SiOx@C

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Energy, Environmental, and Catalysis Applications

Toward Mechanically Stable Silicon-based Anodes using Si/SiOx@C Hierarchical Structures with Well-controlled Internal Buffer Voids Ran Wang, Jing Wang, Shi Chen, Chenglong Jiang, Wurigumula Bao, Yuefeng Su, Guoqiang Tan, and Feng Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16245 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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ACS Applied Materials & Interfaces

Toward Mechanically Stable Silicon-based Anodes using Si/SiOx@C Hierarchical Structures with Well-controlled Internal Buffer Voids Ran Wang1, Jing Wang1,2,3*, Shi Chen1,2,3, Chenglong Jiang4, Wurigumula Bao1, Yuefeng Su1,2,3, Guoqiang Tan1,2* and Feng Wu1,2,3 1 School of Materials Science & Engineering, Beijing Key Laboratory of Environmental Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China 2 National Development Center of High Technology Green Materials, Beijing, 100081, China 3 Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China 4 China Automotive Technology and Research Center Co., Ltd. Tianjin, 300300, China

Abstract: Low conductivity and structural degradation of silicon-based anodes lead to severe capacity fading, which fundamentally hinders their practical applications in Li-ion batteries. Here we report a scalable Si/SiOx@C anode architecture, which is constructed simultaneously by sintering a mixture of SiO/sucrose in an argon atmosphere, followed by acid etching. The obtained structure features highly uniform Si nanocrystals embedding in silica matrices with well-controlled internal nanovoids, and all of them embraced by carbon shells. Because of the improvement on the volumetric efficiency for accommodating Si active spices and electrical properties, this hierarchical anode design enables the promising electrochemical performance, including a high initial reversible capacity (1210 mAh g−1), stable cycling performance (90% capacity retention after 100 cycles), and good rate capability (850 mAh g−1 at 2.0 A g−1 rate). More notably, the compact hetero-structures derived from micro-SiO allow high active mass loading for practical applications, and the facile and scalable fabrication strategy makes this electrode material potentially viable for commercialization in Li-ion batteries.

Keywords: silicon monoxide, disproportionation, acid etching, nanovoid, Li-ion battery.

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Introduction Li-ion batteries (LIBs) with ever-increasing energy densities are becoming more and more critical owing to the rapid development of advanced devices and electric vehicles (EVs).1‒3 However, conventional anodes, primarily based on graphite with a theoretical capacity of 370 mAh g‒1, seem to be far from completely fulfilling this demand. Silicon-based anodes with higher theoretical capacity (Si: 3579 mAh g‒1; SiO: 2670 mAh g‒1 and SiO2: 1680 mAh g‒1) and low lithiation potential (< 0.5 V vs. Li/Li+) offer more available anode candidates for high energy density applications.4‒6 Among the silicon-based anodes, notably, pure Si delivers the highest gravimetric capacity and the most stable operating voltage, but it also suffers from the most serious volume expansion during lithiation. Although great efforts have been devoted to tackle this issue, practical usage of Si anodes has not yet been widely commercialized due to high cost and complex fabrication.7‒10 In contrast, SiO exhibits moderate gravimetric capacity and good cycle-life performance due to relatively small volume change.11,12 More importantly, it has extremely rich resources coupled with low cost, simple fabrication and environmentally benign. Therefore, it has been regarded as one of the most promising anode alternatives for commercialization in LIBs. In an attempt to promote the commercial production of SiO anodes, a primary approach has so far been to fabricate hybrid or composite anode architectures by integrating SiO with conducting agents, such as carbon materials, conductive polymers, and other oxides species, where both electrical conductivity and structural stability are enhanced.13‒16 In addition, a magnesiothermal reduction combined with etching method has been reported to improve the utilization of active materials.17,18 This fabrication technique is suitable for nano-Si used in LIBs, where very high purity of Si is not required. Tremendous progress has also been made towards this direction; however, these traditional strategies still appear far from the practical application due to limited cycling performance and/or complex manufacturing process.19,20 To address all above technical challenges, alternative solutions must be pursued for SiO anode architectures in a simple and scalable manner. Here, we propose a facile and efficient approach to construct capsule hetero-structures of active materials wrapped by conductive carbon layers; meanwhile the internal nanovoids are prior-reserved. As illustrated in Figure 1, Si/SiOx@C hierarchical structures were generated simultaneously via a one-step calcination 2


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process and controllable internal nanovoids were created by a subsequent acid etching. This specific hetero-structure bears several intrinsic advantages. First, due to the capsule structure with sufficient voids space, no volume expansion is expected for Si-based composites during cell operation. It renders the electrode structure stable during cycling, thus enhancing cycling life. Second, carbon frameworks function as electrical highways and mechanical backbones so that particles are electrochemically active. Third, carbon layers completely encapsulate the entire particle, limiting most solid-electrolyte interface (SEI) formation to the outer surface instead of on inside particles, thereby reducing the SEI. Last and most importantly, benefiting from its compact original structure and much improved conductivity, such a composite allows for high mass loading―one of the key factors for practical application of Si-based anodes. As a result, the materials exhibit remarkable electrochemical performance, including large initial charge capacity of 1210 mAh g−1, high capacity retention of 90% after 100 cycles, and good rate capability of 850 mAh g−1 at 2.0 A g−1. In this study, four kinds of Si/SiOx@C heterostructures were investigated for comparison. Hereafter, we refer to the primary Si/SiOx@C materials without acid etching as SSC-0, Si/SiOx@C materials with 10 min acid etching as SSC-10, Si/SiOx@C materials with 60 min acid etching as SSC-60, and the bare Si/SiOx materials with 10 min acid etching followed by carbon coating as BSSC-10, respectively.

Experimental Section Material fabrication As illustrated in Figure 1. Commercially available SiO powder (200 mesh, 99.99%, Aladdin) were ball-milled for 10 h as the baseline SiO materials. On the one route, the baseline SiO materials were stirring with sucrose solution for 3 h, the mixture was dried under vacuum at 150 oC for 12 h then sintered at 1100 oC for 5 h under argon atmosphere. During the sintering process, the amorphous SiO underwent an unusual disproportionation by forming Si- and SiO2-like regions,21 meanwhile, the encompassed sucrose was converted into carbon layers. The resulting products thus had a Si/SiOx@C structure, and we denoted it as SSC. SSC-0, SSC-10, and SSC-60 samples were finally obtained by the HF (10 wt%) etching for 0, 10, and 60 min, respectively. On the other route, the baseline SiO materials were firstly sintering at 1100 oC for 5 h at argon atmosphere and 3



followed by HF etching for 10min, the obtained materials were then stirring with sucrose solution for 3 h and dried under vacuum at 150 oC for 12 h, after that materials were re-heated at 1100 oC for 2 h under argon atmosphere. The final products were denoted as BSSC-10. In addition, two carbon-based materials were prepared as the references to identify the capacity contribution of carbon in above Si/SiOx@C composites. One was synthesized by sintering the pure sucrose at 1100 oC for 5 h under argon atmosphere; the other was obtained by etching a Si/SiOx@C material (20 wt% Si/SiOx and 80 wt% C) for 1 h then drying under vacuum. Material Characterization The crystalline structures were identified by X-ray diffraction (XRD, Rigaku Ultima IV) with Cu-Kα radiation. The micro-morphologies were examined by field-emission scanning electron microscope (FE-SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL 3000F). The chemical microstructures were elucidated by Raman spectroscopy (JY Labram HR800). The thermodynamic characteristics were evaluated by thermogravimetric analysis (TGA, Netzsch TG209F1). N2 sorption measurements were performed on a V-Sorb 2800P analyzer at 77 K. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method, and pore-size distribution was calculated from the adsorption branch of isotherms by the Barrett–Joyner–Halenda (BJH) analysis. Electrochemical measurements To evaluate electrochemical performance of Li-ion batteries, all of the baseline SiO, SSC-0, SSC-10, SSC-60, and BSSC-10 materials were used as active materials for cell tests. The electrodes were prepared by pasting a mixture of 85 wt% active materials and 15 wt% polyacrylonitrile (PAN, Mw=150,000 g mol−1) on copper foils and then heated at 500 oC under argon atmosphere for 12 h, as described in our previous work.14 The obtained electrodes were finally calendared into a thickness of ~ 25 μm with the mass loading of ~ 3.5 mg/cm‒2. Distinguished from the traditional slurry-coating, the cyclized-PAN in the electrodes could act as both the conduction network and binder, thus there was no necessary of additional conductive agents and binder. The electrical conductivities of the materials were measured by the 4-point probe method, as shown in Table S1. Electrochemical measurements were carried out using 2032 coin-type half-cells. The prepared electrode was used as the working electrode, Li foil was used as the counter electrode, and Celgard 2400 separator 4


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absorbing 60 µl solution (1 M LiPF6 in l L ethylene carbonate (EC) + ethyl methyl carbonate (EMC) + dimethyl carbonate (DMC) (1 : 1 : 1 in volume)) as the electrolyte. Cells were assembled in an argon-filled glove-box and then aged overnight before the electrochemical measurements. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured on a CHI 660D electrochemical workstation. Galvanostatic charge-discharge measurements were carried out using a Land CT2001A battery tester. The specific capacity of the electrodes was calculated based on the weight of the Si/SiOx@C active materials.

Results and Discussion Structural Properties Structural properties of the composites were characterized by XRD, Raman, TGA, and N2 sorption measurements, as shown in Figure 2. The XRD patterns (Figure 2A) reveal the crystalline structural characteristics of these materials. Notably, the baseline SiO sample shows a broad diffraction peak in the range of 18o–31o, indicating the primary amorphous structure.13 In addition, it displays several very small characteristic peaks of the quartz-phase SiO2 (PDF#46-1045) (26.6o, 68.1o) and cubic-phase Si (PDF#27-1402) (28.4o, 56.2o). This finding suggests that few of the Si and SiO2 nanocrystals have been generated by the mechanochemical disproportionation of SiO during ball-milling.14 After the high-temperature calcination, all of the SSC and BSSC samples exhibit several much stronger diffraction peaks. It is worth noting that the peaks of the cubic Si (PDF#27-1402) become stronger, but not for the quartz SiO2 (PDF#46-1045), besides, two new peaks of the cristobalite-phase SiO2 (PDF#39-1425) appear at 22.0o and 36.1o. This result indicates that the high-temperature calcination promotes the disproportionation of amorphous SiO in a new manner of SiO → Si (PDF#27-1402) + SiO2 (PDF#39-1425), but not in the manner of SiO → Si (PDF#27-1402) + SiO2 (PDF#46-1045) by ball-milling. Moreover, the average domain size of Si nanocrystals in the composites is estimated to be ~ 10 nm according to the Scherrer equation.22 Raman spectra (Figure 2B) also reflect the crystallinity of composites. Obviously, both SSC and BSSC samples display two broad bands at 1328 and 1598 cm‒1, which are attributed to the D-band (disorder-induced phonon mode) and G-band (graphitic E2g phonon mode) of 5



the carbon, respectively.23 Generally, the intensity ratio of these two bands (IG/ID) reflects the graphitization degree of carbon.24 Note that the IG/ID ratio of all samples is lower than 0.98, indicating a high disorder degree in carbon frameworks, which would afford good volumetric efficiency for accommodating Si-based active materials used in LIBs. In addition, the Raman spectrum reveal a characteristic band of Si at 518 cm‒1, which is due to the single crystal Si.25 This result confirms the high crystallinity of Si nano-species within the heterostructures. Note that the Si band becomes sharper with increasing the etching time; this is mainly caused by the excessive consumption of silica, leading to internal Si nanocrystals to be exposed. TGA profiles (Figure 2C) reveal the thermodynamic characteristics of composites. For the baseline SiO, it shows a highly thermal stability under air atmosphere even heat to 800 oC. However, for the SSC and BSSC samples, they display drastic weight loss start at ~ 560 oC, which is due to the decomposition of carbon layers. The SSC-0 maintains the highest remains of 61.3 wt%, which is almost the same as that of the BSSC-10, indicating that the content of carbon layers in these composites is as high as 38.7 wt%. The high carbon frameworks could provide good electrical conductivity and mechanical flexibility. As the etching time increased to 10 and 60 min, the residues was reduced to 37.4 wt% and 20.3 wt%, respectively. This result reveals that a large amount of silica has been removed from the bulk Si/SiOx matrix, resulting in abundant internal voids. Generally, sufficient voids can help to accommodate the volume expansion of Si-based active materials during cell operation, thus promoting good cycling stability. The void structures were evaluated by 77 K N2 sorption measurements. In Figure 2D, the SSC-10 shows mesoporous characteristics from sorption‒desorption isotherms, with high specific surface area (SSA) of 133.3 m2 g−1 and average pore size of ~ 8.0 nm. Comparatively, the BSSC-10 shows completely different isotherms, where the SSA is only 45.1 m2 g−1, and average pore size is reduced to ~ 1.5 nm. The finding indicates that the mesoporous structures of Si/SiOx matrix in the BSSC-10 sample were refilled by subsequent carbon coating. In addition, we found that both average pore size and pore volume of the SSC samples were significantly increasing as extending the etching time, as shown in Figure S1. The SSC-10 has a high pore volume of 0.98 cm3 g‒1, which is almost twice as that of the SSC-0. The results suggest that the etching creates abundant voids space for the accommodation of the volume expansion of Si/SiOx active materials. 6


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The micro-structures of composites were further characterized by FE-SEM and TEM, as shown in Figure 3. From SEM images, the baseline SiO (Figure 3A) has an irregular shape with smooth surface and micrometer particle size. Differently, SSC samples (Figures 3B-D) show a coarse surface covered by numerous bulge-like nanoparticles, and the coarseness gradually increases with extending the etching time. Similarly, the BSSC-10 (Figure 3E) displays a coarse morphology covered by much larger particles. From TEM images, The SSC samples exhibit a multi-core capsule nanostructure, where nanoparticles are surrounded by a contour coating of carbon. Meanwhile, the SSC-60 (Figure 3G) shows much looser structure than the SSC-10 (Figure 3F) due to the longer etching time. In contrast, the BSSC-10 (Figure 3H) displays much denser structure than the SSC samples; this finding also confirms the above N2 sorption results that the porous structures of bare Si/SiOx matrix were refilled by carbon coating for the BSSC-10 sample. In addition, the HR-TEM image (Figure 3I) further reveals the crystalline structure of Si particles in the SSC-10 bulk composite (Figure 3I insert). obviously, uniform Si nanoparticles, sized of ~ 10 nm, evenly embed into porous silica matrices, and a single Si particle shows highly ordered lattices spaced by 0.31 nm, which corresponds to the (111) plane of Si. Briefly, these specific Si/SiOx@C structures, especially for the SSC-10 sample, feature uniform Si nanocrystals embedding into mesoporous silica matrices, and all of them wrapped by carbon layers, forming porous capsule heterostructures. The most significant advantage of these structures is the extreme volume efficiency for accommodating Si-based active species. As amorphous silica matrices with rich nanovoids provide sufficient space, the capsules allow for the large expansion. In addition, the compact carbon layers afford high electrical conductivity and good mechanical strength; they also help to confine active materials in the capsules and limit the most SEI layers to the outer surface. Therefore, this Si/SiOx@C anode architecture would enable good electrochemical properties.

Electrochemical properties Electrochemical properties of composites were evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge tests using 2032 coin-type half-cells. Figure 4 displays the cyclic voltammograms of the baseline SiO and SSC-10 electrodes. Typically, the baseline SiO (Figure 4A) shows a board cathodic 7



peak at 0.73 V during the first reduction process, corresponding to the SEI formation. Another sharp cathodic peak from 0.25 to 0.01 V represents the alloying process of SiO with Li, while two anodic peaks at 0.31 and 0.51 V correspond to the de-alloying process of Li–Si alloys.26 Note that the cathodic peak at 0.73 V is disappeared during the subsequent cycles, indicating that the SEI was mainly formed during the first reduction process and maintained well during next cycles. This finding was considered to be due to the cyclized-PAN on active materials that hindered the further decomposition of electrolyte. Comparatively, the SSC-10 (Figure 4B) displays a much smaller cathodic peak at 0.73 V but much stronger cathodic peaks in the range of 0.01–0.25 V. The results indicate better structural integrity by carbon coating and higher capacity retention ability during the cycling. We evaluated the charge-discharge cycling of the SSC electrodes in coin-type half-cells. All cells were run between 0.01 and 2.0 V. The voltage profiles (Figures 5A-C) displays the charge/discharge capacity and initial Coulombic efficiency (ICE) of three SSC electrodes. The SSC-0 (Figure 5A) delivers a high initial discharge capacity of 1282.1 mAh g‒1, with a low ICE of only 64.6%. This is mainly caused by the high irreversibility of silica, which occupies the main component of the SSC-0 composite. In contrast, the SSC-10 (Figure 5B) delivers a much higher initial discharge capacity of 1618.7 mA h g−1, with an improved ICE of 75. 0%. In the SSC-10 composite, due to acid etching, the content of silica in the Si/SiOx@C structure was reduced, while the Si was increased, thus both the capacity and ICE were significantly improved. However, as extending the etching time to 60 min, the SSC-60 (Figure 5C) shows a very low initial discharge capacity of only 734.6 mAh g−1, due to the limited content of active materials in the Si/SiOx@C structure. Besides, it exhibits a lowest ICE of 58.6%, which is considered to be attributed to server structural degradation of carbon shells, leading to large amount of SEI formation on active nanoparticles. Additionally, we evaluated the capacity of two carbon-based materials (Figure S2) to identify the capacity contribution of the carbon in Si/SiOx@C composites. Combined with the TGA results, we calculated the capacity contribution of both Si/SiOx and C components in the composites (Table S1), where the SSC-10 shows the best electrochemical activity. Owing to the high carbon content, it shows high conductivity; more importantly, it can be directly used as anodes for LIBs without further treatments. As shown in Table S2, the SSC-10 displays a 8


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much higher energy density of 191.2 Wh kg‒1 than the commercialized graphite (37.6 Wh kg‒1) even after 100 cycles. The result demonstrates that the SSC-10 composite has a great potential to be anode candidate for LIBs. Figure 5D shows the cycling performance of the baseline SiO and SSC electrodes. It can be seen that the baseline SiO exhibits the highest initial capacity, but with a rapid fading upon cycling from 2300 mA h g−1 to below 140 mA h g−1 in just 10 cycles. This was caused by serious volume change during the charge‒discharge processes. In contrast, the SSC electrodes exhibit better cycling performance, where both capacity and retention are obviously enhanced. Especially for the SSC-10, it maintains a very high reversible capacity of 1095.0 mAh g‒1 even after 100 cycles, with high capacity retention of 90%. The corresponding values of the SSC-0 and SSC-60 electrodes are just 324.7 mAh g‒1 with 39.2%, and 288.3 mAh g‒1 with 66.6%, respectively. This result reveals an optimal structure of Si/SiOx@C composites, where well-controlled internal nanovoids within the SSC-10 structure can efficiently accommodate the volume change, and complete carbon coating can maintain the entire structural integrity. Figure 5E displays the rate cycling performance of SSC electrodes at high current densities. In general, the capacity retention decreases with increasing current density from 0.1 to 5.0 A g‒1. Comparatively, among three SSC samples, the SSC-10 has the best stability on cycling with the highest capacity at each current density. A capacity of 850 mAh g−1 is achieved at 2.0 A g−1; even at the highest current rate of 5.0 A g‒1, the capacity is still maintained at about 350 mAh g‒1, which is comparable to the capacity of commercialized graphite anodes. Notably, the capacity almost recovers its original value when the current density is restored to 0.1 A g‒1. This finding reveals that the SSC-10 composite shows high charge transfer kinetics and good structural stability. To further confirm the important role of internal nanovoids on cell performance, we also compared electrochemical performance of the SSC-10 and BSSC-10 electrodes. The voltage profiles (Figures 6A, B) exhibit charge/discharge capacities at the selected cycles. Obviously, the SSC-10 has much higher capacities with more stable plateaus than the BSSC-10 at each selected cycle. The cycling profile (Figure 6C) further shows the long cycling stability, where the SSC-10 also has higher capacity retention than the BSSC-10 (90.0% vs. 56.4% after 100 cycles). This result indicates that sufficient nanovoids of the Si/SiOx@C structures efficiently 9



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accommodate volume change of Si-based active materials to sustain good structural stability during cell operation. Additionally, Figure 6D reveals the rate cycling performance at high current densities. Compared to the BSSC-10, the SSC-10 shows much better stability on cycling with higher capacity at each current rate. On the basis of these results, we believed that well-controlled nanovoids within the SSC-10 structure are beneficial for the stabilization of electrode structure, thus enabling the very stable cycling performance. As described above the electrochemical characterizations reveal different electrochemical performance of the Si/SiOx@C anode composites in Li-ion cells. To further understand their electrochemical behaviors, we analyzed electrode dynamics by AC impedance measurements. Figures 7A, B show EIS profiles of all electrodes before and after the cycling. Note that all the EIS plots are composed of two partially overlapping semicircles and a straight sloping line. In Figure 7C, according to the equivalent circuit model (insert), the first intercept at the Z'real axis corresponds to electrolyte resistance (Rs), the first semicircle at high frequency is ascribed to lithium ion diffusion through the SEI layers (Rsei) and SEI capacitance (Csei), the second semicircle at medium frequency is attributed to charge transfer (Rct) and double-layer capacitance (Cdl), and the sloping line at low frequency is related to lithium ion diffusion impedance (Zw) into the bulk electrode.27 The fitted values of Rsei and Rct of all electrodes are recorded in Table 1. Obviously, the Rsei of all electrodes before cycling are very small and almost the same, ~ 3 . However, the Rsei values are greatly increased after 100 cycles, this finding suggests that SEI layers are formed and can maintain during cycling. Comparatively, among all these electrodes, the SSC-10 shows the smallest Rsei, indicating the most stable SEI structure on the electrode surface. In addition, the Rct of all electrodes before cycling are very close to each other, ~ 50 . After 100 cycles, the Rct values are increased, but the increasing rate is different. The SSC-10 has the smallest Rct of ~ 50  not only before but also after cycling. This result confirms the fastest charge-transfer kinetics within the Si/SiOx@C heterostructures of the SSC-10 composite, thanks to the high electrochemical activity by uniform coating of carbon layers. The plot of Z'real vs. the reciprocal root square of the lower angular frequencies (ω‒0.5) is shown in Figure 7D. The linear relationship of the fitted line is governed by equation: 28 Z'real = Rs + Rsei + Rct + σωω‒0.5 10


(1)

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where ω is the angular frequency in the low frequency range. The slope of the fitted line is the Warburg coefficient σω. Among all the electrodes, the SSC-10 has the lowest σω of 14.3 Ω s‒0.5. The diffusion coefficient of lithium ions (D) in the bulk electrode is calculated from the following equation:28 D = 0.5(RT/AF2n2Cσω)2

(2)

where R is the gas constant, T is the temperature, A is the area of the electrode surface, F is the Faraday's constant, n is the number of electrons per molecule during oxidization, and C is the molar concentration of Li+ ions. The calculated results of σω and D are recorded in Table 1. The SSC-10 electrode shows the highest D of 1.95 × 10−10 cm2 s−1, much higher than that of other three electrodes. The enhancement of the diffusion coefficient is mainly attributed to abundant conductive pathways within Si/SiOx@C heterostructures. The EIS results reveal the electrochemical kinetics of the composite electrodes, where compact capsule heterostructures with the complete carbon coating ensure uniform SEI formation on capsule's surface and fast charge-transfer kinetics and Li+ ions transport in the electrode. It also demonstrates that the capsule heterostructures maintain good structural integrity after cycling because the sufficient nanovoids inside the capsule alleviate the volume changes of Si-based active materials. As expected, the Si/SiOx@C heterostructures exhibited good electrochemical properties, which was largely attributed to the enhanced electrical conductivity and mechanical accommodation. To confirm their mechanical properties, we examined the structural stability of electrodes using SEM imaging. SEM images (Figures 8A-F) display the morphologies of the SSC and BSSC electrodes after cycling. Notably, the cycled SSC-10 electrode has a compact and integral surface morphology without any cracks. It also shows a thin-film cross-sectional structure with thickness of 49.2 μm. In contrast, both cycled SSC-60 and BSSC-10 electrodes show much coarser surface structure with several severe cracks, which is caused by the large volume change during cycling. In addition, the cross-section images reveal a serious volume expansion, since the thickness of the SSC-60 and BSSC-10 electrodes increases to 64.9 and 74.2 μm, respectively. TEM images (Figures 8G-I) further display microstructural changes of particles in these cycled electrodes. The cycled SSC-10 maintains a capsule-like structure, where many dense active nanoparticles are still confined and evenly dispersed in the carbon matrix. This finding indicates that the SSC-10 composite 11



has a high volumetric efficiency for accommodating the Si/SiOx active species. Comparatively, both cycled SSC-60 and BSSC-10 composites show incomplete structures, where serious aggregation of particles and cracks are observed due to the poor mechanical elasticity. Figures 8J-L illustrates the micro-mechanism of the (de)-lithiation process in these composites. Since the SSC-10 composite has the complete capsule structures with sufficient internal void space, it can effectively accommodate volume expansion during cell operation. However, the SSC-60 composite shows incomplete carbon coating due to the cruel acid etching, so a large amount of SEI are deposited directly on the inside nanoparticles. In addition, the very low SiOx is not sufficient to afford the mechanical accommodation, which causes the aggregation of particles, leading to the low CE and severe capacity fade. Differently, the BSSC-10 composite has a very compact heterostructure, where internal voids are almost completely refilled by carbon coating. Therefore, it suffers from the most serious volume changes associated with the lithium (de)-alloying processes. In brief, in our Si/SiOx@C hierarchical electrode design, uniform Si nanocrystals embed into silica matrices with well-controlled internal nanovoids, and all of them are embraced by carbon shells. The Si/SiOx nanoparticles serve as active materials, the silica matrices with nanovoids act as volumetric buffers, while the carbon layers function as electrical highways and mechanical frameworks, and therefore, the structural degradation of Si-based materials during cycling can be under control. Accordingly, the structural properties of the Si/SiOx@C hierarchical electrodes significantly determine their improved electrochemical performance.

Conclusion In conclusion, a highly scalable and cost-effective high-temperature disproportionation and carbonation approach has been shown to form Si/SiOx@C capsule heterostructures. Both crystalline Si nanoparticles and carbon encapsulation coating are nucleated simultaneously, in addition, well-controlled internal nanovoids are created by acid etching, finally to generate composite anode materials with high compactness, good conductivity and excellent structural stability. The Si/SiOx@C hierarchical composites thus exhibit the promising electrochemical performance. This specific combination of composition and conformation addresses stability challenges common to Si-based anodes. Moreover, the facile and scalable fabrication strategy 12


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makes this Si-based material potentially viable for commercialization in LIBs.

Conflicts of interest The authors declare no competing financial interest.

Author Information Corresponding authors*: J.W. ([email protected]); G.T. ([email protected])

Acknowledgments This work was financially supported by the National Key R&D Program of China (2018YFB0104401). References 1.

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Xiao, X.; Liu, G.; Zhao, P.; Zhang, S.; Wang, C.; Lu, Y.; Cai, M. Inward Lithium-ion Breathing of Hierarchically Porous Silicon Anodes. Nat. Commun., 2015, 6, 8844. 19. Wu, P.; Wang, H.; Tang, Y.; Zhou, Y.; Lu, T. Three-dimensional Interconnected Network of Graphene-wrapped Porous Silicon Spheres: In Situ Magnesiothermic-reduction Synthesis and Enhanced Lithium-storage Capabilities. ACS Appl. Mater. Interfaces, 2014. 6, 3546‒3552. 20. Liu, Z.; Chang, X.; Wang, T.; Li, W.; Ju, H.; Zheng, X.; Wu, X.; Wang, C.; Zheng, J.; Li, X. Silica-derived Hydrophobic Colloidal Nano-Si for Lithium-ion Batteries ACS Nano, 2017, 11, 6065‒6073. 21. Hirata, A.; Kohara, S.; Asada, T.; Arao, M.; Yogi, C.; Imai, H.; Tan, Y.; Fujita, T.; Chen, M. Atomic-scale Disproportionation in Amorphous Silicon Monoxide. Nat. Commun., 2016, 7, 11591. 22. Holzwarth, U.; Gibson, N. The Scherrer Equation versus the 'Debye-Scherrer Equation'. Nat. Nanotechnol., 2011, 6, 534. 23. Tan, G.; Bao, W.; Yuan, Y.; Liu, Z.; Shahbazian-Yassar, R.; Wu, F.; Amine, K.; Wang, J.; Lu, J. Freestanding Highly Defect Nitrogen-enriched Carbon Nanofibers for Lithium Ion Battery Thin-film Anodes. J. Mater. Chem. A 2017, 5, 5532‒5540. 24. Tan, G.; Wu, F.; Yuan, Y.; Chen, R.; Zhao, T.; Yao, Y.; Qian, J.; Liu, J.; Ye, Y.; Shahbazian-Yassar, R; Lu, J.; Amine, K. Freestanding Three-dimensional Core–shell Nanoarrays for Lithium-ion Battery Anodes. Nat. Commun., 2016, 7, 11774. 25. Park, M. H.; Kim, M. G.; Joo, J.; Kim, K.; Kim, J.; Ahn, S.; Cui, Y.; Cho, J. Silicon Nanotube Battery Anodes. Nano Lett., 2009, 9, 3844‒3847. 26. Bao, W.; Wang, J.; Chen, S.; Li, W.; Su, Y.; Wu, F.; Tan, G.; Lu, J. A Three-dimensional Hierarchical Structure of Cyclized-PAN/Si/Ni for Mechanically Stable Silicon Anodes. J. Mater. Chem. A, 2017, 5, 24667‒24676. 27. Shim, E. G.; Nam, T. H.; Kim, J. G.; Kim, H. S.; Moon, S. I. Diphenyloctyl Phosphate as a Flame-retardant Additive in Electrolyte for Li-ion Batteries. J. Power Sources, 2008, 175, 533‒539. 28. Cui, Y.; Zhao, X.; Guo, R. High Rate Electrochemical Performances of Nanosized ZnO and Carbon Co-coated LiFePO4 Cathode. Mater. Res. Bull., 2010, 45, 844‒849. 15



Figures

Figure 1 Schematic illustration of synthesis processes of Si/SiOx@C hierarchical structures. (In this and subsequent figures, we refer to the Si/SiOx@C materials prepared via the route І as SSC, and Si/SiOx@C materials prepared via the route ІІ as BSSC, respectively.)

16


Page 16 of 25

Page 17 of 25

A

B

PDF#46-1045 (quartz SiO2) PDF#39-1425 (cristobalite SiO2) PDF#27-1402 (cubic Si)

Si

Intensity (a.u.)

Intensity (a.u.)

SSC-60 SSC-10 SSC-0

10

20

30

40

50

60

70

D-band

G-band

BSSC-10

SSC-0 SSC-10 SSC-60

Baseline SiO

BSSC-10

400

80

800

2 (degree)

80 61.3 61.7

Baseline SiO SSC-0

40

37.4

SSC-10 SSC-60

20 0 200

20.3

BSSC-10

300

400

500

600

700

70 60

1600

1

50 40 30

Temperature ( C)

0.12

SSC-10

0.09 0.06 0.03 0.00

0

10

20

30

40

Pore diameter (nm)

BSSC-10

20 10 0

800

o

D dV/dlog(D) (cm3 g1)

99.0

Quantity Adsorbed (cm3 g1)

100

60

1200

Raman Shift (cm )

C Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Figure 2 (A) XRD patterns of the baseline SiO, SSC and BSSC composites. (B) Raman spectra of the SSC and BSSC composites. (C) TGA profiles of the baseline SiO, SSC and BSSC composites. (D) N2 sorption isotherms of the SSC-10 and BSSC-10 composites.

17



Figure 3 (A-E) SEM images of the baseline SiO (A), SSC-0 (B), SSC-10 (C), SSC-60 (D) and BSSC-10 (E) composites. (F-I) TEM images of the SSC-10 (F), SSC-60 (G) and BSSC-10 (H) composites. (I) HR-TEM image of the SSC-10 composite.

18


Page 18 of 25

0.31 V 0.51 V

0.2

A

Delithiation

0.0 -0.2

Lithiation

0.73 V

-0.4 -0.6 -0.8

1st 2nd 3rd

Baseline SiO

0.3 0.52 V

0.5

1.0

1.5

2.0

Delithiation

0.0 Lithiation

0.72 V

-0.3 -0.6 -0.9

0.0

B

0.32 V 2

2

Current (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current (mA cm )

Page 19 of 25

1st 2nd 3rd

SSC-10 0.0

0.5

1.0

1.5

2.0 +

+

Potential (V vs Li/Li )

Potential (V vs Li/Li )

Figure 4 Cyclic voltammograms of the baseline SiO (A) and SSC-10 (B) electrodes over the first three cycles at the scan rate of 0.1 mV s‒1.

19



A

i = 0.1 A g1

1.5 1.0 0.5

i = 0.1 A g1

1.0

600

900

0.0

1200

1

0

300

2500

600

100

2000 1500

SSC-0

i = 0.1 A g

SSC-10

SSC-60

80

1

60 1000

40

500 0 0

20

10

20

30

40

50

60

70

1st 2nd 3rd

i = 0.1 A g1

1.5 1.0

1200



1500

0.0

1800

0

Capacity (mAh g )

D Baseline SiO

900

80

90

0 100

2000

Capacity (mAh g1)

300

SSC-60

0.5

0.5

0

C

2.0

1.5

Capacity (mAh g )

-1

2.5

1st 2nd 3rd

SSC-10

Coulombic efficiency (%)

0.0

B

2.0

Votage (V)

Votage (V)

2.0

2.5

1st 2nd 3rd

SSC-0

Votage (V)

2.5

Capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

200

600

800

E 0.1 A g1

1500

0.5 A g1

0.1 A g1

1 A g1 2 A g1

1000

5 A g1

SSC-0 SSC-10 SSC-60

500

0

400


0

5

Cycle number

10

15

20

25

30

35

40

Cycle number

Figure 5 (A-E) Voltage profiles of the SSC-0 (A), SSC-10 (B), and SSC-60 (C) electrodes for the first three cycles. (D) Cycling performance of the baseline SiO and SSC electrodes at the current density of 100 mA g‒1. (E) Rate performance of the baseline SiO and SSC electrodes at different current densities.

20


Page 21 of 25

2.5

A

SSC-10

1st 20th 50th 80th 100th

Votage (V)

2.0 1.5 1.0 0.5

0

300

600

900

1200

1500


1st 20th 50th 80th 100th

1.5 1.0

-1

80

1200

60

800

40

SSC-10 BSSC-10 0

20

20 40

60

80

0

2000

100

1600

400

0.0

1800

Coulombic efficiency

C

2000

0

BSSC-10

0.5

Capacity (mAh g-1)

0.0

B

2.0

Votage (V)

2.5

Capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 100

300

600

900

1200


1500

D 0.1 A g1

1500

0.1 A g1

0.2 A g1 0.5 A g1 2 A g1

1000

5 A g1

500

SSC-10 BSSC-10

00

10

20

30

40

Cycle number

Cycle number

Figure 6 (A, B) Voltage profiles of the SSC-10 (A) and BSSC-10 (B) electrodes at the selected cycles. (C) Cycling performance of the SSC-10 and BSSC-10 electrodes at 100 mA g‒1. (D) Rate performance of the SSC-10 and BSSC-10 electrodes at different current densities.

21



300

100

A SSC-0 SSC-10 SSC-60 BSSC-10

60

-Z'' (ohm)

-Z'' (ohm)

B

250

80

40 20

SSC-0 SSC-10 SSC-60 BSSC10

200 150 100 50 0

0 0

20

40

60

80

0

100

50

100

150

200

250

300

350

Z' (ohm)

Z' (ohm) 400

250

C 200

350

Equivalent Circuit

Z' (ohm)

-Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

150 100 50

Rsei

Rct

59.3 Ω

51.5Ω

50

100

D SSC-0 SSC-10 SSC-60 BSSC-10

300 250 200 150

SSC-10 Fitting line

0

100 0

150

200

250

0.6

0.8



Z' (ohm)



1.0

(



1.2

1.4

)

Figure 7 (A, B) Electrochemical impedance spectra of the SSC and BSSC electrodes before (A) and after (B) cycling. (C) Typical Nyquist plots of the SSC-10 electrode after 100 cycles. (D) Relationship between Z’real and ω‒0.5 at low frequencies of the SSC and BSSC electrodes.

22


Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8 (A-C) Surface morphologies of the SSC-10 (A), SSC-60 (B) and BSSC-10 (C) electrodes after 100 cycles. (D-F) Cross-section morphologies of the SSC-10 (D), SSC-60 (E) and BSSC-10 (F) electrodes after 100 cycles. (G-I) TEM images of the cycled SSC-10 (G), SSC-60 (H) and BSSC-10 (I) samples. (J-L) Schematic diagrams showing the (de)-lithiation process of the SSC-10 (J), SSC-60 (K) and BSSC-10 (L) electrodes.

23



Page 24 of 25

Table 1 The fitted impedance parameters of the samples Before cycling

After cycling

Rsei (Ω)

Rct (Ω)

Rsei (Ω)

Rct (Ω)

SSC-0

2.12

56.06

72.34

61.66

24.43

6.70×10

SSC-10

3.05

49.13

59.26

51.51

14.32

1.95×10

SSC-60

3.61

56.62

126.60

96.00

136.16

2.16×10

BSSC-10

2.79

63.47

91.44

84.32

22.84

7.67×10

Samples

24


σω (Ω s

‒0.5

2 ‒1

)

D (cm s ) ‒11

‒10

‒12

‒11

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC

25


SiOx@C

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