SiOx@Cy anodes for high

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Coralloid-like nanostructured c-nSi/SiOx@Cy anodes for high performance lithium ion battery Xianhuan Zhuang, Pingan Song, Guorong Chen, Liyi Shi, Yuan Wu, Xinyong Tao, Hongjiang Liu, and Dengsong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05255 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017

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ACS Applied Materials & Interfaces

Coralloid-like Nanostructured c-nSi/SiOx@Cy Anodes for High Performance Lithium Ion Battery Xianhuan Zhuang,†a Pingan Song,†b,c Guorong Chen,a* Liyi Shi,a Yuan Wu,a Xinyong Tao,d Hongjiang Liu,a and Dengsong Zhanga* a

Research Center of Nanoscience and Nanotechnology, Shanghai University, Shanghai, 200444,

China b

Department of Materials, School of Engineering, Zhejiang A&F University, Hangzhou, 311300,

China c

Centre for Future Materials, University of Southern Queensland, Toowoomba, Queensland 4350,

Australia d

College of Chemical Engineering and Materials Science, Zhejiang University of Technology,

Hangzhou 310014, China †Xianhuan Zhuang and Dr Pingan Song contributed equally to this work. *Correspondence should be addressed at Email: [email protected] (G. Chen) and [email protected] (D. Zhang) ABSTRACT: Balancing the size of primary Si unit and void space are considered to be an effective approach for developing high performance silicon based anode materials, and is vital to create lithium ion battery with high energy density. We herein have demonstrated the facile fabrication of coralloid-like nanostructured silicon composites (c-nSi/SiOx@Cy) via sulfuric acid etching the Al60Si40 alloy and followed by surface growth carbon layer approach. The HRTEM images of pristine and cycled c-nSi/SiOx@Cy show that abundant nanoscale internal pores and the continuous 1

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conductive carbon layer effectively avoid the pulverization and agglomeration of Si units during multiple cycles. It is interesting that the c-nSi/[email protected] anode exhibits a high initial coulombic efficiency of 85.53 %, and typical specific capacity of over 850 mAh g-1 after deep 500 cycles at a current density of 1 A g-1. This work offers a facile strategy to create silicon based anodes consist of highly dispersed primary nano Si units. KEY WORDS: Fabrication, Carbon, Silicon, Lithium Ion Battery, Anode

1. INTRODUCTION Lithium ion battery devices have been applied in people’s daily life, electrical transportation, and large-scale stationary energy storage. Rapid growth of electric vehicles has recently accelerated the development of new generation energy storage system with high gravimetric energy density, specifically over 300 Wh kg-1, to meet the growing power needs. Unfortunately, most of currently commercialized lithium ion batteries only exhibit a gravimetric density of 120 ~ 180 Wh kg-1. This has being driven an exploration of new generation high-performance electrodes materials. As compared with traditional graphite-based anode materials, silicon-based materials are one class of ideal anode materials due to their higher theoretical capacity (4200 mAh g-1 for Li4.4Si), and lower lithiation voltage platform (0.4~0.6V vs Li/Li+) relative to other potential anode materials such as Sn, Ge, Sb, Al etc.1-3 However, the huge volume expansion during repeated charge-discharge cycles, poor electronic conductivity and unstable solid electrolyte interface (SEI) result in a rapidly reduced specific capacity, which extremely restricts their potential applications in lithium ion batteries.4-6

Last two decades have witnessed the great advances in terms of addressing the volume

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expansion issue of the Si-based materials by reducing the particle size of Si down to nanometer scales.7-9 As a result, various nano-porous Si materials such as hollow spheres, core-shell structure,

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nanotube,

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nanowire,

18, 19

nanofiber,

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10, 11

porous Si,

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and silicon alloy21 have been

successfully developed as anode materials for Li-ion battery. These nanostructured silicon materials with voids allow for the free space to adapt to the significant volume change during lithiation/delithaition process, thus showing a prolonged charge-discharge cycle life. Despite that, when it comes to the mass production, there are still many critical issues associated with them such as complicated preparation process, high cost, low tap density, and lower initial Coulombic Efficiency (CE).20, 22-24

Coating nano-silicon materials with carbon has been proven an effective route to enhance the electrical conductive performance of Si-based anode due to the dramatically improved electronic conductivity and the minimization of the electrode/electrolyte interface side reaction.25,26 Although porous Si can provide a stable capacity and high power density, the large surface area of porous Si tends to increase the possibility of side reactions with electrolytes, thus reducing the first-cycle coulombic efficiency.27-31 This makes it difficult to form stable SEI film in the initial cycles and then results in a constant consumption of electrolyte and the fast capacity fading. It is widely accepted that an ideal Si-based anode material should contain uniform nanoscale primary silicon units with a size of below 150 nm, and an intact conductive carbon-coating layer on the surface of Si units as the protective layer32-35 to prevent their gather again. Moreover, there should exit some void space between Si units. Very recently, chemical etching has been widely employed to prepare a variety of nano Si, such as core-shell silicon particle, silicon nanofiber and porous silicon.36-42. Especially, the work on porous Si by using hydrochloric acid (HCl) as etched reagent to etch the SiM alloy were 3



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attracted.37,42,44 For example, Hwang et al 43 reported that the porous Si/Al2O3 foam was synthesized by selective etching AlxSiy alloy via hydrochloric acid and following thermal oxidation. This porous Si/Al2O3 foam exhibits stable cycle life. In our study, we have selected sulfuric acid as etching reagent and poly-dopamine (PDA) as carbon source. Compared with hydrochloric acid etching, sulfuric acid etching reaction is more moderate which easy to control process, environmentally friendly and the etching agent is cheaper, more suitable for the mass production. Therefore, the objective of this work is to prepare one novel class of coralloid-like nanostructured silicon composites (c-nSi/SiOx@Cy) by sulphuric acid etching the eutectic Al60Si40 followed by coating carbon on the surface of coralloid-like nanostructured c-nSi/SiOx. As-prepared c-nSi/SiOx@Cy composites contain nanoscale Si units with average diameters of ~100 nm and many voids due to the removal of Al. More importantly, a thin and dense carbon layer on the surface is expected to effectively isolate primary Si units inside the c-nSi/SiOx@Cy, and prevent them from fracturing and reconstructing largest particles during charge-discharge cycling process. These composites combine dual advantages of porous silicon and nano-silicon. As expected, the c-nSi/[email protected] anode exhibits a very high initial coulombic efficiency of 85.53% and stable cycling performances. This work provides a facile approach to create silicon based anodes consisting of uniform highly mono-disperse nano Si units, which is expected to find wide applications in new energy storage.

2. RESULTS AND DISCUSSION

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Fig. 1 Schematic illustration for the preparation process of the coralloid-like nanostructured Si/SiOx@Carbon composites, c-nSi/SiOx@Cy.

Fig. 1 illustrates the fabrication process of the c-nSi/SiOx@Cy by the combination of cost-effective de-alloying and surface growth carbon coating approach. After the aluminum component is chemically etched completely from eutectic Si40Al60 with H2SO4, the coralloid-like nanostructured silicon skeleton can be successfully obtained and they are composed of nano-sized Si units with an average diameter around 100 nm (Fig. 2 a and b). Evidently, abundant void space is observed inside the skeleton of coralloid-like c-nSi/SiOx, which is mainly attributed to the removal of 57 vol. % Al with the Si skeleton left. To increase the electron conductivity of the bulk of c-nSi/SiOx, the thin poly-dopamine (DA) molecule layer introduced as the carbon source by polymerization on the Si bulk surface are formed and then carbonized. By doing this, the target carbon-coated c-nSi/SiOx, namely c-nSi/SiOx@Cy, can be achieved.

Basically, the strategy for preparing nanostructured c-nSi/SiOx@Cy composites possesses three advantages. First, as compared with the commercialized nano Si ($500-1000/kg), Al60Si40 as the starting material is inexpensive (about $15.00/kg), even considering the cost of sulphuric acid and process, the total cost is estimated to be about $150/kg (Table S1), thus dramatically reducing the 5



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production cost. In addition, c-nSi/SiOx@Cy particles have many voids and space among nanostructured Si skeleton which can accommodate the volume expansion. Thirdly, the primary nano Si units in c-nSi/SiOx@Cy with an uniform diameter of around 100 nm is very suitable for Si-based anode materials since such size is widely reported to be able to effectively suppress the fracture of Si particles during the volume expansion and shrink process.31, 32, 37 In brief, this strategy is low cost and easy for mass production compared to other preparing nano structure Si based anode with high electrochemical performance, including CVD, electronic spinning,

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and template

method.

Fig. 2 Typical SEM images of as-prepared coralloid Si particles (a) pristine and (b) with carbon coating layer; (c, d) HRTEM images of c-nSi/[email protected] composite at various magnification; (e) The EDS element mapping of selected area HRTEM of the edge in the c-nSi/[email protected] composite: 6


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e1) Si, e2) O, e3) C and e4) N elements.

To investigate the microstructure and morphology of both c-nSi/SiOx and c-nSi/[email protected], SEM and TEM were carried out and the results are shown in Fig. 2. It is clear that c-nSi/SiOx displays coralloid-like nanostructure (Fig. 2a) and this hierarchical structure basically sustains after being coated with carbon (Fig. 2b, c and d, Fig.S1). Many open apertures are readily observed inside coralloid-like particles and they can provide free spaces for volume changes during the lithiation/delithiation process. In addition, the open space is also able to absorb and fix electrolyte to ensure the close contact between active materials and electrolyte, which can contribute to decrease Li+ transport resistance and improve the rate capability. The HRTEM image of c-nSi/[email protected] (Fig. 2 c and d) clearly shows a continuous dense carbon layer with a thickness of 5-10 nm covered on the c-nSi/SiOx surface. Notably, the carbon layer (Fig. 2d) can offer excellent electrical conductivity for Si units, acting as the protective layer to prevent the neighboring units from aggregating to form larger Si particles, and prohibit the Si particles from fracturing to tiny pieces b)

during the repeatedly volume expansion and shrink process. Elemental mapping results of the corresponding selected area (Fig. 2e) clearly show the presence of Si, O, C and little heteroatom N atoms in as-prepared c-nSi/[email protected] composites. The N elements are residual after the PDA being carbonized, whereas the O elements are from the bulk of Si owing to the oxidation reaction of Si exposure in the air (Table S2) and residual after carbonized PDA. Interestingly, the presence of SiOx is reported to be beneficial for improving the electrochemical performance.45

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Fig. 3 (a) XRD patterns of c-nSi/SiOx and c-nSi/[email protected]; (b) Raman spectra of c-nSi/SiOx and c-nSi/[email protected]; (c) TGA analysis of c-nSi/SiOx and c-nSi/[email protected] and (d) Pore size distribution of c-nSi/SiOx and c-nSi/[email protected].

The XRD patterns of c-nSi/SiOx and c-nSi/[email protected] (Fig. 3a) evidently show three characteristic peaks at around 2θ = 28.5o, 38o, 57o, corresponding to lattice planes of (111), (220), (331) of Si crystals (JCPDS card No. 27-1402), respectively. Based on the above analysis, both the c-nSi/SiOx and c-nSi/[email protected] are mainly composed of pure silicon phase, and a thin layer of SiOx and carbon basically has an insignificant influence on these three peaks. Similarly, the Raman spectrum of c-nSi/SiOx shows the characteristic band of the silicon at 510 cm-1 in Fig. 3b. By comparison, besides the 510 cm-1 peak of silicon, two absorbance bands respectively located at 8


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around 1350 cm-1 and 1580 cm-1 are also found in the spectrum of c-nSi/[email protected], which respectively corresponds to the D band (sp3-hybridized carbon or structural defects) and G band (sp2-hybridized graphitic domains) of carbon, thus strongly indicating the presence of the carbon layer. Additionally, a high ID/IG value (area ratio) of only 1.51 suggests that the carbon layer on the c-nSi/SiOx surface has rich defects, which indicates the good electrical conductivity of this coated carbon layer.36 Fig. 3c shows the TGA plots of both c-nSi/SiOx and c-nSi/[email protected] in air condition. It is clear that both samples basically have the same mass loss below 270 oC, which is mainly due to the absorbed water. Upon the temperature rising up to over 300 oC, the c-nSi/SiOx sharply gains weight because Si is oxidized to form SiO2. In comparison, for c-nSi/[email protected], the mass continues to reduce since the carbon layer is oxidized in air condition and thus decomposes by releasing CO2 until the carbon degrades completely. After that, the inner Si also undergoes the oxidation process to transform into SiO2, thus making the mass gradually increase. Because of this, the actual carbon content of c-nSi/SiOx@Cy can be easily calculated by taking the difference between the water loss and the weight loss of the carbon layer. For instance, the c-nSi/[email protected] has an actual carbon content of 4.0 wt. %, and similarly, c-nSi/[email protected] and c-nSi/[email protected] contains 1.5 wt. % and 6.2 wt. % of carbon, respectively (Fig. S2).

To investigate the pore structure and size distribution of the c-nSi/SiOx and composites, Brunauer-Emmett-Teller (BET) absorption-desorption measurements were performed (Fig. 2d and Fig. S3). It is understandable that the specific surface area of c-nSi/SiOx reduces from 61.41 to 42.58 m2 g-1 after the surface carbon coating. Likewise, the pore diameter distribution centers at 5 nm for the c-nSi/SiOx, and it slightly shifts to a smaller size of 3.9 nm after carbon coating (c-nSi/[email protected]), which is attributed to the presence of carbon layer on the surface of inner nano 9



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pores. These abundant nanoscale pores are expected to allow for the expansion of the bulk Si units during the charge-discharge process. Moreover, these nanostructured pores can also absorb the electrolyte, which will contribute to better intercalation kinetics of lithium ions, improving cycle life, shortening the transmission distance of Li+ ion and enhancing the high C-rate performance.

Theoretically, for as-prepared c-nSi/SiOx@Cy-based anode materials, there probably is an optimum carbon content where the best electrochemical performances appear. Therefore, the electrochemical performances of c-nSi/SiOx@Cy with different carbon content are investigated to determine the optimum value. Apparently, the c-nSi/[email protected] anode with 4.0 wt. % carbon exhibits much more stable cycle life and better rate capability than the other two c-nSi/SiOx@Cy composites, namely c-nSi/[email protected] and c-nSi/[email protected] (Fig. S4, Fig. S5). This means that 4.0 wt. % of carbon is the optimum content for the c-nSi/SiOx@Cy systems as the anode materials. Therefore, the work will focus on examining in detail the electrochemical performances of c-nSi/[email protected] in the following sections.

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Fig. 4 Electrochemical performances of c-nSi/SiOx and c-nSi/[email protected] electrodes: (a) Charge discharge profiles at 0.1 A g-1 in the voltage range of 0.01-1.5 V for the 1st cycle; (b-c) Reversible delithiation capacity for galvanostatic cycles at 0.1 A g-1, 0.4 A g-1, respectively; (d) Rate capability at various current density (from 0.1 A g-1 to 3 A g-1); (e) Long cycle life at 1A g-1(Insert is the charge-discharge profiles of 500th cycle).

As shown in Fig. 4a, for the c-nSi/[email protected] and c-nSi/SiOx anode, the specific capacity can reach about 2200 mAh g-1 at a current density of 0.1 A g-1 in voltage range of 0.01~1.5 V, but the efficiencies has big difference, as high as 85.53% of CE for c-nSi/[email protected] in the first discharge-charge cycle (Fig. 4a) comparable to the CE of 75% for c-nSi/SiOx and previously reports on nanostructured silicon-based anode.28,39 Generally, the first coulombic efficiency (CE) is a very 11



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crucial parameter for the electrochemical performance. A higher CE normally indicates a better solid electrolyte interface (SEI) film between the electrode and the electrolyte formed and stabilized in the period of initial charge-discharge process, and the consumption of a smaller amount of lithium source to achieve higher energy density.46, 47 Thus, the high CE is a very important parameter for the design of commercialized battery with higher energy density. It should be mentioned that the lithiation potential shows a sloping profile between 0.3 and 0.01 V (Fig. 4a) during first discharge period, which is well consistent with the Li insertion to form amorphous LixSi as previously reported.1,

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Moreover, the cyclic voltammogram (CV) results only show one peak at 0 V,

corresponding to the first lithiation process of the crystalline silicon (Fig. S6). In the following scans, the cathodic peaks basically locate at around 0.15 V and 0.21 V, whereas anodic peaks appear at around 0.6 V and 0.55 V for c-nSi/SiOx and c-nSi/[email protected], respectively. Therefore, the potential difference between lithiation and delithiation of c-nSi/[email protected] is smaller than that of c-nSi/SiOx, indicating that the button cell internal polarization of c-nSi/[email protected] is relatively low. This means that Li ions can rapidly pass through the thin carbon layer to reach the silicon active material.

Fig. 4b shows the specific capacity fading of both samples with cycles at 0.1A g-1. Although the initial specific capacities of both of c-nSi/SiOx and c-nSi/[email protected] are almost same, but the specific capacity of c-nSi/SiOx@C retains over 2000 mAh g-1 within the initial 10 cycles at a small current density of 0.1 A g-1, and then decreases to 1600 mAh g-1 after 50 cycles (Fig. 4b). By contrast, the specific capacity of the c-nSi/SiOx anode fades rapidly to 1000 mAh g-1 after 50 cycles. When the current density increases up to 0.4A g-1 and 1A g-1, (Fig. 4 c and e), the c-nSi/[email protected] still exhibits stable cycling performance. More importantly, the specific capacity can stay above 850 mAh g-1 and cycle efficiency of 99.98% after 500 cycles under a constant current density of 1 A g-1 12


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(insert of Fig. 4e). These results strongly demonstrate that as-fabricated c-nSi/[email protected] anode materials exhibit excellent electrochemical performances including long stable cycle life and good rate capability.

Fig. 4d further shows that the c-nSi/[email protected] anode displays a much superior rate performance to the c-nSi/SiOx from 0.1 A g-1 to 3 A g-1. Clearly, the c-nSi/[email protected] anode delivers a stable capacity of 2200 mAh g-1 at 0.1 A g-1, 1700 mAh g-1 at 0.4 A g-1, 1200 mAh g-1 at 1 A g-1, and 700 mAh g-1 at 3 A g-1. By comparison, the much lower average specific capacity of 1750 mAh g-1 at 0.1 A g-1, 1000 mAh g-1 at 0.4 A g-1, 500 mAh g-1 at 1 A g-1, 50 mAh g-1 at 3 A g-1, respectively, are observed for the c-nSi/SiOx counterpart. When the current density returns to 0.1 A g-1, the specific capacity rebounds back to 2000 mAh g-1 and 1500 mAh g-1 for c-nSi/[email protected] and c-nSi/SiOx, respectively. The better rate and cycle performance of c-nSi/[email protected] is primarily ascribed to the uniform distribution of nano size units (diameter of around ~100 nm), suitable void space distribution and dense carbon coating layer. This coralloid-like nanostructure can tolerate the volume change, prevent the fracture of Si units, and improve the electrochemical performance. To evaluate the interfacial stability of c-nSi/[email protected] after cycles, the electrochemical impedance spectra (EIS) was performed for cells after 50 cycles, compared with the c-nSi/SiOx (Fig. 5a). Both plots of AC impedance show clear depressed semicircles in the high frequency region and behave nonlinear at low frequency region. The depressed semicircle (Rct) in the high frequency region is related close to the charge transfer impedance and solid electrolyte interface impedance (RSEI),

33, 6

the sloping line of low frequency corresponds to Li+ ion diffusion impedance.49 Via

Z-view fitting software, the corresponding equivalent circuit and value of Rs, Rct are listed in the table inserted in Fig. 5a. Rs represents bulk resistance of electrode corresponding to intersection of 13



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high frequency oblique line and horizontal axis. Rs of both of c-nSi/SiOx and c-nSi/[email protected] is similar, but the Rct of c-nSi/[email protected] is 105.2 which much smaller than 188.4 of c-nSi/SiOx. It is probably because the carbonized PDA layer leads to a lower and stable interfacial resistance which is beneficial to the prolonged cycle life. To further illustrate the problem of interfacial stability of electrode and electrolyte, the charge-discharge efficiencies with cycles in first 12 cycles were listed in Fig. 5b at a current density of 0.1A g-1. Clearly, for c-nSi/[email protected], the first cycle efficiency is 85.53%, the efficiency of second cycle rapid rises to near 98%, and retains about 98% in subsequent cycles. By contrast, the cycle efficiency of sample c-nSi/SiOx is about 98% after 10 cycles. The big difference of cycle efficiency both samples also indicated that the interfacial stability of c-nSi/[email protected] is much better than c-nSi/SiOx.7,49

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Fig. 5 a) Nyquist plots for after 50 cycles at a current density of 0.1 A g-1 (inset is the enlarged EIS plot in the high-frequency region), the AC impedance fitted to an equivalent circuit for a1) c-nSi/SiOx and a2) c-nSi/[email protected], (b) the efficiencies of c-nSi/SiOx and c-nSi/[email protected] within the first few cycles.

High resolution transmission electron microscopy HRTEM (Fig. 6) is used to investigate the structure stability of c-nSi/[email protected] which is collected from the cycled electrode via multiple washing with DMC and DI water. The morphology of c-nSi/[email protected] composite has some changed after 10 cycles mainly due to the structure transfer of c-nSi/[email protected] from the crystal phase into the amorphous phase after the action of charge-discharge cycles, and the void space inside the nanoparticle reduces to some extent, which is probably caused by the volume expand and shirk during repeated charge discharge cycles.49,

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Despite that, abundant interconnecting free

apertures remain (Fig. 6 c, d, e), and the carbon layer as the protective layer still is also observed to completely cover the surface of Si units. Moreover, no broken carbon layer is found which well confirms the stability of c-nSi/SiOx@Cy against volume expansion and shrink during repeatedly charge-discharge process (Fig. 6f). Similar to the published work,

51, 52

the thickness of the

coated-carbon layer on the c-nSi/[email protected] seems to only marginally change and maintain the structural integrity after 10 cycles, this means that the preserved void space is enough to buffer the volume change during the repeated expansion and shrink process. In contrast, because of the lack of the protective carbon layer the morphology of c-nSi/SiOx has significantly changed after 10 cycles, and the majority of nanosize apertures basically disappear and some free tiny silicon pieces are also found on the edge of particle due to the large volume change (Fig. S7). Therefore, The carbonized PDA coating layer basically plays triple roles in the c-nSi/SiOx@Cy anode materials, namely 15



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enhancing the electrical conductivity of c-nSi/SiOx, serving as a buffer layer to alleviate the mechanical fractures of Si units during charge-discharge process, and effectively preventing primary silicon nanoparticles from aggregating which may result in poor electrochemical performance.

Fig. 6 HRTEM images of c-nSi/[email protected] with different magnification: (a-c) before cycles; (d-f) after deep 10 charge discharge cycles at a current density of 0.1A g-1.

3. CONCLUSIONS In this work, we have designed and successfully constructed a novel coralloid-like c-nSi/SiOx@Cy-based anode material for lithium ion battery via a facile approach by combining chemical etching and surface growth carbon coating technology. The coralloid-like c-nSi/SiOx@Cy is composed of nano sized silicon units with an intact carbon coating layer, and the c-nSi/SiOx@Cy with 4.0 wt. % of carbon (c-nSi/[email protected]) shows the optimum electrochemical performances, namely excellent cycling stability and high rate performance. The c-nSi/[email protected] can release a specific capacity of as high as 2200 mAh g-1 at a current density of 0.1A g-1 in the voltage range of 0.01~1.5 V, and maintain a high specific capacity of 1600 mAh g-1 after 50 cycles with a high initial 16


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coulombic efficiency of 85.53%. The specific capacity of c-nSi/[email protected] is still able to maintain over 850 mAh g-1 after 500 deep charge-discharge cycles even at a current density of 1.0 A g-1. These impressive performances are attributed to the hierarchical coralloid-like nanostructure and dense carbon coating layer of c-nSi/[email protected] with enough capability to buffer the volume change. This work offers a facile, low cost but effective strategy to fabricate Si-based anode materials for lithium ion battery with high energy density.

EXPERIMENTAL SECTION Materials Eutectic aluminum-silicon alloy and concentrated sulphuric acid (H2SO4) were purchased from the Changsha Tianjiu Metal Materials Co., Ltd and the Aladdin Chemical Agents Co., Ltd, respectively. Other chemical agents including dopamine (DA), the HCl/Tris-buffer solution, LiPF6, ethylene carbonate (EC), diethylcarbonate

(DEC), and dimethylcarbonate (DMC) and fluorinated

ethylene carbonate (FEC) were bought from Sigma-Aldrich Co., Ltd of and used without further purification. Synthesis of coralloid-like c-nSi/SiOx@Cy As for the fabrication of coralloid-like nanostructured Si/SiOx (noted as c-nSi/SiOx) particles, the eutectic aluminum-silicon alloy (Al/Si: 60/40, w/w) was immersed in an aqueous solution of 2M H2SO4 and stirred for 48 h at room temperature to etch the aluminum component. The residual acid and Al3+ absorbed on the surface of silicon was thoroughly removed by repeatedly washing followed by filtering with deionized (DI) water. As-prepared coralloid-like c-nSi/SiOx particles with abundant nano pores were used as the primary material.

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The target product, carbon-coated c-nSi/SiOx (designated as c-nSi/SiOx@Cy) was prepared via coating c-nSi/SiOx with poly-dopamine (PDA) followed by carbonization. The c-nSi/SiOx particles were first coated with the poly-dopamine (PDA) by adding into the HCl/Tris-buffer solution (pH = 8.5) containing a predesigned concentration of DA via the self-polymerization with the aid of vigorous stirring for 48 h. The PDA-coated c-nSi/SiOx, c-nSi/SiOx@PDA, was obtained by repeatedly washing with DI water and then ethanol for at least five times to completely remove the unreacted DA monomer, followed by drying in a vaccum oven at 60 oC for 12 h. In addition, the content of the carbon on the c-nSi/SiOx could be tailored by adjusting the DA concentration. After that, the target c-nSi/SiOx@Cy composites were achieved by sintering c-nSi/SiOx@PDA at 400 oC for 2 h and then 800 oC for 3 h with a heating rate of 5 oC min-1 in the argon condition. As for the designation of c-nSi/SiOx@Cy with different carbon contents, the y in the c-nSi/SiOx@Cy refers to the mass fraction of content (wt. %). For instance, c-nSi/[email protected] and c-nSi/[email protected] represent the c-nSi/SiOx@Cy composite containing 1.5 wt. % and 4.0 wt. % of carbon, respectively. Material characterization Raman spectrum (in the range of 200 to 2000 cm-1) was recorded on a Raman Spectrometer (JY H800UV) equipped with an optical microscope at room temperature using the 532 nm excited wave of an Ar ion laser. X-ray diffraction (XRD) patterns of c-nSi/SiOx and c-nSi/SiOx@Cy samples were tested by using a Rigaku D/MAX-RB X-ray diffractometer (CuKa, 40 kV, 20 mA) in a 2θ range from 10 to 85°. The actual amount of carbon in the c-nSi/SiOx@Cy composites was analysed by thermogravimetric analysis (TGA) on a Netzsch STA409PC (Germany), the temperature rose to 800 oC with a heating rate of 5 oC min-1 under air condition. Nitrogen absorption-desorption isotherms were conducted by an Autosorb-IQ2 (Quantachrome Corporation) with degassed 18


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overnight at 593 K under vacuum condition. Based on the Brunauer-Emmett-Teller (BET) theory and the Barrett Joyner Halenda method the surface characteristics (SSA) and pore structure were characterized. The morphology was observed by using a field emission scanning electron microscopy (FESEM, JEOL JEM-700F) and a field emission transmission electron microscopy (FETEM, JEOL JEM-200CX). High resolution transmission electron microscopy (HRTEM) images and selected area elements mapping were recorded on a JEOLJEM-2010F. For HRTEM analysis of cycled active materials, the cell was disassembled in a glove box with full argon filled, active materials after cycling were collected from electrode, then washed repeatedly with DEC and DI water. Electrochemical behaviors Electrochemical behaviors were assessed by using two-electrode model CR2032. Button cells were assembled in an argon filled glove box with controlled H2O and O2 contents

SiOx@Cy anodes for high

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