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Apr 23, 2018 - School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. •S Supporting Information. ABSTRACT: Silicon ...
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Green, Scalable, and Controllable Fabrication of Nanoporous Silicon from Commercial Alloy Precursors for High-Energy Lithium-Ion Batteries Yongling An,† Huifang Fei,† Guifang Zeng,† Lijie Ci,† Shenglin Xiong,‡ Jinkui Feng,*,† and Yitai Qian‡ †

SDU & Rice Joint Center for Carbon Nanomaterials, Key Laboratory for Liquid−Solid Structural Evolution & Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, China ‡ School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China S Supporting Information *

ABSTRACT: Silicon is considered as one of the most favorable anode materials for next-generation lithium-ion batteries. Nanoporous silicon is synthesized via a green, facile, and controllable vacuum distillation method from the commercial Mg2Si alloy. Nanoporous silicon is formed by the evaporation of low boiling point Mg. In this method, the magnesium metal from the Mg2Si alloy can be recycled. The pore sizes of nanoporous silicon can be secured by adjusting the distillated temperature and time. The optimized nanoporous silicon (800 °C, 0.5 h) delivers a discharge capacity of 2034 mA h g−1 at 200 mA g−1 for 100 cycles, a cycling stability with more than 1180 mA h g−1 even after 400 cycles at 1000 mA g−1, and a rate capability of 855 mA h g−1 at 5000 mA g−1. The electrochemical properties might be ascribed to its porous structure, which may accommodate large volume change during the cycling process. These results suggest that the green, scalable, and controllable approach may offer a pathway for the commercialization of high-performance Si anodes. This method may also be extended to construct other nanoporous materials. KEYWORDS: silicon, vacuum distillation, nanoporous structure, anodes, lithium-ion batteries tures, such as nanotubes,16,17 nanowires,18,19 and hollow20,21 and nanoporous structures,22,23 have been proven with enhanced electrochemical properties. For example, Du et al. reported silicon nanotubes with an initial discharge capacity of 2400 mAh g−1 and a 50% capacity retention after 100 cycles, which was attributed to the hollowness of Si nanotubes that can accommodate a large volume expansion during charge/ discharge processes.24 Peled et al. provided crystallineamorphous core−shell silicon-nanowire-based electrodes with a loading of 3−15 mAh/cm2, which also exhibited an enhanced electrochemical performance.25 Silicon-based electrodes with porous structure display outstanding and promising electrochemical performance due to the much open channels in the

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ilicon is considered as one of the most favorable anode materials for next-generation lithium-ion batteries (LIBs) with a lower voltage potential (about 0.2−0.3 V vs Li/ Li+), abundant resources, and high specific capacity (4200 mA h g−1 for Li4.4Si, 3579 mAh g−1 at mild temperature).1−4 However, several key issues hinder its practical application.5−7 The huge volume change (more than 400%) during the charge/ discharge process leads to the cracking and pulverization of the silicon anode, resulting in loose contact between current collectors and active materials accompanied by everlasting side reactions between the silicon anode and electrolyte.8−10 Moreover, the continuous growth of the solid electrolyte interface (SEI) layer depletes electrolyte and lithium ions, causing high cell impedance, finally leading to fast capacity loss, low Coulombic efficiency, and poor electrochemical performance.11−13 These crucial issues are expected to be relieved by designing suitable nanostructures.14,15 Recently, several silicon nanostruc© 2018 American Chemical Society

Received: March 24, 2018 Accepted: April 23, 2018 Published: April 23, 2018 4993

DOI: 10.1021/acsnano.8b02219 ACS Nano 2018, 12, 4993−5002

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Figure 1. (a) Schematic of the evolution of Mg2Si alloy by the vacuum distillation method. (i) Initially, the Mg2Si alloy has an atomic structure. (ii) When the Mg2Si alloy is distillated at 600 °C, only limited Mg atoms are sublimated. (iii) More and more Mg atoms are sublimated when the Mg2Si precursor is distillated at 700 °C, and a large number of voids or vacancies can be composed in the Mg2Si alloy. (iv) When the temperature is increased to 800 °C, the Mg atoms are entirely eliminated, and the pure NP-Si are obtained. (v) When the heat treatment temperature reaches 900 °C, the pores of NP-Si are reduced. (b,g) SEM images of the Mg2Si alloy precursor. (c−f) SEM images of evolution structure of the Mg2Si alloy, distillated by the vacuum distillation method at (c) 600, (d) 700, (e) 800, and (f) 900 °C for 0.5 h. (h−k) SEM images of evolution structure of the Mg2Si alloy, distillated by the vacuum distillation method at (h) 600, (i) 700, (j) 800, and (k) 900 °C for 2 h.

with a common corrosion process, which dissolves the element from the original alloys to form the successive 3D porous meshwork structure with interconnected ligaments.15,31,32 Dealloying reaction can be performed at room temperature and is facile to scale as the alloys are commercially available, which can compose such porous structures with a higher specific surface area.33 Our group has studied the porous Si,28 Ge,34 Bi,35 and Sb36 by a chemical dealloying method. The porous silicon and reduced graphene oxide nanocomposites were prepared by a chemical dealloying method; this porous structure can deliver a discharge capacity of 2280 mAh g−1 and a 85% capacity retention after 100 cycles and a 1521 mAh g−1 discharge capacity at 4000 mA g−1.28 However, most of these procedures require polluted chemicals to dissolve the component, which is harmful to the environment and workers. In this work, we explore a green and facile vacuum distillation technique to provide nanoporous silicon (NP-Si) from commercial Mg2Si alloy. The vacuum distillation method is a common process by subliming one or more low boiling point

porous structure that can act as both volume expansion buffers and the ion pathway.15 Yoon et al. demonstrated that the compound of mesoporous Si hollow nanocubes was received from the metal−organic framework (MOF) as an electrode with better electrochemical performance.26 However, the stateof-the-art preparation methods of nanoporous silicon generally demand several complex steps, such as depositing a thin film or providing a template, which leads to expensive preparation and complicated scale up.15,27 For the mass fabrication of porous silicon in LIBs, the preparation of such porous silicon by a lowcost, high-output, scalable, and simple approach is imperative.28 Vrankovic et al. produced a nano Si, which showed a 1459 mAh g−1 retention capacity after 200 cycles.29 Wu et al. provided a solution process to prepare a 3D network of silicon nanoparticles, which displayed cycle properties of 550 mAh g−1 after 5000 cycles.30 Dealloying is a general corrosion procedure in which the alloy is “separated” by the decidable removal of its elements.31 Chemical dealloying has been studied to provide porous metal 4994

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Figure 2. (a) XRD and (c) Raman evolution of the Mg2Si alloy, distillated by the vacuum distillation method at 600, 700, 800, and 900 °C for 0.5 h. (b) XRD and (d) Raman evolution of the Mg2Si alloy, distillated by the vacuum distillation method at 600, 700, 800, and 900 °C for 2 h.

suggest that the green, scalable, and controllable approach may offer a pathway for the commercialization of high-performance Si anodes. This method may also be extended to construct other nanoporous materials.

components in the precursor to obtain a pure high boiling point product.37 The vacuum temperature must be below the melting point of the alloy to form pores, and the theoretical basis of the vacuum distillation is the Kirkendall voids and sublimation.38 The continuous NP-Si skeleton could be obtained in a temperature below the melting point of the Mg2Si alloy (1085 °C) via vacuum bumping. In this method, the magnesium metal from the Mg2Si alloy can be recycled. The pore size of the NP-Si can be easily controllable by adjusting the distillated temperature and time. The optimized NP-Si (800 °C, 0.5 h) shows electrochemical capabilities (a discharge capacity of 2034 mA h g−1 after 100 cycles at 200 mA g−1, a discharge capacity of 855 mA h g−1 at 5000 mA g−1, a discharge capacity of 1180 mA h g−1 after 400 cycles at 1000 mA g−1), whereas the commercial nano-silicon shows poor cycling properties and a worse rate property. These results

RESULTS AND DISCUSSION The Mg2Si alloy contains the Mg element with a low boiling point (1107 °C) and the Si element with a high boiling point (2900 °C). The matrix metal (Mg) will undergo sublimation and diffusion to produce voids and eventually form the porous structure.37,38 Figure 1a illustrates the schematic of the evolution of the Mg2Si alloy via the vacuum distillation method. The degree of the vacuum is set to 10 Pa. Initially, the Mg2Si alloy has an atomically distributed structure (i). When the Mg2Si alloy is distillated at 600 °C, only limited Mg atoms 4995

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Figure 3. (a−d) Nitrogen adsorption−desorption isotherm and the corresponding pore size distribution (inset) of the Mg2Si alloy, distillated by the vacuum distillation method at 800 °C for (a) 0.5 h and (b) 2 h and at 900 °C for (c) 0.5 h and (d) 2 h.

pattern (Figure 2a, 800 °C) shows that the pure silicon is obtained and the Mg content in the Mg2Si alloy is completely removed. The intensity of the XRD pattern is weak, owing to the porous structure of NP-Si, as illustrated in the transmission electron microscopy (TEM) results, which may provide perfect electrochemical properties.28 The Raman spectra in Figure 2c (800 °C) also reveal that pure silicon is attained. Figure 1f reveals the SEM image of the Mg2Si distillated at 900 °C. Compared to Figure 1e, the pores of NP-Si are gradually fused due to the pores partially merging together at higher temperature.37,38 The XRD pattern in Figure 2a (900 °C) indicates that only the Si atoms remain in the Mg2Si alloy at 900 °C. The Raman spectra in Figure 2c (900 °C) also confirm that the Mg content in the Mg2Si alloy is completely removed. The successful synthesis of NP-Si with interknitted pores may promote the Li+ insertion and extraction and buffer volume change of a silicon anode during the charge/discharge process. Thus, the promising electrochemical performance of NP-Si can be expected. Moreover, the evolution structures of the Mg2Si alloy by the vacuum distillation method for a longer time (2 h) are also explored. Figure 1g−k, corresponding to Figure 1a, shows the SEM images of the structure evolution of the Mg2Si alloy using the vacuum distillation method for 2 h, which is accord with that for 0.5 h. The XRD (Figure 2b) evolution of Mg2Si distillated for 2 h reveals that the intensity of Mg is weaker with increased vacuum heat temperature from 600 to 800 °C. When the temperature reaches 800 °C, the Mg2Si alloy changes into pure NP-Si. The Raman spectra in Figure 2d also show that the pure NP-Si is obtained when the temperature increases to 800 °C, which is accord with the XRD results. In addition, the evolution structure of the Mg2Si distillated for 4 h is also studied, as shown in Figures S1 and S2. Moreover, the energydispersive spectroscopy (EDS) results and mapping images of the original and the vacuum distilled Mg2Si alloy at different

are sublimated (ii). When the distillated temperature is set at 700 °C, more and more Mg atoms are sublimated, and a large number of voids or vacancies can be composed in the Mg2Si alloy (iii). When the distillated temperature is increased to 800 °C, the Mg element is entirely eliminated, and pure NP-Si is obtained (iv). However, when the heat treatment temperature reaches 900 °C, the pores of NP-Si are reduced (v). The reduced pore value may be ascribed to the growth of silicon crystals at high temperature. Figure 1b−f shows the scanning electron microscopy (SEM) images of the Mg2Si alloy in its origin and vacuum distillation conditions at different temperature for 0.5 h. Figure 1b shows the SEM image of the Mg2Si alloy precursor. The X-ray diffraction (XRD) pattern of the Mg2Si alloy precursor in Figure 2a fits well with its pure structure (JCPDS no. 35-0773),39,40 which is in accord with the result of the Raman spectra in Figure 2c (Mg-Si).41 Figure 1c shows the SEM of the Mg2Si distillated at 600 °C. Compared to that in Figure 1b, a rough surface is created; however, the XRD pattern in Figure 2a (600 °C) is nearly unchanged because only a limited part of the Mg element is sublimated. The Raman spectra in Figure 2c (600 °C) are also consistent with the XRD result. Figure 1d reveals the SEM images of the evolution structure of the Mg2Si distillated at 700 °C. More and more nanopores are formed because a majority of the Mg elements are sublimated at this temperature, compared to that in Figure 1c. The XRD pattern (Figure 2a, 700 °C) illustrates that the intensity of the (111) peak of pure silicon increased, and the Mg2Si peaks were greatly reduced, demonstrating that most of the Mg atoms are sublimated. This result is consistent with the Raman spectra in Figure 2c (700 °C). Figure 1e illustrates the SEM image of the Mg2Si distillated at 800 °C. Compared to Figure 1d, the uniform pores of the porous structure are obtained, as shown in Figure 1e. The interconnected nanopores can facilitate the lithiation and delithiation to buffer the volume expansion of silicon during the cycling process.28,42 The XRD 4996

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Figure 4. (a)TEM, (b) HRTEM, and (c) SAED of the Mg2Si alloy, distillated by the vacuum distillation method at 800 °C for 0.5 h. (d) TEM, (e) HRTEM, and (f) SAED of the Mg2Si alloy, distillated by the vacuum distillation method at 800 °C for 2 h. (g) TEM, (h) HRTEM, and (i) SAED of the Mg2Si alloy, distillated by the vacuum distillation method at 900 °C for 0.5 h. (j) TEM, (k) HRTEM, and (l) SAED of the Mg2Si alloy, distillated by the vacuum distillation method at 900 °C for 2 h.

crystal planes.11 The corresponding selected area electron diffraction (SAED) patterns (Figure 4c) can be ascribed to the silicon with a diamond cubic structure, which is in accord with the result of Figure 4b.15 With the extension of time and the increasing of temperature, the number of pores is reduced, and the aperture of the pores is smaller, as shown in Figure 4d (800 °C, 2 h), Figure 4g (900 °C, 0.5 h), and Figure 4j (900 °C, 2 h). The lattice fringe of 0.31 nm (Figure 4e,h,k) and the SAED (Figure 4f,i,l) are also observed, which can be ascribed to the cubic structure of silicon. Moreover, the EDS results, TEM images, and corresponding electron energy loss spectroscopy maps of the original and the vacuum distilled Mg2Si alloy at different conditions are also studied, as illustrated in Figure S8. These results illustrate that the Mg2Si alloy changes into pure NP-Si when the temperature reaches 800 °C, which is accord with the results of XRD and Raman spectroscopy. The electrochemical performance of NP-Si is explored by cyclic voltammetry (CV) and static−current charge/discharge cycling using 2016-coin cells with lithium foil as both the reference electrode and the counter electrode. The electrolyte is 1 M LiPF6 in EC/DEC (v/v, 1:1), as illustrated in Figure 5. The initial three CV curves of NC-Si (Figure 5a) and NP-Si (800 °C, 0.5 h, Figure 5b) are executed at a scanning speed of 0.1 mV s−1 between 0.01 and 3 V. The CV curves of NP-Si are in accord with those of NC-Si, which indicates the identical charge−discharge mechanism. In the first cycle, the cathodic peak between 0.01 and 0.18 V corresponds to the cathodic lithiation of crystalline Si to form LixSi.43,44 The wide oxidation

conditions are also presented, as shown in Figures S3−S5. From these figures, we can see that the Mg2Si alloy changes into pure NP-Si when the temperature reaches 800 °C. The porous ability of NP-Si at different temperature and time is explored by Brunauer−Emmett−Teller (BET), as shown in Figure 3. Compared to the Mg2Si alloy (Figure S6) of 1.9 m2 g−1, the specific surface area of NP-Si at 800 °C for 0.5 h (Figure 3a) and 2 h (Figure 3b) and at 900 °C for 0.5 h (Figure 3c) and 2 h (Figure 3d) is about 10.9, 9.8, 9.5, and 8.4 m2 g−1, respectively. The distribution of pore size reveals that the majority of pore diameters are less than 100 nm, and 2−20 nm pore diameters are the majority. These pores may be ascribed to the void space remanent after subliming the Mg component in the Mg2Si to develop the NP-Si. The void spaces of various sizes will be beneficial for the transport of lithium ions and more electrolyte−electrode touchs during the cycling process, accommodating volume expansion and alleviating stress of the silicon electrode during the cycling process.8,11,15 Figure 4 provides the high-resolution transmission electron microscopy (HRTEM) image with details of the structure of the Mg2Si alloy by vacuum distillation at different conditions, which shows a typical nanoporous structure. Compared to Figure S7 with TEM images of the Mg2Si alloy precursor, Figure 4a illustrates the continuous structure of the nanocrystalline silicon (NC-Si) and the porous structure with uniformly distributed pores. Figure 4b reveals the HRTEM image of the NP-Si (800 °C, 0.5 h). The 0.31 nm lattice fringe is commonly perceived, which can be attributed to the (111) 4997

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Figure 5. Electrochemical performance of the NC-Si and NP-Si as anode materials between 0.01 and 3 V (vs Li+/Li). Cyclic voltammetry profiles at 0.1 mV s−1 of (a) NC-Si and (b) NP-Si (800 °C, 0.5 h) anodes. Galvanostatic charge/discharge voltage curves at 200 mA g−1 current density of (c) NC-Si and (d) NP-Si (800 °C, 0.5 h) anodes. (e) Cycling capability at 200 mA g−1 of NC-Si and NP-Si (800 °C, 0.5 h) anodes. (f) Rate property at different current densities from 1000 to 5000 mA g−1 of NC-Si and NP-Si (800 °C, 0.5 h) anodes. (g) Long-term cycling capability at 100 mA g−1 of NP-Si (800 °C, 0.5 h) anode.

electrode shows a 76% Coulombic efficiency with a 2569 mAh g−1 discharge capacity. The irreversible capacity can be attributed to the irreversible lithium ion in the anode and the formation of SEI.8 The irreversible capacity mainly comes from the formation and deformation of LixSi, and a few side reactions, such as sorption and desorption of the lithium ion on the electrode surface, can also provide capacity.11,15 In the subsequent cycles, compared to the charge/discharge profiles of the NC-Si anode, the profiles of the NP-Si anode nearly overlap, illustrating the stable cycling performance. The reversible capacity and Coulombic efficiency of every cycle of NC-Si and NP-Si electrodes are distinctly shown in Figure 5e.

peak that appears between 0.1 and 0.53 V, corresponding to the delithiation of LixSi and formation of amorphous silicon, is obviously observed.3 The increased peak area can be ascribed to the gradual activation of the silicon anode at rapid CV measurement.3,43,44 Figure 5c,d illustrates the charge/discharge curves of NC-Si (Figure 5c) and NP-Si (Figure 5d) between 0.01 and 3 V at 200 mA g−1. Both display typical silicon charge/ discharge profiles, illustrating a similar energy storage mechanism, which is in accord with the CV conclusions.43 Specifically, in the initial cycle, compared with a high 85% Coulombic efficiency with a 3412 mA h g−1 discharge capacity of the cell with the NP-Si electrode, the cell with the NC-Si 4998

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ACS Nano After 100 cycles, the discharge capacity of the NC-Si decreases from 2008 to 274 mA h g−1, which is in accord with a capacity retention of as low as 13.65%. The large volume expansion of the NC-Si anode causes the capactiy loss during the cycling process.43 In comparison, for the cell with NP-Si, after 100 cycles, the discharge capacity still remains at 2034 mA h g−1 with a 100% Coulombic efficiency. In addition, the cycling capabilities of the NP-Si (800 °C for 0.5 h, 900 °C for 0.5 h, and 900 °C for 2 h) at 200 mA g−1 between 0.01 and 3 V are studied, as shown in Figure S9. As a result, the NP-Si (800 °C, 0.5 h) exhibits a cycling performance better than that of the NP-Si in the other conditions. Except for the favorable cycling property, the NP-Si (800 °C, 0.5 h) anode also indicates the enhanced rate performance, as shown in Figure 5f. The NC-Si anode is tested at different current densities of 1000, 2000, 3000, 4000, and 5000 mA g−1, and the corresponding discharge capacities can be attained at 1235, 420, 130, 25, and 10 mA h g−1, respectively. A discharge capacity of 520 mA h g−1 can be obtained at 1000 mA g−1. For the cell with NP-Si (800 °C, 0.5 h), a discharge capacity of 1715, 1277, 1070, 954, and 855 mA h g−1 can be obtained at the current density of 1000, 2000, 3000, 4000, and 5000 mA g−1, respectively. Importantly, a discharge capacity of 1560 mA h g−1 can be attained when the current density returns to 1000 mA g−1. The most outstanding characteristic of the NP-Si electrode is the ultrasteady cycling performance, as illustrated in Figure 5g. After 400 cycles at 1000 mA g−1, there is no obvious capacity depression, corresponding to a 1180 mA h g−1 discharge capacity and about a 100% Coulombic efficiency. The enhanced electrochemical capabilities of the NP-Si anode may be ascribed to its special porous structure, which could accommodate large volume change and facile ion transport of silicon electrodes during the cycling process, as illustrated in Figure 6.22,23 To further explore the outstanding capabilities of the NP-Si anode, the charge transfer kinetics measurement (electrochemical impedance spectroscopy, EIS) and ex situ SEM are executed, as presented in Figure 7. The ex situ SEM of the NCSi (Figure 7a,c) and NP-Si (Figure 7b,d) anodes that are removed from the cells after 100 cycles is explored. From Figure 7a,c, we can see cracks and aggregations in the NC-Si electrode, which may result in the disconnection of the active material from the current collector, which will lead to the capacity fading.28,45 Nevertheless, no disassembly or cracking (Figure 7b,d) is observed in the NP-Si anode, implying that the NP-Si is robustly anchored on the anode even after 100 cycles. That is the reason why the NP-Si anode shows such enhanced electrochemical properties compared to those of the NC-Si anode.46−49 To further explore the mechanism of excellent electrochemical properties of the NP-Si anode, the charge transfer kinetics of the NC-Si (Figure 7e) and NP-Si (Figure 7f) anodes after different cycles at the full delithiation situation are tested by EIS. In this EIS investigation, the NC-Si and NPSi anodes are applied to the working electrode and lithium foil as the reference electrode and the counter electrode. The consequences are the distinctive Nyquist plots, which comprise the flattened semicircle at high- and middle-frequency space due to surface impedance and charge transfer resistance, and the straight line at low-frequency space is ascribed to the diffusion of Li+.28,46 As shown in Figure 7e, the resistance of the NC-Si anode sharply increases after 100 cycles. Compared to the NC-Si anode, no obvious impedance change of the NP-Si anode is observed after 100 cycles, as illustrated in Figure 7f, which indicates restricted growth of the SEI layer during

Figure 6. Schematic of the anode design exhibiting a large silicon particle and nanoporous silicon with different porosities. The volume expansion can be accommodated by the porous structure of NP-Si.

charge/discharge processes.11 Thus, the NP-Si anode illustrates considerably improved electrochemical properties with excellent rate capacity and cycle stability.

CONCLUSION In conclusion, nanoporous silicon is successfully synthesized via a green vacuum distillation method from commercial Mg2Si alloys. The nanoporous silicon is formed by the evaporation of low boiling point Mg. The pore sizes of nanoporous silicon could be secured by adjusting the distillated temperature and time. The optimized nanoporous silicon (800 °C, 0.5 h) delivered a discharge capacity of 2034 mA h g−1 after 100 cycles at 200 mA g−1, a cycling stability with more than 1180 mA h g−1 at 1000 mA g−1 even after 400 cycles, and a rate capability of 855 mA h g−1 at the rate of 5000 mA g−1. The electrochemical properties might be acribed to its porous structure, which could accommodate the large volume change during the cycling process. These results suggest that the green, scalable, and controllable approach may offer a pathway for the commercialization of high-performance Si anodes. This method may also be extended to construct other nanoporous materials. EXPERIMENTAL SECTION Material Synthesis. A commercial Mg2Si alloy (99.5%) was first purchased from J&K Chemical and used without further purification. 4999

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static programmable battery charger between 0.01 and 3 V (vs Li+/Li) at different current densities. Electrochemical impedance spectroscopy was performed on an electrochemical workstation (CHI 660E, Shanghai China) with a frequency of 100 kHz to 0.01 Hz.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b02219. Detailed experimental methods, additional XRD, Raman, SEM, EDS, BET and electrochemical performance (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Yongling An: 0000-0002-2666-3051 Lijie Ci: 0000-0002-1759-105X Shenglin Xiong: 0000-0002-8324-4160 Jinkui Feng: 0000-0002-5683-849X Author Contributions Figure 7. (a−d) SEM images. The NC-Si anode (a) before and (b) after 100 cycles. The NP-Si anode (c) before and (d) after 100 cycles. EIS spectra of (e) NC-Si and (f) NP-Si anodes before cycle and after the 10th cycle, 20th cycle, 50th cycle, and 100th cycle.

J.F. conceived the work. Y.A., H.F., and G.Z. performed the synthesis, characterization, and electrochemical performance tests. Y.A. wrote the manuscript. J.F., L.C., S.X., and Y.Q. polished the manuscript. Notes

The commercial silicon powder was purchased from the Aladdin Company, and its SEM was tested (Figure S10). Then the vacuum distillation experiments were performed at 600, 700, 800, and 900 °C in dynamic high vacuum conditions for 0.5, 2, and 4 h. The vacuum environment was kept at 10 Pa or lower during the experiment. Finally, the nanoporous silicon was obtained. The experiment was handled using a tubular furnace (OTF-1200X-S-II, MTI) in this work. Characterization Methods. The crystallographic phases of the assynthesized NP-Si were checked by power X-ray diffraction on a Rigaku Dmaxrc diffractometer at a scanning speed of 10 °C min−1 from 10 to 90 °C using Cu Kα radiation (V = 50 kV, I = 100 mA). The morphology of the as-prepared NP-Si was characterized by field emission scanning electron microscopy (HITACHI SU-70) and highresolution transmission electron microscopy (JEOL JEM-2100). The structure and composition were tested using a JY HR800 spectrometer with a laser spot size of about 1 μm for the Raman spectra. The porous property based on the nitrogen adsorption−desorption isotherms was obtained by BET theory (ASAP 2020). Electrochemical Measurements. The as-synthesized active materials were mixed with Super P carbon and polyacrylic acid binder in the weight ratio of 6:2:2 to form a homogeneous slurry, which was coated on a copper foil as a current collector and then dried at 100 °C under vacuum conditions for 12 h to form the electrodes. The electrode of nanocrystalline silicon is also fabricated by the above method. The mass loading of the active material on the NP-Si and NC-Si is about 1.2−1.5 mg/cm2. To measure the electrochemical performance, the 2016-coin cells were assembled using the NP-Si anode as the working electrode, the Li foil as the reference electrode and counter electrode, a Celgard 2400 as the separator, and 1 M lithiumhexafluorophosphate (LiPF6) in cosolvent of ethylene carbonate/diethyl carbonate (EC/DEC, 1:1, v/v) as the electrolyte. To standardize the measurement, a fixed amount (about 100 μL) of electrolyte was used in each coin battery. All the cells were installed in a glovebox with water and oxygen content lower than 1 ppm and measured at room temperature. Cyclic voltammetry measurements were executed using a CHI 660E electrochemical workstation at a scanning rate of 0.2 mV s−1 between 0.01 and 3 V (vs Li+/Li). Galvanostatic discharge/charge cycles were measured on a galvano-

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Shandong Provincial Natural Science Foundation (China, ZR2017MB001), Independent Innovation Foundation of Shandong University, The Young Scholars Program of Shandong University (2016WLJH03), The State Key Program of National Natural Science of China (Nos. 61633015, 51532005), 1000 Talent Plan program (No. 31270086963030), The National Natural Science Foundation of China (No. 21371108), and the Project of the Taishan Scholar (No. ts201511004). REFERENCES (1) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for Li−Ion Batteries. Chem. Rev. 2014, 114, 11444−11502. (2) Ogata, K.; Salager, E.; Kerr, C. J.; Fraser, A. E.; Ducati, C.; Morris, A. J.; Hofmann, S.; Grey, C. P. Revealing Lithium-Silicide Phase Transformations in Nano-Structured Silicon-Based Lithium Ion Batteries via in Situ NMR Spectroscopy. Nat. Commun. 2014, 5, 3217. (3) Yin, S.; Zhao, D.; Ji, Q.; Xia, Y.; Xia, S.; Wang, X.; Wang, M.; Ban, J.; Zhang, Yi.; Metwalli, E.; Wang, X.; Xiao, Y.; Zuo, X.; Xie, S.; Fang, K.; Liang, S.; Zheng, L.; Qiu, B.; Yang, Z.; Lin, Y.; Chen, L.; Wang, C.; Liu, Z.; Zhu, J.; Müller-Buschbaum, P.; Cheng, Y.-J. Si/Ag/C Nanohybrids with in Situ Incorporation of Super-Small Silver Nanoparticles: Tiny Amount, Huge Impact. ACS Nano 2018, 12, 861−875. (4) Gao, S.; Su, J.; Wei, X.; Wang, M.; Tian, M.; Jiang, T.; Wang, Z. Self-Powered Electrochemical Oxidation of 4-Aminoazobenzene Driven by a Triboelectric Nanogeneratortor. ACS Nano 2017, 11, 770−778. (5) Liu, S.; Xia, X.; Zhong, Y.; Deng, S.; Yao, Z.; Zhang, L.; Cheng, X.; Wang, X.; Zhang, Q.; Tu, J. 3D TiC/C Core/Shell Nanowire Skeleton for Dendrite-Free and Long-Life Lithium Metal Anode. Adv. Energy Mater. 2018, 8, 1702322. 5000

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ACS Nano

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DOI: 10.1021/acsnano.8b02219 ACS Nano 2018, 12, 4993−5002

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DOI: 10.1021/acsnano.8b02219 ACS Nano 2018, 12, 4993−5002