Simultaneous Perforation and Doping of Si Nanoparticles for Lithium

Dec 6, 2017 - Silicon nanostructures have served as promising building blocks for various applications, such as lithium-ion batteries, thermoelectrics...
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Simultaneous perforation and doping of Si nanoparticles for Lithium ion battery anode Guangxin Lv, Bin Zhu, Xiuqiang Li, Chuanlu Chen, Jinlei Li, Yan Jin, Xiaozhen Hu, and Jia Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12898 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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Simultaneous perforation and doping of Si nanoparticles for Lithium ion battery anode

Guangxin Lv†, Bin Zhu†, Xiuqiang Li, Chuanlu Chen, Jinlei Li, Yan Jin, Xiaozhen Hu, 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, P. R. China. † These authors contributed equally to this work. * Email: [email protected] (J.Z.)

ABSTRACT: Silicon nanostructures have served as promising building blocks for various applications, such as lithium ion battery, thermoelectrics and solar energy conversions. Particularly, controls of porosity and doping are critical for fine tuning the mechanical, optical and electrical properties of these silicon nanostructures. However, perforation and doping are usually separated processes, both of which are complicated and expensive. Here, we demonstrate that the porous nano-Si particles with controllable dopant can be massively produced through a facile and scalable method, combining ball milling and acid etching. Nano-Si with porosity as high as 45.8% can be achieved with 9 orders of magnitude of conductivity changes compared to intrinsic silicon. As an example of demonstration, the obtained nano-Si particles with 45.8% porosity and 3.7 at% doping can serve as a promising anode for lithium ion batteries with 2000 mA h/g retained over 100 cycles at the current density of 0.5 C, excellent rate performance with 1600 mA h/g at the current density of 5 C and a stable cycle performance of above 1500 mA h/g retained over 940 cycles at the current density of 1 C with carbon coating. KEYWORDS: silicon, perforation, doping, lithium ion, battery

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INTRODUTCION Silicon (Si), the second rich element in the crust of the earth, serves as foundation for various applications, such as integrated circuit,1, 2 thermoelectrics3, 4 and photovoltaics.5 Particularly, porous Si nanoparticles have shown significant advantages in many fields, such as anode of lithium ion battery,6-20 water splitting,21 chemical sensors,22 photoluminescence23-25 and cancer thermotherapy.26 Because of low conductivity (1.6×10-3 S m-1) of intrinsic silicon,27 doping of other elements such as phosphorus (P) and boron (B) into Si has been an effective method to tune the conductivity of nano-Si particles by several magnitude of orders.27 While perforation and doping are both critical for fine tuning properties of nano-Si, they are typically separated processes. There have been several methods, such as anodization,28 stain etching,18, 20, 29 bottomup synthesis12, 21 and magnesiothermic reduction, 30 to produce porous nano-Si particles. Effective doping has also been successfully achieved by chemical vapor deposition,31, 32

ball milling,3 laser ablation31 and so on. It usually takes complicated and expensive

processes to achieve perforation and doping separately. EXPERIMENTAL SECTION Here for the first time we demonstrate a facile and scalable method to produce nano-Si particles with simultaneous control of porosity and doping (Figure 1a). This process essentially contains two steps, ball milling and hydrofluoric (HF) acid treatment. As shown in Figure 1a, firstly, a mixture of metallurgical silicon powders (~ 98 wt%) and phosphorus pentoxide (P2O5) powders, with different weight ratios, was ball-milled with a rotation rate of 900 rpm for 3h to obtain nano-Si particles with P-dopant and

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SiO2. Then the particles were treated with HF for 2 hours to remove SiO2 to produce the porous nano-Si particles with P-dopant. Carbon-coated porous nano-Si particles with P-dopant for lithium ion battery were produced by mixing the obtained powders after ball milling which contain Si and SiO2, and citric acid powders with a weight ratio of 1:2 in deionized water. After 3h ultrasonic treatment, the obtained suspension was dried at 100 °C in a vacuum oven and then heat-treated at 1000 °C under a flowing Ar/H2 atmosphere to get the carbon-coated nanoparticles. Then the particles were treated with HF for 2 hours to remove SiO2. The control electrode was prepared by dissolving the mixture of 60 wt% active materials, 20 wt% carbon black and 20 wt% CMC binder in deionized water to make slurry. Then slurry was cast onto a thin copper foil after stirring for 8 hours and dried in a vacuum oven at 110 °C overnight. Coin-type cells (2032) were fabricated inside an Ar-filled glovebox using Celgard 2250 separator and Li metal foil as counter/reference electrode. The electrolyte utilized was 1.0 M LiPF6 in 1:1 vol/vol ethylene carbonate/diethyl carbonate with 2 wt% vinylene carbonate (Guotai Huarong) added to improve the cycling stability. The electrochemical tests were performed by a LANHE CT2001A. RESULTS AND DISCUSSION As shown in Figure 1b, the color of powders obtained by the process in Figure 1a presents dark red because of P doping. Therefore, by controlling the ratio of the Si particles and P2O5 powders, we can obtain Si nanoparticles with various porosities and P-dopant concentrations. Figure 1c shows a table of P concentration, conductivity and porosity of these nano-Si particles produced by different ratios of Si to P2O5. It is clear

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that as more P2O5 mixed with Si, P concentration of nano-Si increases and the conductivity of these nano-Si particles dramatically increases as well, up to 8-9 orders of magnitude higher than that of intrinsic silicon.27 It is clear that the particles produced by a weight ratio of 10:2 have larger porosity than those produced by a weight ratio of 10:1 of Si to P2O5 because more SiO2 was produced during ball milling. If more P2O5 is added in (as in the case of 10:4), there are so much SiO2 content that the structure became not stable and collapse to smaller particles after HF treatment. There are several unique features related to this process. (1) the process enables the simultaneous perforation and doping of nano-Si particles; (2) the porosity of nano-Si particles and the amount of dopant P can be controlled through adjusting ratio of P2O5 and Si sources; (3) the entire process only contains ball milling and acid treatment.

The morphology and structures of these nano-Si particles were carefully examined through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 2a and Figure 2b are the SEM images of the metallurgical Si powders (~ 98 wt%) and P2O5 powders. It is shown that the sizes of the Si powders and P2O5 powders are about ~1μm and ~20μm, respectively. Figure 2c presents the morphology of the obtained Si particles after ball milling with containing P-dopant and SiO2 produced by a weight ratio of 10:2, as a typical example. The size of the particles decreases to about 150 nm. The morphology of the Si particles after acid treatment showed almost no change from SEM image (Figure 2d). Energy dispersive spectroscopy (EDS) mapping of porous nano-Si particles with P-dopant produced by

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the weight ratio 10:2 of Si to P2O5 is shown in Figure 2e and Figure 2f. The green and blue bright regions correspond to the element Si and P and the two elements are distributed uniformly throughout the whole area as we expected, which suggests the uniform P doping in the nano-Si particles. EDS mapping results present that the atomic ratios of P in the nano-Si particles produced by different weight ratios of 10:1 and 10:4 of Si to P2O5 are 2.0 at% and 5.3 at%, respectively (see Supporting Information, Figure S1). It indicates that P concentration increases as more P2O5 powders were added at the beginning. Those P2O5 powders not reacted with Si during ball milling were dissolved by the solution during HF acid treatment. TEM image in Figure 2g clearly shows the mesoporous structure of porous nano-Si particles with P-dopant, which is beneficial to overcome the problem of volume expansion of Si anode during cycling of lithium ion batteries, as discussed in more details later. The lattice fringes of 0.31 nm labelled in the high resolution TEM (HRTEM) image (Figure 2h) is consistent with the d-spacing values of the (1 1 1) plane of Si.32, 33

To confirm the successful P-dopant in the obtained nano-Si particles, X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) were utilized to probe the particles. Figure 3a presents the crystal structure of porous nano-Si particles with Pdopant, which were produced by different weight ratios of Si to P2O5 (10:1, 10:2, 10:4). It is observed that the diffraction peaks (see Figure 3a, yellow area) shift to smaller angle when more P2O5 mixed with Si, confirming the increased concentrations of P doping in the nano-Si particles. Figure 3b exhibits Raman spectra of the samples. The

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Si reference peak (520 cm-1) shifts to 515.3, 510.2 and 496.6 cm-1 for the 10:1, 10:2 and 10:4 samples, respectively.34 This shift is accompanied by the increase of intensity of the peak around 300 cm-1 and a broad low-energy tail that extends from 350–500 cm-1 , which suggests that more P atoms doped in the Si.35 It is found that the XPS peaks of 10:2 sample appear around 98.5 eV and 102.5 eV, which are assigned primarily to Si0 and Si4+, respectively (Figure 3c). The peak of Si4+ is caused by the existence of the commonly observed surface oxides of Si nanoparticles.21 It is shown that the Si0 reference peak shifts from 99.4 eV to 98.5 eV and the Si4+ reference peak shifts from 103.2 eV to 102.5 eV (the yellow shade region in the Figure 3c), further suggesting the successful P doping.36 Figure S2 also presents the XPS peak of P0 which shifts from 129.9 eV to 131.75 eV.

To probe the exact porosities of the different products, the porous nano-Si were characterized by Brunauer−Emmett−Teller (BET) nitrogen (N2) adsorption (Figure 3d). It is found that these from weight ratio of 10:2 absorb the highest amount of N2. The BET surface areas of porous nano-Si particles from weight ratios of 10:1, 10:2 and 10:4 are calculated to be 18.817 m2/g, 45.596 m2/g and 21.423 m2/g, with the porosities of 19.0%, 45.8% and 33.4%, respectively. The porous nano-Si from the weight ratio of 10:2 has the largest porosity, which may be beneficial for the cycle performance of the battery. As mentioned earlier, when adding too much P2O5 (as in the case of weight ratio of 10:4) the generated SiO2 content was so much that the structure became not stable or collapsed to smaller particles after HF treatment.

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Si has been intensively pursued as an attractive anode material for next-generation lithium-ion batteries because of its high theoretical capacity (4200 mA h/g, ten times higher than commercial graphite anodes).37-40 However, the grave problems, such as mechanical fracture and unstable solid electrolyte interface (SEI) due to the tremendous volume expansion (~300%) during cycling,37, 41-47 restrict its large-scale applications. Many Si nanostructures have been proposed to solve the problem.6-20, 48-52 The obtained nano-Si particles with high porosity and high electrical conductivity can serve as excellent building blocks for electrochemical performance. Two-electrode coin cells using porous nano-Si particles with P-dopant as the anode and lithium metal as the counter electrode were fabricated. Figure 4a exhibits the typical cyclic voltammetry (CV) curves of porous nano-Si particles with P-dopant (10:2) at the first and second cycle at a scanning rate of 0.1 mV/s between 0.01 and 1.5 V versus Li/Li+. The peak at 0.15 V in the first cycle represents the transition from crystalline structure to an amorphous structure. In the delithiation process, two typical redox peaks between 0.3 and 0.6 V appeared which suggests the conversion from amorphous α-LixSi to α-Si. There are no other peaks like the reaction between P and Li observed,53 further confirming that P was exactly doped in the Si. Figure 4b presents the cycle performance of nano-Si particles and three samples with different weight ratios (10:1, 10:2, 10:4) at charging/discharging rate of 0.1 C for the first five cycles and 0.5 C for the additional 95 cycles. It is obvious that the porous nano-Si particles with P-dopant demonstrated more stable cycle performance than that of nano-Si particles, which decayed below 1000 mA h/g rapidly after only 60 cycles. It is clear that the porous structure of nano-

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Si particles provides the void space for the volumetric expansion during cycling. Moreover, it is found that the product with 10:2 ratio has the most stable cycling performance, which is as expected because of its highest porosity. Figure 4c shows the rate performance of the different nano-Si particles. It is observed that the nano-Si particles from weight ratio of 10:2 ratio presented great performance with 1600 mA h/g even at a current density of 5 C. As shown in the Figure 4d, with carbon coating, these nano-Si particles present a considerably stable cycle performance with above 1500 mA h/g remained after 940 cycles at the current density of 1 C. The initial coulombic efficiency (CE) was about 85.4% and after 5 cycles the CE quickly increased above 99.1%. To further demonstrate the feasibility of our nano-Si particles with P-dopant for commercially used cell, full cells with LiCoO2 (LCO) cathode and obtained Si nanoparticles anode were assembled to examine the electrochemical performance. It is found from Fig. S3 that LCO electrode demonstrates relatively stable cycles with 98.6 mA h/g retained after 100 cycles. Therefore, the obtained nano-Si particles with Pdopant is highly promising for the lithium-ion battery anode. CONCLUSION In summary, we have demonstrated a new facile and scalable method to produce nanoSi particles with simultaneous control of porosity and doping. Nano-Si particles with porosity as high as 45.8% and doping of 3.7 at% can be produced. This type of porous nano-Si particles can serve as high performance lithium battery anode with a stable cycle performance of 2000 mA h/g retained over 100 cycles at the current density of 0.5 C, excellent rate performance with 1600 mA h/g at the current density of 5 C and a

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stable cycle performance of above 1500 mA h/g over 940 cycles at the current density of 1 C with carbon coating. It is anticipated that this method will open tremendous opportunities for cost-effective production of controllable porous nano-Si particles with dopant for energy storage, as well as other applications such as thermoelectrics and photovoltaics. ASSOCIATED CONTENT Supporting Information SEM and EDS mapping of porous nano-Si particles with P-dopant produced by different weight ratios of Si to P2O5 (10:1 and 10:4) are shown in Supporting Information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Z.). Author Contributions † G.L. and B.Z. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the micro-fabrication center of National Laboratory of Solid State Microstructures (NLSSM) for technique support. This work is jointly supported by the

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National Key Research and Development Program of China (No. 2017YFA0205700) and the State Key Program for Basic Research of China (No. 2015CB659300), National Natural Science Foundation of China (Nos. 11621091, 11574143), Natural Science Foundation of Jiangsu Province (Nos. BK20150056), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Fundamental Research Funds for the Central Universities (Nos. 021314380068, 021314380089, 021314380091).

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(41) Sun, Y.; Liu, N.; Cui, Y. Promises and Challenges of Nanomaterials for Lithium-Based Rechargeable Batteries. Nat. Energy 2016, 1, 16071. (42) Su, X.; Wu, Q.; Li, J.; Xiao, X.; Lott, A.; Lu, W.; Sheldon, B. W.; Wu, J. Silicon-Based Nanomaterials for Lithium-Ion Batteries: A Review. Adv. Energy Mat. 2014, 4,1300882. (43) Obrovac, M.; Christensen, L. Structural Changes in Silicon Anodes During Lithium Insertion/Extraction. Electrochem. Solid-State Lett. 2004, 7, A93-A96. (44) Verma, P.; Maire, P.; Novák, P. A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55, 6332-6341. (45) Nie, M.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L. Silicon Solid Electrolyte Interphase (SEI) of Lithium Ion Battery Characterized by Microscopy and Spectroscopy. J. Phys. Chem. C 2013, 117, 1340313412. (46) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano 2012, 6, 1522-1531. (47) He, Y.; Piper, D. M.; Gu, M.; Travis, J. J.; George, S. M.; Lee, S.-H.; Genc, A.; Pullan, L.; Liu, J.; Mao, S. X. In Situ Transmission Electron Microscopy Probing of Native Oxide and Artificial Layers on Silicon Nanoparticles for Lithium Ion Batteries. ACS Nano 2014, 8, 11816-11823. (48) McDowell, M. T.; Lee, S. W.; Ryu, I.; Wu, H.; Nix, W. D.; Choi, J. W.; Cui, Y. Novel Size and Surface Oxide Effects in Silicon Nanowires as Lithium Battery Anodes. Nano Lett. 2011, 11, 4018-4025. (49) Lee, W. J.; Hwang, T. H.; Hwang, J. O.; Kim, H. W.; Lim, J.; Jeong, H. Y.; Shim, J.; Han, T. H.; Kim, J. Y.; Choi, J. W. N-Doped Graphitic Self-Encapsulation for High Performance Silicon Anodes in Lithium-Ion Batteries. Energy Environ. Sci. 2014, 7, 621-626. (50) Roy, A. K.; Zhong, M.; Schwab, M. G.; Binder, A.; Venkataraman, S. S.; Tomović, Z. e. Preparation of

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a Binder-Free Three-Dimensional Carbon Foam/Silicon Composite as Potential Material for Lithium Ion Battery Anodes. ACS Appl. Mater. Interfaces 2016, 8, 7343-7348. (51) Chen, X.; Li, X.; Ding, F.; Xu, W.; Xiao, J.; Cao, Y.; Meduri, P.; Liu, J.; Graff, G. L.; Zhang, J.-G. Conductive Rigid Skeleton Supported Silicon as High-Performance Li-Ion Battery Anodes. Nano Lett. 2012, 12, 41244130. (52) Jin, Y.; Tan, Y.; Hu, X.; Zhu, B.; Zheng, Q.; Zhang, Z.; Zhu, G.; Yu, Q.; Jin, Z.; Zhu, J. Scalable Production of the Silicon–Tin Yin-Yang Hybrid Structure with Graphene Coating for High Performance Lithium-Ion Battery Anodes. ACS Appl. Mater. Interfaces 2017, 9, 15388-15393. (53) Li, W.; Yang, Z.; Jiang, Y.; Yu, Z.; Gu, L.; Yu, Y. Crystalline Red Phosphorus Incorporated with Porous Carbon Nanofibers as Flexible Electrode for High Performance Lithium-Ion Batteries. Carbon 2014, 78, 455-462.

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Figure 1. (a) Schematics of the process containing two steps: high energy mechanical milling of Si and P2O5 and hydrofluoric (HF) acid treatment. (b) Optical image of P2O5 particles, Si particles, porous nano-Si particles with P-dopant. (c) P concentration, conductivity and porosity of nano-Si particles produced by different weight ratios of Si to P2O5.

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Figure 2. (a) SEM image of metallurgical Si powders (~ 98 wt%). (b) SEM image of P2O5 powders. (c) SEM image of nano-Si particles with P-dopant and SiO2 produced after ball milling (weight ratio of Si to P2O5: 10:2). (d-f) SEM image and EDS mapping of the porous nano-Si particles with P-dopant after acid etching (weight ratio of Si to P2O5: 10:2), the green (e) and blue bright (f) regions correspond to the elements of Si and P. (g) TEM images of porous nano-Si particles with P-dopant (weight ratio of Si to P2O5: 10:2). (h) HRTEM image of the porous nano-Si particles with P-dopant (weight ratio of Si to P2O5: 10:2). The well-resolved lattice spacing of 0.31 nm corresponds to the (1 1 1) plane of silicon.

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Figure 3. Characterizations of porous nano-Si particles with P-dopant from different weight ratios of Si to P2O5. (a) XRD patterns of nano-Si particles, the diffraction peaks in yellow area shift to smaller angle when more P2O5 mixed with Si. (b) Raman spectrum of nano-Si particles. (c) XPS spectra (Si2p) of the porous nano-Si particles with P-dopant produced by a weight ratio of 10:2, dashed lines correspond to the reference peak of Si0 and Si4+. (d) Nitrogen isotherm plots of different porous nano-Si particles with P-dopant produced by different weight ratios.

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Figure 4. Electrochemical performance: (a) Voltage curves of nano-Si particles (weight ratio of Si to P2O5: 10:2) for the first and second discharge/charge cycles at constant current (0.1 C, 0.01–1.5 V/Li+/Li). (b) Cycling performance of porous nano-Si particles with P-dopant produced by different weight ratios of Si to P2O5 (0.1 C for the first cycle and 0.5 C for the following cycles). (c) Cycling performance at different rates from 0.05 C to 5 C of porous nano-Si particles with P-dopant produced by different weight ratios of Si to P2O5. (d) Cycling performance of carbon-coated porous nano-Si particles with P-dopant produced (weight ratio of Si to P2O5: 10:2) (0.1 C for the first 5 cycles and 1 C for the following cycles).

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THE TABLE OF CONTENTS ENTRY

Simultaneous perforation and doping of Si nanoparticles for Lithium ion battery anode

Guangxin Lv†, Bin Zhu†, Xiuqiang Li, Chuanlu Chen, Jinlei Li, Yan Jin, Xiaozhen Hu, Jia Zhu*

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