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Surface Binding of Polypyrrole on Porous Silicon Hollow Nanospheres for Li-Ion Battery Anodes with High Structure Stability Fei-Hu Du, Bo Li, Wei Fu, Yi-Jun Xiong, Kai-Xue Wang,* and Jie-Sheng Chen* Lithium-ion batteries (LIBs) with long cycling life, high energy and power densities are of critical importance for a large variety of technological applications such as portable electronics and (hybrid) electric vehicles.[1,2] As a key component, electrode material dominates the electrochemical properties of LIBs. In order to satisfy the ever-growing demand for LIBs with high energy density, many electrochemically active materials with high capacities have been developed to replace the commercial graphite anode which has a theoretical capacity of only 372 mA h g−1.[3,4] Silicon with many merits, including high theoretical specific capacity (ca. 4200 mA h g−1 based on the Si weight), low Li-uptake voltage (ca. 0.5 V vs Li/Li+), natural abundance, and environmental benignity, has been proposed as one of the most promising next-generation anode materials for LIBs.[5] However, the low structure stability due to the huge volume expansion (ca. 400%) upon alloying with lithium would result in fracture of Si active particles, loss of electrical contact, and an unstable solid electrolyte interphase (SEI) growth on the Si surface, hindering the practical implementation of silicon anodes.[6,7] Two main strategies are used for improving the structure stability and electrochemical performance of Si anodes. The first strategy is based on the exploitation of the structuredependent properties of silicon. Si nanostructures, such as Si nanowires,[8,9] nanotubes,[10,11] hollow nanospheres,[12] and porous structures,[13,14] have been designed and prepared. Particularly, Si hollow nanospheres with large inner void space can well accommodate the volume change and relieve the diffusion-induced stress during cycling. The thin shell of the hollow nanospheres can significantly decrease the diffusion distance for the electrolyte and Li+. For example, interconnected Si hollow nanospheres prepared by a chemical vapor deposition (CVD) method achieved a high initial reversible capacity of 2725 mA h g−1 and good stability, retaining more than 1400 mA h g−1 after 700 cycles.[12] Unfortunately, the whole CVD process which involves the use of silica hard templates and toxic SiH4 is quite complicated for mass production. In 2007, Bao et al. developed a magnesiothermic reduction method for the preparation of porous silicon material by using diatom frustules as a silicon source.[15] Since then, a variety of F. H. Du, B. Li, W. Fu, Y.-J. Xiong, Prof. K.-X. Wang, Prof. J.-S. Chen School of Chemistry and Chemical Engineering Shanghai Jiao Tong University Shanghai 200240, China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201401937
Adv. Mater. 2014, DOI: 10.1002/adma.201401937
nanostructured silicon materials were fabricated by adopting silicon sources, including silica nanotubes,[16] nanosheets,[17] nanofibers,[18] hollow nanospheres,[19] and mesoporous silica materials (SBA-15,[20] KIT-6[21] and MCM-41[22]. However, it is still challenging for this method to prepare porous Si hollow nanospheres with well controlled morphology and mesoporosity. The other strategy is to apply electronically conductive coatings to Si. Such conductive coatings can not only improve the electrical conductivity of the Si, but also act as soft media to buffer the stress of volume expansion. Thermal decomposition of carbon precursors,[23] chemical reduction of AgNO3 via a silver-mirror reaction,[24,25] and magnetron sputtering of a copper target[26] are well-established methods for the generation of conductive carbon, silver, and copper coatings, respectively. Nevertheless, the thermal decomposition of carbon precursors to generate the carbon coatings may cause environmental issues due to the release of CO and CO2, volatile organic compounds, and sometimes change the inferior structure of active materials.[27] For metal coatings, high-cost metal precursors and complicated instruments are involved.[28] Conducting polymers possess unique properties, including excellent electronic conductivity, chemical stability and structural flexibility.[29–31] Therefore, it is envisaged that the incorporation of a thin and robust conducting polymer coating is a simple and feasible way to enhance the electrochemical performance of Si anodes. Herein we developed a simple approach for the fabrication of ploypyrrole@porous silicon hollow spheres (PPy@PHSi) nanocomposite through the magnesiothermic reduction of mesoporous silica hollow nanospheres (MHSiO2) and subsequent in situ chemical polymerization of PPy on the PHSi surface. By galvanostatic discharge/charge measurements, we demonstrated the excellent electrochemical characteristics of the nanocomposite as an anode for LIBs, such as improved large reversible capacity, rate capability, and long-term cycling stability, compared with the pure PHSi. The preparation procedure for the PPy@PHSi nanocomposite is illustrated in Figure 1. Typically, uniform MHSiO2 nanospheres with ordered and radially oriented mesoporous channels were prepared via a spontaneous self-transformation approach by dispersing the silica spheres generated by the Stöber method in water.[32] The preparation of the nanospheres reported in this work is highly reproducible and does not require any sacrificial templates, emulsion droplets, or surface protective agents. Then, PHSi nanospheres were generated through the reduction of MHSiO2 by Mg at a temperature of 650 °C in H2/Ar. After the removal of magnesia and residual SiO2 in acid solutions, the PHSi nanospheres were dispersed in distilled water and then wrapped by a soft layer of sodium
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Figure 1. A schematic illustration of the preparation of PPy@PHSi nanocomposite.
dodecylsulfate (SDS). Finally, the polymerization of pyrrole monomers initiated by (NH4)2S2O8 oxidant occurred between the region of the soft SDS layer and the PHSi, resulting in the formation of PPy@PHSi nanocomposite. The X-ray diffraction (XRD) patterns of the samples are shown in Figure 2a. The diffractions of the acid-treated sample can be readily indexed to a cubic phase of well-crystallized Si (JCPDS card No. 27–1402), indicating that SiO2 is successfully converted into Si by the magnesiothermic reduction reaction. After PPy coating, a broad peak at ca. 22° is observed, attributing to the amorphous PPy in PPy@PHSi nanocomposite. The Raman spectra of the samples are shown in Figure 2b. The strong peak of PHSi nanospheres at around 512 cm−1 is ascribed to the characteristic scattering of the first-order optical phonon of Si. Additionally, two broad peaks are observed at 304 and 947 cm−1, which can be assigned to the scattering of two transverse acoustic phonons and two transverse optical phonons, respectively.[33] Compared with pure PHSi, significant decrease is observed in the intensity of the Si peaks of the PPy@PHSi nanocomposite, attributable to the PPy coating on the surface of PHSi nanospheres. The strong peak at 1561 cm−1 and weak peaks at 1404 and 1349 cm−1 of the PPy@PHSi nanocomposite are assigned to C=C backbone stretching and ring stretching of PPy, respectively. The peak at 1045 cm−1 belongs to the C–H in-plane bending of PPy, while those at 967 and 926 cm−1 are attributed to in-plane deformation of the ring.[34] Compared with those of pure PPy and PHSi, the bands of PPy@PHSi nanocomposite are shifted slightly. This might be due to the chemical interaction between the PHSi and PPy. The morphology and structure of the samples were determined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM image shows that the silica is spherical and uniform, giving an average diameter of approximately 600 nm (Figure 3a). TEM image demonstrates the hollow structure of the silica nanospheres (Figure 3d). The shell of the nanospheres with a thickness of about 70 nm is mesoporous ordered in short range (Figure S1a, Supporting Information). As determined by N2 adsorption-desorption analyses, the Brunauer–Emmett–Teller (BET) surface area of the MHSiO2 is approximately 934 m2 g−1 and the Barrett– Joyner–Halenda (BJH) pore-size distribution is about 2.9 nm
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Figure 2. a) XRD patterns and b) Raman spectra of the PPy@PHSi nanocomposite, PHSi, and PPy.
in diameter (Figure S2a, Supporting Information). The PHSi nanospheres generated by the magnesiothermic reduction have a rough surface (Figure 3b,e). The shell of PHSi nanospheres is composed of silicon crystallites of 10–20 nm in diameter (Figure S1b, Supporting Information). The diffraction rings of Si (111), (220) and (311) planes observed in the selected area electron diffraction pattern suggest the polycrystalline nature of the PHSi nanospheres (inset of Figure 3e), consistent with the TEM observation. Moreover, the BET surface area of the PHSi is approximately 221 m2 g−1 and the BJH pore-size distribution is about 9.6 nm in diameter (Figure S2b, Supporting Information). The SEM and TEM images of the PPy@PHSi nanocomposite are shown in Figure 3. It is observed that the PPy nanoparticles with an average particle size of 50 nm are uniformly coated on the surface of PHSi nanospheres (Figure 3c,f,g). In the high-resolution TEM (HRTEM) image of the nanocomposite, the meas-
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scan agrees well with the Li-insertion process of crystalline Si to form an amorphous LixSi phase. This peak disappears from the second cycle onwards indicating the complete amorphization of Si in the first cycle. During the first charge process, two distinct peaks are revealed at 0.37 V and 0.58 V, which can be ascribed to the phase transition between LixSi and amorphous silicon. The peak current density and integrated area intensity are nearly unchanged in the subsequent discharge and charge cycles, indicating good cycling stability of the nanocomposite. For comparison, the CV curves of pure PPy are also shown in the same figure. The current density is one order of magnitude smaller than that of the nanocomposite electrode, indicating the negligible contribution from PPy to the capacity of the whole electrode. These results are in good agreement with that reported in the literature.[2,25] Figure 4b shows the discharge– charge profiles of the PPy@PHSi nanocomposite cycled at a current density of 1.0 A g−1 between the potential range of 0.01–1.2 V vs Li+/Li. A long plateau observed below 0.1 V in the first discharge curve is ascribed to the Li-alloying process of crystalline Si, leading to the formation of an amorphous LixSi phase. Afterwards, the discharge and charge curves show the characteristics of amorphous Si. The Figure 3. a–c) SEM images of the MHSiO2 nanospheres (a), PHSi nanospheres (b), and PPy@ initial discharge and charge capacities of the PHSi nanocomposite (c). d–f) TEM images of the MHSiO2 nanospheres (d), PHSi nano−1 spheres (e), and PPy@PHSi nanocomposite (f). g,h) Magnified TEM (g) and HRTEM images nanocomposite are 2603 and 1772 mA h g , respectively, giving a Coulombic effi ciency (h) of the PPy@PHSi nanocomposite. of 68%. The irreversible capacity can be assigned to the decomposition of the electrolyte, forming a SEI on the electrode surface, and to the irreversured interplanar distance of 0.31 nm is ascribed to the (111) ible insertion of Li+ into silicon nanoparticles.[24] plane of the diamond-structured Si and an amorphous PPy layer is obviously covered on the surface of Si (Figure 3h). The The cycling stability of the PPy@PHSi nanocomposite and content of PPy in the nanocomposite is approximately 27.4 wt% pure PHSi is evaluated at a current density of 1.0 A g−1 (Figure 4c). based on the elemental analysis. After the modification by PPy, The pure PHSi electrode delivers a high initial discharge the nanocomposite still possesses a large BET surface area of capacity of 3445 mA h g−1, while the capacity drops dramatiapproximately 172 m2 g−1 and an average BJH pore size of 9.5 nm, cally after several cycles. The capacity retention of the 100th indicating that the porous channels in the Si shell are still cycle is only 45% vs the 2nd cycle. Although it is unsatisfactory, retained and accessible (Figure S2c, Supporting Information). the cycling stability of the PHSi electrode is clearly much supeEnergy dispersive X-ray (EDX) spectroscopy and elemental rior over the commercial Si nanoparticles.[16,20,25] In contrast, mapping analyses were performed to further reveal the structhe PPy@PHSi electrode demonstrates an excellent cycling statural information of the PPy@PHSi nanocomposite. The EDX bility with 88% capacity retention against at the 2nd cycle after analysis shows that the composite is composed of Si, C, N, and 250 cycles. Moreover, we note that the specific capacity of the O (Figure S3a, Supporting Information). Combined with the PPy@PHSi nanocomposite slightly increases in the initial bright/dark field scanning TEM (STEM) images (Figure S3b,c, 15 cycles, which is associated with the gradual activation of the Supporting Information), elemental mappings of the Si, C, and Si host. It is reported in the literature that the Si core remains N in the nanocomposite clearly demonstrate a homogeneous until the particle cracks and only then fully lithiates.[35,36] The distribution of PPy nanoparticles throughout the PHSi nanogradual exposure of Si core upon cycling contributes to the spheres (Figure S3d, Supporting Information). In addition, capacity increase in the initial cycles. Discharged/charged oxygen signals cover the entire sample area, attributing to the at high rates of 4.0 A g−1 for 100 cycles and 8.0 A g−1 for surface oxidation of the Si nanospheres. 70 cycles, large capacities of 1161 and 680 mA h g−1, respectively, are still maintained, indicating the excellent cycling Typical cyclic voltammetry (CV) curves of the PPy@PHSi stability of the PPy@PHSi nanocomposite. High Coulombic nanocomposite at a scanning rate of 1.0 mV s−1 are shown in efficiencies of over 95% are well maintained after the 15th cycle Figure 4a. The sharp peak at 0.5 V during the first discharge
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obviously superior to that of the pure PHSi which delivers a discharge capacity of less than 210 mA h g−1 at 8.0 A g−1. The remarkably high rate capability can be attributed to the following reasons. First, the hollow structure of the nanocomposite and porous channels in the shell form a three-dimensional mass transportation pathway, facilitating the transportation of the electrolyte and Li+ within the electrode. The high specific surface area of the nanocomposite is favorable for the fast electrode kinetics. Second, the surface PPy coating improves the surface electrical conductivity of the Si, providing an electron percolation path from the current collector to the whole surface area of each individual PHSi nanoparticle. The chargedischarge profiles of the nanocomposite provide stable voltage plateaus around 0.2 V for Li insertion at different current densities (Figure S5, Supporting Information), higher than that of the graphite and Si nanoparticles reported in the literature.[20] The high lithium ion intercalation potential for the nanocomposite may prevent the formation of metallic lithium or lithium dendrites on the electrode surface, ensuring the high safety of the electrode. In order to understand the superior cycling performance, electrochemical impedance spectroscopy (EIS) measurements of the PPy@PHSi electrode were conducted after the 1st, 100th, and 250th cycles at a current density of 1.0 A g−1 (Figure 4e). No obvious resistance increase is observed upon Figure 4. a,b) CV curves (a) and galvanostatic charge–discharge profiles (b) of the PPy@PHSi cycling, indicating the structure stability of nanocomposite. c,d) Cycling (c) and rate performance (d) of the PHSi and PPy@PHSi nanothe nanocomposite. As confirmed by the composite at a variety of current densities. e) Nyquist plots of the PPy@PHSi nanocomposite st th th −1 SEM and TEM observation of the PPy@ after the 1 , 100 and 250 cycles at a current density of 1.0 A g . PHSi electrode after 250 cycles (Figure 5), the spherical morphology and hollow structure of the PPy@PHSi nanocomposite is well retained. An increase at the rates of 4.0 and 8.0 A g−1 (Figure S4, Supporting Inforfrom approximately 700 to over 900 nm in the diameter of the mation). The outstanding cycling stability of the PPy@PHSi nanocomposite is observed. Although serious volume expannanocomposite electrode is ascribed to the synergetic action of sion exists, no obvious cracks of Si and exfoliation of PPy the porous hollow structure and the surface PPy coating. The coating are detected. This observation indicates that the porous free volume in the hollow interior and the porosity in the shell hollow structure and PPy coating can well reduce the diffusioncan accommodate the huge volume change and buffer the large induced stress and buffer the volume change during lithiation mechanical stress. The PPy coating can enhance the electrical and delithiation processes, contributing to the excellent strucconductivity of the Si by forming a conductive network and stature stability and good electric contact between PPy and PHSi. bilize the whole structure by preventing the pulverization of Si. The rate performance of the PPy@PHSi nanocomposite and In summary, we have successfully prepared PPy@PHSi pure HPSi at different current densities is shown in Figure 4d. nanocomposite using a simple approach involving the magThe discharge capacities of the PPy@PHSi nanocomposite are nesiothermic reduction of MHSiO2 and in situ chemical poly2594, 1610, 1125, and 661 mA h g−1 at current densities of 1.0, merization of PPy on the PHSi surface. The nanocomposite exhibits excellent structure stability and attractive electrochem2.0, 4.0, and 8.0 A g−1, respectively. When the current density ical performance for LIBs, such as large reversible capacity, is decreased stepwise to 1.0 A g−1, a rebound in capacity with a high rate capability, and outstanding cycleability. These highly slight decrease can be observed for the PPy@PHSi. The nanodesirable electrochemical characteristics are attributed to the composite still retains a discharge capacity of 467 mA h g−1 synergetic effect of the porous hollow structure and the surwhen the current density increases stepwise again from face PPy coating. The hollow structure of the nanocomposite 1.0 to 8.0 A g−1. The rate capability of the nanocomposite is
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were acquired using an inVia-reflex micro-Raman spectrometer (Renishaw, UK) with a 532 nm wavelength incident laser. The morphology of the samples was observed using a NOVA NanoSEM 230 field-emission scanning electron microscope (FESEM) (FEI, USA).The microstructure of the samples was characterized using a JEM-2100F transmission electron microscope (TEM) (JEOL, Japan) operating at 200 kV. The specific surface area and pore-size distribution were measured using a NOVA2200e analyzer (Quantachrome, USA). Elemental analysis (EA) was carried out on a Vario EL cube elemental analyzer (Elementar, Germany). Electrochemical measurements: The PPy@PHSi Figure 5. a,b) SEM (a) and TEM (b) images of PPy@PHSi electrode after 250 cycles at a curnanocomposite (80 wt%), Super-P carbon black rent density of 1.0 A g−1. (5 wt%, Timcal), and sodium carboxymethyl cellulose (CMC) (15 wt%) were mixed in water solution to form a slurry. The slurry was spread onto a Cu foil by a doctor and porous channels in the shell can not only buffer the huge blade method, followed by drying in a vacuum at 70 °C for 10 h. CR2016 volume change and reduce the diffusion-induced stress, but coin cells were assembled in a glove box filled with ultra-high purity argon + also facilitate the diffusion of Li and electrolyte into the elecusing polypropylene membrane (UBE Industries Ltd.) as the separator, trode. The surface PPy coating can significantly enhance the Li metal as the anode, and 1 M LiPF6 in ethyl carbonate/dimethyl carbonate (EC/DEC) (1:1 v/v) as the electrolyte. The galvanostatic charge surface electronic conductivity of the PHSi nanospheres and and discharge experiment was performed with a battery tester LANDstabilize the whole structure. Moreover, given the scalable and CT2001A in the potential range of 0.01–1.2 V at room temperature. facile nature of the synthesis procedure, the PPy@PHSi nanoElectrochemical impedance spectroscopy (EIS) measurements were composite can be exploited as a practical anode material for performed on a CHI660B electrochemical workstation. Cyclic voltammetry LIBs. (CV) was conducted on the workstation at a scanning rate of 1 mV s−1 in a potential range of 10 mV to 2.0 V (vs Li/Li+).
Experimental Section Preparation of MHSiO2: The preparation of MHSiO2 was following a procedure reported in the literature.[32] Typically, cetyltrimethylammonium bromide (CTAB) (0.3 g) was dissolved in ethanol aqueous solution (60 mL C2H5OH/100 mL H2O) containing concentrated ammonia aqueous solution (2 mL, 25 wt%). Under vigorous stirring, tetraethoxysilane (TEOS) (2 mL) was added quickly to the above solution. After stirring at 35 °C for 24 h, the white silica precipitation was collected by centrifugation and washed with ethanol. Then, the precipitation was incubated in distilled water (320 mL) and kept at 90 °C for 48 h. The product collected by centrifugation was dispersed into an ethanol solution (240 mL) containing concentrated HCl (480 µL, 37%) and stirred at 60 °C for 3 h, generating the templatefree MHSiO2. Preparation of PHSi: Magnesium powder (0.3 g) was mixed with MHSiO2 (0.3 g) by grinding. The mixture was then heated in a tube furnace at 650 °C for 4 h under an argon atmosphere containing 5 vol% H2. The ramp rate was kept at 5 °C min−1. The obtained brown powder was first immersed in a 2 M HCl solution for 6 h to remove MgO, followed by the treatment in a 10 wt% HF solution to etch off the residual SiO2. Finally, the resulting powder was washed with ethanol and vacuum-dried at 80 °C for 10 h. Fabrication of PPy@PHSi Nanocomposite: The obtained PHSi nanoparticles (0.08 g) were dispersed into 50 mL of distilled water containing sodium dodecylsulfate (SDS) (5 mg) under ultrasonication for 5 min and stirring overnight at room temperature. Later, pyrrole monomer (0.035 mL) and HCl solution (1 M, 0.5 mL) were successively added into the suspension. Then, (NH4)2S2O8 (0.114 g in 5 mL of H2O) was added as an oxidant into the system to start the polymerization. After stirring the mixture in an ice/water bath for approximately 3 h, the PPy@PHSi nanocomposite was obtained by filtration, and rinsed with water for several times. The final product was dried in vacuum at 50 °C for 12 h. Pure PPy was also prepared by the same procedure for comparison. Characterization: The X-ray diffraction (XRD) patterns were recorded on a D/max 2550VL/PC X-ray diffractometer (Rigaku, Japan) equipped with Cu Kα radiation (λ = 1.5418 Å, 40 kV, 30 mA). The Raman spectra
Adv. Mater. 2014, DOI: 10.1002/adma.201401937
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the National Basic Research Program of China (2013CB934102, 2014CB932102) and the National Natural Science Foundation of China. Received: April 29, 2014 Revised: June 9, 2014 Published online:
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Surface Binding of Polypyrrole on Porous Silicon Hollow Nanospheres for Li-Ion Battery Anodes with High Structure Stability Fei-Hu Du, Bo Li, Wei Fu, Yi-Jun Xiong, Kai-Xue Wang,* and Jie-Sheng Chen* Lithium-ion batteries (LIBs) with long cycling life, high energy and power densities are of critical importance for a large variety of technological applications such as portable electronics and (hybrid) electric vehicles.[1,2] As a key component, electrode material dominates the electrochemical properties of LIBs. In order to satisfy the ever-growing demand for LIBs with high energy density, many electrochemically active materials with high capacities have been developed to replace the commercial graphite anode which has a theoretical capacity of only 372 mA h g−1.[3,4] Silicon with many merits, including high theoretical specific capacity (ca. 4200 mA h g−1 based on the Si weight), low Li-uptake voltage (ca. 0.5 V vs Li/Li+), natural abundance, and environmental benignity, has been proposed as one of the most promising next-generation anode materials for LIBs.[5] However, the low structure stability due to the huge volume expansion (ca. 400%) upon alloying with lithium would result in fracture of Si active particles, loss of electrical contact, and an unstable solid electrolyte interphase (SEI) growth on the Si surface, hindering the practical implementation of silicon anodes.[6,7] Two main strategies are used for improving the structure stability and electrochemical performance of Si anodes. The first strategy is based on the exploitation of the structuredependent properties of silicon. Si nanostructures, such as Si nanowires,[8,9] nanotubes,[10,11] hollow nanospheres,[12] and porous structures,[13,14] have been designed and prepared. Particularly, Si hollow nanospheres with large inner void space can well accommodate the volume change and relieve the diffusion-induced stress during cycling. The thin shell of the hollow nanospheres can significantly decrease the diffusion distance for the electrolyte and Li+. For example, interconnected Si hollow nanospheres prepared by a chemical vapor deposition (CVD) method achieved a high initial reversible capacity of 2725 mA h g−1 and good stability, retaining more than 1400 mA h g−1 after 700 cycles.[12] Unfortunately, the whole CVD process which involves the use of silica hard templates and toxic SiH4 is quite complicated for mass production. In 2007, Bao et al. developed a magnesiothermic reduction method for the preparation of porous silicon material by using diatom frustules as a silicon source.[15] Since then, a variety of F. H. Du, B. Li, W. Fu, Y.-J. Xiong, Prof. K.-X. Wang, Prof. J.-S. Chen School of Chemistry and Chemical Engineering Shanghai Jiao Tong University Shanghai 200240, China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201401937
Adv. Mater. 2014, DOI: 10.1002/adma.201401937
nanostructured silicon materials were fabricated by adopting silicon sources, including silica nanotubes,[16] nanosheets,[17] nanofibers,[18] hollow nanospheres,[19] and mesoporous silica materials (SBA-15,[20] KIT-6[21] and MCM-41[22]. However, it is still challenging for this method to prepare porous Si hollow nanospheres with well controlled morphology and mesoporosity. The other strategy is to apply electronically conductive coatings to Si. Such conductive coatings can not only improve the electrical conductivity of the Si, but also act as soft media to buffer the stress of volume expansion. Thermal decomposition of carbon precursors,[23] chemical reduction of AgNO3 via a silver-mirror reaction,[24,25] and magnetron sputtering of a copper target[26] are well-established methods for the generation of conductive carbon, silver, and copper coatings, respectively. Nevertheless, the thermal decomposition of carbon precursors to generate the carbon coatings may cause environmental issues due to the release of CO and CO2, volatile organic compounds, and sometimes change the inferior structure of active materials.[27] For metal coatings, high-cost metal precursors and complicated instruments are involved.[28] Conducting polymers possess unique properties, including excellent electronic conductivity, chemical stability and structural flexibility.[29–31] Therefore, it is envisaged that the incorporation of a thin and robust conducting polymer coating is a simple and feasible way to enhance the electrochemical performance of Si anodes. Herein we developed a simple approach for the fabrication of ploypyrrole@porous silicon hollow spheres (PPy@PHSi) nanocomposite through the magnesiothermic reduction of mesoporous silica hollow nanospheres (MHSiO2) and subsequent in situ chemical polymerization of PPy on the PHSi surface. By galvanostatic discharge/charge measurements, we demonstrated the excellent electrochemical characteristics of the nanocomposite as an anode for LIBs, such as improved large reversible capacity, rate capability, and long-term cycling stability, compared with the pure PHSi. The preparation procedure for the PPy@PHSi nanocomposite is illustrated in Figure 1. Typically, uniform MHSiO2 nanospheres with ordered and radially oriented mesoporous channels were prepared via a spontaneous self-transformation approach by dispersing the silica spheres generated by the Stöber method in water.[32] The preparation of the nanospheres reported in this work is highly reproducible and does not require any sacrificial templates, emulsion droplets, or surface protective agents. Then, PHSi nanospheres were generated through the reduction of MHSiO2 by Mg at a temperature of 650 °C in H2/Ar. After the removal of magnesia and residual SiO2 in acid solutions, the PHSi nanospheres were dispersed in distilled water and then wrapped by a soft layer of sodium
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Figure 1. A schematic illustration of the preparation of PPy@PHSi nanocomposite.
dodecylsulfate (SDS). Finally, the polymerization of pyrrole monomers initiated by (NH4)2S2O8 oxidant occurred between the region of the soft SDS layer and the PHSi, resulting in the formation of PPy@PHSi nanocomposite. The X-ray diffraction (XRD) patterns of the samples are shown in Figure 2a. The diffractions of the acid-treated sample can be readily indexed to a cubic phase of well-crystallized Si (JCPDS card No. 27–1402), indicating that SiO2 is successfully converted into Si by the magnesiothermic reduction reaction. After PPy coating, a broad peak at ca. 22° is observed, attributing to the amorphous PPy in PPy@PHSi nanocomposite. The Raman spectra of the samples are shown in Figure 2b. The strong peak of PHSi nanospheres at around 512 cm−1 is ascribed to the characteristic scattering of the first-order optical phonon of Si. Additionally, two broad peaks are observed at 304 and 947 cm−1, which can be assigned to the scattering of two transverse acoustic phonons and two transverse optical phonons, respectively.[33] Compared with pure PHSi, significant decrease is observed in the intensity of the Si peaks of the PPy@PHSi nanocomposite, attributable to the PPy coating on the surface of PHSi nanospheres. The strong peak at 1561 cm−1 and weak peaks at 1404 and 1349 cm−1 of the PPy@PHSi nanocomposite are assigned to C=C backbone stretching and ring stretching of PPy, respectively. The peak at 1045 cm−1 belongs to the C–H in-plane bending of PPy, while those at 967 and 926 cm−1 are attributed to in-plane deformation of the ring.[34] Compared with those of pure PPy and PHSi, the bands of PPy@PHSi nanocomposite are shifted slightly. This might be due to the chemical interaction between the PHSi and PPy. The morphology and structure of the samples were determined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM image shows that the silica is spherical and uniform, giving an average diameter of approximately 600 nm (Figure 3a). TEM image demonstrates the hollow structure of the silica nanospheres (Figure 3d). The shell of the nanospheres with a thickness of about 70 nm is mesoporous ordered in short range (Figure S1a, Supporting Information). As determined by N2 adsorption-desorption analyses, the Brunauer–Emmett–Teller (BET) surface area of the MHSiO2 is approximately 934 m2 g−1 and the Barrett– Joyner–Halenda (BJH) pore-size distribution is about 2.9 nm
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Figure 2. a) XRD patterns and b) Raman spectra of the PPy@PHSi nanocomposite, PHSi, and PPy.
in diameter (Figure S2a, Supporting Information). The PHSi nanospheres generated by the magnesiothermic reduction have a rough surface (Figure 3b,e). The shell of PHSi nanospheres is composed of silicon crystallites of 10–20 nm in diameter (Figure S1b, Supporting Information). The diffraction rings of Si (111), (220) and (311) planes observed in the selected area electron diffraction pattern suggest the polycrystalline nature of the PHSi nanospheres (inset of Figure 3e), consistent with the TEM observation. Moreover, the BET surface area of the PHSi is approximately 221 m2 g−1 and the BJH pore-size distribution is about 9.6 nm in diameter (Figure S2b, Supporting Information). The SEM and TEM images of the PPy@PHSi nanocomposite are shown in Figure 3. It is observed that the PPy nanoparticles with an average particle size of 50 nm are uniformly coated on the surface of PHSi nanospheres (Figure 3c,f,g). In the high-resolution TEM (HRTEM) image of the nanocomposite, the meas-
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scan agrees well with the Li-insertion process of crystalline Si to form an amorphous LixSi phase. This peak disappears from the second cycle onwards indicating the complete amorphization of Si in the first cycle. During the first charge process, two distinct peaks are revealed at 0.37 V and 0.58 V, which can be ascribed to the phase transition between LixSi and amorphous silicon. The peak current density and integrated area intensity are nearly unchanged in the subsequent discharge and charge cycles, indicating good cycling stability of the nanocomposite. For comparison, the CV curves of pure PPy are also shown in the same figure. The current density is one order of magnitude smaller than that of the nanocomposite electrode, indicating the negligible contribution from PPy to the capacity of the whole electrode. These results are in good agreement with that reported in the literature.[2,25] Figure 4b shows the discharge– charge profiles of the PPy@PHSi nanocomposite cycled at a current density of 1.0 A g−1 between the potential range of 0.01–1.2 V vs Li+/Li. A long plateau observed below 0.1 V in the first discharge curve is ascribed to the Li-alloying process of crystalline Si, leading to the formation of an amorphous LixSi phase. Afterwards, the discharge and charge curves show the characteristics of amorphous Si. The Figure 3. a–c) SEM images of the MHSiO2 nanospheres (a), PHSi nanospheres (b), and PPy@ initial discharge and charge capacities of the PHSi nanocomposite (c). d–f) TEM images of the MHSiO2 nanospheres (d), PHSi nano−1 spheres (e), and PPy@PHSi nanocomposite (f). g,h) Magnified TEM (g) and HRTEM images nanocomposite are 2603 and 1772 mA h g , respectively, giving a Coulombic effi ciency (h) of the PPy@PHSi nanocomposite. of 68%. The irreversible capacity can be assigned to the decomposition of the electrolyte, forming a SEI on the electrode surface, and to the irreversured interplanar distance of 0.31 nm is ascribed to the (111) ible insertion of Li+ into silicon nanoparticles.[24] plane of the diamond-structured Si and an amorphous PPy layer is obviously covered on the surface of Si (Figure 3h). The The cycling stability of the PPy@PHSi nanocomposite and content of PPy in the nanocomposite is approximately 27.4 wt% pure PHSi is evaluated at a current density of 1.0 A g−1 (Figure 4c). based on the elemental analysis. After the modification by PPy, The pure PHSi electrode delivers a high initial discharge the nanocomposite still possesses a large BET surface area of capacity of 3445 mA h g−1, while the capacity drops dramatiapproximately 172 m2 g−1 and an average BJH pore size of 9.5 nm, cally after several cycles. The capacity retention of the 100th indicating that the porous channels in the Si shell are still cycle is only 45% vs the 2nd cycle. Although it is unsatisfactory, retained and accessible (Figure S2c, Supporting Information). the cycling stability of the PHSi electrode is clearly much supeEnergy dispersive X-ray (EDX) spectroscopy and elemental rior over the commercial Si nanoparticles.[16,20,25] In contrast, mapping analyses were performed to further reveal the structhe PPy@PHSi electrode demonstrates an excellent cycling statural information of the PPy@PHSi nanocomposite. The EDX bility with 88% capacity retention against at the 2nd cycle after analysis shows that the composite is composed of Si, C, N, and 250 cycles. Moreover, we note that the specific capacity of the O (Figure S3a, Supporting Information). Combined with the PPy@PHSi nanocomposite slightly increases in the initial bright/dark field scanning TEM (STEM) images (Figure S3b,c, 15 cycles, which is associated with the gradual activation of the Supporting Information), elemental mappings of the Si, C, and Si host. It is reported in the literature that the Si core remains N in the nanocomposite clearly demonstrate a homogeneous until the particle cracks and only then fully lithiates.[35,36] The distribution of PPy nanoparticles throughout the PHSi nanogradual exposure of Si core upon cycling contributes to the spheres (Figure S3d, Supporting Information). In addition, capacity increase in the initial cycles. Discharged/charged oxygen signals cover the entire sample area, attributing to the at high rates of 4.0 A g−1 for 100 cycles and 8.0 A g−1 for surface oxidation of the Si nanospheres. 70 cycles, large capacities of 1161 and 680 mA h g−1, respectively, are still maintained, indicating the excellent cycling Typical cyclic voltammetry (CV) curves of the PPy@PHSi stability of the PPy@PHSi nanocomposite. High Coulombic nanocomposite at a scanning rate of 1.0 mV s−1 are shown in efficiencies of over 95% are well maintained after the 15th cycle Figure 4a. The sharp peak at 0.5 V during the first discharge
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obviously superior to that of the pure PHSi which delivers a discharge capacity of less than 210 mA h g−1 at 8.0 A g−1. The remarkably high rate capability can be attributed to the following reasons. First, the hollow structure of the nanocomposite and porous channels in the shell form a three-dimensional mass transportation pathway, facilitating the transportation of the electrolyte and Li+ within the electrode. The high specific surface area of the nanocomposite is favorable for the fast electrode kinetics. Second, the surface PPy coating improves the surface electrical conductivity of the Si, providing an electron percolation path from the current collector to the whole surface area of each individual PHSi nanoparticle. The chargedischarge profiles of the nanocomposite provide stable voltage plateaus around 0.2 V for Li insertion at different current densities (Figure S5, Supporting Information), higher than that of the graphite and Si nanoparticles reported in the literature.[20] The high lithium ion intercalation potential for the nanocomposite may prevent the formation of metallic lithium or lithium dendrites on the electrode surface, ensuring the high safety of the electrode. In order to understand the superior cycling performance, electrochemical impedance spectroscopy (EIS) measurements of the PPy@PHSi electrode were conducted after the 1st, 100th, and 250th cycles at a current density of 1.0 A g−1 (Figure 4e). No obvious resistance increase is observed upon Figure 4. a,b) CV curves (a) and galvanostatic charge–discharge profiles (b) of the PPy@PHSi cycling, indicating the structure stability of nanocomposite. c,d) Cycling (c) and rate performance (d) of the PHSi and PPy@PHSi nanothe nanocomposite. As confirmed by the composite at a variety of current densities. e) Nyquist plots of the PPy@PHSi nanocomposite st th th −1 SEM and TEM observation of the PPy@ after the 1 , 100 and 250 cycles at a current density of 1.0 A g . PHSi electrode after 250 cycles (Figure 5), the spherical morphology and hollow structure of the PPy@PHSi nanocomposite is well retained. An increase at the rates of 4.0 and 8.0 A g−1 (Figure S4, Supporting Inforfrom approximately 700 to over 900 nm in the diameter of the mation). The outstanding cycling stability of the PPy@PHSi nanocomposite is observed. Although serious volume expannanocomposite electrode is ascribed to the synergetic action of sion exists, no obvious cracks of Si and exfoliation of PPy the porous hollow structure and the surface PPy coating. The coating are detected. This observation indicates that the porous free volume in the hollow interior and the porosity in the shell hollow structure and PPy coating can well reduce the diffusioncan accommodate the huge volume change and buffer the large induced stress and buffer the volume change during lithiation mechanical stress. The PPy coating can enhance the electrical and delithiation processes, contributing to the excellent strucconductivity of the Si by forming a conductive network and stature stability and good electric contact between PPy and PHSi. bilize the whole structure by preventing the pulverization of Si. The rate performance of the PPy@PHSi nanocomposite and In summary, we have successfully prepared PPy@PHSi pure HPSi at different current densities is shown in Figure 4d. nanocomposite using a simple approach involving the magThe discharge capacities of the PPy@PHSi nanocomposite are nesiothermic reduction of MHSiO2 and in situ chemical poly2594, 1610, 1125, and 661 mA h g−1 at current densities of 1.0, merization of PPy on the PHSi surface. The nanocomposite exhibits excellent structure stability and attractive electrochem2.0, 4.0, and 8.0 A g−1, respectively. When the current density ical performance for LIBs, such as large reversible capacity, is decreased stepwise to 1.0 A g−1, a rebound in capacity with a high rate capability, and outstanding cycleability. These highly slight decrease can be observed for the PPy@PHSi. The nanodesirable electrochemical characteristics are attributed to the composite still retains a discharge capacity of 467 mA h g−1 synergetic effect of the porous hollow structure and the surwhen the current density increases stepwise again from face PPy coating. The hollow structure of the nanocomposite 1.0 to 8.0 A g−1. The rate capability of the nanocomposite is
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were acquired using an inVia-reflex micro-Raman spectrometer (Renishaw, UK) with a 532 nm wavelength incident laser. The morphology of the samples was observed using a NOVA NanoSEM 230 field-emission scanning electron microscope (FESEM) (FEI, USA).The microstructure of the samples was characterized using a JEM-2100F transmission electron microscope (TEM) (JEOL, Japan) operating at 200 kV. The specific surface area and pore-size distribution were measured using a NOVA2200e analyzer (Quantachrome, USA). Elemental analysis (EA) was carried out on a Vario EL cube elemental analyzer (Elementar, Germany). Electrochemical measurements: The PPy@PHSi Figure 5. a,b) SEM (a) and TEM (b) images of PPy@PHSi electrode after 250 cycles at a curnanocomposite (80 wt%), Super-P carbon black rent density of 1.0 A g−1. (5 wt%, Timcal), and sodium carboxymethyl cellulose (CMC) (15 wt%) were mixed in water solution to form a slurry. The slurry was spread onto a Cu foil by a doctor and porous channels in the shell can not only buffer the huge blade method, followed by drying in a vacuum at 70 °C for 10 h. CR2016 volume change and reduce the diffusion-induced stress, but coin cells were assembled in a glove box filled with ultra-high purity argon + also facilitate the diffusion of Li and electrolyte into the elecusing polypropylene membrane (UBE Industries Ltd.) as the separator, trode. The surface PPy coating can significantly enhance the Li metal as the anode, and 1 M LiPF6 in ethyl carbonate/dimethyl carbonate (EC/DEC) (1:1 v/v) as the electrolyte. The galvanostatic charge surface electronic conductivity of the PHSi nanospheres and and discharge experiment was performed with a battery tester LANDstabilize the whole structure. Moreover, given the scalable and CT2001A in the potential range of 0.01–1.2 V at room temperature. facile nature of the synthesis procedure, the PPy@PHSi nanoElectrochemical impedance spectroscopy (EIS) measurements were composite can be exploited as a practical anode material for performed on a CHI660B electrochemical workstation. Cyclic voltammetry LIBs. (CV) was conducted on the workstation at a scanning rate of 1 mV s−1 in a potential range of 10 mV to 2.0 V (vs Li/Li+).
Experimental Section Preparation of MHSiO2: The preparation of MHSiO2 was following a procedure reported in the literature.[32] Typically, cetyltrimethylammonium bromide (CTAB) (0.3 g) was dissolved in ethanol aqueous solution (60 mL C2H5OH/100 mL H2O) containing concentrated ammonia aqueous solution (2 mL, 25 wt%). Under vigorous stirring, tetraethoxysilane (TEOS) (2 mL) was added quickly to the above solution. After stirring at 35 °C for 24 h, the white silica precipitation was collected by centrifugation and washed with ethanol. Then, the precipitation was incubated in distilled water (320 mL) and kept at 90 °C for 48 h. The product collected by centrifugation was dispersed into an ethanol solution (240 mL) containing concentrated HCl (480 µL, 37%) and stirred at 60 °C for 3 h, generating the templatefree MHSiO2. Preparation of PHSi: Magnesium powder (0.3 g) was mixed with MHSiO2 (0.3 g) by grinding. The mixture was then heated in a tube furnace at 650 °C for 4 h under an argon atmosphere containing 5 vol% H2. The ramp rate was kept at 5 °C min−1. The obtained brown powder was first immersed in a 2 M HCl solution for 6 h to remove MgO, followed by the treatment in a 10 wt% HF solution to etch off the residual SiO2. Finally, the resulting powder was washed with ethanol and vacuum-dried at 80 °C for 10 h. Fabrication of PPy@PHSi Nanocomposite: The obtained PHSi nanoparticles (0.08 g) were dispersed into 50 mL of distilled water containing sodium dodecylsulfate (SDS) (5 mg) under ultrasonication for 5 min and stirring overnight at room temperature. Later, pyrrole monomer (0.035 mL) and HCl solution (1 M, 0.5 mL) were successively added into the suspension. Then, (NH4)2S2O8 (0.114 g in 5 mL of H2O) was added as an oxidant into the system to start the polymerization. After stirring the mixture in an ice/water bath for approximately 3 h, the PPy@PHSi nanocomposite was obtained by filtration, and rinsed with water for several times. The final product was dried in vacuum at 50 °C for 12 h. Pure PPy was also prepared by the same procedure for comparison. Characterization: The X-ray diffraction (XRD) patterns were recorded on a D/max 2550VL/PC X-ray diffractometer (Rigaku, Japan) equipped with Cu Kα radiation (λ = 1.5418 Å, 40 kV, 30 mA). The Raman spectra
Adv. Mater. 2014, DOI: 10.1002/adma.201401937
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the National Basic Research Program of China (2013CB934102, 2014CB932102) and the National Natural Science Foundation of China. Received: April 29, 2014 Revised: June 9, 2014 Published online:
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