Elsevier

Chemical Engineering Journal

Volume 327, 1 November 2017, Pages 297-306
Chemical Engineering Journal

Tailored silicon hollow spheres with Micrococcus for Li ion battery electrodes

https://doi.org/10.1016/j.cej.2017.06.103Get rights and content

Highlights

  • Porous Si hollow spheres were synthesized using Micrococcus bacteria templates.

  • Excellent Li diffusion coefficient can be provided by carbon coating of Si spheres.

  • Carbon coated Si spheres exhibited superior cycle stability for Li ion batteries.

Abstract

The porous Si hollow spheres (p-Si HSs), which feature interconnected Si nanostructures decorated with spherical-type Micrococcus bacteria, were synthesized by a combination of magnesiothermic reduction and byproduct removal and were subsequently studied as an anode material for lithium-ion batteries (LIBs). The p-Si HSs offer a high lithium-ion storage capacity because of their numerous active sites and large electrolyte contact area stemming from their large specific surface area (∼313.7 m2 g−1); in addition, their large pore volume (∼0.927 cm3 g−1) buffers large volume changes during the lithiation/delithiation processes, which is important for improving the cycle stability of anode materials. Furthermore, carbon coating resulted in the formation of a stable solid electrolyte interface through minimization of the Si/electrolyte contact area and also offered an efficient electronic conduction pathway, corresponding with improved lithium reactivity of the active Si materials. The lithium-ion diffusion coefficient of the non-clogging carbon-coated p-Si HSs was approximately five times greater than that of the p-Si HSs. As a result, the designed composite nanostructured electrodes demonstrated excellent cycle stability and superior rate capability.

Introduction

Template-assisted materials engineering has multipurpose applications in the design and production of advanced materials with modulated nanostructures and hybridized functions [1]. In general, the template-assisted approach can be simply classified into hard templates and soft templates [2]. Hard templates, such as anodized alumina membranes and MCM-41 silica, include preformed porous or channel structures, which have been useful in fabricating nanorods and nanotubes [3]. Soft templates, which range from biological matter to polymeric materials, assist in the assembly of the reacting species [4]. These templates are beneficial for the construction of nanometer-scale architectures with controlled morphology [5], [6], [7]. Biological templates, which include bacteria, DNA, viruses, and proteins, have been used as soft templates to fabricate 3D hierarchical and hollow nano/microstructures [8], [9], [10], [11], [12], [13]. Compared with conventional methods of materials fabrication, bacteria-template-assisted syntheses feature easy handling, eco-friendliness, and cost effectiveness [14].

Bacteria templates have several characteristics that render them suitable for nanomaterial fabrication. First, bacteria such as bacillus, coccus, spirillum, fusiform bacilli, and Escherichia coli exhibit well-ordered and various morphologies [15], [16]. Therefore, bacteria-driven strategies are being widely explored for the synthesis of nano/microhierarchical structures. Second, bacteria templates serve as nanoparticle stabilizers by providing effective isolations of interparticles, thereby exploiting the advantages of the structural and dimensional uniformity of bacteria. In addition, the templates are easily retrieved from the reaction system for subsequent processes [17]. For example, the synthesis of metal-oxide hollow nanostructures via bacteria-template-controlled assembly has been reported [18], [19].

Micrococcus is one of the widely studied Gram-positive bacteria with a multilayered peptidoglycan sacculus that includes proteins, teichoic acids, and polysaccharides, which are surrounded by an outer shell of proteins packed in a paracrystalline layer in some species. Teichoic acids feature carboxyl groups, which are negatively charged and chemically reactive moieties on the biomolecules and can be exploited to attract and react with a mineral source. As a result, such bacteria can function as templates for natural mineralization by providing a platform to assemble and align biomolecules in a specific pattern. Third, mass production of bacteria can easily be achieved by growing them in infected hosts in large quantities; this approach can be extended to the development of unique high-order inorganic architectures for various engineering applications.

The development of electrode materials for lithium-ion batteries (LIBs) has been mainly focused on fast lithium-ion diffusion and structural flexibility/stability for energy storage, together with low costs and environmental benignity [20], [21], [22], [23], [24]. The major problems concerning electrode materials are the pulverization and loss of electronic conductivity caused by enormous volumetric changes during lithiation/delithiation. Si has a theoretical capacity of approximately 4,198 mA h g−1 (volumetric capacity of approximately 9781 mA h cm−3) based on the fully alloyed form of Li22Si4; this capacity ranks highest among all LIBs anode materials reported to date. However, Si anode undergoes huge volume expansion (>420%) during lithiation, and the slow ion/electron transport kinetics lead to dramatic capacity fading after discharge-charge cycling [25]. The volumetric expansion of crystalline Si is critically dependent on the reaction, including ion diffusion and expansion, at the two-phase boundary between the crystalline Si and the amorphous LixSi [26]. The highly anisotropic lithiation strain ({1 1 1} plane > {1 1 0} plane) in c-Si is correlated with the orientation-dependent lithiation rate at the reaction front due to the fastest Li diffusivity of the {1 1 0} plane in crystalline Si. The result is that the surface cracking occurs, and the primary Si structure is fractured. Also, the huge volume expansion causes a short circuit from the current collector [25].

To solve these problems, researchers have focused on designing new types of electrode materials, for which bacteria-template-assisted synthesis is very effective. For example, bacteria-template-assisted syntheses of transition-metal-oxide-based electrode materials and Sn-based anode materials have been explored; this work resulted in composites with a porous, hollow hierarchical structure and carbon derived from carbonization of bacterial templates through thermal decomposition [27], [28]. Such strategies have resulted in unique electrode materials that provide substantially greater specific capacity compared to that of commercial graphite.

In this work, high-capacity Si was hybridized with spherical Micrococcus, which resulted in porous Si hollow spheres (p-Si HSs) with a high surface area. We systematically fabricated p-Si HSs in two steps: precipitation within a Micrococcus bacteria solution and magnesiothermic reduction. Furthermore, we applied a carbon coating to the p-Si HSs, which opened efficient electronic pathways for active materials to the current collector. When used as anodes for LIBs, these bacteria-template-assisted Si anodes exhibited excellent rate capability and enhanced cycle stability.

Section snippets

Bacteria culture and template preparation

The Micrococcus lylae bacterial cells were grown from a single colony in a liquid medium of the Luria–Bertani broth (LB broth, Sigma-Aldrich) at 37 °C and 200 rpm in a shaking incubator. When the optical density (OD 600 nm) of the culture reached 1.8–2.0, the culture was transferred into Erlenmeyer flasks for subculturing. After sub-culturing for 12 h, the bacterial cells were centrifuged at 8000 rpm for 8 min and resuspended in distilled water; the optical density of the resulting solution (OD 600 

Synthesis procedures via Micrococcus bacteria

Fig. 1 schematically illustrates the procedure used to synthesize uniform p-Si HSs. First, the Micrococcus lylae bacterial templates were grown from single colony using the sustainable cultivation technique (Fig. 1a). Second, the silica@coccus SPs (Fig. 1b) were synthesized using a silica deposition process. Third, the silica HSs were assembled from numerous primary silica nanocrystals by preheating treatment as purification process (Fig. 1c). Fourth, the uniform p-Si HSs were obtained through

Conclusions

We developed an assembly route for p-Si HSs using Micrococcus bacteria as a synthesis template. The magnesiothermic reduction and etching processes provide sufficient void space and a large specific surface area for large volume changes and an expanded electrolyte contact area during battery operation. In addition, we designed carbon-coated p-Si HSs to address the issues of low electronic conductivity and stable SEI layer formation in Si anodes. As a result, the C-p-Si HSs exhibited high

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT, and Future Planning (Nos. 2016R1A2B2012728 and 2016M3A7B4909318).

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    Y. Yi and G.-H. Lee contributed equally to this work and should be regarded as co-first authors.

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