Scalable Production of Si Nanoparticles Directly from Low Grade

Aug 10, 2015 - However, low cost and scalable processes to produce these Si nanostructures remained as a challenge, which limits the widespread applic...
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Scalable Production of Si Nanoparticles Directly from Low Grade Sources for Lithium-Ion Battery Anode Bin Zhu,†,∥ Yan Jin,†,∥ Yingling Tan,† Linqi Zong,† Yue Hu,† Lei Chen,‡ Yanbin Chen,§ Qiao Zhang,‡ and Jia Zhu*,† †

National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ‡ Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China § School of Physics, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: Silicon, one of the most promising candidates as lithium-ion battery anode, has attracted much attention due to its high theoretical capacity, abundant existence, and mature infrastructure. Recently, Si nanostructures-based lithium-ion battery anode, with sophisticated structure designs and process development, has made significant progress. However, low cost and scalable processes to produce these Si nanostructures remained as a challenge, which limits the widespread applications. Herein, we demonstrate that Si nanoparticles with controlled size can be massively produced directly from low grade Si sources through a scalable high energy mechanical milling process. In addition, we systematically studied Si nanoparticles produced from two major low grade Si sources, metallurgical silicon (∼99 wt % Si, $1/kg) and ferrosilicon (∼83 wt % Si, $0.6/kg). It is found that nanoparticles produced from ferrosilicon sources contain FeSi2, which can serve as a buffer layer to alleviate the mechanical fractures of volume expansion, whereas nanoparticles from metallurgical Si sources have higher capacity and better kinetic properties because of higher purity and better electronic transport properties. Ferrosilicon nanoparticles and metallurgical Si nanoparticles demonstrate over 100 stable deep cycling after carbon coating with the reversible capacities of 1360 mAh g−1 and 1205 mAh g−1, respectively. Therefore, our approach provides a new strategy for cost-effective, energy-efficient, large scale synthesis of functional Si electrode materials. KEYWORDS: lithium-ion battery, anode, Si nanoparticles, low grade, ferrosilicon, metallurgical Si

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nanoparticles is below 150 nm.20 Second, Si nanoparticles are compatible with the traditional slurry coating manufacturing processes. In addition, Si nanoparticles can serve as elements for advanced structure constructions such as yolk shell and pomegranate,9,10 which prevent the instability of solid electrolyte interface (SEI). However, the existing processes for producing Si nanoparticles are still largely limited to a few expensive, complex and energy intensive processes, such as laser ablation of bulk Si and high temperature or high energy pyrolysis of silane/polysilane/halosilane precursors. To enable widespread application, it is desirable to develop scalable, costeffective, and energy-efficient processes to produce Si nanoparticles. It is well known that there are two major low grade silicon sources, metallurgical silicon (M-Si, ∼99 wt % Si) and ferrosilicon (F-Si, ∼ 83 wt % Si, ∼ 13 wt % Fe) in industry, with annual production of million tons globally,21,22 at a very

o meet the increasing demand of electric vehicles and portable electronics, the development of high performance lithium-ion battery (LIB) is of great importance.1−4 In the case of anodes, silicon is considered to be one of the most promising materials due to its abundance, environmentally benignity, low working potential, and highest theoretical capacity of 4200 mA h g−1 (corresponding to the fully lithiated composition of Li4.4Si), which is ten times higher than that of traditional graphite anode (∼372 mAh g−1).5,6 However, the stress caused by the large volume change (∼400%) during cycling, resulting in electrode pulverization and unstable solidelectrolyte interphases (SEI), seriously affects long-time stability and lifetime of Si anode. In recent years, a variety of nanosized Si such as nanoparticles, nanowires, nanotubes and nanoporous networks have demonstrated enhanced mechanical integrity, which can effectively address this issue by accommodating the large volume change.7−19 Among them, Si nanoparticles are perceived as a promising candidate for several important reasons. First, it is found that the surface cracking and particle fracture associated with volume expansion of Si particles can be alleviated when the size of crystal Si © XXXX American Chemical Society

Received: April 30, 2015 Revised: July 21, 2015

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DOI: 10.1021/acs.nanolett.5b01698 Nano Lett. XXXX, XXX, XXX−XXX

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capacities of 1360 and 1205 mAh g−1 after 100 deep electrochemical cycles, respectively. The silicon pieces (ferrosilicon and metallurgical Si) were first smashed into ∼1 mm diameter size, and then were ball milled for 25 h under Ar atmosphere by the planetary ball mill (MSK-SFM, MTI) with a rotation speed of 500 rpm. A ball-topowder weight ratio of 20:1 was used. In addition, the micron sized powders were taken into the HEMM (PULVERISETTE 7, Fritsch) with a rotation speed of 800 rpm to obtain nanosized Si powders, the ball-to-powder weight ratio was 30:1. The nanosized Si powders and citric acid powders were mixed in deionized water with a weight ratio of 1:2.5 and followed by a 3 h ultrasonic treatment. Then the obtained nanosized Si and citric acid suspensions were dried at 110 °C in a vacuum oven and then heat-treated at 800 °C for 2 h under a flowing Ar atmosphere to carbonize the citric acid, forming the carbon coated Si powders. The Si electrodes were prepared by coating the slurries that mixed Si powders with CMC and acetylene black (3:1:1) on a copper foil using a blade coater. Then the electrodes were dried at 110 °C in a vacuum oven for 8 h. The mass loading of the Si electrode was ∼0.5 mg cm−2. All reported capacities in this study are based on the total mass of the powders (Si powders for M-Si and Si/FeSi2 powders for FSi). The 2032 coin-type half cells were assembled in an Ar-filled glovebox with the as-prepared mixture coated on copper foil as the working electrode and lithium foil (MTI) as the counter/ reference electrode. The electrolyte for all the tests was 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 vol/vol, Guotai Huarong) with 2 wt % VC addictive. The cells were galvanostatic cycled with voltage cut-offs of 0.01 and 1 V vs Li/ Li+. Cyclic voltammetry (CV) measurement was performed at a scan rate of 0.1 mV s−1 and a potential range of 0.01−1 V (vs Li/Li+). One of advantages of this process is that the size of particles can be tuned from ∼10 μm to ∼150 nm by the speed (500− 800 rmp) and time (3−10 hours) of ball milling. Figure 2a and c show the morphology of metallurgical Si (M-Si) and

low cost (∼$1000/ton for metallurgical silicon, and ∼$600/ton for ferrosilicon). In this work, we demonstrate that nanosized Si powders, with size around ∼150 nm, smaller than the critical size to alleviate the pulverization during cycling, can be massively produced from these two different low grade Si sources through one step and scalable high energy mechanical milling (HEMM) (Figure 1).23−25 We systematically studied

Figure 1. Schematics of fabrications and electrochemical cycling of low grade silicon nanoparticles. (a) Si nanoparticles from metallurgical Si (M-Si) sources. (b) Si nanoparticles from ferrosilicon (F-Si) sources.

the crystal structures and electrochemical performances of these two kinds of nanoparticles. We found that both metallurgical Si (M-Si) and ferrosilicon (F-Si) nanoparticles maintain good crystal quality after ball milling. Interestingly, because of excessive iron in ferrosilicon sources, F-Si nanoparticles contain both crystalline Si and FeSi2 after ball milling. As illustrated in Figure 1b, because FeSi2 is not reactive to Li+, it can effectively buffer the volume expansion of Si particles during cycling, which is beneficial for electrochemical cycling performance.26−28 On the other hand, the metallurgical Si (M-Si) nanoparticles electrode has higher capacity (∼3200 mAh g−1), in comparison with 2200 mAh g−1 in the case of ferrosilicon (FSi), due to the higher purity of silicon as active material in the source. Both M-Si and F-Si nanoparticles demonstrated a stable electrochemical performance after carbon modification, with

Figure 2. (a) SEM image of M-Si nanoparticles. (b) XRD characterizations of M-Si before ball milling, after ball milling and after carbon coating. (c) SEM image of F-Si nanoparticles. (d) XRD characterizations of F-Si before ball milling, after ball milling, and after carbon coating. B

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Figure 3. TEM characterizations of low grade Si nanoparticles. (a) TEM image of M-Si nanoparticles with carbon coating, (b) high resolution TEM image of a M-Si nanoparticle with uniform carbon coating, (c) TEM image of F-Si nanoparticles with coexistence of Si and FeSi2, (d) high resolution TEM image of a F-Si nanoparticle with coexistence of Si and FeSi2.

M-Si nanoparticles after carbon coating. It is noted that a uniform layer of carbon is coated on the surface of M-Si particles. In the high resolution TEM image (Figure 3b), about ∼5 nm thickness carbon layer can be identified along the surface of M-Si nanoparticles. Figure 3c shows a typical TEM image of F-Si nanoparticles with around 100 nm diameter. As seen from the high resolution TEM (Figure 3d), two kinds of lattice fringes with basal distances of 0.314 and 0.513 nm can be observed, which reflect the (111) plane of cubic silicon and the (001) plane of tetragonal FeSi2, therefore confirming the coexistence of Si and FeSi2. To evaluate the obtained nanosized M-Si and F-Si powders as electrodes for lithium-ion battery, we systematically studied their electrochemical properties, as presented in Figure 4. Figure 4a shows the capacity change with cycling of M-Si nanoparticles and F-Si nanoparticles. For comparative purposes, we also tested the micron sized Si powders after the first planetary ball milling. We can see that the capacity of the two micron-sized powders decayed rapidly during cycling. In contrast, the capacity of nanosized M-Si and F-Si remained comparatively stable and still reached 1354 and 980 mAh g−1 after 50 cycles, respectively. This result can be mainly attributed to the enhanced mechanical integrity of small size nanoparticles (∼150 nm), which can effectively avoid fracture of Si particles. It is notable that the nanosized M-Si powders possess a higher initial capacity (3262 mAh g−1) than that of F-Si powders (2198 mAh g−1) due to the higher Si purity in the metallurgical Si source. On the other hand, in both cases of micron-sized and nanosized powders, the capacity retentions of F-Si are higher than these of M-Si. It indicates that the presence of crystalline FeSi2 among Si particles is beneficial as FeSi2 can effectively buffer the expansion of Si during cycling.28 In addition, the initial Coulombic efficiencies of nanosized M-Si and F-Si are 79% and 72%, respectively. The loss of the first reversible capacities can be mainly attributed to the formation of solid

ferrosilicon (F-Si) nanoparticles, respectively. The size of nanoparticles typically decreases to ∼150 nm after HEMM (800 rpm for 3 h). The statistical analysis of size distribution is provided in the Figure S3 (Supporting Information). Through the careful characterizations of X-ray fluorescence (XRF), it is found that the purity of metallurgical Si is 98.8%, with a small amount of Fe and Al as the main impurities (Table S1, Supporting Information). In the case of ferrosilicon, silicon typically accounts for 83.4 wt % and the main impurity Fe accounts for 12.8 wt %. In addition, X-ray photoelectron spectra (XPS) was performed to probe the ferrosilicon source before HEMM where we find the existence of FeSi2 (Figure S2, Supporting Information). X-ray diffraction (XRD) is used to examine the crystal structure of M-Si (Figure 2b) and F-Si (Figure 2d) at each step of the processes. Black, red, and blue lines represent particles before ball milling (BM), after BM, and after carbon coating, respectively. It is clear that in both cases, the crystal structure does not compromise after BM and carbon coating processes. In the case of metallurgical Si, as shown in Figure 2b, the XRD pattern shows sharp peaks of Si before BM without other peaks. After BM and carbon coating process, the peaks of Si still have no shift and just become broader because of the decreased grain size. From the XRD pattern of ferrosilicon (Figure 2d), we can see that before BM typical peaks of Si show up accompanied by some peaks of FeSi2 which is consistent with the result of XPS (Figure S2, Supporting Information). After BM, the diffraction peaks of Si become much weaker and broader and the peaks of FeSi2 become more obvious, implying a decrease in its size. When the particles were further carbon coated, the diffraction peaks of Si became sharper and there were no new phases appeared in the XRD pattern. We also used transmission electron microscope (TEM) to characterize crystal structures of the M-Si and F-Si nanoparticles. Figure 3a shows the low-magnification TEM image of C

DOI: 10.1021/acs.nanolett.5b01698 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 4. Electrochemical characterizations of the M-Si and F-Si electrodes. (a) Cycling test of nanosized and micron-sized M-Si, F-Si. (b) Cyclic voltammetry tests of nanosized M-Si and F-Si after the initial cycle. (c) Charge−discharge cycling test of M-Si nanoparticles after carbon coating. (d) Capacity retention of carbon coated M-Si nanoparticles cycled at different current densities ranging from 0.2 A g−1 to 8 A g−1. (e) Charge−discharge cycling test of F-Si nanoparticles after carbon coating. (f) Capacity retention of carbon coated F-Si nanoparticles cycled at different current densities ranging from 0.2 A g−1 to 4 A g−1.

cycle, the current density was increased to 4 A g−1, the charge capacity remained stable above 1360 mA h g−1 after 100 cycles with almost no capacity decay. As seen in the Figure 4c, the first cycle Coulombic efficiency (CE) was 86%, which is quite high for Si nanoparticle anode, and from the fourth cycle, the CE reached above 99.3%. The galvanostatic charge/discharge curves of the first two cycles are presented in the Figure S4 (Supporting Information). At charge/discharge current densities ranging from 0.2 A g−1 to 8 A g−1, the capacities of 2500 to 900 mAh g−1 in the electrode were obtained (Figure 4d). This is mainly due to the thin and uniform carbon layer coating on the nanosized Si particles. Additionally, Figure 4e and f present the battery performance of carbon coated F-Si nanoparticles. The initial charge capacity reached 1250 mAh g−1 at 0.4 A g−1 and remained 1205 mAh g−1 in the 100th cycle with just 3% capacity decay. Its first two cycles’ galvanostatic charge/discharge curves are also provided in the Figure S5

electrolyte interface. Figure 4b shows the typical cyclic voltammograms of the M-Si and F-Si electrodes in the first cycle at a scanning rate of 0.1 mV/s between 0 and 1 V versus Li/Li+. A sharp redox peak below 0.1 V appeared during the lithium insertion process in each curve, indicating the initial lithiation of crystalline Si. In the process of delithiation, the two curves both showed two typical redox peaks between 0.3 and 0.6 V, corresponding to the process of amorphous α-LixSi converted to α-Si. No other peaks can be observed in the Figure 4b, demonstrating that other impurities (such as Fe, Al) did not react with Li+. To further improve the battery performance, we also coated these two kinds of nanoparticles with uniform carbon coating. Figure 4c and d show the electrochemical performances of M-Si nanoparticles after carbon coating. With the carbon coating, the first reversible capacity of M-Si nanoparticles electrode can reach 2628 mAh g−1 at the 0.4 A g−1 rate. From the fourth D

DOI: 10.1021/acs.nanolett.5b01698 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters (Supporting Information), from which we can see a flat curve below 0.1 V, which is consistent with the behavior of crystal Si. No other platform in the initial cycle appears, indicating that FeSi2 is inactive. Figure 4f shows the capacity change of electrode at different current densities. At 0.2 A g−1 rate, the capacity was approximately 1550 mAh g−1, when at 4 A g−1 rate, the capacity still reached above 500 mAh g−1. It is notable that the carbon-coated M-Si still presents a better rate performance than that of F-Si@C by comparing Figure 4d and f. For the rate ranging from 0.4 to 4 A g−1, the capacity retention ratio of M-Si was 70%, whereas for the F-Si, the retention ratio was just 36% in the same condition. The main reason is that the low impurities level in the M-Si powders can lead to a better electronic transport property. Conclusion. In summary, we have demonstrated a very simple and scalable fabrication method of producing nanosized Si electrode materials directly from the low cost metallurgical Si and ferrosilicon using high energy mechanical milling. The obtained M-Si and F-Si nanoparticles with further coating carbon both showed 100 stable deep cycling with the reversible capacity above 1200 mAh g−1 with less than 3% capacity decay. Thus, we anticipate that this method would open up a new path for scalable and cost-effective production of Si nanoparticles for Li-ion batteries, as well as other energy conversion devices, such as photovoltaics and thermoelectrics. Methods. Characterizations. The samples were characterized by scanning electron microscopy (SEM; Hitachi S4800), X-ray diffraction (XRD; Philips X’pert Pro X-ray diffractometer) and high-resolution TEM (JEM-2010, 200 kV) equipped with energy dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). The purity of the metallurgical Si was analyzed by X-ray fluorescence (XRF; Rigaku).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b01698. XRF data of the two low-grade Si source, XPS spectra of ferrosilicon powders before HEMM, statistical analysis of nanosized M-Si and F-Si, the galvanostatic charge/ discharge curves of the first two cycles for M-Si and F-Si with carbon coating. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is jointly supported by the State Key Program for Basic Research of China (No. 2015CB659300), National Natural Science Foundation of China (NSFC No. 11321063), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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DOI: 10.1021/acs.nanolett.5b01698 Nano Lett. XXXX, XXX, XXX−XXX