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Novel synthesis of SiOx/C composite as high-capacity lithium-ion battery anode from silica-carbon binary xerogel

https://doi.org/10.1016/j.cjche.2019.11.003Get rights and content

Abstract

Micro/nanostructured SiOx/C composite was firstly synthesized by carbothermal reduction of silica-carbon binary xerogel. The homogeneous dispersion feature of the two components in binary xerogel contributes to effectively carbothermally reduce the O/Si atomic ratio, enhancing the electrochemical activity of the SiOx component. The micron-sized SiOx/C spheres are composed of many near-spherical nanoparticles. The synthesized SiOx/C exhibits a stable and high reversible capacity of 830 mA·h·g−1 for 100 cycles, and excellent rate-capability. The homogeneous dispersion structure of phases, the micro/nanostructure and the high electrochemical activity of SiOx component combinedly contribute the excellent electrochemical performance.

Introduction

With the increasing market for high-energy-density lithium-ion batteries, there is a significant increase in developing high-capacity electrode materials. Nonstoichiometric silicon oxide (SiOx), possessing high theoretical specific capacity (2100 mA·h·g−1 for SiO) has attracted great attentions as next-generation promising anode material. Compared to Si, SiOx exhibits relatively better cyclability due to its special electrochemical mechanism [1,2]. During the first lithiation, inert components (Li2O and Li4SiO4), generated with the active nano-Si formation, can buffer a certain degree of volume expansion to improve the cycling stability. Nevertheless, non-negligible volume changes (~ 200% of the initial volume) [3] and the intrinsically low electrical conductivity still hinders the practical application of SiOx anodes.

Great efforts were made to advance the SiOx-anode techniques. One strategy is utilizing commercial SiO to directly prepare SiO/C composites [[4], [5], [6]]. For example, mix SiO with graphite through physical mechanical method and pyrolyze organic carbon sources to prepare coated SiO/C composites. However, the rigid preparation technology leads to high cost of commercial SiO [7]. In view of this, many researchers tend to focus on the synthesis of SiOx/C through carbothermal reduction of SiO2. In the carbothermal reduction process, the high chemical reactivity of SiO2 is the key factor. Conventional crystalline SiO2 is difficult to be reduced due to its high chemical stability from strong Sisingle bondO bond [8]. It is reasonable to know that reducing the particle size to nanoscale can enhance the chemical reactivity. Thus carbothermal reduction of nano-sized SiO2 to prepare nonstoichiometric SiOx seems to be another effective strategy to promote the practical application of SiOx anodes.

Wet chemistry routes [[9], [10], [11]] including Stöber method, sol–gel and colloidal solution are frequently applied to synthesize SiOx-based composites, which exhibit satisfying cycling capacity and stability. In our previous works, different structured SiOx/C composites were synthesized using the above wet chemistry routes. Nano-sized SiOx/C was synthesized via a modified Stöber route [9]. The generated inert Li2O and Li4SiO4, as well as the carbon shell, effectively improved the cyclability. Micro-sized dual-phase glass-like SiOx-C was synthesized through a simple sol–gel route [10] to elevate the energy density. The SiOx and carbon components were uniformly dispersed at nanoscale, developing a specific dual-phase structure. Thus the composite showed a high and stable cycling capacity. To promote high yield synthesis of SiOx/C, a modified colloidal route was adopted [11]. The specific core–shell (active SiOx-carbon layer) structure ensured excellent electrochemical properties.

It could be summed up that preparation of the mixed precursor, including nano-sized SiO2 and homogenously dispersed carbon, is a key factor for carbothermal reduction. Inspired by the above-mentioned composite designs and aimed at preparing high-dispersed and uniform SiO2-C precursor, we conceived a novel preparation route of SiOx/C composite based on the SiO2-C binary xerogel. As the key point in the preparation route, SiO2-C binary gel was firstly prepared, of which the two components are homogenously dispersed at atomic scale, promoting the carbothermal reduction. After carbothermal reduction, the O/Si ratio decreases to 1.2, leading to active SiOx. Material characterizations and electrochemical properties were systematically discussed.

Section snippets

Experimental Methods

In a typical experiment, tetraethoxysilane (20 ml) and ethanol (30 ml) were added into distilled water (8 ml) under stirring. Acetic acid was introduced to adjust pH to 6 to form solution A. Meanwhile, resorcinol (6.6 g), formaldehyde (10 ml) and Na2CO3 (0.2 g) were mixed in distilled water (10 ml) to form solution B. Then the two solutions were mixed together and placed in a water bath (60 °C), forming a homogeneous gel. After gelation, the gel was aged in ethanol for 24 h under ambient

Results and Discussion

Material characterizations of SiOx/C were systematically conducted. Fig. 1a shows the XRD pattern of SiOx/C. Two distinct broad diffraction peaks are detected ranging in 20–27° and around 43°, indicating that SiOx/C contains nonstoichiometric SiOx and graphitic carbon, respectively. Fig. 1b presents the FTIR spectrum of SiOx/C. Three characteristic peaks located at 470, 802 and 1101 cm−1 correspond to different stretching types of Sisingle bondOsingle bondSi bond, representing the SiOx phase in the composite. The

Conclusions

A simple sol–gel and heat-treatment process was proposed to synthesize micro/nanostructured SiOx/C composite based on carbothermal reduction of silica-carbon binary xerogel. The silica-carbon binary xerogel precursor has homogeneously dispersed components, promoting carbothermal reduction and thus gaining low O/Si ratio of SiOx. The SiOx/C composite has a micro/nanostructure, the micron-sized spheres of which are composed of many near-spherical nanoparticles. Thanks to the combined effects of

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51602313 and 51764008), Science and Technology Project of Guizhou Province (Qiankehe No. 2016, 7439).

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    If its mechanical strength is insufficient, Si with a nano-engineered porous structure may collapse during electrode calendering; as a result, the built-in porosity in Si particles will be lost, which defeats the original purpose of the design [29]. Although the use of micron-sized SiOx has achieved limited success in industrial applications, it still carries pressing issues such as low initial coulombic efficiency (CE), so prelithiation is required to maximize its potential in high-energy-density LIBs [30–33]. It has been demonstrated that a micron-sized, carbon/porous-Si composite is more suitable for practical applications and scale-up than nano-silicon [34–38].

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