C nanocomposite anode by ball milling for highly reversible sodium storage

    A Si/C nanocomposite anode by ball milling for highly reversible sodium storage Qianjin Zhao, Yunhui Huang, Xianluo Hu PII: DOI: Reference:

S1388-2481(16)30140-0 doi: 10.1016/j.elecom.2016.06.012 ELECOM 5724

To appear in:

Electrochemistry Communications

Received date: Revised date: Accepted date:

21 April 2016 26 May 2016 22 June 2016

Please cite this article as: Qianjin Zhao, Yunhui Huang, Xianluo Hu, A Si/C nanocomposite anode by ball milling for highly reversible sodium storage, Electrochemistry Communications (2016), doi: 10.1016/j.elecom.2016.06.012

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A Si/C nanocomposite anode by ball milling for highly reversible

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sodium storage Qianjin Zhao, Yunhui Huang, Xianluo Hu*

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State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China Corresponding author. Tel: + 86 27 87558245 E-mail address: [email protected] (X.L. Hu)

ABSTRACT

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Si/C nanocomposites have been successfully prepared by grinding Si powders in the presence of graphite. When used as anodes for sodium-ion batteries, the as-prepared Si/C nanocomposites show

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significantly enhanced sodium-storage properties. In particular, the nanostructuring and close contact

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outstanding cyclability.

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of the Si nanodomains and amorphous carbon enable the Si/C nanocomposites with high capacity and

Key words: Si/C; anode; sodium-ion batteries; nanocomposite 1. Introduction

The ability to chemically store energy has enabled a wide variety of energy applications, from portable electronics to grid storage [1–4]. High-energy lithium-ion batteries (LIBs) are anticipated to drive the advances, but the high cost and low stability will limit wide applications of LIBs in this field. One hope is that sodium-ion batteries (SIBs) can replace Li resources, because of the abundance and low cost of Na in the earth [5–11]. However, the crystalline graphite that is suitable for Li-insertion chemistry exhibits poor Na-storage capabilities [12,13]. Therefore, considerable efforts are made to seek out new chemical configurations to achieve high-performance Na-storage materials. Silicon is a unique anode material for rechargeable energy-storage batteries, due to its high theoretical capacity (nearly ten times that of the state-of-the-art carbonaceous materials) [14–20]. However, Si-based anodes for SIB applications are seldom reported. The storage of Na+ ions in Si 1

ACCEPTED MANUSCRIPT leads to a NaSi stoichiometry [21,22]. Theory calculations by Han et al. suggest that amorphous Si can accommodate 0.76 Na atoms per Si atom, corresponding to a specific capacity of 725 mA h g–1 [23]. It

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is highly desirable to develop cost-effective and scalable methods for the preparation of high-performance anode materials in room-temperature SIBs. Here, we report a facile route to prepare

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Si/C nanocomposites through direct grinding the mixture of commercial micrometer-sized Si and

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graphite powders. We show that the nanostructuring of Si and graphite granules plays a crucial role in electrochemical Na-storage performances. Benefiting from nanostructuring and surface passivation by

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covalent Si-C bonds on amorphous Si/C nanohybrids, enhanced capability and cyclability are achieved.

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2. Experimental 2.1 Materials synthesis

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Commercial Si (Aladdin, micrometer-sized particles) and graphite powders (Aladdin, 750-850

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mesh) were mixed in a weight ratio of 6 : 3, and 0.9 g of the mixture was ball milled in a stainless steel

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jar. The ball milling was carried out at a speed of 1200 rpm for different time (0, 0.5, 1, 5, and 10 h). 2.2 Characterization

The morphology and microstructure were characterized by scanning electron microscopy (SEM, FEI, Sirion 200), and transmission electron microscopy (TEM, JEOL 2100F). X-ray diffraction (XRD) studies were carried out using a X'Pert PRO (PANalytical B.V., 5 Holland) diffractometer with high-intensity Cu K irradiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG MultiLab 2000 system with a monochromatic Al Kα X-ray source (Thermo VG Scientific). 2.3 Electrochemical measurement The Si/C active materials were mixed with a binder of poly (vinylidene fluoride) (PVDF, dissolved in N-methyl-2-pyrrolidone) to achieve a slurry. The weight ratio of Si : graphite : PVDF is 6:3:1. The mass loading of the active material is about 0.5–0.8 mg cm–2. The 2032-type coin cells were 2

ACCEPTED MANUSCRIPT assembled in an Ar-filled glove box. The sodium metal piece and glass fiber membrane were used as the counter electrode and the separator. The electrolyte composed of 1 M NaClO4 in ethylene

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carbonate–propylene carbonate (V/V, 1:1). The galvanostatic electrochemical tests were conducted between 0.01 and 3.0 V on a Land Testing System at room temperature. CV curves were recorded at

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0.1 mV s−1 using an electrochemical workstation (CHI, 660E) at room temperature.

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3. Results and discussion

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In a typical procedure, the mixture of Si and graphite in a weight ratio of 6:3 was treated directly by ball milling for 1 h. The commercial Si and graphite are in the micrometer scale. Fig. 1 displays the

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XRD patterns of the Si/C products obtained by grinding the mixtures of Si and graphite at various periods of time. Clearly, both the pristine Si and graphite particles possess a high crystallinity before

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milling. The diffraction peaks belonging to graphite disappear after milling for 0.5 h. Meanwhile, the

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intensity of XRD peaks for Si decreased gradually with the milling time prolonged. After milling the mixture for 10 h, the XRD peaks for both Si and graphite are hardly detectable, suggesting the

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complete amorphization of the Si/C hybrid. SEM observations (Fig. 2) show that the milled products are composed of nanoparticles, although some of them are in sub-micrometer aggregates. The original features of commercial micro-sized Si and graphite granules disappear after ball milling. Fig. 3a displays the TEM image for the pristine composite of Si and graphite particles. The corresponding high-resolution TEM (HRTEM) images for the individual Si and graphite particles are shown in Fig. 3b and 3c, respectively. The periodic fringe spacing of 0.31 nm for (111) planes of Si and 0.34 nm for (002) planes of graphite is clearly observed. The electron diffraction (ED) pattern in the inset of Fig. 3a for a single Si particle indicates the highly crystalline nature of the starting Si granules. As revealed in Fig. 3d, the micro-structure of the pristine powders has evolved into randomly distributed nanosized domains after ball milling for 1 h. The corresponding ED pattern (inset of Fig. 3d) suggests that a large part of the Si/C hybrid has transformed to be amorphous. Nevertheless, there still exist a small amount of Si nanocrystals 3

ACCEPTED MANUSCRIPT with weakened crystallinity in the final product. Meanwhile, the dislocation and deformation of crystal lattice in those Si nanocrystals are evidently identified, which may be helpful to the

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electrochemical interfacial storage of Na + ions. If the milling time is prolonged to 10 h, the crystal nature of the pristine Si and graphite granules is thoroughly destroyed, and the final

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Si/C nanocomposite becomes amorphous (Fig. 3e). The time-dependent structural evolution

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processes agree well with the XRD results (Fig. 1).

The sodium insertion/extraction performances of the Si/C composites were studied by

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galvanostatic charge/discharge measurements at room temperature. The first specific discharge capacities of the Si/C hybrid obtained after milling for 1 h are approximately 650 mA h g –1 at

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100 mA g–1 (Fig. 4a), and 740 mA h g–1 at 50 mA g–1, respectively. The corresponding Coulombic efficiency is about 54.4%, where the initial capacity loss may result from the

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formation of solid electrolyte interphase (SEI) layer and the incomplete Na-extraction reaction

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[22]. This is also reflected in the cyclic voltammetry (CV) profiles (Fig. 4b). From the second cycle onwards, the specific discharge capacity does not decay evidently and retains 280 mA h

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g–1 over 100 cycles at 100 mA g–1 with a Coulombic efficiency of nearly 100% (Fig. 4c). At a same rate of 100 mA g –1, the present reversible Na-storage capacity is much higher than that of the previous report (< 200 mA h g –1) on the Si/C hybrids [22]. Such a Si/C nanocomposite by grinding exhibits significantly improved electrochemical performances, compared to those of the physically mixed Si/graphite granules without grinding, and even the milled graphite (Fig. 4c). Importantly, the as-obtained Si/C nanohybrid from 1-h milling still exhibits a remarkable cyclability at a much higher current density of 1000 mA g –1, and the capacity reaches 170 mA h g–1 after 500 cycles (Fig. 4d). The mechanical treatment by grinding could ignite the sodiation activity of the Si/C hybrids, which could be assigned to the formation of numerous nanodomains of Si and amorphization of graphite. The high-energy grinding process could prompt the destruction of the crystalline graphite, leading to amorphous carbon substances. Meanwhile, the numerous Si nanodomains are well encapsulated or embedded in the carbonaceous nanostructures. The close contact 4

ACCEPTED MANUSCRIPT between Si and C may help to improve the electric conductivity of the whole electrode . As the grinding time increased from 10 min to 1 h, the Na-storage ability of the Si/C hybrids is

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gradually improved. During the grinding process, mechanochemical reactions between Si and C may happen, as suggested by XPS analyses (Fig. 5). The peaks at around 97.5 and 101.6 eV

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(Fig. 5a) are assigned to Si 0 and Sin+, where the binding energies in between correspond to

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SiOx (0 < x < 2) [24–26]. The signals for the O elements originate from the oxidation of Si nanoparticles, which are also detected by the energy dispersive X-ray results [2.76 wt% (0 h),

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5.21 wt% (1 h), and 3.81 wt% (10 h)]. The shift of Si0 binding energy is likely resulted from different oxidation states. After ball milling for 1 h, the peaks corresponding to Si and SiO x

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shift to 99.89 and 103.69 eV (Fig. 5c), suggesting that the grinding could alter the chemical bonding states of Si [27–29]. The chemical bonds of Si-C appear (Fig. 5c), possibly arising

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from the formation of Si-C bonds between the Si nanomains and amorphous carbon as well as

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silicon carbide. Fig. 5e demonstrates a remaining peak at ~101.5 eV, corresponding to SiC, if the grinding time is prolonged to 10 h. It is believed that the strongly-coupled interaction

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between Si nanodomains and amorphous carbon plays a crucial role in the Na-cycling stability and capacity. If the grinding time is further prolonged to exceed 1 h, it is found that the capacity fades rapidly. This is because a large amount of Si has transformed into amorphous SiC that is difficult to deliver the Na-storage capacity. 4. Conclusion

In summary, we have successfully demonstrated the scalable preparation of Si/C nanocomposites by a ball-milling method. When evaluated as anodes for SIBs, the resulting Si/C electrodes exhibit high reversible capacities and remarkable cyclability. The superior electrochemical performances of the Si/C nanocomposites benefit from the nanostructuring effects of the active materials.

Acknowledgements

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ACCEPTED MANUSCRIPT This work is supported by the National High-tech R&D Program of China (863 Program, No. 2015AA034601), and National Natural Science Foundation of China (No. 51522205,

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51472098 and 21271078).

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Fig. 1 XRD patterns of the Si/C products obtained by grinding the mixtures of Si and graphite at various periods of time (0, 0.5, 1, 5, and 10 h).

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Fig. 2 SEM images for the Si/C products obtained by grinding the mixtures of Si and graphite at various periods of time. (a) 0 h, (b) 1 h, and (c) 10 h.

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Fig. 3 TEM images of the Si/C products obtained by grinding the mixtures of Si and graphite at various periods of time. (a–c) 0 h, (d) 1 h, and (e) 5 h. Insets of (a, d, and e): corresponding to ED patterns. Fig. 4 Electrochemical properties of the Si/C and C products obtained by grinding for different time. (a)

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Charge/discharge profiles of the Si/C sample (1 h) for the 1st, 20th, and 50th cycles at 100 mA g1 over 0.013.0 V. (b) Cycling performance for the Si/C samples (0.5 and 10 h) at 100 mA g1. (c) Comparison of cycling performance for the Si/C and C samples obtained by grinding for 1 h at 100 mA g1. (d) Cycling performance of the Si/C electrode at a higher current density of 1000 mA g1.

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Fig. 5 High-resolution XPS spectra of Si 2p for the Si/C composites obtained by ball milling for different time: 0, 1, and 10 h.

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Graphical abstract

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Highlights: Si/C nanocomposites obtained by ball milling are for Na-ion storage.

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Nanostructuring of Si/C facilitates the Na-ion diffusion and improves the capacity.

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Surface passivation by covalent Si-C bonds enables the enhanced cyclability.

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