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(S-doped-carbon nanowire network) as anode material for lithium-ion batteries
Journal of Power Sources 297 (2015) 344e350
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Nano-structured composite of Si/(S-doped-carbon nanowire network) as anode material for lithium-ion batteries Dan Shao a, b, Daoping Tang a, Jianwen Yang c, Yanwei Li c, Lingzhi Zhang a, * a
Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, 510640, China University of Chinese Academy of Sciences, Beijing, 100049, China c College of Chemistry & Bioengineering, Guilin University of Technology, 12 Jiangan Road, Guilin, Guangxi, 541004, China b
h i g h l i g h t s Si/(S-doped-C nanowire-network) prepared by carbonizing Si/(PEDOT nanowire-network). Deliver capacity of 820 mAh g1 after 400 cycles with capacity fade of 0.09%/cycle. S-doped-C nanowire-network constructs robust conducting network in the composite. S-doped-C nanowire-network improves structural stability of the composite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2015 Received in revised form 30 June 2015 Accepted 10 August 2015 Available online xxx
Novel nanostructured silicon composites, Si/Poly(3,4-ethylenedioxythiophene) nanowire network (Si/ PNW) and Si/(S-doped-carbon nanowire network) (Si/S-CNW), are prepared by a soft-template polymerization of 3,4-ethylenedioxythiophene (EDOT) using sodium dodecyl sulfate (SDS) as surfactant with the presence of Si nanoparticles and a subsequent carbonization of Si/PNW, respectively. The presence of Si nanoparticles in the soft-template polymerization of EDOT plays a critical role in the formation of PEDOT nanowire network instead of 1D nanowire. After the carbonization of PEDOT, the S-doped-carbon nanowire network matrix shows higher electrical conductivity than PNW counterpart, which facilitates to construct robust conductive bridges between Si nanoparticles and provide large electrode/electrolyte interfaces for rapid charge transfer reactions. Thus, Si/S-CNW composite exhibits excellent cycling stability and rate capability as anode material, retaining a specific capacity of 820 mAh g1 after 400 cycles with a very small capacity fade of 0.09% per cycle. © 2015 Elsevier B.V. All rights reserved.
Keywords: Sulfur-doped carbon nanowire network Nano-Si composite Anode Lithium-ion battery
1. Introduction Silicon (Si)/carbon composite is one of the most promising next generation anode materials for lithium-ion batteries due to their unique advantages, such as (i) ultrahigh capacity provided by Si component (theoretical value: 4200 mAh g1), as compared with 372 mAh g1 of the commercial graphite anode [1e6]; (ii) the carbon matrix can not only accommodate the severe volume changes (~400%) of Si particles over cycling, but also act as a conductive medium to compensate for the intrinsic low conductivity of Si, thus improving the cycling stability and rate performance [7e10]. A variety of carbonaceous materials (e.g. graphite
* Corresponding author. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2015.08.037 0378-7753/© 2015 Elsevier B.V. All rights reserved.
[11], graphene [12], porous carbon [13] and carbon nano-fiber/tube [14e16]) have been used as matrix to make Si composites so far. Among these, carbon nano-fiber/-tube with one-dimensional (1D) nano-structure have attracted particular interest as carbon matrix to prepare these composites in view of their unique advantages such as the high aspect ratio, stable chemical property and excellent mechanical strength [14e16]. Their 1D nanostructure can not only sustain large lithiation/delithiation strain, but also shorten the diffusion length of Li ions and provide large electrode/electrolyte interfaces for rapid charge transfer reactions [14e16]. The common strategy in literature to prepare these Si composites includes: (i) mixing or embedding Si particle in the nano-carbon materials [14,17,18]; (ii) growing Si particle/film on the surface of carbon nano-fiber/-tube; or conversely growing carbon nanofiber/-tube on the surface of Si particle [15,16,19]. These composites showed significantly improved cycling performance as anode
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materials compared with the baer Si material. However, the cycling stability is not satisfied enough for practical applications, due to the poor distribution of Si particles and the insufficient binding between Si particle and 1D-carbon matrix. To tackle this binding issue, we reported novel nano-Si/(multi-wall carbon-nanotube) composite as flexible anode prepared by a facial filtration method and subsequently thermal sintering process, using sodium carboxymethyl cellulose (CMC) as a dispersing/binding agent [20]. The excess CMC provides extra binding capability to tightly anchor the Si nanoparticles in MWCNT matrix after thermal treatment, due to the structural shrinkage caused by the carbonization of CMC. Besides, these carbon components in Si composites usually cause a high irreversible capacity in the first cycle and correspondingly a low initial Coulombic efficiency (CE), which limits the practical use of these Si/carbon composites in industry [20,21]. Heteroatom (e.g. nitrogen [22], phosphorous [23] and sulfur [24,25]) doped carbon materials with enhanced electrochemical performance have been exploited for various applications, such as electronics, sensors, batteries, and catalysts. The heteroatomdoping can manipulate the electronic structures and improve the overall properties of carbon materials (e.g. electro-negativity, electronic conductivity, lithium electro-activity and workfunction) [25e27]. Such heteroatom-doping carbon materials as anode materials have shown high specific capacity (even exceeding the theoretical capacity of graphitic carbon) and improved cyclability [22,25]. For example, Lee et al. prepared Si/(N-doped carbon nanotube) composite by electrostatic self-encapsulation method using N-doped graphitic carbon as encapsulant which showed a high CE of 84.8% and a reversible capacity of 941.7 mAh g1 with 79.4% capacity retention after 100 cycles [27]. We previously reported nanosized Si/3,4-ethylene-dioxythiophene:poly(styrenesulfonate) (PEDOT:PSS) and Si/S-doped C composite as anode materials prepared by an in situ polymerization of EDOT in PSS aqueous solution with dispersed Si nanoparticles and subsequent carbonization of Si/PEDOT:PSS [28,29]. Interestingly, these Si/Sdoped-carbon composites exhibited extraordinary high CE and enhanced cycling stability [28]. In this paper, we report a novel Si/S-doped-carbon nanowire composite (Si/S-CNW) prepared by carbonizing Si/(PEDOT nanowire network) composite (Si/PNW) which was synthesized by a soft-template polymerization of EDOT in sodium dodecyl sulfate (SDS) aqueous micellar solution with the presence of Si nanoparticles. The structural characterization and electrochemical properties of these composites have been investigated in detail.
and the subsequent carbonization process. In a typical synthesis, SDS (0.178 g) was dissolved in deionized (DI) water (20 mL) followed by addition of a small amount of CTAB (0.007 g) with stirring. To this solution, Si nanoparticles (0.05 g) was added and dispersed by stirring for 1 h. EDOT monomer (0.042 g) was then added into the solution with continuous stirring. Finally, FeCl3 (0.034 g) was slowly introduced into the above suspension, and the polymerization reaction proceeded for 10 h at 50 C under an Ar atmosphere. A solid precipitate (Si/PNW) was recovered after filtration, washed with water and ethanol, and dried under vacuum at 60 C for 24 h. Si/S-CNW composite was obtained by carbonizing Si/PNW at 800 C for 3 h under Ar gas protection at a heating rate of 3 C min1.
2. Experimental section
2.4. Electrochemical characterization
2.1. Materials
The coin-type half cells (CR2025) were assembled to test the electrochemical performance of Si, Si/PNW and Si/S-CNW (active material). The working electrode was prepared by coating the slurry of active material (70 wt.%), carbon black (20 wt.%), and carboxymethyl cellulose (10 wt.%) in an aqueous solution onto a copper foil. The obtained electrodes were then dried at 60 C overnight under vacuum and then pressed to obtain the electrode sheets; the loading level of the active material is 0.7, 0.5 and 0.5 mg cm2 for Si, Si/PNW and Si/S-CNW electrode, respectively. The half cells were assembled in an Ar filled glove-box, using 1 M LiPF6 EC/DEC/DMC (v/v/v ¼ 1:1:1) with 5% FEC as the electrolyte, Li foil as the counter electrode and celgard 2400 as the separator. The cells were then galvanostatically charged and discharged at 25 C on a Shenzhen Neware battery cycler (china) at various rates between cut-off voltages of 0.01 and 1.5 V (vs. Li/Liþ). The specific capacity was calculated based on the active mass in the electrode. Electrochemical impedance spectroscopic (EIS) and cyclic voltammetry (CV) measurements were carried out on an IM6e
EDOT (99%), FeCl3 (>99.9%) and sodium dodecyl sulfate (>99%) was purchased from SigmaeAldrich corporation. Cetyltrimethylammonium bromide was purchased from Tianjin Fuchen Chemicals Reagent Factory (China). Si nanoparticles with average particle sizes of 30e80 nm was purchased from Xuzhou Jiechuang New Material Technology Company (China). All of these materials were used as received. The Celgard 2400 was used as a separator. The electrolyte of 1 M LiPF6 and 5% fluoroethylene carbonate (FEC) in ethylene carbonate (EC, >99.9%)/diethylene carbonate (DEC, >99.9%)/dimethyl carbonate (DMC, >99.9%) (v/v/v ¼ 1:1:1, water content <10 ppm) was purchased from Guotai-Huarong New Chemical Materials Co. (China). 2.2. Preparation of Si/S-CNW Si/S-CNW was prepared through a soft template polymerization
2.3. Material characterization To evaluate the interactions of SDS with Si particles, Si nanoparticles (0.05 g) was dispersed in a dilute SDS solution (0.17 g in 20 mL DI water) and stirred for 1 h. After removing water and dried in vacuum at 60 C for 12 h, SDS/Si sample was obtained. For a comparative experiment, Si powder was treated with HF (8 wt.%) for several seconds to generate Si nanoparticles with a hydrophobic surface. Following the above procedures, this pretreated Si particles was also stirred with SDS solution; and the resulted sample was noted as SDS/Si-H after drying. Fourier transform infrared spectra were recorded on a tensor 27 spectrometer from 4000 to 400 cm1 (Bruker, Germany). The morphology and microstructure of samples were observed using scanning electron microscope (Hitachi S-4800, Japan) and transmission electron microscope (TEM) (JEOL JEM 2100F, Japan). The conductivity of the sample was measured through a conventional 2-wire Ohms method using the IM6e electrochemical workstation (Zahner, Germany) at 25 C. The composition and crystal structures of samples were studied using HR800 Confocal Raman system (HORIBA Jobin Yvon, France) and X-ray diffraction (XRD) (PANALYTICAL, the Netherlands). The XPS spectra were obtained with ESCALAB250 XPS (Thermo Fisher Scientific, USA) at 2 109 mba, using Al K (1486.6 eV) radiation at 15 keV of anode voltage. Thermogravimetric analysis measurements (TGA) were conducted on a STA409C/PC-PFEIFFER VACUUMTGA-7 analyzer (NETZSCH-Gertebau GmbH, Germany) in either air or an Ar atmosphere with a flow rate of 30 mL min1 from 30 C to 800 C. The BrunauereEmmetteTeller (BET) (SI-MP-10/Pore Master 33, Quantachrome Instruments, USA) test was performed to analyze the porosities and specific surface area of the samples.
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electrochemical workstation (Zahner, Germany) at 25 C. EIS was measured by applying an oscillating voltage of 5 mV over the frequency ranging from 102 to 105 Hz. CV was conducted in cells at different scan rates from 10 mV to 1.5 V. 3. Results and discussion The synthesis procedure of nanostructured Si/S-CNW composite is schematically illustrated in Fig. 1. To synthesize Si/PEDOTnanowire composite, Si nanoparticles were firstly dispersed in SDS aqueous solution. Cetyltrimethylammonium bromide (CTAB) was added to facilitate to form the rod-like structure micelles of SDS [30]. EDOT monomer was then introduced into the suspension, and the mixture was stirred slowly for 20 min FeCl3 was added finally as a catalyst to initialize the polymerization of EDOT; and the reaction mixture was stirred for 10 h. After removing the excess surfactant and FeCl3, a deep greeneblack color (in web version) precipitate was obtained (noted as Si/PNW). The Si/S-CNW composite was obtained by subsequently subjecting Si/PNW for carbonizing at 800 C in an Ar atmosphere. The synthesis of PEDOT nanowire has been reported before, using a similar soft-template procedure described above [31]. Interestingly, we found that PEDOT formed a nanowire network structure instead of 1D nanowire when Si nanoparticle was present in the soft-template reaction system (Fig. 2a/b). To clarify the effect of Si particle on the morphology difference of PEDOT, a comparative experiment was conducted according to the same synthetic procedures without the presence of Si particles in the reaction (the obtained sample noted as PNW and its carbonization product noted as S-CNW). SEM results confirmed the nanowire structure of PEDOT for PNW (not shown). For Si/PNW, Si nanoparticles were homogeneously dispersed in the PEDOT nanowire network. For Si/SCNW, Si nanoparticles were tightly embedded in the S-doped-carbon nanowire network (wire diameter, ~25 nm) due to the structural shrinkage of PEDOT carbonization (Fig. 2cee). The selected area electron diffraction (SAED) pattern of Si/S-CNW (Fig. 2f) reveals, (i) the three diffused ring-pattern corresponding to the (002), (101), and (110) planes of the amorphous carbon structure [32]; (ii) the spot patterns corresponding to the planes of the silicon crystallites: (111), (220), (311), (400), and (331) [33]. The hydrogen bonding between the sulfonate group of SDS and the surface of Si may play a critical role in the formation of PEDOT nanowire network instead of PEDOT nanowire in the case of the soft template polymerization of EDOT without the presence of Si nanoparticles. FTIR analysis was carried out to investigate the interaction of SDS with Si nanoparticles. For SDS/Si-H, FTIR characteristic peaks of sulfonate group of SDS at 1225 and 1086 cm1 were slightly shifted to lower wave numbers as compared with 1216 and 1077 cm1 for SDS/Si, evidencing a chemical interaction between SDS and Si nanoparticles (Fig. 3a) [34,35]. After
Fig. 1. Schematic illustration of the synthesis of Si/S-CNW composite.
Fig. 2. SEM images of the obtained Si/PNW (a,b), Si nanoparticles (insert, a) and Si/SCNW (c,d). TEM image (e) and the selected area electron diffraction pattern (f) of Si/SCNW.
polymerization, the ]CeH deformation vibration of EDOT at 891 cm1 disappeared. Correspondingly, the eCeSe bond at 980 cm1, the stretching vibration of the eCeOeCe bond at 1090 cm1, and the CeC and C]C stretches of quinoidal structure from thiophene ring at 1335 cm1 were observed for Si/PNW sample, confirming the formation of Si/PNW composite (Fig. 3b) [35,28]. To prepare the Si/S-CNW composite, Si/PNW was subjected for carbonization at 800 C for 3 h under an Ar atmosphere. The typical bands of the thiophene ring at 1335 cm1 from PEDOT disappeared after carbonization, indicating that PEDOT was transformed into carbon. Raman spectra of Si, Si/PNW and Si/S-CNW are presented in Fig. 4a. For all the samples, the characteristic peak of Si was observed at 520 cm1 [36e38]. For both Si/PNW and Si/S-CNW, the peak at around 520 cm1 was somewhat broadened and downshifted to lower wavenumber compared with Si sample, probably due to a phonon confinement effect and/or a masking effect induced by PEDOT-nanowire/S-doped-carbon nanowire matrix (Fig. 4a, insert i) [13,37]. The characteristic peaks of PEDOT, the CeC inter-ring stretching vibration at 1267 cm1, the C]C symmetrical stretching vibration at 1433 cm1 and the C]C asymmetrical stretching vibration at 1510 cm1, were observed in the spectrum of Si/PNW [38]. After carbonization, these characteristic peaks of PEDOT disappeared for Si/S-CNW sample. Furthermore, the weak peaks at 1340 and 1580 cm1 corresponding to the D and G band of carbon, typical for amorphous carbon and crystalline graphite, were observed in the spectrum of Si/S-CNW (Fig. 4a, insert ii) [38,39]. X-ray diffraction (XRD) measurement was conducted to further examine the structures of these samples. All of Si, Si/PNW and Si/SCNW showed typical diffraction peaks of Si crystals at 2q of about 28.4 , 47.4 , 56.2 , 69.2 and 76.5 (Fig. 4b). The characteristic
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Fig. 3. (a) FTIR spectra of the SDS, SDS/Si and SDS/Si-H and the proposed hydrogen bonding between the sulfonate group of SDS and Si surface (insert). (b) FTIR spectra of the Si, EDOT, Si/PNW and Si/S-CNW.
diffraction peak of PEDOT at 26.0 , corresponding to the (020) crystalline plane, was clearly observed in the patterns of both Si/ PNW and PEDOT nanowire [40]. For Si/S-CNW, two broad and weak diffraction peaks were observed at about 23 and 43 corresponding to the amorphous carbon in the composite [13]. The content of Si in the Si/PNW and Si/S-CNW composites is estimated to be 49.9 and 68.7 wt.% respectively, based on the thermal gravimetric curves (TG) (Fig. 4c), which was higher than that of Si/nano1D carbon composite in the literature [17,18]. The energy dispersive X-ray spectroscopy (EDS) analysis showed that the weight ratio of S in the Si/S-CNW composite was 3.2 wt.% (Fig. 4c, insert). For Si/SCNW, 10.2 wt.% of the S element related to the carbon weight was doped in the carbon layer after the carbonization of Si/PNW. X-ray photoelectron spectroscopy (XPS) experiments were carried out to investigate the bonding state of C and S atom in Si/PNW (Fig. 4d) and Si/S-CNW (Fig. 4e). The C 1s spectrum for Si/PNW appeared in two components: (i) one at low binding energy at around 284.8 eV related to carbon atoms directly bonded to other carbons; (ii) another at higher binding energies at 286.1 and 287.3 eV related to the carbon atoms bonded to oxygen (CeO) and sulfur (CeS), respectively [41]. For Si/PNW, the S 2p spectrum shows four peaks at 163.9, 165.0, 168.4 and 170.0 eV, corresponding
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to the sulfur atoms in the neutral PEDOT, the positively charged PEDOT, doped (DS-PEDOTþ) and undoped dodecyl sulfate (DS-Naþ), respectively [31,42]. For Si/S-CNW, the C 1s spectrum is composed of four peaks, the two at 284.6/285.7 eV corresponding to carbon atoms with sp2 hybridization and sp3 hybridization and another two at higher binding energy of 287.2/290.4 eV corresponding to CeS and the p-p* shake-up peak, respectively [43]. The S 2p spectrum shows three peaks at 164.0, 165.1 and 168.5 eV, corresponding to the sulfide bridge (eCeSeCe) for the first two peaks and to the sulfone bridge (eCeSO2eCe) for the third peak [43,44]. The sulfide bridge represents 63.7% of the total S content in Si/SCNW, indicating that most of the sulfur in the carbon-nanowire matrix exits in a state of sulfide. This doped sulfur in the carbon materials can increase the charge capacity of the cells [25,45], which may have a favorable effect on improving the electrochemical performance of Si/S-CNW. Electrical conductivities of PNW, S-CNW, Si, Si/PNW and Si/SCNW were measured by employing 2-wire Ohms measurement apparatus (Fig. 5a). The conductivity of PNW and S-CNW was 24.5 and 48.0 S cm1, respectively. Due to the insulating effect of the embedded Si nanoparticles with very low conductivity (2.7E6 S cm1), the conductivity was correspondingly decreased to 6.7E3 and 0.8 S cm1 for Si/PNW and Si/S-CNW, respectively. Interestingly, Si/S-CNW showed a much higher conductivity (above 100 times) than Si/PNW. The specific surface area of Si nanoparticles, Si/ PNW, and Si/S-CNW was 31.0, 53.1 and 97.7 m2 g1, respectively (Fig. 5b). The increased surface areas of both composite are mainly due to the intrinsic large surface areas of the nanowire-network structured matrix. Notably, the specific surface area of Si/S-CNW was larger than that of Si/PNW which can be attributed to the formation of mesopores during the carbonization process (Fig. 5b, insert). The cycling performance of S-CNW, Si, Si/PNW and Si/S-CNW as anode at a current density of 0.4 A g1 is displayed in Fig. 6a. S-CNW showed an initial reversible capacity of 557 mAh g1 with a CE of 64.0%, better than the carbon nanowires without doping reported in literature [20,21]. Si, Si/PNW and Si/S-CNW electrode exhibited an initial reversible capacity of 2262, 1406 and 1292 mAh g1, with an initial CE of 70.8, 78.2 and 83.0%, respectively. The initial CE of Si is lower than those of Si/PNW and Si/S-CNW composite, due to the severe surface reactions with the electrolyte [46e48]. The initial voltage profile for all the samples showed a typical charge/ discharge plateau of Si around 0.15 and 0.4 V (Fig. 6b) [11e16]. For all the samples, the short discharge plateau at about 1.2 V in the first cycle can be ascribed to the decomposition of FEC additive in the electrolyte (Fig. 6b). Compared with Si and Si/PNW, Si/S-CNW sample showed an extra short charge plateau at about 0.75 V in the first cycle which can be ascribed to the formation of SEI on the surface of S-doped-carbon nanowire network matrix. The capacity of Si electrode degraded quickly even at a low number of cycles due to the cracking and pulverization of the Si nanoparticles [46e48]. Compared with Si, Si/PNW showed an improved cycling performance, retaining a reversible capacity of 838 mAh g1 after 50 cycles. Unlucky, the capacity decreased to 680 mAh g1 at 100th cycle. Notably, Si/S-CNW exhibited the best cycling performance, showing a reversible capacity of 1258 mAh g1 (97.3% capacity retention) after 100 cycles and a capacity of 820 mAh g1 even after 400 cycles with a small capacity fading rate of 0.09% per cycle (Fig. 6d). The remarkable cycling stability can be attributed to the confinements of the S-CNW matrix which suppressed Si interparticles contact and kept good electric conductivity over cycling. Fig. 6c shows the rate capability of Si/PNW and Si/S-CNW anode. Si/ PNW showed a relatively fast capacity decrease from 1460 mAh g1 at 0.2 A g1 to 654 mAh g1 at 10 A g1 (44.7% capacity retention). As a comparison, Si/S-CNW showed better rate performance,
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Fig. 4. (a) Raman spectrum of Si, Si/PNW and Si/S-CNW, and (insert) partial magnification Raman shift. (b) XRD patterns of Si, PEDOT-nanowire, Si/PNW and Si/S-CNW. (c) TGA curves of Si, Si/PNW and Si/S-CNW under air atmospheres, and EDS spectrum of Si/S-CNW (insert). C 1s and S 2p XPS spectra of Si/PNW (d) and Si/S-CNW (e).
Fig. 5. (a) IeV curves of Si, Si/PNW and Si/S-CNW, insert: IeV curves of the PNW and S-CNW sample. (b) N2 adsorptionedesorption isotherms for Si, Si/PNW and Si/S-CNW, insert: BJH pore size distributions of the samples (c) Table for conductivity and specific surface area of the samples.
retaining a reversible capacity of 1423 and 953 mAh g1 at 0.2 A g1 and 10 A g1 and thus a higher capacity retention of 66.9%. When the current density returned back to 0.2 A g1, a reversible capacity of 1238 mAh g1 was retained (86.9% of the initial charge capacity), indicating that the integrity of Si/S-CNW electrode was maintained even after high rate charge/discharge test. This excellent rate
capability of Si/S-CNW can be attributed to the robust conductive network constructed by S-CNW matrix which provides an efficient pathway for the transport of electrons (Fig. 6d, insert). Fig. 7a and b shows the Nyquist plots of Si and Si/S-CNW electrode, respectively. Compared with Si anode, the high frequency semicircle during the first charge/discharge for Si/S-CNW,
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Fig. 6. (a) Cycling performance and coulombic efficiency of the samples. (b)Galvanostatic charge/discharge profiles of the initial cycle of the Si, Si/PNW and Si/S-CNW electrode. (c) Rate capability of the Si, Si/PNW and Si/S-CNW electrode. (d) Long-term cycling performance of Si/S-CNW electrode, (insert) proposed mechanism of electronic conductivity effect of S-CNW matrix in the Si/S-CNW composite.
Fig. 7. Nyquist plots of (a) the Si and (b) Si/S-CNW electrodes at different charge/discharge states during the first cycles. (c) Nyquist plots of the Si and Si/S-CNW electrodes before and after 20 cycles at a current density of 0.4 A g1.
reflecting the formation of the SEI film, changed little with the change of the potentials due to the formation of stable SEI layer during the first cycle (Fig. 7a/b) [40]. After 20 cycles, the impedance semicircles showed a much smaller change compared with Si (Fig. 7c). This result demonstrates that S-CNW matrix is effective to reduce the charge-transfer resistance. To better understand the effect of S-CNW matrix on the electrochemical kinetics during the charge/discharge process of Si/SCNW composite, the cyclic voltammetry measurements (CV) were performed. Fig. 8a/b shows the CV curves of Si and Si/S-CNW electrode recorded at different scan rate (0.2e0.6 mV s1) from 10 mV to 1.5 V. For both samples, the large reduction peak at 0.01 V can be assigned to the insertion of Li ions into Si [13,48e51]. Two oxidation peaks at 0.39 and 0.53 V can be attributed to the dealloying reactions of LixSi and the extraction of Li ions from the silicon host, respectively [13]. The reduction peak at 0.2 V corresponded to the generation of LieSi alloy phase [13,50,51]. Compared with Si anode, the two redox pairs in the CV curves of Si/S-CNW overlapped and showed less change even at high scan rates, suggesting an improved cycling reversibility (Fig. 8a/b). And, Si/S-CNW also showed a smaller potential difference between the oxidation and reduction peak and a better linear relationship between the peak current and the scan rate (Fig. 8c/d), which demonstrates that SCNW matrix effectively reduced the polarization of Si nanoparticles
Fig. 8. CV curves of Si (a) and Si/S-CNW (b) electrode at different scan rates from 1.5 V to 0.01 V (vs. Li/Liþ). (c) The variation of potential difference between the oxidation and the reduction peak (marked by arrow in Fig. 8a/b) as a function of scan rate. (d) The relationship between the peak current and the scan rate.
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and improved the electrochemical kinetics during the charge/ discharge process [52]. 4. Conclusions We reported Si/Poly(3,4-ethylenedioxythiophene) nanowire network (Si/PNW) and Si/(S-doped-carbon nanowire network) (Si/ S-CNW) composite which were prepared by a soft-template polymerization of EDOT with the presence of Si nanoparticles and subsequent carbonization of Si/PNW, respectively. Poly(3,4ethylenedioxythiophene) in Si/PNW appeared as a nanowire network instead of 1D nanowire structure, due to the interaction between the sulfonate group of template agent SDS and the surface of Si nanoparticle. Both Si/PNW and Si/S-CNW showed higher initial Coulombic efficiency and better cycling performance than Si nanoparticle as anode in lithium-ion cells. Highly conductive SCNW constructed robust conductive network and structural stability in Si/S-CNW composite. Therefore, Si/S-CNW exhibited excellent cycling and rate performance, retaining a specific capacity of 820 mAh g1 after 400 cycles with a very small capacity fade of 0.09% per cycle. 5. Acknowledgments This work was supported by Guangdong Provincial Program of Special Support for High-Level Talents (2014TX01N014), National Natural Science Foundation of China, Collaboration Project of CASGuangdong Province (2013B091300017), and Guangzhou Municipal Project for Science & Technology (201423/2014Y2-00219). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Nano-structured composite of Si/(S-doped-carbon nanowire network) as anode material for lithium-ion batteries Dan Shao a, b, Daoping Tang a, Jianwen Yang c, Yanwei Li c, Lingzhi Zhang a, * a
Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, 510640, China University of Chinese Academy of Sciences, Beijing, 100049, China c College of Chemistry & Bioengineering, Guilin University of Technology, 12 Jiangan Road, Guilin, Guangxi, 541004, China b
h i g h l i g h t s Si/(S-doped-C nanowire-network) prepared by carbonizing Si/(PEDOT nanowire-network). Deliver capacity of 820 mAh g1 after 400 cycles with capacity fade of 0.09%/cycle. S-doped-C nanowire-network constructs robust conducting network in the composite. S-doped-C nanowire-network improves structural stability of the composite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2015 Received in revised form 30 June 2015 Accepted 10 August 2015 Available online xxx
Novel nanostructured silicon composites, Si/Poly(3,4-ethylenedioxythiophene) nanowire network (Si/ PNW) and Si/(S-doped-carbon nanowire network) (Si/S-CNW), are prepared by a soft-template polymerization of 3,4-ethylenedioxythiophene (EDOT) using sodium dodecyl sulfate (SDS) as surfactant with the presence of Si nanoparticles and a subsequent carbonization of Si/PNW, respectively. The presence of Si nanoparticles in the soft-template polymerization of EDOT plays a critical role in the formation of PEDOT nanowire network instead of 1D nanowire. After the carbonization of PEDOT, the S-doped-carbon nanowire network matrix shows higher electrical conductivity than PNW counterpart, which facilitates to construct robust conductive bridges between Si nanoparticles and provide large electrode/electrolyte interfaces for rapid charge transfer reactions. Thus, Si/S-CNW composite exhibits excellent cycling stability and rate capability as anode material, retaining a specific capacity of 820 mAh g1 after 400 cycles with a very small capacity fade of 0.09% per cycle. © 2015 Elsevier B.V. All rights reserved.
Keywords: Sulfur-doped carbon nanowire network Nano-Si composite Anode Lithium-ion battery
1. Introduction Silicon (Si)/carbon composite is one of the most promising next generation anode materials for lithium-ion batteries due to their unique advantages, such as (i) ultrahigh capacity provided by Si component (theoretical value: 4200 mAh g1), as compared with 372 mAh g1 of the commercial graphite anode [1e6]; (ii) the carbon matrix can not only accommodate the severe volume changes (~400%) of Si particles over cycling, but also act as a conductive medium to compensate for the intrinsic low conductivity of Si, thus improving the cycling stability and rate performance [7e10]. A variety of carbonaceous materials (e.g. graphite
* Corresponding author. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2015.08.037 0378-7753/© 2015 Elsevier B.V. All rights reserved.
[11], graphene [12], porous carbon [13] and carbon nano-fiber/tube [14e16]) have been used as matrix to make Si composites so far. Among these, carbon nano-fiber/-tube with one-dimensional (1D) nano-structure have attracted particular interest as carbon matrix to prepare these composites in view of their unique advantages such as the high aspect ratio, stable chemical property and excellent mechanical strength [14e16]. Their 1D nanostructure can not only sustain large lithiation/delithiation strain, but also shorten the diffusion length of Li ions and provide large electrode/electrolyte interfaces for rapid charge transfer reactions [14e16]. The common strategy in literature to prepare these Si composites includes: (i) mixing or embedding Si particle in the nano-carbon materials [14,17,18]; (ii) growing Si particle/film on the surface of carbon nano-fiber/-tube; or conversely growing carbon nanofiber/-tube on the surface of Si particle [15,16,19]. These composites showed significantly improved cycling performance as anode
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materials compared with the baer Si material. However, the cycling stability is not satisfied enough for practical applications, due to the poor distribution of Si particles and the insufficient binding between Si particle and 1D-carbon matrix. To tackle this binding issue, we reported novel nano-Si/(multi-wall carbon-nanotube) composite as flexible anode prepared by a facial filtration method and subsequently thermal sintering process, using sodium carboxymethyl cellulose (CMC) as a dispersing/binding agent [20]. The excess CMC provides extra binding capability to tightly anchor the Si nanoparticles in MWCNT matrix after thermal treatment, due to the structural shrinkage caused by the carbonization of CMC. Besides, these carbon components in Si composites usually cause a high irreversible capacity in the first cycle and correspondingly a low initial Coulombic efficiency (CE), which limits the practical use of these Si/carbon composites in industry [20,21]. Heteroatom (e.g. nitrogen [22], phosphorous [23] and sulfur [24,25]) doped carbon materials with enhanced electrochemical performance have been exploited for various applications, such as electronics, sensors, batteries, and catalysts. The heteroatomdoping can manipulate the electronic structures and improve the overall properties of carbon materials (e.g. electro-negativity, electronic conductivity, lithium electro-activity and workfunction) [25e27]. Such heteroatom-doping carbon materials as anode materials have shown high specific capacity (even exceeding the theoretical capacity of graphitic carbon) and improved cyclability [22,25]. For example, Lee et al. prepared Si/(N-doped carbon nanotube) composite by electrostatic self-encapsulation method using N-doped graphitic carbon as encapsulant which showed a high CE of 84.8% and a reversible capacity of 941.7 mAh g1 with 79.4% capacity retention after 100 cycles [27]. We previously reported nanosized Si/3,4-ethylene-dioxythiophene:poly(styrenesulfonate) (PEDOT:PSS) and Si/S-doped C composite as anode materials prepared by an in situ polymerization of EDOT in PSS aqueous solution with dispersed Si nanoparticles and subsequent carbonization of Si/PEDOT:PSS [28,29]. Interestingly, these Si/Sdoped-carbon composites exhibited extraordinary high CE and enhanced cycling stability [28]. In this paper, we report a novel Si/S-doped-carbon nanowire composite (Si/S-CNW) prepared by carbonizing Si/(PEDOT nanowire network) composite (Si/PNW) which was synthesized by a soft-template polymerization of EDOT in sodium dodecyl sulfate (SDS) aqueous micellar solution with the presence of Si nanoparticles. The structural characterization and electrochemical properties of these composites have been investigated in detail.
and the subsequent carbonization process. In a typical synthesis, SDS (0.178 g) was dissolved in deionized (DI) water (20 mL) followed by addition of a small amount of CTAB (0.007 g) with stirring. To this solution, Si nanoparticles (0.05 g) was added and dispersed by stirring for 1 h. EDOT monomer (0.042 g) was then added into the solution with continuous stirring. Finally, FeCl3 (0.034 g) was slowly introduced into the above suspension, and the polymerization reaction proceeded for 10 h at 50 C under an Ar atmosphere. A solid precipitate (Si/PNW) was recovered after filtration, washed with water and ethanol, and dried under vacuum at 60 C for 24 h. Si/S-CNW composite was obtained by carbonizing Si/PNW at 800 C for 3 h under Ar gas protection at a heating rate of 3 C min1.
2. Experimental section
2.4. Electrochemical characterization
2.1. Materials
The coin-type half cells (CR2025) were assembled to test the electrochemical performance of Si, Si/PNW and Si/S-CNW (active material). The working electrode was prepared by coating the slurry of active material (70 wt.%), carbon black (20 wt.%), and carboxymethyl cellulose (10 wt.%) in an aqueous solution onto a copper foil. The obtained electrodes were then dried at 60 C overnight under vacuum and then pressed to obtain the electrode sheets; the loading level of the active material is 0.7, 0.5 and 0.5 mg cm2 for Si, Si/PNW and Si/S-CNW electrode, respectively. The half cells were assembled in an Ar filled glove-box, using 1 M LiPF6 EC/DEC/DMC (v/v/v ¼ 1:1:1) with 5% FEC as the electrolyte, Li foil as the counter electrode and celgard 2400 as the separator. The cells were then galvanostatically charged and discharged at 25 C on a Shenzhen Neware battery cycler (china) at various rates between cut-off voltages of 0.01 and 1.5 V (vs. Li/Liþ). The specific capacity was calculated based on the active mass in the electrode. Electrochemical impedance spectroscopic (EIS) and cyclic voltammetry (CV) measurements were carried out on an IM6e
EDOT (99%), FeCl3 (>99.9%) and sodium dodecyl sulfate (>99%) was purchased from SigmaeAldrich corporation. Cetyltrimethylammonium bromide was purchased from Tianjin Fuchen Chemicals Reagent Factory (China). Si nanoparticles with average particle sizes of 30e80 nm was purchased from Xuzhou Jiechuang New Material Technology Company (China). All of these materials were used as received. The Celgard 2400 was used as a separator. The electrolyte of 1 M LiPF6 and 5% fluoroethylene carbonate (FEC) in ethylene carbonate (EC, >99.9%)/diethylene carbonate (DEC, >99.9%)/dimethyl carbonate (DMC, >99.9%) (v/v/v ¼ 1:1:1, water content <10 ppm) was purchased from Guotai-Huarong New Chemical Materials Co. (China). 2.2. Preparation of Si/S-CNW Si/S-CNW was prepared through a soft template polymerization
2.3. Material characterization To evaluate the interactions of SDS with Si particles, Si nanoparticles (0.05 g) was dispersed in a dilute SDS solution (0.17 g in 20 mL DI water) and stirred for 1 h. After removing water and dried in vacuum at 60 C for 12 h, SDS/Si sample was obtained. For a comparative experiment, Si powder was treated with HF (8 wt.%) for several seconds to generate Si nanoparticles with a hydrophobic surface. Following the above procedures, this pretreated Si particles was also stirred with SDS solution; and the resulted sample was noted as SDS/Si-H after drying. Fourier transform infrared spectra were recorded on a tensor 27 spectrometer from 4000 to 400 cm1 (Bruker, Germany). The morphology and microstructure of samples were observed using scanning electron microscope (Hitachi S-4800, Japan) and transmission electron microscope (TEM) (JEOL JEM 2100F, Japan). The conductivity of the sample was measured through a conventional 2-wire Ohms method using the IM6e electrochemical workstation (Zahner, Germany) at 25 C. The composition and crystal structures of samples were studied using HR800 Confocal Raman system (HORIBA Jobin Yvon, France) and X-ray diffraction (XRD) (PANALYTICAL, the Netherlands). The XPS spectra were obtained with ESCALAB250 XPS (Thermo Fisher Scientific, USA) at 2 109 mba, using Al K (1486.6 eV) radiation at 15 keV of anode voltage. Thermogravimetric analysis measurements (TGA) were conducted on a STA409C/PC-PFEIFFER VACUUMTGA-7 analyzer (NETZSCH-Gertebau GmbH, Germany) in either air or an Ar atmosphere with a flow rate of 30 mL min1 from 30 C to 800 C. The BrunauereEmmetteTeller (BET) (SI-MP-10/Pore Master 33, Quantachrome Instruments, USA) test was performed to analyze the porosities and specific surface area of the samples.
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electrochemical workstation (Zahner, Germany) at 25 C. EIS was measured by applying an oscillating voltage of 5 mV over the frequency ranging from 102 to 105 Hz. CV was conducted in cells at different scan rates from 10 mV to 1.5 V. 3. Results and discussion The synthesis procedure of nanostructured Si/S-CNW composite is schematically illustrated in Fig. 1. To synthesize Si/PEDOTnanowire composite, Si nanoparticles were firstly dispersed in SDS aqueous solution. Cetyltrimethylammonium bromide (CTAB) was added to facilitate to form the rod-like structure micelles of SDS [30]. EDOT monomer was then introduced into the suspension, and the mixture was stirred slowly for 20 min FeCl3 was added finally as a catalyst to initialize the polymerization of EDOT; and the reaction mixture was stirred for 10 h. After removing the excess surfactant and FeCl3, a deep greeneblack color (in web version) precipitate was obtained (noted as Si/PNW). The Si/S-CNW composite was obtained by subsequently subjecting Si/PNW for carbonizing at 800 C in an Ar atmosphere. The synthesis of PEDOT nanowire has been reported before, using a similar soft-template procedure described above [31]. Interestingly, we found that PEDOT formed a nanowire network structure instead of 1D nanowire when Si nanoparticle was present in the soft-template reaction system (Fig. 2a/b). To clarify the effect of Si particle on the morphology difference of PEDOT, a comparative experiment was conducted according to the same synthetic procedures without the presence of Si particles in the reaction (the obtained sample noted as PNW and its carbonization product noted as S-CNW). SEM results confirmed the nanowire structure of PEDOT for PNW (not shown). For Si/PNW, Si nanoparticles were homogeneously dispersed in the PEDOT nanowire network. For Si/SCNW, Si nanoparticles were tightly embedded in the S-doped-carbon nanowire network (wire diameter, ~25 nm) due to the structural shrinkage of PEDOT carbonization (Fig. 2cee). The selected area electron diffraction (SAED) pattern of Si/S-CNW (Fig. 2f) reveals, (i) the three diffused ring-pattern corresponding to the (002), (101), and (110) planes of the amorphous carbon structure [32]; (ii) the spot patterns corresponding to the planes of the silicon crystallites: (111), (220), (311), (400), and (331) [33]. The hydrogen bonding between the sulfonate group of SDS and the surface of Si may play a critical role in the formation of PEDOT nanowire network instead of PEDOT nanowire in the case of the soft template polymerization of EDOT without the presence of Si nanoparticles. FTIR analysis was carried out to investigate the interaction of SDS with Si nanoparticles. For SDS/Si-H, FTIR characteristic peaks of sulfonate group of SDS at 1225 and 1086 cm1 were slightly shifted to lower wave numbers as compared with 1216 and 1077 cm1 for SDS/Si, evidencing a chemical interaction between SDS and Si nanoparticles (Fig. 3a) [34,35]. After
Fig. 1. Schematic illustration of the synthesis of Si/S-CNW composite.
Fig. 2. SEM images of the obtained Si/PNW (a,b), Si nanoparticles (insert, a) and Si/SCNW (c,d). TEM image (e) and the selected area electron diffraction pattern (f) of Si/SCNW.
polymerization, the ]CeH deformation vibration of EDOT at 891 cm1 disappeared. Correspondingly, the eCeSe bond at 980 cm1, the stretching vibration of the eCeOeCe bond at 1090 cm1, and the CeC and C]C stretches of quinoidal structure from thiophene ring at 1335 cm1 were observed for Si/PNW sample, confirming the formation of Si/PNW composite (Fig. 3b) [35,28]. To prepare the Si/S-CNW composite, Si/PNW was subjected for carbonization at 800 C for 3 h under an Ar atmosphere. The typical bands of the thiophene ring at 1335 cm1 from PEDOT disappeared after carbonization, indicating that PEDOT was transformed into carbon. Raman spectra of Si, Si/PNW and Si/S-CNW are presented in Fig. 4a. For all the samples, the characteristic peak of Si was observed at 520 cm1 [36e38]. For both Si/PNW and Si/S-CNW, the peak at around 520 cm1 was somewhat broadened and downshifted to lower wavenumber compared with Si sample, probably due to a phonon confinement effect and/or a masking effect induced by PEDOT-nanowire/S-doped-carbon nanowire matrix (Fig. 4a, insert i) [13,37]. The characteristic peaks of PEDOT, the CeC inter-ring stretching vibration at 1267 cm1, the C]C symmetrical stretching vibration at 1433 cm1 and the C]C asymmetrical stretching vibration at 1510 cm1, were observed in the spectrum of Si/PNW [38]. After carbonization, these characteristic peaks of PEDOT disappeared for Si/S-CNW sample. Furthermore, the weak peaks at 1340 and 1580 cm1 corresponding to the D and G band of carbon, typical for amorphous carbon and crystalline graphite, were observed in the spectrum of Si/S-CNW (Fig. 4a, insert ii) [38,39]. X-ray diffraction (XRD) measurement was conducted to further examine the structures of these samples. All of Si, Si/PNW and Si/SCNW showed typical diffraction peaks of Si crystals at 2q of about 28.4 , 47.4 , 56.2 , 69.2 and 76.5 (Fig. 4b). The characteristic
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Fig. 3. (a) FTIR spectra of the SDS, SDS/Si and SDS/Si-H and the proposed hydrogen bonding between the sulfonate group of SDS and Si surface (insert). (b) FTIR spectra of the Si, EDOT, Si/PNW and Si/S-CNW.
diffraction peak of PEDOT at 26.0 , corresponding to the (020) crystalline plane, was clearly observed in the patterns of both Si/ PNW and PEDOT nanowire [40]. For Si/S-CNW, two broad and weak diffraction peaks were observed at about 23 and 43 corresponding to the amorphous carbon in the composite [13]. The content of Si in the Si/PNW and Si/S-CNW composites is estimated to be 49.9 and 68.7 wt.% respectively, based on the thermal gravimetric curves (TG) (Fig. 4c), which was higher than that of Si/nano1D carbon composite in the literature [17,18]. The energy dispersive X-ray spectroscopy (EDS) analysis showed that the weight ratio of S in the Si/S-CNW composite was 3.2 wt.% (Fig. 4c, insert). For Si/SCNW, 10.2 wt.% of the S element related to the carbon weight was doped in the carbon layer after the carbonization of Si/PNW. X-ray photoelectron spectroscopy (XPS) experiments were carried out to investigate the bonding state of C and S atom in Si/PNW (Fig. 4d) and Si/S-CNW (Fig. 4e). The C 1s spectrum for Si/PNW appeared in two components: (i) one at low binding energy at around 284.8 eV related to carbon atoms directly bonded to other carbons; (ii) another at higher binding energies at 286.1 and 287.3 eV related to the carbon atoms bonded to oxygen (CeO) and sulfur (CeS), respectively [41]. For Si/PNW, the S 2p spectrum shows four peaks at 163.9, 165.0, 168.4 and 170.0 eV, corresponding
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to the sulfur atoms in the neutral PEDOT, the positively charged PEDOT, doped (DS-PEDOTþ) and undoped dodecyl sulfate (DS-Naþ), respectively [31,42]. For Si/S-CNW, the C 1s spectrum is composed of four peaks, the two at 284.6/285.7 eV corresponding to carbon atoms with sp2 hybridization and sp3 hybridization and another two at higher binding energy of 287.2/290.4 eV corresponding to CeS and the p-p* shake-up peak, respectively [43]. The S 2p spectrum shows three peaks at 164.0, 165.1 and 168.5 eV, corresponding to the sulfide bridge (eCeSeCe) for the first two peaks and to the sulfone bridge (eCeSO2eCe) for the third peak [43,44]. The sulfide bridge represents 63.7% of the total S content in Si/SCNW, indicating that most of the sulfur in the carbon-nanowire matrix exits in a state of sulfide. This doped sulfur in the carbon materials can increase the charge capacity of the cells [25,45], which may have a favorable effect on improving the electrochemical performance of Si/S-CNW. Electrical conductivities of PNW, S-CNW, Si, Si/PNW and Si/SCNW were measured by employing 2-wire Ohms measurement apparatus (Fig. 5a). The conductivity of PNW and S-CNW was 24.5 and 48.0 S cm1, respectively. Due to the insulating effect of the embedded Si nanoparticles with very low conductivity (2.7E6 S cm1), the conductivity was correspondingly decreased to 6.7E3 and 0.8 S cm1 for Si/PNW and Si/S-CNW, respectively. Interestingly, Si/S-CNW showed a much higher conductivity (above 100 times) than Si/PNW. The specific surface area of Si nanoparticles, Si/ PNW, and Si/S-CNW was 31.0, 53.1 and 97.7 m2 g1, respectively (Fig. 5b). The increased surface areas of both composite are mainly due to the intrinsic large surface areas of the nanowire-network structured matrix. Notably, the specific surface area of Si/S-CNW was larger than that of Si/PNW which can be attributed to the formation of mesopores during the carbonization process (Fig. 5b, insert). The cycling performance of S-CNW, Si, Si/PNW and Si/S-CNW as anode at a current density of 0.4 A g1 is displayed in Fig. 6a. S-CNW showed an initial reversible capacity of 557 mAh g1 with a CE of 64.0%, better than the carbon nanowires without doping reported in literature [20,21]. Si, Si/PNW and Si/S-CNW electrode exhibited an initial reversible capacity of 2262, 1406 and 1292 mAh g1, with an initial CE of 70.8, 78.2 and 83.0%, respectively. The initial CE of Si is lower than those of Si/PNW and Si/S-CNW composite, due to the severe surface reactions with the electrolyte [46e48]. The initial voltage profile for all the samples showed a typical charge/ discharge plateau of Si around 0.15 and 0.4 V (Fig. 6b) [11e16]. For all the samples, the short discharge plateau at about 1.2 V in the first cycle can be ascribed to the decomposition of FEC additive in the electrolyte (Fig. 6b). Compared with Si and Si/PNW, Si/S-CNW sample showed an extra short charge plateau at about 0.75 V in the first cycle which can be ascribed to the formation of SEI on the surface of S-doped-carbon nanowire network matrix. The capacity of Si electrode degraded quickly even at a low number of cycles due to the cracking and pulverization of the Si nanoparticles [46e48]. Compared with Si, Si/PNW showed an improved cycling performance, retaining a reversible capacity of 838 mAh g1 after 50 cycles. Unlucky, the capacity decreased to 680 mAh g1 at 100th cycle. Notably, Si/S-CNW exhibited the best cycling performance, showing a reversible capacity of 1258 mAh g1 (97.3% capacity retention) after 100 cycles and a capacity of 820 mAh g1 even after 400 cycles with a small capacity fading rate of 0.09% per cycle (Fig. 6d). The remarkable cycling stability can be attributed to the confinements of the S-CNW matrix which suppressed Si interparticles contact and kept good electric conductivity over cycling. Fig. 6c shows the rate capability of Si/PNW and Si/S-CNW anode. Si/ PNW showed a relatively fast capacity decrease from 1460 mAh g1 at 0.2 A g1 to 654 mAh g1 at 10 A g1 (44.7% capacity retention). As a comparison, Si/S-CNW showed better rate performance,
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Fig. 4. (a) Raman spectrum of Si, Si/PNW and Si/S-CNW, and (insert) partial magnification Raman shift. (b) XRD patterns of Si, PEDOT-nanowire, Si/PNW and Si/S-CNW. (c) TGA curves of Si, Si/PNW and Si/S-CNW under air atmospheres, and EDS spectrum of Si/S-CNW (insert). C 1s and S 2p XPS spectra of Si/PNW (d) and Si/S-CNW (e).
Fig. 5. (a) IeV curves of Si, Si/PNW and Si/S-CNW, insert: IeV curves of the PNW and S-CNW sample. (b) N2 adsorptionedesorption isotherms for Si, Si/PNW and Si/S-CNW, insert: BJH pore size distributions of the samples (c) Table for conductivity and specific surface area of the samples.
retaining a reversible capacity of 1423 and 953 mAh g1 at 0.2 A g1 and 10 A g1 and thus a higher capacity retention of 66.9%. When the current density returned back to 0.2 A g1, a reversible capacity of 1238 mAh g1 was retained (86.9% of the initial charge capacity), indicating that the integrity of Si/S-CNW electrode was maintained even after high rate charge/discharge test. This excellent rate
capability of Si/S-CNW can be attributed to the robust conductive network constructed by S-CNW matrix which provides an efficient pathway for the transport of electrons (Fig. 6d, insert). Fig. 7a and b shows the Nyquist plots of Si and Si/S-CNW electrode, respectively. Compared with Si anode, the high frequency semicircle during the first charge/discharge for Si/S-CNW,
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Fig. 6. (a) Cycling performance and coulombic efficiency of the samples. (b)Galvanostatic charge/discharge profiles of the initial cycle of the Si, Si/PNW and Si/S-CNW electrode. (c) Rate capability of the Si, Si/PNW and Si/S-CNW electrode. (d) Long-term cycling performance of Si/S-CNW electrode, (insert) proposed mechanism of electronic conductivity effect of S-CNW matrix in the Si/S-CNW composite.
Fig. 7. Nyquist plots of (a) the Si and (b) Si/S-CNW electrodes at different charge/discharge states during the first cycles. (c) Nyquist plots of the Si and Si/S-CNW electrodes before and after 20 cycles at a current density of 0.4 A g1.
reflecting the formation of the SEI film, changed little with the change of the potentials due to the formation of stable SEI layer during the first cycle (Fig. 7a/b) [40]. After 20 cycles, the impedance semicircles showed a much smaller change compared with Si (Fig. 7c). This result demonstrates that S-CNW matrix is effective to reduce the charge-transfer resistance. To better understand the effect of S-CNW matrix on the electrochemical kinetics during the charge/discharge process of Si/SCNW composite, the cyclic voltammetry measurements (CV) were performed. Fig. 8a/b shows the CV curves of Si and Si/S-CNW electrode recorded at different scan rate (0.2e0.6 mV s1) from 10 mV to 1.5 V. For both samples, the large reduction peak at 0.01 V can be assigned to the insertion of Li ions into Si [13,48e51]. Two oxidation peaks at 0.39 and 0.53 V can be attributed to the dealloying reactions of LixSi and the extraction of Li ions from the silicon host, respectively [13]. The reduction peak at 0.2 V corresponded to the generation of LieSi alloy phase [13,50,51]. Compared with Si anode, the two redox pairs in the CV curves of Si/S-CNW overlapped and showed less change even at high scan rates, suggesting an improved cycling reversibility (Fig. 8a/b). And, Si/S-CNW also showed a smaller potential difference between the oxidation and reduction peak and a better linear relationship between the peak current and the scan rate (Fig. 8c/d), which demonstrates that SCNW matrix effectively reduced the polarization of Si nanoparticles
Fig. 8. CV curves of Si (a) and Si/S-CNW (b) electrode at different scan rates from 1.5 V to 0.01 V (vs. Li/Liþ). (c) The variation of potential difference between the oxidation and the reduction peak (marked by arrow in Fig. 8a/b) as a function of scan rate. (d) The relationship between the peak current and the scan rate.
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and improved the electrochemical kinetics during the charge/ discharge process [52]. 4. Conclusions We reported Si/Poly(3,4-ethylenedioxythiophene) nanowire network (Si/PNW) and Si/(S-doped-carbon nanowire network) (Si/ S-CNW) composite which were prepared by a soft-template polymerization of EDOT with the presence of Si nanoparticles and subsequent carbonization of Si/PNW, respectively. Poly(3,4ethylenedioxythiophene) in Si/PNW appeared as a nanowire network instead of 1D nanowire structure, due to the interaction between the sulfonate group of template agent SDS and the surface of Si nanoparticle. Both Si/PNW and Si/S-CNW showed higher initial Coulombic efficiency and better cycling performance than Si nanoparticle as anode in lithium-ion cells. Highly conductive SCNW constructed robust conductive network and structural stability in Si/S-CNW composite. Therefore, Si/S-CNW exhibited excellent cycling and rate performance, retaining a specific capacity of 820 mAh g1 after 400 cycles with a very small capacity fade of 0.09% per cycle. 5. Acknowledgments This work was supported by Guangdong Provincial Program of Special Support for High-Level Talents (2014TX01N014), National Natural Science Foundation of China, Collaboration Project of CASGuangdong Province (2013B091300017), and Guangzhou Municipal Project for Science & Technology (201423/2014Y2-00219). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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