Si@SiO₂ nanowires/carbon textiles cable-type anodes for high-capacity reversible lithium-ion batteries

Abstract

To fulfil the increasing demands for high-efficiency next-generation energy storage systems, different electrode matrixes have been designed to achieve the desired batteries with high energy density and long cycle life. Herein, we fabricated novel binder-free Si@SiO₂ nanowires/carbon textiles cable-type anodes for Li-ion batteries applications. The half-cells based on the above electrodes delivered high reversible capacity of 2247 mA h g⁻¹ at 800 mA g⁻¹, good rate capability, and excellent cycle stability of as long as 1000 cycles at 8 A g⁻¹. Full batteries including the as-synthesized Si@SiO₂ anodes and commercial LiCoO₂ cathodes were also fabricated, which exhibited greatly enhanced performance, thus revealing that these novel Si@SiO₂ nanowires/carbon textiles cable-type electrodes can be applied in next-generation energy storage devices.

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Introduction

The rechargeable lithium-ion battery is identified as one of the most important power sources, and has been applied widely in various applications.^1–5 Although previous Li-ion batteries have achieved developments from their original ideas to industrialization, the battery techniques may encounter a bottleneck, because the currently existing batteries with low energy density and poor rate-capability will not fulfill future higher requirements, such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and stationary power grids, etc. Thus, the efficient strategies to develop alternative active materials and fabricate advanced electrode-matrix have been concerned in recent years.^6–15 Among lots of anodes, silicon anodes with low discharge potential and the highest theoretical specific capacity (4200 mA h g⁻¹) have attracted more and more attentions.^16–20 However, their 400% upon volume changes for Li⁺ insertion/extraction result in the pulverization and capacity fading, which hinder further potential energy-storage applications of silicon anodes. It is still a huge challenge to accommodate this large strains related to volume changes and improve silicon-based anode electrochemical performance.

Developing silicon-based nanostructures is an efficient way to enhance their corresponding capacity and cycling stability.^21–24 For example, Yao et al. represented that Si NWs coated with conductive polymer (PEDOT) lead to a considerable improvement of cycling performance.²⁵ Jia et al. fabricated a novel 3D mesoporous lotus-root-like silicon-based anodes with a stable cycling ability and good rate capability.²⁶ Zhou et al. successfully inserted silicon nanoparticles into graphene sheets, which can contribute remarkably to enhanced cycling stability and rate performance compared with bare Si nanoparticles.²⁷ Although significant progresses have been achieved, there is still great room for improvements in cycling stability and rate capability in a complete silicon-based storage unit. Only in this unit can we replace the existing graphite anode with silicon anode.

In this paper, we report on the synthesis of Si@SiO₂ nanowires/carbon textiles cable-type composite electrodes via a conventional chemical vapor deposition (CVD) process. As-synthesized silicon-based anodes delivered high capacity (2247 mA h g⁻¹ at 800 mA g⁻¹), good rate performance, and excellent cycle stability (less than 8% capacity degradation per 100 cycles for 1000 total cycles at big current density of 8 A g⁻¹). The full cells also display desirable performance. All the excellent electrochemical performances are attributed to the unique cable-type matrix consisting of the core–shell Si@SiO₂ nanowires and the high conductive carbon textiles.

Experiment

Materials synthesis

Commercially available carbon textiles were first cleaned by sonication consequently in acetone, ethanol and deionized (DI) water for 30 min each. After dried at 70 °C for 10 hours, the cleaned carbon textiles were transferred into the Al₂O₃ tube downstream as substrates to collect the product. In a typical process, SiO powders (purity: 99.99%; Aladdin) were placed at the center of the Al₂O₃ tube. Meanwhile high-purity argon was used as the carrier gas at 100 sccm. The SiO powers were heated to 1350 °C and hold at that temperature for 3–5 min under ambient gas pressure of ∼100 torr. The mass loading for the products was measured to be 0.6–0.8 mg cm⁻².

Materials characterization

The X-ray diffraction (XRD) patterns were tested using X-ray diffractometer (X'Pert PRO, PANalytical B.V., The Netherlands) with Cu Kα radiation (λ = 0.154 nm). The morphologies of the samples were characterized by using scanning electron microscopy (SEM, Hitachi S4800) coupled with an energy-dispersive X-ray spectrometer (EDX) and transmission electron microscopy (TEM, JEOL JEM-3000F). Raman spectrum was collected using JY Horiba under 514 nm Ar laser. XPS measurements were performed on a VG Multi-lab 2000 system with a monochromatic Al Kα X-ray source.

Electrochemical measurement

The electrochemical behaviors of the as-synthesized cable-type Si@SiO₂ NWs/carbon textiles electrodes were measured using CR2032 coin-type cells at room temperature. The cells were laboratory-assembled without conductive carbon or any other binder in an argon-filled glove box where moisture and oxygen concentrations were strictly limited to below 1 ppm. Celgard 2300 was used as the separator membrane. The electrolyte was 1 mol L⁻¹ LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v/v = 1 : 1). Lithium foils were used as the counter electrode in coin-type half-cells, while the commercial LiCoO₂ were used as the cathode in coin-type full-cells. The galvanostatic charge–discharge tests were carried out on a Land Battery Measurement System (Land, China) at various current densities with a cutoff voltage window of 0.01–1.5 V vs. Li⁺/Li for half-cells and 2.5–4.0 V for full-cells.

Results and discussion

Core–shell Si@SiO₂ nanowires twining on carbon fibers were synthesized via a conventional chemical vapor deposition (CVD) process following the equations.²⁸

Si_xO → Si_x−1 + SiO (x > 1) (1)

2SiO → Si + SiO₂ (2)

Fig. 1 shows the schematic illustration of Si@SiO₂ nanowires/carbon textiles cable-type anodes and the experimental facility was shown in Fig. S1.† Fig. 1 illustrates change details of carbon textiles before and after the reaction. Compared with the cleaned carbon fibers in Fig. 1a, the product in Fig. 1b reveal that every carbon fiber was uniformly coated with numerous nanowires to form the final Si@SiO₂ nanowires/carbon textiles cable-type matrix. In large scale, they contact with each others to constitute this unique cable-type anodes.

Fig. 1 Schematic illustration of the fabrication process of the cable-type Si-based nanostructure.

The composition of the as-synthesized product was studied by XRD and the corresponding XRD pattern is shown in Fig. 2a. All peaks in this pattern, except those from carbon textiles (black curve), can be indexed to pure Si (JCPDS no. 1-791). The morphologies of the samples were characterized by using scanning electron microscopy and the corresponding SEM images were shown in Fig. 2b–d and S2† and the cross section SEM image in Fig. S3.† From Fig. 2b and c we can see that, compared with the bare carbon textiles with smooth surface (Fig. S4†), the carbon fibers are orderly covered with numerous ultrafine nanowires. Fig. 2d gives more details about the deposited nanowires, which have diameters of 10–30 nm and length of micrometers, and nanowires interweave each other to form three-dimensional uniform network on a large scale. The corresponding energy dispersive spectroscopy (EDS) analysis is illustrated in Fig. 2d inset, which confirms the constitution of Si, O elements from the as-prepared samples. In this spectrum, the carbon signal comes from the carbon textiles substrate.

Fig. 2 (a) XRD pattern, (b–d) SEM images of the cable-type Si@SiO₂ NWs/carbon textiles. Inset: the EDS of the as-prepared samples.

To get more clear information about the as-synthesized samples, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectrum were further performed. Fig. 3a shows a typical TEM image of the synthesized core–shell Si@SiO₂ nanowires. From this image, we can see that the diameters of the nanowires are about 20 nm, which is in consistent with SEM results.

Fig. 3 (a) TEM and (b) HRTEM image of the as-prepared core–shell Si@SiO₂ NWs. (c) XPS spectrum and (d) Raman spectrum of the Si@SiO₂ NWs/carbon textiles cable-type anodes.

The high-resolution TEM (HRTEM) images shown in Fig. 3b and S5† demonstrate the core–shell structure of Si@SiO₂ NWs that very thin amorphous SiO₂ layers were coated on the surface of the Si nanowires. The dashed inset is the boundary between crystalline Si part and the amorphous SiO₂ part. The marked interplanar d-spacings of 0.19 nm and 0.31 nm match well with the (220) and (111) planes of monoclinic Si shown in Fig. 3b. X-ray photoelectron spectroscopy (XPS) was further used to investigate the local structure and bonding states of the as-synthesized core–shell Si@SiO₂ nanowires. Fig. 3c shows the corresponding result, where the peak at 98.6 eV corresponds to the zero-valent silicon, and the peak at 103.3 eV coincides with the four-valent silicon representing the out SiO₂ surface. The XPS spectrum of O 1s was shown in Fig. S6† and the observed peak at 532.65 eV can be referred to SiO₂.²⁹ Fig. 3d is the Raman spectrum of the sample. The sharp peak located at ∼516.7 cm⁻¹ is related to the Si–Si stretching mode, while the little shoulder to the extent of 938.5–977.5 cm⁻¹ is due to the stretching mode of amorphous Si–Si, inferring the existing of amorphous Si, which we can't distinguish it from outer surface amorphous SiO₂ layer accurately in HRTEM images. There is not an obvious peak in the range of 1000–1100 cm⁻¹ for silicon oxide, suggesting that the SiO₂ shell is very thin, according to previous report.³⁰ Besides, no peaks located at either ∼1360 cm⁻¹ or ∼1580 cm⁻¹, advising that the carbon fibers are wrapped with Si@SiO₂ nanowires tightly.

The unique cable-type anodes design of Si@SiO₂ NWs/carbon textiles makes it possible to directly assemble these materials as binder-free electrodes for lithium-ion batteries. Therefore, electrochemical measurements of the as-prepared Si-based anodes were performed by using laboratory-based CR2032 coin cells at room temperature. The electrochemical reaction mechanism of Li with Si has been well investigated and can be described as the following equations:⁵

xLi + Si ↔ Li_xSi (0 ≤ x ≤ 4.4) (3)

The cyclic voltammetry (CV) was carried out of the system shown in Fig. S7.† Fig. 4a displays the galvanostatic discharge–charge voltage profiles of the Si@SiO₂ NWs/carbon textiles anodes at a current density of 800 mA g⁻¹ (0.2 C) with a voltage window of 0.01–1.5 V. The initial discharge and charge specific capacities are found to be 2851 and 2214 mA h g⁻¹, respectively, with the coulombic efficiency of 77.7%. The large capacity loss in the first cycle is mainly attributed to the initial irreversible formation of Li₂O and evitable formation of SEI layer and electrolyte decomposition, which is a common phenomenon for most anode materials.³¹ The second discharge capacity is a little higher than the first cycle, involving the activation of the whole system, which is also a common phenomenon according to previous reports.^32–35 After 50 charge–discharge cycles, these electrodes can still deliver a desired retention of ∼99.6% and an enhanced specific capacity of ∼2247 mA h g⁻¹ in average, which is still six times larger than that of commercial graphite (Fig. 4b). Besides, discharge curves of the Si-based anodes and the pure carbon textiles substrate were also provided in Fig. S8.† It reveals that the capacitance of pure carbon textiles is negligible compared to that of the Si-based anodes. Thus, the total capacity of the as-assembled cells is contributed greatly by the Si@SiO₂ nanowires products.^36–42 The rate performance of the cable-type anodes was also investigated at various current densities between 0.01–1.5 V. Fig. 4c shows the discharge–charge potential profiles curves at the different currents. The specific capacity decreases from 2293, 1929, 1419, and 862 to 364 mA h g⁻¹ with increasing currents ranging from 0.8 A g⁻¹ (0.2 C), 2 A g⁻¹ (0.5 C), 4 A g⁻¹ (1 C), and 8 A g⁻¹ (2 C) to 16 A g⁻¹ (4 C). Fig. 4d presents the good cycling stability of these electrodes at different rates for as long as 60 cycles and highly desired coulombic efficiency (over 96.6%).

Fig. 4 (a) Galvanostatic discharge and charge curves and (b) cycling performance and coulombic efficiency of the Si@SiO₂ NWs/carbon textiles cable-type anode cycled in the cutoff voltage window of 0.01–1.5 V vs. Li⁺/Li at a current density of 800 mA g⁻¹ for 50 cycles. (c) Galvanostatic discharge and charge curves at different current densities, and (d) cycling stability and coulombic efficiency for 10 cycles each at the current densities of 0.8 A g⁻¹, 2 A g⁻¹, 4 A g⁻¹, 8 A g⁻¹, 16 A g⁻¹, in the cutoff voltage window of 0.01–1.5 V vs. Li⁺/Li.

From the above results, we can see that the cable-type structure electrode of the as-synthesized Si@SiO₂ NWs/carbon textiles possesses superior capacity of energy storage, good rate capability and excellent electrochemical stability. This may be attributed to the following aspects:

(i) The ultrafine 1D silicon nanowires are propitious to the insertion/extraction of Li ion and shorten the Li-ion diffusion paths, which enhances the rate capability; and among the discharge and charge processes, the ultrafine nanowires produce less stresses, which can lessen the wreck of materials.⁴³

(ii) The ultrathin outer SiO₂ layer can suffer the mechanical stresses created by the core silicon nanowire, which reduces the mechanical breaking and allows for development of a stable solid-electrolyte interphase (SEI), contributing to the excellent electrochemical stability.⁴⁴

(iii) The ultrafine Si@SiO₂ NWs directly grow and wrap on the carbon textiles, which makes it possible to use them as battery anodes without any conducting additives or binders.

(iv) The nanowires interweave to form the continuous 3D network, providing great mechanical and electrical interconnects throughout the entire network.

(v) The numerous interspaces involving the continuous 3D network provide enough exposed surface, which promotes the diffusion of the electrolyte.

Considering the excellent electrochemical performance above, the stability of the as-fabricated Si-based cable-type anodes was researched in two aspects. Fig. 5a and b show the typical SEM image of the Si@SiO₂ NWs/carbon textiles cable-type anodes after 50 charge–discharge cycles at 800 mA g⁻¹ between 0.01 and 1.5 V. From these figures, it can be seen that, even after 50 cycles, the Si@SiO₂ NWs still tightly wrapped around the carbon fibers. The morphology of the nanowires remains unchanged and orderly and their 3D continuous network keeps well. There are abundant spaces between neighbouring nanowires, which facilitate the enhanced electrochemical features of electrodes. On the other aspect, the cycling stability of the as-prepared anodes at a higher current rate for ultra-long cycles was tested, and a current of 8 A g⁻¹ (2 C) was applied to the cell. Fig. 5c shows the curves of the discharge–charge capacity and coulombic efficiency of the half-cells over 1000 cycles. It is noticed that the capacity of the cable-type anodes only degrades less than 8% per 100 cycles for such a long cycling process, indicating their prefect stability and greatly desired coulombic efficiency (approximately 98%). Even after 1000 cycles at such a high rate of 8 A g⁻¹, the specific capacity still remains a reasonable value of 466 mA h g⁻¹, which is still higher than that of currently used graphite anode (372 mA h g⁻¹).

Fig. 5 (a and b) SEM images of the Si@SiO₂ NWs/carbon textiles cable-type electrode after 50 cycles at the current density of 800 mA g⁻¹; (c) reversible Li discharge–charge capacity and coulombic efficiency of the cable-type anodes versus cycle number. The discharge capacity degrades less than 8% per 100 cycles during the 1000 total cycles, in the cutoff voltage window of 0.01–1.5 V vs. Li⁺/Li.

To reflect the feasibility of the as-prepared Si@SiO₂ NWs/carbon textiles cable-type anodes, full cells were further fabricated with the Si-based cable-type anodes and the commercial LiCoO₂ as the cathode. The anode-limited full batteries were tested at 2 A g⁻¹ (0.5 C) with the cutoff voltage window of 2.5–4.0 V. Fig. 6a and b show the voltage profiles and the corresponding cycle performance for 50 cycles of these full cells, respectively. The initial discharge and charge specific capacities are found to be 2371.9 and 1949.4 mA h g⁻¹, respectively, with the coulombic efficiency of 82.2%. The full cell maintains ∼93.4% of its initial capacity after 50 cycles with the average specific capacity of ∼1879 mA h g⁻¹. It provides a great possibility to replace the existing graphite electrodes with the as-fabricated Si@SiO₂ NWs/carbon textiles cable-type anodes. Fig. S9† illustrates the practical applications of these cable-type Si@SiO₂ NWs based full cells, which can be easily used to lighten LEDs with different colours.

Fig. 6 (a) Galvanostatic discharge and charge curves of the full cells cycled in the cutoff voltage window of 2.5–4.0 V at a current density of 800 mA g⁻¹, and (b) cycling performance and coulombic efficiency for 50 cycles.

Conclusions

In summary, we have successfully synthesized a unique cable-type structure electrode containing Si@SiO₂ NWs on carbon textiles current collectors via a conventional CVD approach. The as-prepared Si@SiO₂ NWs/carbon textiles cable-type anodes delivered improved specific capacity, good rate capability, and excellent cycling stability. Interestingly, it exhibits less than 8% capacity degradation per 100 cycles for as long as 1000 cycles at high rate of 8 A g⁻¹. The full cells based on the Si@SiO₂ NWs/carbon textiles composites anodes displayed superior performance compared to commercial graphite electrodes and desired practical applications. The Si@SiO₂ NWs/carbon textiles cable-type electrodes make it possible to be applied for next-generation advanced energy storage components.

Acknowledgements

We acknowledge the support from the National Natural Science Foundation (91123008, 61377033), the 973 Program of China (2011CB933300), the Program for New Century Excellent Talents of the University in China (grant no. NCET-11-0179), the Fundamental Research Funds for the Central Universities (HUST: 2013NY013) and Wuhan Science and Technology Bureau (20122497).

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Footnote

† Electronic supplementary information (ESI) available: Cross section SEM of Si@SiO₂ nanowires/carbon fiber, SEM of pure carbon textiles, HRTEM of ultrafine Si@SiO₂ nanowires, galvanostatic discharge curves of samples/carbon textiles and carbon textiles. See DOI: 10.1039/c4ra01363h

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