphite band) of carbon, respectively. The relatively peak-integrated intensity of D-band to G-band is 0.98 indicating the existence of short-rangeordered carbon structure and faulty long-range-ordered carbon crystalline, which is consistent with the above XRD result. To visualize the detailed internal structure of Si/C microspheres, TEM characterizations are used. Figure 5a clearly shows the marked contrast between crystalline silicon and carbon in which the dark core corresponds to the silicon nanoparticles. One can clearly see that silicon nanoparticles are highly dispersed in the carbon matrix. No significant aggregation of silicon nanoparticles is observed in the carbon matrix. The result indicates that the SA
molecule is able to favor the dispersion of silicon nanoparticles during electrospraying. It has been reported that carboxylmethylcellulose (CMC) can be used as an efficient binder to enhance the stability of the Li Si reactions resulting from the nature of SiCMC chemical binding.33 35 In view of a structural similarity to CMC, similar chemical binding might occur between Si and SA, which may be useful for disperse Si nanoparticles in the aqueous SA solution. This point is supported by the fact that the mixture of SA and silicon nanoparticles retains no apparent precipitation but possesses a yellowish brown solution even after storing over one month. High-resolution TEM (HRTEM) image shows that silicon nanoparticles are well encapsulated in the carbon matrix (Figure 5b). The lattice spacing of silicon fringe in the HRTEM image is 14151
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Figure 6. The first three cyclic voltammograms of Si/C microspheres in the potential window of 0 3 V cycled at a rate of 1 mV s 1.
Figure 5. (a c) TEM images of the as-prepared Si/C microspheres recorded at different magnifications and (d) EDX pattern collected from a.
determined to be 0.31 nm, which corresponds to the (111) plane of the diamond cubic Si phase (Figure 5c). The carbon matrix exhibits a relatively disordered microstructure with curved and bent graphenelike sheets (see Figure S1 of the Supporting Information), which could be derived from SA as reported by our previous study.36 From the HRTEM images shown in Figure 5b and c, one can also clearly see that silicon nanoparticles are well covered by a carbon layer with a thickness of 2 3 nm. The results indicate that SA is also a good carbon precursor for uniform carbon coating besides being a good disperser for Si nanoparticles in aqueous solution. The results of energy-dispersive X-ray spectroscopy (EDX) of the product evidence the presence of the compositions of Si, Cu, and C besides the composition of Ni and O for the grid holder (Figure 5d). Except for the Ni and O from the holder, the mass percentage of Si, Cu3Si, and C in the Si/C nanoporous microsphere is 66.8%, 9.2%, and 24%, respectively. On the basis of the above determined contents, the theoretical capacity of Si/C microspheres was calculated to be 2480 mA h g 1 taking into account no contribution from Cu3Si fraction to the total capacity. Electrochemical measurements were performed for the Si/C nanoporous microspheres and the starting Si nanoparticles used for electrospraying. Figure 6 shows the cyclic voltammogtam (CV) curves for Si/C nanoporous microspheres measured at a scan rate of 1 mV s 1. In the case of the first cathodic half-cycle (lithium uptake), a broad cathodic peak is observed at around 0.75 V, which could be attributed to the formation of SEI film.37,38 The peak disappears in the subsequent cycles. One additional cathodic peak appears at 0.19 V, which can be ascribed to the formations of a series of Li Si alloys.39,40 In the case of the first anodic scan (lithium ion release), two broader anodic peaks occur at 0.34 and 0.52 V, respectively, corresponding to the phase transition between amorphous LixSi and amorphous silicon.41 On the second and third cycles, no significant shift was observed suggesting good cycling performance. In addition, the anodic current peaks in the second scan exhibit a slightly enhanced intensity in comparison with the first scan. This phenomenon can be attributed mainly to the gradual breakdown of the crystalline silicon structure that depends on the migration rate of lithium ions
into the silicon host and the rate of amorphous Li Si alloy formation42 reported in previous studies of silicon-based anode materials.39,43 45 Figure 7 shows charge discharge voltage profiles for the electrodes of Si/C nanoporous microspheres and nano-Si. In the case of the nano-Si electrode, one can see that the initial discharge and the charge specific capacities are 3517 and 2003 mA h g 1, respectively, under a current density of 50 mA g 1 (Figure 7a). Although the initial discharge capacity is close to the theoretical capacity of Li15Si4 (3579 mA h g 1), a rapid decay was observed upon further cycling. The reversible capacity is only 324 mA h g 1 in the fifth cycle. This performance of nano-Si is in good agreement with nanocrystalline Si electrode reported perviously.46,47 In the case of the Si/C nanoporous microspheres, the first discharge and charge specific capacities are 2482 and 1614 mA h g 1, respectively, at a current density of 50 mA g 1. The reversible capacity of the Si/C nanoporous microspheres remains almost unchanged through the subsequent four discharge/charge cycles as shown in Figure 7b in which the discharge/charge curves of these cycles overlap. The initial discharge capacity is close to the theoretical value of the Si/C composite with 66.8 wt % Si and 24 wt % C. In marked contrast to the nano-Si electrode, the Si/C microsphere electrode exhibits a good match from the first to the fifth cycle. This strongly indicates no obvious decrease in charge capacity throughout the first five cycles. The reversible capacity of the Si/C nanoporous microspheres is still as high as ∼1000 mA h g 1 after 50 cycles revealing the significantly improved cycle stability of the Si/C nanoporous microspheres in comparison with the pristine Si nanoparticles. It has been reported that the total specific capacity of Li-ion battery can obtain a noticeable improvement when the anodes have capacity about 1000 1200 mA h g 1, and it shows a slow increase when the capacity of anode materials exceeds 1200 mA h g 1.6,48 The present Si/C nanoporous microspheres with a high capacity of ∼1000 mA h g 1 are right in the favorable range and promise to be a good candidate for high-energy Li-ion batteries. The cyclability is further evaluated under a current density of 200 mA g 1. After 30 cycles, the reversible specific capacity is still about 800 mA h g 1 indicating a good cycling performance and rate performance (Figure 8). The initial Coulombic efficiencies of Si/C microspheres are 65% and 68% at 50 and 200 mA g 1, respectively (see Figure S2 of the Supporting Information). The irreversible capacity ratio can be assigned to the decomposition of the electrolyte forming SEI on the electrode surface because 14152
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Figure 7. Charge discharge profiles of (a) nano-Si particles used for electrospray and (b) Si/C microspheres cycled between 0 and 1.1 V under a current density of 50 mA g 1.
Figure 8. Cycling behaviors of (a) Si/C microspheres and nano-Si particles under a current density of 50 mA g 1 and (b) Si/C microspheres under a current density of 200 mA g 1.
of the slightly large surface areas (ca. 57 m 2 g 1),4 which requires further investigation to increase the initial Coulombic efficiency. One possibility of the enhanced behavior may be accounted for by the fact that the nanoporous structures have increased pores sizes as shown by the above N2 adsorption desorption isotherms results. These may alleviate the volume changes during lithium alloying and dealloying.12 14 The pore distribution of Si/C microsphere shows that HF etching leads to not only removal of the SiO2 layer on the surface of Si nanoparticles but also to the dissolution of the part of Si nanoparticles creating more void to fit the volume variation of Si during cycling. In addition, the porous nano/microspherical structures are not destroyed after 50 cycles and still consist of intact granules (see Figure S3 of the Supporting Information), which are able to favor retaining the void for Li-ion insertion and extraction during discharge and charge. The present procedure is a simple but necessary way to obtain nanoporous microspheres and could be extended to other porous materials (not just limited to carbon matrix). Another possible reason for the improvement could be assigned to the in situ interfacial formation of Cu3Si, which can improve the surface electrical conductivity of silicon as reported previously.27
4. CONCLUSIONS We have demonstrated a facile method to prepare Si/C nanoporous microspheres by electrospraying the aqueous solution containing silicon nanoparticles and SA into CuCl2 solution. The designed strategy benefits the good dispersion of Si nanoparticles in SA solution and forms an excellent carbon coating layer on Si nanoparticles. In addition, the introduction of Cu2+ into the Si/C composite is crucial to immobilize the Si/C microspheres and results in its subsequent conversion into Cu3Si, which enhances structural stability and electron transportation. In addition, HF etching enlarges pore size to accommodate the volume change of Si particles. The Si/C nanoporous microspheres exhibit a high reversible specific capacity of larger than 1000 mA h g 1 and much improved cycling performance in comparison with pure nano-Si. The results demonstrate the importance of nanopores and electronically conducting coating layers for enhancing the electrochemical properties of Si-based electrode materials. Both the structure design and the synthesis method may be extended to other high-capacity lithium-storage materials with a large volume variation and poor electronic conductivity. ’ ASSOCIATED CONTENT
bS
Supporting Information. TEM image of the as-prepared carbon originated from SA, Coulombic efficiency profiles of Si/C microsphere and nano-Si particle under different current densities, and SEM image of a Si/C nanoporous microsphere after being used in a lithium-ion battery cycled 50 cycles at a rate of 50 mA g 1. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully thank the National Natural Science Foundation of China (Grant Nos. 50730005 and 20821003), the National Key Project on Basic Research (Grant Nos. 2011CB935700 and 2009CB930400), and the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KJCX2-YW-W26) for financial support. 14153
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