目录结构

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参考文献

1 Introduction▲
2 Experimental▲
2.1 Preparation of Si nanoparticles
2.2 Materials characterization and electrochemical test
3 Results and discussion▲
3.1 Characterizations of Si nanoparticles
3.2 Electrochemical performance
4 Conclusion▲
图表▲

Rare Metals2019年第3期

A scalable synthesis of silicon nanoparticles as high-performance anode material for lithium-ion batteries

Jin Li Juan-Yu Yang Jian-Tao Wang Shi-Gang Lu

China Automotive Battery Research Institute Co., Ltd.,General Research Institute for Nonferrous Metals

作者简介：*Shi-Gang Lu,e-mail:190586718@qq.com;

收稿日期：21 July 2016

基金：financially supported by the National Natural Science Foundation of China (No. 51404030);the National Key Technologies Research and Development Program (No. 2016YFB0100400);the Natural Science Foundation of Beijing Municipality (No. 3154043);the Beijing Science and Technology Plan (No. Z151100000115015);the Beijing Nova Program (No. Z161100004916096);

A scalable synthesis of silicon nanoparticles as high-performance anode material for lithium-ion batteries

Jin Li Juan-Yu Yang Jian-Tao Wang Shi-Gang Lu

China Automotive Battery Research Institute Co., Ltd.,General Research Institute for Nonferrous Metals

Abstract：

In this work, a scalable and cost-effective method including mechanical milling, centrifugation and spray drying was developed to fabricate Si nanoparticles.The synthesized Si nanoparticles show an average size of 62 nm and exhibit a narrow particle size distribution. The influence of particle sizes on electrochemical performance of Si-based electrode was investigated, and it is found that as the particle size decreases in the studied range, the Si particles show a lower specific capacity and a higher irreversible capacity loss(ICL). Furthermore, an oxide layer with thickness of ～3 nm was detected on the surface of the as-received Si nanoparticles, and this layer can be effectively removed by hydrofluoric acid(HF) etching,resulting in much improved electrochemical performance over the as-received samples.

Keyword：

Lithium-ion batteries; Anode; Silicon nanoparticles; Wet grinding mill;

Received： 21 July 2016

1 Introduction

Rechargeable lithium-ion batteries (LIBs) are key electrical energy storage devices for next-generation portable electronics,electric vehicles and utility grid applications because of their high energy density [ 1, 2] .Graphite as anode material for the majority of the marketed LIBs is limited by its theoretical specific capacity (372 mAh.g^-1).In the past decade,great attention has been paid to introduce Si into the anode materials of LIBs because of its high theoretical specific capacity (4200 mAh·g^-1 for Li₂₂Si₅).It is worth noting that the Li₂₂Si₅ alloy is only obtained at high temperature of 415℃,while Li₁₅Si₄ is the highest lithiated phase achievable at room temperature for the lithiation of silicon,corresponding to a capacity of3579 mAh·g^-1 [ 3, 4] .However,the large volume expansion (about 300%) that accompanies Li insertion and extraction during cycling generates enormous mechanical stress,with resulting electrode pulverization and loss of electrical contact.In addition,the low electrical conductivity and ionic diffusivity of pristine Si have also restrained the achievement of a high capacity at high rates.

To mitigate the adverse mechanical effects accompanying during lithiation and delithiation,various Si nanostructures including nanoparticles [ 5, 6] ,nano wires [ 7, 8] ,nanotubes [ 9, 10] and porous structures [ 11, 12] have been intensively studied.They have shown improved performance,presumably due to the small sizes that enable fast Li transportation and facile stress relaxation.Liu et al. [ 13] reported that the Si nanoparticles showed a strong size dependence of fracture and there existed a critical particle diameter of～150 nm,below which the particles neither cracked nor fractured upon the first lithiation.Another strategy is constructing Si-based composite structures in which the active Si is dispersed in or coated by other less active/inactive materials [ 14, 15, 16, 17] to alleviate the mechanical stress induced by the large volume change in active materials and to prevent aggregation of the active domains.

Several methods including chemical vapor deposition [ 18, 19] ,laser ablation [ 20] ,rapid metathesis [ 21] ,molten salt electrolysis [ 22] ,magnesiothermic reduction [ 11, 23] and solution synthesis routes [ 5, 24] have been reported to synthesize the Si nanoparticles.However,most of these methods rely on processes which are neither economically viable for commercial utilization nor easily scalable for volume production.Furthermore,the synthesized Si nanoparticles show strong aggregation or involve the undesired surface impurities,which play a significant role in the electrochemical performance of the Si nanoparticles.In the case of rapid metathesis reaction method,though the synthesis is rapid and scalable,the yield of silicon nanoparticles is low (～6 wt%),resulting in the consumption of large quantities of the reactants [ 21] .Epur et al. [ 25] successfully synthesized silicon nanoparticles by a mechano-chemical reduction of Mg2Si and SiO using high-energy mechanical milling (HEMM) technique followed by acid leaching.However,the particle size distribution is rather wide due to the agglomerates of few micrometers in diameter,and thus,the synthesized silicon particles may not be suitable for practical application in LIBs.

This paper reported a scalable and cost-effective method based on wet grinding mill to fabricate the Si nanoparticles as high-performance anode materials for LIBs.The synthesized Si nanoparticles show a narrow particle size distribution (PSD) and high initial reversible capacity.In addition,the influences of particle sizes and surface oxide layer on electrochemical performance of Si were discussed in details.

2 Experimental

2.1 Preparation of Si nanoparticles

The bulk Si powder in micrometer sizes (Beijing Huawei Ruike Chemical Co.Ltd.,China) was firstly milled in ethanol by a wet grinding mill (SM-230,Wuxi Xinguang Powder Technology Co.Ltd.,China) with using yttriumstabilized zirconia (ZrO₂) milling beads of 0.3 mm.The rotational speed and the solid content of Si suspension were1800 r·min^-1 and 10 wt%,respectively.Then,the as-obtained Si suspension was centrifuged at 11,000 r·min^-1 for5 min to remove some large particles in sub-micrometer sizes.After spray drying and hydrofluoric acid (HF) etching,the high-performance Si nanoparticles were obtained.

2.2 Materials characterization and electrochemical test

The particle size distribution (PSD) of the synthesized Si nanoparticles was measured by means of laser diffraction on a Malvern Mastersizer 3000 device.Brunauer-EmmettTeller (BET) measurements to determine the specific surface area of the Si nanoparticles were carried out on a Quadras orb SI automated surface area and pore size analyzer.The phase components of the materials were in vestigated by X-ray diffractometer (XRD,PANalytical/X’Pert PRO) using Cu Kαradiation.The morphology of the particles was examined by scanning electron microscope(SEM,Hitachi S-4800) operated at 10 kV.The micros tructures of Si nanoparticles were observed by a transmission electron microscope (TEM,Tecnai,G2 F20),and attached energy-dispersive X-ray spectroscope (EDX)was used for elemental analysis.

The electrochemical experiments were performed using coin cells with pure Li as the counter electrode,a Celgard2400 film as the separator and a solution of 1 mol·L^-1LiPF₆ in EC/EMC/DMC (1:1:1 by volume) as electrolyte(Zhangjiagang Guotai Huarong Chemical New Material Co.Ltd.).The working electrodes were prepared by pasting a homogeneous aqueous slurry of 30.0 wt%Si,12.5 wt%graphite (KS6) 37.5 wt%Super-P and 20.0 wt%polymer binder onto copper foil (10μm).An aqueous binder containing carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) at weight ratio of 1:3 was selected as the binder of the electrode.After drying at 80℃C in vacuum oven for 4 h,the electrodes were pressed and punched in the form of 14-mm-diameter disks.The average mass loading of the electrodes was approximately 3 mg·cm^-2.The cells were fabricated in an Ar-filled glove box and galvanostatically cycled between 0.005 and 2.500 V (vs.Li/Li⁺) at room temperature on a multi-channel cell tester(Land CT2001A system).Cyclic voltammetry (CV) was carried out using a CHI660A electrochemical workstation at a scan rate of 0.1 mV.s^-1 with a voltage range of0.005-2.500 V (vs.Li/Li⁺) at room temperature.

3 Results and discussion

3.1 Characterizations of Si nanoparticles

Figure 1 presents the variation of particle sizes as the milling time prolongs.A fast decrease in particle size from several micrometers to 174 nm is observed in the first 6 h.Then,the decrease in particle size becomes slow and no obvious variation of particle sizes can be detected after grinding mill for 11 h,indicating that the grinding limit in particle size is reached [ 26, 27] .It is known that a Si suspension with average particle size (d₅₀) of 126 nm was fabricated after grinding mill for 11 h.

Fig.1 Variation of particle sizes as a function of milling time with using ZrO₂ milling beads of 0.3 mm

Figure 2 shows PSD curves of Si suspensions obtained before and after centrifugation.It is found that the PSD of the as-obtained Si suspension shows a bimodal distribution and it shifts to a narrower unimodal distribution showing smaller particle sizes after centrifugation.The d₅₀ value of the synthesized Si nanoparticles is 62 nm and the d₉₉ (the value of the particle diameter at 99%in the cumulative distribution) value is 147 nm which is lower than the critical fractured size of 150 nm for Si particles.The yield of the Si nanoparticles is about 50 wt%,and the removed Si particles of sub-micrometer sizes can be reused for next wet grinding mill process.

The morphologies of the pristine Si particles and the synthesized Si nanoparticles are displayed in Fig.3a,b,respectively.As observed in SEM images,the pristine Si has a particle size of several micrometers and the synthesized Si shows an irregular shape and greatly decreases particle size.The statistical PSD of the synthesized Si nanoparticles (Fig.3c) is obtained by directly measuring the sizes of 150 particles randomly chosen from Fig.3b.The Si nanoparticles have a narrow PSD from 20to 160 nm and an average size of 80 nm,which is close to the result obtained by laser diffraction test.This indicates that Si suspension has a fine dispersion without agglomerates.

Fig.2 PSD curves of Si suspension before and after centrifuged at11,000 r·min^-1 for 5 min

Figure 4 shows XRD patterns of the pristine Si particles and the synthesized Si nanoparticles.The Si phase is indexed with a single fcc crystal structure (JCPDS Card File 27-1402).Compared with the characteristic peaks of the pristine Si particles,obvious broadening and decrease in peak intensity are observed in XRD pattern of Si nanoparticles,which can be attributed to the decreasing amount of crystal areas and increasing lattice deformations due to enormous stresses acting on the particles [ 28] .

Figure 5a,b presents high-resolution transmission electron microscopy (HRTEM) images of as-received Si nanoparticles and the sample etched by a 5 wt%HF solution for 30 min,respectively.An obvious amorphous layer with a thickness of～3 nm is observed on the surface of the as-received Si nanoparticles,and it is removed after HF etching.According to EDX results (Fig.5c),the amorphous layer is silicon oxide (SiO_x) layer formed during grinding mill and spray drying [ 29] .In addition,small amount of zirconium impurities resulted from grinding media can be detected and completely removed after HF-etching process (Fig.5d).

Fig.4 XRD patterns of pristine Si particles and synthesized Si nanoparticles

Fig.3 SEM image of a pristine Si particles and b synthesized Si nanoparticles and c PSD of Si nanoparticles obtained by directly measuring size of 150 particles randomly chosen from b

Fig.5 HRTEM images and corresponding EDX results of a,c as-received Si nanoparticles and b,d HF-etched Si nanoparticles

3.2 Electrochemical performance

Figure 6 shows the initial galvanostatic charge/discharge curves of Si-based electrodes using Si particles with different sizes (d₅₀) as active materials,and the results are summarized in Table 1.It should be noted that the Si particles with sizes of 126,162 and 628 nm are directly obtained by grinding mill for different durations and spray drying,without treated by centrifugation and HF etching.It is worth noting that a short plateau at 0.45 V is observed on the delithiation curve of the electrode fabricated by the 628-nm Si particles and it is regarded as an indication for the formation of Li₁₅Si₄ phase during the lithiation process.However,the plateau at 0.45 V is absent when the particle sizes of Si decrease to 162 nm and below.This implies that the formation of Li₁₅Si₄phase is avoided during the lithiation process,due to the self-limiting effect of Si nanostructures [ 30] .Moreover,it is found that as the particle size decreases,Si particles show lower initial reversible capacities,which can be ascribed to the existence of SiO_x layer which leads to a lower content of Si metal and enhanced self-limiting effect.Furthermore,as the particle size of Si decreases,the BET surface area (S_BET) increases,resulting in lower first columbic efficiency.

Fig.6 Initial galvanostatic charge/discharge curves of Si particles with different sizes in voltage range of 0.005-2.500 V (vs.Li/Li⁺) at a rate of 0.05 C

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Table 1 BET surface areas and electrochemical performances of Si particles with different sizes (initial delithiation capacities of Si calculated from Fig.6 after discounting contribution of graphite)

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Table 2 Electrochemical performances of HF-etched Si nanoparticles (62 nm) obtained at different lithiation/delithiation rates (initial delithiation capacities of Si calculated from Fig.7 after discounting contribution of graphite)

Fig.7 Initial galvanostatic charge/discharge curves of HF-etched Si nanoparticles (62 nm) at different charge/discharge rates in voltage range of 0.005-2.500 V (vs.Li/Li⁺)

As for the as-received Si nanoparticles with d₅₀ of62 nm,the initial delithiation capacity at 0.05 C is1485 mAh·g^-1 and the first columbic efficiency is 55.0%.After HF etching,the Si nanoparticles show a higher initial delithiation capacity of 2801 mAh.g^-1 and a higher columbic efficiency of 68.9%(Table 2).Therefore,it is concluded that the surface SiO_x layer negatively affects the electrochemical performance of the Si nanoparticles.The initial irreversible capacity can be attributed to the formation of solid electrolyte interphase (SEI) layer on the surface of the electrode and side reactions between Li⁺and surface oxide layers of Si particles [ 31] .Figure 7 shows the initial galvanostatic charge/discharge curves of HF-etched Si nanoparticles at different charge/discharge rates,and the results are summarized in Table 2.It is noted that as the charge/discharge rate decreases,the initial delithiation capacity of the Si nanoparticles increases and the Si-based electrode shows lower first columbic efficiency due to the aggravated electrolyte decomposition.

Figure 8 a displays the typical CV curves of the HF-etched Si nanoparticles for the first three cycles.In the first cathodic half-cycle (lithiation),a broad cathodic peak centered at 0.74 V is ascribed to the formation of SEI film by decomposition of the electrolyte,it disappears from the subsequent cycles.The cathodic peak from 0.3 V to cutoff potential can be attributed to the lithiation of Si.During the first delithiation process,two broad peaks are observed at0.36 and 0.52 V,which are attributed to the phase transition between amorphous Li_xSi and amorphous Si [ 32] .Upon the third lithiation process,a new cathodic peak is observed at 0.16 V as opposed to the first lithiation,and it results from the reaction of lithium with Si to form amorphous Li_xSi,which needs a possible activation process [ 33] .The CV curves of the as-received Si nanoparticles are shown in Fig.8b.Compared with the CV curve of the HF-etched Si,a new anodic peak at 0.25 V is detected,which is supposed to have a relationship with the reversible reaction between SiO_x and lithium [ 34] .An obvious activation process occurs over the first three cycles,as indicated by the increased peak intensity of current.This is because the SiO_x layer slows the transportation of Li⁺and electron,preventing the Si core from being fully lithiated.As the cycles proceed,the SiO_x gradually reacts with lithium and converts to silicate form which improves the transportation of Li⁺ [ 35, 36] .

Fig.8 Typical cyclic voltammograms of a HF-etched Si nanoparticles and b as-received Si nanoparticles (d₅₀=62 nm) in voltage range of0.005-2.500 V (vs.Li/Li⁺) at a scan rate of 0.1 mV·s^-1

4 Conclusion

In summary,a scalable and cost-effective method including mechanical milling,centrifugation and spray drying was developed to synthesize the Si nanoparticles.The synthesized Si nanoparticles show an average particle size of62 nm and a narrow particle size distribution of30-110 nm.The influence of particle sizes on electrochemical performance of Si-based electrode was investigated,and it is found that as the particle size decreases in the studied range,the Si particles show lower specific capacity and first columbic efficiency.Furthermore,the surface SiO_x layer of Si nanoparticles is proved to negatively affect the electrochemical performance of the Sibased electrode.After HF etching,the Si nanoparticles with d₅₀ at 62 nm show a greatly improved initial reversible capacity of 2801 mAh·g^-1 and a first columbic efficiency of 68.9%at a rate of 0.05 C.

This work provides an excellent nanostructured Si for fabricating Si-based anode for LIBs and an insight for physical and electrochemical properties of Si nanoparticles.However,the continuous formation of SEI layers and degradation of electrolyte cannot be prohibited during the cycles,due to the high specific surface area of Si nanoparticles.More efforts should be paid to fabricate Sibased composite structures that meet the requirements of practical application in LIBs.