Morphology- and Porosity-Tunable Synthesis of 3D Nanoporous SiGe

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Morphology- and Porosity-Tunable Synthesis of 3D-Nanoporous SiGe Alloy as High-Performance Lithium-Ion Battery Anode Yinghui Yang, Shuai Liu, Xiu-Fang Bian, Jinkui Feng, Yongling An, and Chao Yuan ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00426 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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Morphology- and Porosity-Tunable Synthesis of 3D-Nanoporous SiGe Alloy as High-Performance Lithium-Ion Battery Anode

Yinghui Yang, Shuai Liu, Xiufang Bian∗, Jinkui Feng∗, Yongling An, Chao Yuan

Key Laboratory for Liquid-Solid Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, China

∗

Corresponding authors:

E-mail addresses: [email protected] (X. Bian), [email protected] (J. Feng)

ABSTRACT The lithium-storage performance of silicon (Si) can be enhanced by alloyed with germanium (Ge) because of its good electronic and ionic conductivity. Here we synthesized three-dimensional nanoporous (3D-NP) SiGe alloy as high performance lithium-ion battery (LIB) anode using a dealloying method with ternary AlSiGe ribbon served as precursor. The morphology and porosity of as-synthesized SiGe alloy can be controlled effectively by adjusting the sacrificial Al content of the precursor. With a Al content of 80%, the 3D-NP SiGe presents uniformly coral-like structure with continuous ligaments and hierarchical micropores and mesopores, which leads to a high reversible capacity of 1158 mA h g-1 after 150 cycles at a current density of

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1000 mA g-1 with excellent rate capacity. The strategy might provide guidelines for nanostructures optimization and mass production of energy storage materials.

KEYWORDS: chemical dealloying, Li-ion battery, anode, SiGe alloy, silicon, germanium

Lithium-ion battery (LIB) has captured the market of portable electronic over the past few decades.1,2 In modern society, LIBs are gaining popularity in the fields of electric vehicles, hybrid-electric vehicles and renewable sources storage systems.3-7 To cope with the ever-growing demands of high-efficient and large-scale energy storage systems, low-cost electrode materials with unique lithium storage performance and manufacturing processes for mass production are urgently needed.8-10 Group IV elements (Si, Ge, Sn, Pb) are considered as appealing alternatives to commercially graphite as LIB anode because of their high energy capacity.2 Among them, Si is in focus by virtue of highest known theoretical capacity (3579 mA h g-1 for Li15Si4)11,12 with a low working potential,13-15 environmental friendliness, resource abundance and low cost.2,16 However, the rate capacity and cycling performance of Si anode are limited by the intrinsic low electron conductivity and poor Li+ diffusivity.17,18 In addition, Ge has also stood out due to not merely the high capacity (1384 mA h g-1 for Li15Ge4) but also its better rate capacity and cycleability,19,20 which is dependent on its favorable electronic conductivity (100 times higher than Si) and


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rapid Li+ mobility (400-fold faster than in Si).21 Nevertheless, high price and low abundance obstruct the commercialization of Ge.19 As an essential semiconductor material,19,21 SiGe alloy has recently been reported to perform well as LIBs anode.22-31 The optimized performance is a synergistic result from the combined contributions: (i) The integration of Ge in Si can accelerate the kinetics of lithium transport and boost the rate capability.23,24 (ii) Si and Ge react with Li at different onset potentials, so Si and Ge don’t swell simultaneously and then the strain-stress can be released gradually.27,28 (iii) When Li insert into one component, the other will alleviate the volume change as a buffer.32 Despite these advantages, there are still issues to be addressed. Both Si and Ge suffer from drastic volume change during the lithiation/delithiation process,21 which leads to anode pulverization, failure and eventual capacity fading.23,27 Designing complex nanoarchitecture with additional free space which can accommodate the volume expansion has been a well-adopted strategy to remain structural integrity and robust cycling of the electrode.33-38 It has been reported that SiGe alloys with nanoporous structure (such as porous nanorod,22 three dimensional nanoporous arrays,24 hollow particles,26 nanotube26,30 and nanowire arrays29) show favorable cycleability. However, the porosity is usually obtained by complex multistep template-assisted methods with poor productivity,22-24,29,30 which hampers the large-scale production of nanoporous SiGe alloys. Wang and co-workers reported a one-pot electrolytic process to synthesize Si/Ge alloy nanotubes and hollow particles,24 yet the low production efficiency and high electricity consumption retard


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its industrial application. In recent years, dealloying has been regarded as a facile, low-cost and practical method to fabricate three-dimensional nanoporous (3D-NP) materials for use in batteries, capacitors, catalysis and so on.39,40 As the dealloying proceeded, the less-noble component is selectively removed from the alloy,41 meanwhile the noble component re-organizes into a bicontinuous ligament-and-pores structure with a high specific surface area.42-44 Herein, we synthesized morphology- and porosity-tunable 3D-NP SiGe alloy by dealloying ternary AlSiGe ribbon. Their electrochemical performance as LIB anode was measured. With an appropriate Al content (80%) in precursors, SiGe alloy shows uniformly coral-like structure with hierarchical microand meso-pores, leading to good cycling performance and rate capability. The strategy provides a sample structure-predictable approach for mass production of high-performance LIB electrodes and other energy storage materials. RESULTS AND DISCUSSION Structure Evolution of SiGe Alloy In previous study, it was reported that Si0.6Ge0.4 alloys show balanced electrochemical performance in both cycle stability and rate capability.22 In addition, Al can dissolve in

hydrochloric

acid

(HCl)

while

Si

and

Ge

cannot,

so

we

select

Al100-x-Si0.6x-Ge0.4x-HCl system (x = 10, 15, 20, 25, 30) to obtain SiGe alloys, in which Al acts as sacrificial element, Si and Ge as ligament-forming elements and HCl as dealloying medium. With a ultra high cooling rate of 104 to 108 K s-1, the melt spinning is the most commonly used technique to get rapid solidification.45,46 The


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caused large supercooling can lead to constitutional and potentially beneficial changes in solid phase, such as the extension in equilibrium phase solubilities, the refinement of the solid microstructure, the refinement of the segregation scale and the formation of metastable crystalline and amorphous phase.40,47 The melt spinning technique was used to prepare the AlSiGe ribbon precursors. During the process of dealloying, Al elements are selectively dissolved from the precursors by HCl while Si and Ge re-organize into 3D-NP structures. For convenience, the sample obtained from Al100-xSi0.6xGe0.4x precursor was marked as Si0.6xGe0.4x (x = 10, 15, 20, 25, 30). The structural evolution of AlSiGe alloys during the dealloying process is outlined in Figure 1a. The SEM images of the samples, take Al80Si12Ge8 as an example, with different duration (i.e., 0.5 h, 2 h, 5 h and 10 h) are demonstrated in Figure 1b-e. Al, Si and Ge atoms are mixed uniformly in the Al80Si12Ge8 ribbon precursors, as shown in Figure 1a-i and Figure S1. When the precursors are immersed in 5 wt% HCl aqueous, Al atoms dissolve into the acid incessantly, while Si and Ge atoms left in the solid cohere into Si-Ge islands at the solid/liquid interface (Figure 1a-ii). Figure 1b displays the surface morphology of the ribbon after etched for 0.5 h. It shows that several holes form on the surface of the ribbon. Over time, the holes growing bigger and deeper (Figure 1a-iii), and the Si-Ge islands develop into gully-like (Figure 1c) and canyon-like (Figure 1d). After 10 h, almost all Al elements are removed away from the precursor, leaving the Si-Ge alloys with a coral-like 3D-NP structure (Figure 1a-iv and Figure 1e). The XRD patterns in Figure 2a display the evolutionary crystallographic phases


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of Al80Si12Ge8 ribbon with dealloying. According to the results, the peaks of Al phases almost disappear after 0.5 h. Then the peak intensity of Al4Si and Al21Ge8 falls and that of Si and Ge increases over time. It turns out that when the precursor is immersed in the HCl aqueous, Al phases are firstly etched, and then Al atoms in intermetallics (Al4Si and Al21Ge8) dissolve gradually until only Si and Ge are left. All the characteristic peaks of Si12Ge8 are located between those of Ge and Si, revealing the successful transformation of AlSiGe ribbon into the crystalline SiGe alloy.26 In the ex-situ Raman spectrum (Figure 2b) of Si12Ge8, the three peaks near 290 cm-1, 390 cm-1 and 490 cm-1 are corresponding to the region of Ge-Ge, Si-Ge and Si-Si bonds, respectively,31,41 which proves that Si and Ge coexist in 3D-NP SiGe particles. Figure 2c shows an SEM image of Si12Ge8 corresponded with EDS mappings of element Si (Figure 2d) and element Ge (Figure 2e). The results indicate uniform distribution of Si and Ge in SiGe alloys on a large scale. Figure 2f-g show the Si 2p and Ge 3d XPS spectra and their fitting patterns of Si12Ge8 after two-month of exposure in air. The results show that only small part of the particles are oxidized into Si+4 and Ge+2, Ge+3, Ge+4.48,49 The antioxidant activity may facilitate the mass production and storage of dealloying nanoporous Si-Ge alloys. Controlled Morphology and Porosity of 3D-NP SiGe Alloy We compared the constituents, microstructures and porosities of Si0.6xGe0.4x with different x value. According to the XRD and EDS patterns (Figure S2), Al concentration in precursor rarely affects the element and phase components. However, the morphology and porosity of Si-Ge alloy can be controlled by tuning the sacrificial


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Al content in precursor. The nitrogen adsorption-desorption isotherms and corresponding pore size distribution curves obtained by the density functional theory (DFT) method for Si0.6xGe0.4x (x = 10, 15, 20, 25, 30) alloys are shown in Figure 3. As shown in Figure 3a, all the five samples display type IV adsorption isotherms with an H3-type hysteresis loop (based on IUPAC classification) which indicates the existence of mesopores.50,51 Meanwhile the adsorption at relative pressure lower than 0.1 suggests that there is also a certain amount of micropores in the SiGe alloys.52 In addition, with increasing Al contents in precursors, the adsorption amounts of Si0.6xGe0.4x alloy increase at first and then turn to decreasing after reaching a peak value when x = 25, which illustrates that Si15Ge10 has the biggest porosity among the five samples and that of Si12Ge8 takes the second place. Based on the N2 adsorption profile in the relative pressure range of 0.05-0.35,43 the total specific surface areas of the samples are assayed with Brunauer-Emmett-Teller (BET) method, as shown in Table S1. Si15Ge10 has a big specific surface areas of ~114 m2 g-1 and that of Si12Ge8 is ~99 m2 g-1. The big specific surface area can facilitate the electrode/electrolyte contacts during the electrochemical process. The DFT method is applied to analyze the consecutive micro- and mesopores size distribution.53 Figure 3b-f show the morphology (the insets) and the corresponding pore size distribution curves of Si18Ge12, Si15Ge10, Si12Ge8, Si9Ge6 and Si6Ge4, respectively. It is proved that micropores and mesopores coexist in the samples, but the pores distribute differently in different samples due to the microstructure changes.


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As shown in the SEM images of Si18Ge12 (the insets of Figure 3b), thick dendrite arms with thickness of hundreds of nanometers grow from the chunky trunk with gaps of uneven sizes between the arms. According to the pore size distribution, the porous sizes are mainly distributed in ranges of smaller than ~2 nm and bigger than ~8 nm. As the Al content in precursor increases from 70% to 75%, the blocky microstructure disappears and some tiny coralloid architectures form at the outer layers (Figure 3c), which may contribute to the big specific surface areas. However, there are still coarse dendrites internally (the small inset). According to the pore size distribution in Figure 3c, the number of micropores in Si15Ge10 decreases while that of mesopores increases dramatically, especially those at size ranging from 2.5-8 nm. Si12Ge8 presents uniform coral-like morphology (Figure 3d), with the nanopores, including micropores and mesopores, continuously distribute at size scale bigger than ~0.5 nm. However, the integrated coral-like architecture will be broken and smashed if we keep adding Al into the precursor alloys, as shown in Figure 3e (Si9Ge6) and Figure 3f (Si6Ge4). This might result from either the high dispersion degree of Si and Ge in the precursors or the low strength of the dendrites caused by small size. The fragments may pad the interspace and clog the channels, leading to a small porosity. As shown in Figure 3e, when the Al content of precursor is increased to 85%, the number of pores reduces markedly. For Si6Ge4 (Figure 3f), there exist merely few micropores and no mesopores. As stated above, Si12Ge8 alloys possess uniformly coral-like structure with hierarchical micro- and mesopores. Microstructure Characterization for Si12Ge8


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The morphology and crystal structure of Si12Ge8 were further characterized by SEM, TEM, STEM-XEDS and HRTEM. Figure 4a displays the feature on continuous ligaments of Si12Ge8, which illustrates that the thickness of the dendrites is ~50 nm and there are openings and channels between the dendrites. The thin dendrites may facilitate the Li+ ion diffusion into the alloy. Meanwhile, these openings and channels offer a buffer of volume change for the dendrites upon repeated lithiation/delithiation. TEM images (Figure 4b and the insert) show that there are many irregularly scattered dumps on the surface of ligaments which might be related to the growth mechanism of dealloyed SiGe alloy. The elements distribution in the dendrites on a minor scale was analyzed by STEM-XEDS which can be more sensitive than SEM-EDS, as shown in Figure 4c. The results show that Si and Ge distribute very evenly in the ligaments, which provides evidence for the generation of cubic crystalline SiGe alloy as proved by XRD analyses (Figure 2a). Figure 4d-e show the HRTEM images for different parts of Si12Ge8. Figure 4d indicates that there are trace amounts of crystalline Si and Ge phase segregations in the edge of the dendrites. The crystalline Si and Ge exhibit lattice spacing of 0.314 nm and 0.326 nm, respectively, correlating well with (111) planes of cubic Si and Ge. In addition, the local ambiguous lattice presents the low crystallinity of the dealloyed SiGe alloys. Figure 4e shows that the lattice spacing of (111) planes for cubic crystalline SiGe is ~0.32 nm, which agree well with previous study.26 The selected area electron diffraction (SAED) pattern of SiGe alloy shows ring features that can be indexed as different crystal planes of (111), (220), (311), (400), (331), (422), and (333), which is consistent with the result of


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XRD analyses (Figure 2a). Lithium Storage Performance Characterization The lithium storage performance of Si0.6xGe0.4x (x = 10, 15, 20, 25, 30) and Si20 are characterized by cyclic voltammetry (CV) and galvanostatic charge-discharge tests. Figure 5a shows the initial three cycles CV curves for Si12Ge8 in the potential window of 0.01-2.5 V (vs. Li+/Li) at a scan rate of 0.1 mV s-1. In the first cathodic scan, the broad shoulder located between 1.8 to 0.9 V, which disappears in the following cycles, can be related to the irreversible formation of solid electrolyte interface (SEI). The sharp cathodic peaks begin at around 0.4 V can be associated with the reversible Li-alloying reactions of Ge and Si.22 In the anodic scans, the peaks at 0.18 V and the sharp peaks at around 0.57 V can be ascribed to the delithiation reactions of LixSi33 and LixGe,26,27 respectively. Fig 5b exhibits the 1st, 2nd, 10th, 20th and 50th charge-discharge voltage profiles of Si12Ge8 anode between 0.01-1.0 V at a current density of 100 mA g-1. The results correlate well with the cyclic voltammetry test. The potential plateaus located between 0.4-0.2 V and below 0.2 V may correspond to the alloying reactions of Ge and Si components with lithium, respectively.27,30 In the first cycle, the discharge (lithium insertion) capacity and charge (lithium removal) capacity are 2582 mA h g-1 and 1951 mA h g-1, respectively, holding an initial Coulombic efficiency of 75.6%. The irreversible capacity loss is mainly ascribed to the decomposition of electrolyte and the irreversible formation of the solid/electrolyte interphase (SEI) on the surface of electrode.29 For comparison, the discharge capacity versus cycle numbers for Si and SiGe


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electrodes at a constant current density of 100 mA g-1 between 0.01-1V are displayed in Figure 5c. As shown, the retention capacity of Si18Ge12, Si15Ge10, Si9Ge6 and Si6Ge4 are 157 mA h g-1, 1167 mA h g-1, 1032 mA h g-1 and 999 mA h g-1, respectively. Despite the big specific surface areas, the partial coarse dendrites weaken the cycleability of Si15Ge10. Moreover, Si12Ge8 anode still delivers a high capacity of 1372 mA h g-1 at 80th cycle owing to the uniform coral-like morphology with favorable porosity. However, the microstructure is not the only decisive factor in improving the cycle performance of Si12Ge8. The specific capacity of Si20 anode experiences a rapid fading within the initial 10 cycles, although Si20 presents honeycomb-like morphology (Figure S3) that is similar to Si12Ge8 due to the same sacrificial Al content in precursors. At a big current density of 1000 mA g-1, Si12Ge8 anode still maintains a reversible capacity of 1158 mA h g-1 after 150 cycles, exhibiting a reversibility far better than Si anode (as shown in Figure 5d), which indicates that benefiting from the favorable electronic conductivity and rapid Li+ mobility of Ge,21 alloying Si with Ge can significantly improve the cycling performance of Si electrodes. In addition, The Si12Ge8 anode shows a notably high rate capacity besides the high capacity and cycling stability. The rate performance of Si12Ge8 anode was measured by increasing the current density from 1000 to 8000 mA g-1, as shown in Figure 5e. The Si12Ge8 electrode can deliver capacities of 1558, 1281, 952 and 577 mA h g-1, at the current density of 1000, 2000, 4000 and 8000 mA g-1, respectively. When the rate return back to 1000 mA g-1 after 20 cycles, a reversible capacity of


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1582 mA h g-1 can be achieved, indicating the good rate performance for Si12Ge8. To further understand the optimization mechanism of Ge component in SiGe electrodes, the comparison of appearance between Si12Ge8 and Si20 electrodes after 50 cycles is displayed in Figure S4. The results confirm that Si20 electrode suffers from much more severe crack and pulverization (Figure S4b) than Si12Ge8 electrode (Figure S4a). Furthermore, the electrochemical impedance spectroscopy (EIS) studies were performed on Si12Ge8 and Si20 electrodes after 5 cycles in a full-charged state. As shown in Figure 5f, both spectra display a depressed semicircle in the high-to-medium frequency range and a straight line in the low frequency range. The high frequency semicircle is corresponding to the charge-transfer resistance (Rct) and the double-layer capacitance at the electrode/electrolyte interface. The inclined straight line is associated with the rate of lithium ion diffusion process.43 According to the spectra, the Rct of Si12Ge8 electrode is about 615 Ω and that of Si20 electrode is about 1670 Ω, which indicates that Ge can accelerate the electron transfer and kinetics of Li+ transport of the electrode significantly. Therefore, the Si12Ge8 anode exhibits dramatically enhanced electrochemical performance with outstanding cycleability and rate capability. So in summary, the prominent cycling stability of Si12Ge8 anode can be attributed to the joint effect of Ge addition and the coral-like structure with uniformly continuous ligaments and hierarchical micro- and meso-pores. A schematic illustration of lithiation process for Si particle anode and 3D-NP SiGe anode is shown in Figure 6. Owing to the favorable electronic conductivity and rapid Li+ mobility, Ge can


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significantly accelerate the electron and lithium transport kinetics of the electrode and then boost the cycle and rate performance. Besides, the nanopores can efficiently accommodate the volume change and also accelerate the kinetics of Li+ transport by offering more electrode/electrolyte contacts during the lithiation/delithiation process. CONCLUTION In summary, by adjusting the sacrificial Al content in precursor, the morphology and porosity of as-synthesized SiGe alloy can be controlled effectively. With 80% Al content in precursor, Si12Ge8 presents coral-like structure with uniformly continuous ligaments and hierarchical micropores and mesopores that can efficiently accommodate the volume change during lithiation/delithiation process. Benefiting from the joint effect of the coral-like architecture and Ge addition in Si which can accelerate the electron transfer and Li+ transport kinetics, the Si12Ge8 electrode delivers a high reversible capacity of 1158 mA h g-1 after 150 cycles at a current density of 1000 mA g-1 with favorable rate capacity. The strategy provides guidelines for mass production of battery electrodes and other energy storage materials with predictable and controllable nanoarchitecture. EXPERIMENTAL Synthesis of 3D-NP SiGe Alloy The AlSiGe alloys were prepared by melting a atomistic mixture of pure Al (99.99%), pure Si (99.99%) and pure Ge (99.99%) in a quartz crucible with high-frequency induction furnace (1023 K for 8 min) and then casting into ingots in an iron chill mold. After that, the AlSiGe ribbons were prepared by a single roller melt spinning


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apparatus (Model number: SP009A, as shown in Figure S5). The AlSiGe ingots were first remelted in a inert-gas-protected quartz tube by high-frequency induction, and then the melts were spun onto a copper roller at a circumferential speed of about 27 m s-1 and rapidly solidified into ribbons. The AlSiGe ribbons were typically 20-50 µm thick, 2-4 mm wide and several decimeters long. Then the AlSiGe ribbons were dealloyed in a 5 wt% HCl aqueous solution. After the materials steadily reacted, the solution was heated in water bath at 333 K with constant magnetic stirring for 24 h until no bubbles up. The SiGe particles were then centrifuged and washed with deionized water to neutrality and dried in vacuum for 12 h at 353 K. Materials Characterization The as-prepared samples were characterized by X-ray diffraction (XRD, Rigaku Dmax-rc diffractometer) to identify the crystal phases. Roman spectra were recorded on a Horiba Jobin-YVON co-focal laser Raman system with He-Ne 632 nm laser as the excitation source. The microstructures of the samples were characterized by scanning emission microscope (SEM, Zeiss SUPRA 55), transmission electron microscope (TEM, JEOL JEM-1011) and high resolution TEM (HRTEM, JEOL JEM-2100). Element was studied on X-ray photo-electron spectroscopy (XPS, ESCALAB 250). The texture properties were analyzed by measuring nitrogen adsorption-desorption isotherms (Micromeritics Automatic Surface Area Analyzer Gemini 2360, Shimadzu) at 77 K. Electrochemical Characterization


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Electrochemical performance was measured in 2016 coin-type half-cells. The working electrode composing of active materials (70 wt%), Super P carbon (20 wt%) and carboxymethyl cellulose (CMC) binder (10 wt%) was loaded on a Cu-foil current collector, which were then dried in vacuum at 353 K for 12 h. The half coin cells were assembled in a glove box full of high-purity argon with Li sheet as counter electrode, Celgard 2400 as separator, and 1M LiPF6 with 1:1:1 (by volume) ethylene carbonate: dimethyl carbonate: diethyl carbonate as electrolyte. Galvanostatic discharge/charge cycles were carried out on a channels battery cycler (Neware CT-4008, Shenzhen China) between 0.01-1 V. The rate capacity of the active materials was measured after activated at a current density of 100 mA g-1. The electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI 660E (Shanghai China) in a frequency range of 0.01Hz to 100 KHz. Cyclic voltammetry (CV) was performed in the voltage range of 0.01 V to 2.5 V (versus Li/Li+) at a sweep rate of 0.1 mV s-1.

FIGURE CAPTIONS Figure 1. (a) Schematic of the evolution of 3D-NP SiGe structure via chemical dealloying. (i) Initially, AlSiGe ribbon has an atomically homogeneous structure. (ii) As the ribbon is immersed in 5 wt% HCl aqueous, only Al atoms dissolves into the solution, while the Si/Ge atoms left at the solid/liquid interface cohere into SiGe islands. (iii) More and more Al atoms dissolve into the HCl solution. (iv) Al is etched away to form the 3D-NP SiGe. (b-e) SEM images of Al80Si12Ge8 ribbon immersed in the HCl solution for 0.5 h, 2 h, 5 h and 10 h, respectively.


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Figure 2 (a) The XRD patterns of Al80Si12Ge8 ribbons and the samples after etched for 0.5 h, 2 h, 5 h and 10 h. (b) The ex-situ Raman spectrum of Si12Ge8. (c) The SEM image of 3D-NP Si12Ge8. (d-e) The EDS mappings of elements Ge and Si for the zone in (c), respectively. The scales in the images are 500 nm. (f-g) The Si 2p and Ge 3d XPS spectra (the red solid lines) and the fitting patterns of Si12Ge8 after two-month of exposure in air.

Figure 3 (a) The nitrogen adsorption-desorption isotherms of Si0.6xGe0.4x (x = 10, 15, 20, 25, 30). (b-f) The pore size distribution curves obtained by DFT method and the SEM images (the insets) of Si18Ge12, Si15Ge10, Si12Ge8, Si9Ge6 and Si6Ge4, respectively. The scales in the small insets of (b) and (c) are 500 nm.

Figure 4 (a) The SEM image at high magnification of Si12Ge8. (b) The TEM images of Si12Ge8. (c) The element distribution mapping images for Si, Ge, both Si and Ge in Si12Ge8 by STEM-XEDS. (d-e) The HRTEM images of Si12Ge8. (f) The SAED pattern of Si12Ge8.

Figure 5 Electrochemical characterization of the 3D-NP SiGe alloys. (a) The cyclic voltammetry curves of Si12Ge8 at 0.1 mV s-1 within 0.01-2.5 V versus Li/Li+. (b) The voltage profiles of Si12Ge8 between 0.01-1.0 V at 100 mA g-1. (c) Cycle performance of the 3D-NP SiGe anodes and Si20 anode at 100 mA g-1 from 0.1 V to 1 V. (d)


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Comparative cycle performance of Si12Ge8 anode and Si20 anode at 1000 mA g-1 from 0.1 V to 1 V. (e) Rate capability of Si12Ge8 anode at different current densities from 1000 to 8000 mA g-1. (f) Nyquist plots of Si12Ge8 anode and Si20 anode after 5 cycles.

Figure 6 Schematic illustration of lithiation for Si particle and 3D-NP SiGe alloy. SiGe alloy has batter electronic conductivity and faster Li+ mobility than Si. The nanopores can accommodate the volume change during the conversion reaction. Big special surface area offers more electrode/electrolyte contacts.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51571130) and the Young Scholars Program of Shandong University (No. 2016WLJH03).

ASSOCIATED CONTENT The authors declare no competing financial interest. Supporting Information Available: The elements distribution in precursor. The XRD patterns and EDS spectra of Si18Ge12, Si15Ge10, Si12Ge8, Si9Ge6 and Si6Ge4. The morphology of Si20. The SEM images of Si12Ge8 electrode and Si20 electrode after 50 cycles. The specific surface areas of Si18Ge12, Si15Ge10, Si12Ge8, Si9Ge6 and Si6Ge4 assayed with BET method. The schematic diagram of the single roller melt spinning apparatus.


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AUTHOR INFORMATION Author Contribution: Y. Yang, S. Liu, X. Bian and J. Feng conceived and designed the experiments. Y. Yang and C. Yuan fabricated the samples. Y. Yang conducted the characterization of SEM, TEM, XRD and electrochemical performance tests. Y. An conducted the characterization of Raman, XPS and BET. All authors commented on the final manuscript. ORCID: Yinghui Yang: 0000-0002-0849-7096; Xiufang Bian: 0000-0001-6680-3891; Jinkui Feng: 0000-0002-5683-849X

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Table of Contents figure


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Figure 1. (a) Schematic of the evolution of 3D-NP SiGe structure via chemical dealloying. (i) Initially, AlSiGe ribbon has an atomically homogeneous structure. (ii) As the ribbon is immersed in 5 wt% HCl aqueous, only Al atoms dissolves into the solution, while the Si/Ge atoms left at the solid/liquid interface cohere into SiGe islands. (iii) More and more Al atoms dissolve into the HCl solution. (iv) Al is etched away to form the 3D-NP SiGe. (b-e) SEM images of Al80Si12Ge8 ribbon immersed in the HCl solution for 0.5 h, 2 h, 5 h and 10 h, respectively. 80x34mm (300 x 300 DPI)


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Figure 2 (a) The XRD patterns of Al80Si12Ge8 ribbons and the samples after etched for 0.5 h, 2 h, 5 h and 10 h. (b) The ex-situ Raman spectrum of Si12Ge8. (c) The SEM image of 3D-NP Si12Ge8. (d-e) The EDS mappings of elements Ge and Si for the zone in (c), respectively. The scales in the images are 500 nm. (f-g) The Si 2p and Ge 3d XPS spectra (the red solid lines) and the fitting patterns of Si12Ge8 after two-month of exposure in air. 89x56mm (300 x 300 DPI)


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Figure 3 (a) The nitrogen adsorption-desorption isotherms of Si0.6xGe0.4x (x = 10, 15, 20, 25, 30). (b-f) The pore size distribution curves obtained by DFT method and the SEM images (the insets) of Si18Ge12, Si15Ge10, Si12Ge8, Si9Ge6 and Si6Ge4, respectively. The scales in the small insets of (b) and (c) are 500 nm. 114x146mm (600 x 600 DPI)


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Figure 4 (a) The SEM image at high magnification of Si12Ge8. (b) The TEM images of Si12Ge8. (c) The element distribution mapping images for Si, Ge, both Si and Ge in Si12Ge8 by STEM-XEDS. (d-e) The HRTEM images of Si12Ge8. (f) The SAED pattern of Si12Ge8. 90x97mm (300 x 300 DPI)


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Figure 5 Electrochemical characterization of the 3D-NP SiGe alloys. (a) The cyclic voltammetry curves of Si12Ge8 at 0.1 mV s-1 within 0.01-2.5 V versus Li/Li+. (b) The voltage profiles of Si12Ge8 between 0.01-1.0 V at 100 mA g-1. (c) Cycle performance of the 3D-NP SiGe anodes and Si20 anode at 100 mA g-1 from 0.1 V to 1 V. (d) Comparative cycle performance of Si12Ge8 anode and Si20 anode at 1000 mA g-1 from 0.1 V to 1 V. (e) Rate capability of Si12Ge8 anode at different current densities from 1000 to 8000 mA g-1. (f) Nyquist plots of Si12Ge8 anode and Si20 anode after 5 cycles. 114x144mm (600 x 600 DPI)


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Figure 6 Schematic illustration of lithiation for Si particle and 3D-NP SiGe alloy. SiGe alloy has batter electronic conductivity and faster Li+ mobility than Si. The nanopores can accommodate the volume change during the conversion reaction. Big special surface area offers more electrode/electrolyte contacts. 68x52mm (600 x 600 DPI)


Morphology- and Porosity-Tunable Synthesis of 3D Nanoporous SiGe

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