Cu Composite Films as an

Copper-Nanoparticle-Induced Porous Si/Cu Composite Films as an Anode for Lithium Ion Batteries Liang Lin, Yating Ma, Qingshui Xie,* Laisen Wang, Qinfu Zhang, and Dong-Liang Peng* Collaborative Innovation Center of Chemistry for Energy Materials, Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: “Welcome-mat”-like porous Si/Cu composite amorphous films are fabricated by applying the predeposited Cu-nanoparticle-assembled film as the growth direction template for the subsequent deposition of a Si active layer with the cluster beam deposition technique. When used as the binder-free anodes for lithium ion batteries, the acquired single-layer porous Si/Cu composite film exhibits a large reversible capacity of 3124 mA h g−1 after 1000 cycles at 1 A g−1. Even when cycled at 20 A g−1 for 450 cycles, the porous Si/Cu composite film still delivers a decent reversible capacity of 2086 mA h g−1. Also, multilayer porous Si/Cu composite films are synthesized through layer-by-layer sputtering and exhibit outstanding cyclability and relatively high specific capacity and initial Coulombic efficiency irrespective of increasing the layer number from two to four layers. The reasons for the excellent electrochemical properties of single-layer and multilayer porous Si/Cu composite films are discussed in detail. KEYWORDS: silicon, copper granular films, multilayer films, porous structure, binder-free anode, lithium storage properties

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the drastic volume expansion/contraction of Si anodes during lithium insertion/extraction would cause large internal strains and severe pulverization of the electrode, giving rise to detachment of electrical contact between the active material and the active material or the active material and the current collector, eventually inducing rapid capacity fading and poor cycling stability.5,6 Moreover, the solid electrolyte interphase (SEI) layer, derived from the decomposition of the organic electrolyte, would thicken gradually due to the re-exposed fresh Si surface. The thick SEI layer inevitably results in two side effects, poor conductivity and irreversible capacity loss, which prevent the Si anodes from achieving excellent electrochemical performance.7 Another shortcoming of Si active material that cannot be ignored is its poor electrical conductivity, which leads

ith the rapid development of information technologies, portable devices, electric vehicles, and largescale energy storage, the dramatically increasing demand for high-performance power supply has made lithium ion batteries (LIBs) one of the most promising technologies.1,2 In order to satisfy the requirement of LIBs in high energy consumption areas, a higher specific capacity, longer cycling lifetime, and greater rate capability have become the major selection principles of electrode materials in LIBs. During the past decade, silicon has become an attractive anode material for up-to-date battery research among various burgeoning electrode materials because of its ultrahigh reversible capacity of 3579 mA h g−1, since 3.75 Li ions are absorbed per unit of silicon at room temperature, which is approximately 10 times higher than that of a commercial graphite anode (372 mA h g−1).3,4 In addition, a moderate working potential prevents short circuits and resultant explosions from the growth of dendritic crystals. However, © 2017 American Chemical Society

Received: March 23, 2017 Accepted: July 6, 2017 Published: July 6, 2017 6893

DOI: 10.1021/acsnano.7b02030 ACS Nano 2017, 11, 6893−6903

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Figure 1. Front-view (a) and side-view (b) schematic drawing of the plasma-gas-aggregation-type cluster beam deposition apparatus.

means of the improved electronic conductivity in the absence of any binder, the reduced volume variation in the vertical direction of thin film, and the number of active particles cracked, as well as the enhanced adhesion between the active materials and the current collector.20−24 Consequently, taking all the above discussions into account, it can be expected to further increase the performance of Si electrode materials in the aspects of higher specific capacity, better rate capability, and longer cycle lifetime when combining the aforementioned strategies intimately to ingeniously fabricate porous Si-based composite films on rough matrices or substrates, which offers the opportunity to address the severe volume expansion/ contraction during the lithiation−delithiation process and poor electronic conductivity shortcomings of Si electrodes simultaneously and is of paramount significance. In this report, “welcome-mat”-like porous Si/Cu composite amorphous films were successfully fabricated on a current collector directly through a plasma-gas-aggregation-type cluster beam deposition technique (Figure 1), wherein the predeposited porous Cu-nanoparticle-assembled film acts as the growth direction template for the subsequent deposition of a Si layer.25,26 By adjusting the sputtering time, the microstructure and the content ratio of Si/Cu of the composite films can be regulated controllably. When applied as the binder-free anodes for lithium ion batteries, the obtained porous Si/Cu composite

to poor electrochemical reaction kinetics and resultantly a limited rate capability. Consequently, although Si active materials exhibit distinct merits and great potential as anodes for high-performance LIBs, the two above-mentioned main drawbacks significantly hinder their practical commercialization applications in LIBs.5 Numerous attempts have been made to overcome the above challenges of Si electrodes, such as nanostructuring,8−10 hollow or porous construction,11,12 amorphization design,13 and conductive additive modification14,15 by versatile methods.16−18 These elaborately synthesized Si-based anode materials have presented higher reversible capacity and more stable cycle performance than the bulk counterpart. For example, Coleman’s group reported that the electrode containing 20 wt % conductive binder and 80 wt % Si nanoparticles represents an impressive reversible capacity of 1950 mA h g−1 after 100 cycles at a current density of 1 A g−1.19 Unfortunately, most of the above-mentioned strategies employed binders to assemble the button-type battery, which could be regarded as electrical insulators and inevitably deteriorate the electronic conductivity of the overall electrode materials and thus degrade the lithium storage properties. Recently, Si-based thin films have been prepared directly on diversified rough conductive substrates and functioned as binder-free anodes for lithium ion batteries in an effort to obtain excellent electrochemical performance by 6894


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Figure 2. Schematic illustration of the design of welcome-mat-like single-layer and multilayer porous Si/Cu composite amorphous films.

amorphous films reveal exceptional electrochemical properties. After cycling 1000 times, the single-layer porous Si/Cu composite electrodes show a high specific capacity of about 3124 mA h g−1 at a current density of 1 A g−1. A reversible capacity beyond 2086 mA h g−1 can still be retained even when charged and discharged at a very large current density of 20 A g−1, which is more than 5 times that of the theoretical capacity of a commercial graphite anode. Furthermore, the multilayer construction of porous Si/Cu composite films is employed to improve the mass loading of Si active materials while keeping the specific capacity unchanged. Interestingly, when the layer number of multilayer porous Si/Cu composite films increases from two to four layers, the changes in specific capacity and cyclability of the electrodes are negligible. Finally, the excellent lithium storage properties of the obtained multilayer porous Si/ Cu composite amorphous films are interpreted in detail.

Table 1. Summary of Experiment Parameters for SingleLayer Si/Cu Composite Amorphous Films with Different Deposition Times of Cu and Si group A sample time/s

Cu Si

B

a1

a2

a3

a4

b1

b2

b3

29 263

171 263

214 263

286 263

171 526

171 1052

171 1578

lengthened. Figure 3a shows a plane-view SEM image of the produced Cu-nanoparticle-assembled film with a sputtering

RESULTS AND DISCUSSION The deposition procedures of the welcome-mat-like single-layer and multilayer porous Si/Cu composite amorphous films are schematically exhibited in Figure 2. First, the porous Cunanoparticle-assembled film, composed of Cu nanoparticles and the interparticle voids, is deposited on the Cu substrate and then used as the growth direction template for the subsequent deposition of a Si layer, inducing the generation of the welcome-mat-like Si/Cu composite film. At this stage, part of the void spaces are reserved because of the shadowing effect.27 As a result, a single-layer porous Si/Cu composite amorphous film is gained, whose morphology is similar to the welcome mat in the top-right inset. To construct the multilayer Si/Cu composite films, the obtained single-layer porous Si/Cu composite film acts as the in situ substrate for the second deposition of the Cu-nanoparticle-assembled film and the subsequent deposition of a Si layer. Through this layer-by-layer construction, multilayer porous Si/Cu composite films with a controllable layer number could be successfully produced. To explore the influence of the sputtering time of the predeposited Cu-nanoparticle-assembled film on the electrochemical properties of the Si/Cu composite thin film, a series of predeposited Cu-nanoparticle-assembled films with different sputtering times were prepared, and the corresponding synthetic parameters are depicted in Table 1. As exhibited in Figure S1, one can clearly observe that the number of Cu nanoparticles increases rapidly as the sputtering time is

Figure 3. SEM (a) and TEM (b) micrographs of the predeposited Cu-nanoparticle-assembled film (sample a2). SEM image (c) and XRD pattern (d) of single-layer porous Si/Cu composite film (a2).

time of 171 s (sample a2), which demonstrates rough surface morphology and abundant well-dispersed interparticle pores, implying the porous feature of the Cu-nanoparticle-assembled film. On further increasing the sputtering time to 286 s (a4), the significantly increased Cu nanoparticles make the Cunanoparticle-assembled film denser and the surface roughness seems to decrease because more interparticle pores are filled. The transmission electron microscopy (TEM) investigations of various Cu-nanoparticle-assembled films are presented in Figure 3b and Figure S2, and the void spaces between Cu 6895


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Figure 4. Cycling performances at 1 A g−1 of the prepared single-layer Si/Cu composite amorphous films with different Cu deposition times and the same Si deposition times (a) and with the same Cu deposition times and different Si deposition times (b).

in Figure S6. Careful observation suggests that the film surface of sample a2 is rougher than those of the other samples (a1, a3, and a4), relating to the porous configuration of the initial Cunanoparticle-assembled film. The predeposited Cu-nanoparticle-assembled films with too short (a1) or too long (a4) sputtering times cannot act as the effective growth direction templates to fabricate porous Si/Cu composite films due to their smoother or denser surfaces compared to that of the Cunanoparticle-assembled film obtained with a sputtering time of 171 s (Figure 3, Figures S1 and S2). Consequently, a singlelayer porous Si/Cu composite amorphous film (sample a2) could be successfully fabricated by applying the predeposited Cu-nanoparticle-assembled film as the growth direction template. The porous and amorphous features as well as the modification of Cu will benefit the improvement of the electrochemical properties of Si active materials, which will be discussed elaborately below. Figure 4a exhibits the charge and discharge capacities versus the cycle numbers for these Si/Cu composite film electrodes: plane Si film (black squares) and Si/Cu electrodes including sample a1 (red circles), sample a2 (green triangles), sample a3 (blue downtriangles), and sample a4 (pink stars). The plane Si film possesses the largest initial reversible capacity of 3783 mA h g−1 under a current density of 1 A g−1. However, the specific capacity plummets to 2521 mA h g−1 only after 100 cycles with 66.6% capacity retention. Although all Si/Cu composite film anodes present slightly lower reversible capacities at the second cycle (3042, 3305, 3763, and 3070 mA h g−1 for a1, a2, a3, and a4, respectively), the higher capacity retentions of 76.3%, 86.8%, 81.2%, and 77.0% are attained after 100 cycles. Moreover, the higher and more stable Coulombic efficiency of all single-layer Si/Cu composite film anodes than that of the plane Si film electrode can be observed distinctly (Figure S7a− e). It is apparent that the deposition time (morphology) of the predeposited Cu-nanoparticle-assembled film exerts an obvious influence on the specific capacity and cycling efficiency of the Si/Cu composite film electrodes. Compared with moderate deposition times (a2 or a3), the specific capacity of the composite film is relatively lower when the deposition time of the Cu-nanoparticle-assembled film is too short (a1) or too long (a4). This behavior is easily understood. At the short deposition time, the number of predeposited Cu nanoparticles on the substrate is too few to act as the growth direction template, while when the deposition time is increased to 286 s (sample a4), the Cu nanoparticles on the substrate are crowded together and most of the pores or void spaces are filled, which also could not function well as the growth direction template. Because of the well-designed porous features of samples a2 and

nanoparticles can be discerned distinctly, which is in good agreement with the SEM results. Using the various predeposited Cu-nanoparticle-assembled films as the growth direction templates, the Si layer is sputtered directly on the Cu-nanoparticle-assembled films with the given sputtering time of 263 s to synthesize Si/Cu composite films. Taking sample a2 as an example, the SEM image of the Si/Cu composite film is revealed in Figure 3c, wherein the rough and particle-like surface of the composite film is visible and the void space between the interconnected Si nanoparticles can be observed carefully. This phenomenon is quite different with the smooth surface of the plane Si film acquired through sputtering Si on the substrate (current collector) directly without using the porous Cu-nanoparticle-assembled film as the growth direction template (Figure S3). The interparticle pores of the predeposited Cu-nanoparticle-assembled film (sample a2) and the corresponding Si/Cu composite film have been confirmed by the side-view SEM images shown in Figure S4. There is an obvious Si layer with an average thickness of 70 nm on the surface of the Cu-nanoparticle-assembled film, and the porous configuration of the Si/Cu composite film is evidenced by the void cavities indicated by white arrows (Figure S4b). Figure 3d demonstrates the XRD pattern of the single-layer Si/Cu composite film, wherein no other diffraction peak can be assigned except for the bump derived from the glass slide, which validates the amorphous feature of such-derived Si/Cu composite film. The X-ray photoelectron spectroscopy (XPS) investigations of the fabricated single-layer porous Si/Cu composite film (sample a2) are carried out and shown in Figure S5. The full XPS spectrum (Figure S5a) implies the presence of Cu and Si elements in the porous composite film. In the high-resolution Cu 2p XPS spectrum (Figure S5b), two peaks located at 933.4 and 953.2 eV, respectively, originate from Cu 2p3/2 and Cu 2p1/2 of Cu0 and/or Cu+ components.28 In the high-resolution Si 2p XPS spectrum (Figure S5c), two peaks located at 99.5 and 103.2 eV relate to the components of monatomic Si and Si/SiOx (x < 2), respectively.29 The Cu+ and SiOx components may derive from the natural oxidation of freshly fabricated Si/ Cu composite film, and no intermediate phases can be found between Cu0 and Si0 components, suggesting that the nature of the binding between Cu nanoparticles and the Si active layer is physical binding. Namely, the fabricated single-layer porous Si/ Cu film is a physical mixture of Cu nanoparticles and a Si active layer. The scanning electron microscopy (SEM) images of other Si/Cu composite films using different Cu nanoparticleassembled films as the growth direction templates are shown 6896


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Figure 5. CV curves at 0.1 mV s−1 between 0.01 and 3.0 V (a), galvanostatic discharge−charge profiles (b) for first two cycles, long-term cycling performances at 1 A g−1 (c), rate capability at various current densities (d), and cycling performances at a large current density of 20 A g−1 (e) of the single-layer porous Si/Cu composite amorphous film (sample a2).

indicating the inferior cycling performance. The fluctuation in Coulombic efficiency of samples b1, b2, and b3 is more obvious than that of sample a2, indicating the reduced electrochemical reactions’ reversibility and cycling efficiency on increasing the deposition time of the Si layer from 263 s to 1278 s (Figure S7c,f−h). It is well accepted that the excessive Si would fill up almost all the interparticle pores as the deposition time of the Si layer increases, which makes the Si active layer denser and denser and results in damage of the porous structure of Si/Cu composite films and thus rapid capacity fading. According to the above investigations and discussions, the optimal deposition times for the Cu-nanoparticle-assembled film and the subsequent Si layer are identified as 171 and 263 s, respectively (sample a2). Figure 5a shows the first three cyclic voltammograms (CVs) of sample a2 measured at a scan rate of 0.1 mV s−1 between 0.01 and 3.0 V, by which Li+ insertion/extraction reactions are investigated. In the first cathodic scanning, a weak bump at 0.6 V marked by a red arrow can be clearly distinguished, which presumably derives from the formation of an SEI layer, and disappears after the first cycle. Two strong reduction peaks around 0.18 and 0.04 V are attributed to the alloying reaction between Si and Li+ to generate amorphous Li−Si alloys and the

a3, the Si/Cu composite film electrodes reasonably show higher specific capacity and greater cycling stability. Considering the close reversible capacity and cycling stability of samples a2 and a3, the optimal deposition time of the Cu-nanoparticleassembled film for the single-layer porous Si/Cu composite film is identified as 171 s (sample a2) due to the lower mass loading of inactive materials (Cu nanoparticles), which is helpful for the improvement of energy density of the singlelayer porous Si/Cu composite film anode. Figure 4b shows the cycling properties of the Si/Cu composite film electrodes at a current density of 1 A g−1, which were prepared by fixing the deposition time of the Cunanoparticle-assembled film (171 s) and gradually increasing the deposition time of subsequent Si layers from 263 s to 1278 s (detailed synthetic parameters are shown in Table 1). The initial reversible capacity for sample a2 is about 3305 mA h g−1 and stays stable beyond 3000 mA h g−1 after 200 cycles. When increasing the sputtering time of the Si layer to 526 s (sample b1), the initial reversible capacity is about 3694 mA h g−1 and degrades to 2900 mA h g−1 after 110 cycles and to 1869 mA h g−1 after 200 cycles with a capacity retention of 50.6%. As the deposition time further increases (samples b2 and b3), the reversible capacities decay quickly for both of the samples, 6897


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current densities, further hinting at the exceptional electrochemical reaction reversibility. Figure S9 shows the discharge− charge capacity profiles of the single-layer porous Si/Cu composite film (sample a2) at different current densities. Similarly, the lithiation potential increases slightly and the delithiation potential increases obviously, indicating the gradually enhanced electrode polarization at progressively increased current densities.34 Although the delithiation potential moves up as the current density increases, the similar curves in shape indicate the same lithium storage mechanism. More importantly, the produced porous Si/Cu composite film delivers a relatively high specific capacity of more than 2000 mA h g−1 even when tested at a very large current density of 20 A g−1 after 450 cycles (Figure 5e), manifesting the superior rate capability and cycling stability. The EIS measurements of a single-layer porous Si/Cu composite film (sample a2) and the plane Si film are carried out and shown in Figure S10. The values of electrolyte resistance (Re), SEI resistance (Rs), charge transfer resistance (Rct), and lithium ion diffusion coefficient of the single-layer porous Si/ Cu composite film and the plane Si film are calculated and shown in Table S1. Distinctly, the Re, Rs, and Rct values of the single-layer porous Si/Cu composite film are much smaller than those of the plane Si film, indicating the significantly enhanced electronic conductivity due to the modification of the Cu conductive network. The diffusion coefficient (D) of lithium ions of the single-layer porous Si/Cu composite film is higher than that of the plane Si film, relating to the porous construction of the single-layer Si/Cu composite film. These results suggest that the porous construction and the modification of the Cu conductive network play a positive role in the lithium storage properties of the produced porous Si/Cu composite film. In addition, Si active materials on the outermost surface of the Si/Cu composite film interact with the electrolyte, as there is no surface coating on the Si/Cu composite film, consequently leading to the formation and growth of an SEI layer. Indeed, the SEI layer can be seen clearly on the surface of the single-layer Si/Cu composite film after cycling (Figure S11b). This behavior would decay the cycling Coulombic efficiency of the porous Si/Cu composite film anode and is disadvantageous for practical applications in lithium ion batteries. Further work on the in situ surface modification of the porous Si/Cu composite film in order to prohibit direct contact between Si active materials and the electrolyte is of great significance and is in progress. Detrimental and inhomogeneous volume changes during cycling as well as poor electronic conductivity are the main issues that need to be solved urgently for large-scale practical applications of Si electrode materials in LIBs. The prepared porous Si/Cu composite amorphous film offers an effective solution to skillfully overcome these challenges through structural design and composite modification. First, the porous construction of the composite film can offer extra room to effectively accommodate the drastic volume expansion of Si material and guarantee the access of the electrolyte into the inner Si active materials, which is favorable to enhance the cyclability and reversible capacity, respectively.36,37 In order to interpret this point more clearly, the single-layer porous Si/Cu composite film and plane Si film are collected after 100 cycles and subjected to SEM measurements. As illustrated in Figure S11a, there are a lot of cracks on the plane Si film after cycling, which are caused by the large inner stress due to the huge volume change of Si active materials during the repeated

subsequent recrystallization reaction to form crystalline Li−Si alloys, respectively. In the anodic scanning for the first cycle, two intense peaks near 0.31 and 0.49 V are respectively caused by the reduction of the crystalline phase to form amorphous Li−Si alloys and the subsequent dealloying of the Li−Si alloys. This phenomenon is prevalent in many other amorphous Sibased anodes.30 The high superposition of peaks in terms of the sharpness, intensity, and potential position in the following cycles sheds light on the excellent electrochemical reaction reversibility. The voltage versus discharge−charge capacity profiles of the first two cycles for sample a2 under a current density of 1 A g−1 between 0.01 and 3.0 V are shown in Figure 5b. Two weak and inconspicuous plateaus located at 0.26 and 0.08 V can be carefully discerned on account of the alloying reaction between Si and Li+ as well as the recrystallization of Li−Si alloys, respectively. Upon the first delithiation process, two conspicuous plateaus centered at 0.25 and 0.5 V are associated with the amorphization of Li−Si alloys and the dealloying reaction to regenerate Li+ and amorphous Si reversibly. The results correspond well with the above CV characterizations. The porous Si/Cu composite amorphous film electrode displays initial discharge and charge capacities of 4104 and 3305 mA h g−1 with an initial Coulombic efficiency of about 80.5%. The initial irreversible capacity loss can be attributed to irreversible formation of an SEI layer derived from the decomposition of the electrolyte. The enhanced electrical conductivity associated with the incorporation of Cu and the surface homogeneity of the electrode is helpful to bring a thin and stable SEI layer to ensure decent initial Coulombic efficiency (above 80%).31−33 The structural advantages of the obtained porous Si/Cu composite amorphous film are identified intuitively by the longterm cycling test at a current density of 1 A g−1, and the corresponding result is shown in Figure 5c. Apparently, the porous Si/Cu composite amorphous film demonstrates excellent cyclability, and a high reversible capacity of about 3124 mA h g−1 is retained after 1000 cycles, relating to a high capacity retention of 95% compared to the specific capacity of the second cycle. Furthermore, the Coulombic efficiency of the porous Si/Cu composite film increases by 99.0% at the first cycle and then stays stable at this level in the following cycles, indicating the good electrochemical reaction reversibility and the stable SEI layer. The above observations unambiguously confirm the remarkable lithium storage properties, high reversible capacity, and notable long-term cyclability of porous Si/Cu composite film anodes. Figure S8 shows the discharge− charge capacity profiles of the single-layer porous Si/Cu composite film (sample a2) at the 500th and 1000th cycles. For both cycles, the lithiation potential increases very slightly and the delithiation potential increases obviously in comparison with that of the discharge−charge capacity profiles collected at the second cycle (Figure 5b), indicating the increased electrode polarization. This behavior possibly relates to the improved internal resistance of the battery due to the formation and growth of an SEI layer on the surface of the Si/Cu composite film during cycling.34,35 Figure 5d depicts the rate performance of a single-layer porous Si/Cu composite amorphous film (sample a2). As the current density ranges from 1 to 10 A g−1, the average specific capacity decreases from 3535, 3529, 3359, and 3183 to 3094 mA h g−1. When the current density is restored to 1 A g−1, the reversible capacity could recover the initial level. Moreover, the Coulombic efficiency remains stable up to 99.0% at various 6898


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Figure 6. Top-view (a) and side-view (b) SEM images of the four-layer porous Si/Cu composite amorphous film. The inset is a highresolution SEM image with clear surface morphology. TEM (c), HRTEM (d), and STEM (e) images of the four-layer porous Si/Cu composite amorphous film. The corresponding element mappings of Pt (f), Si (g), and (h) Cu elements.

insertion and extraction of lithium ions. Differently, in the case of the single-layer porous Si/Cu composite film, the granularlike surface is well maintained after cycling, and no obvious cracks or particle aggregation can be observed (Figure S11b), illustrating the greatly enhanced structural tolerance during the lithiation−delithiation process. Second, the predeposited Cu nanoparticle-assembled film not only acts as the growth direction template but also can strengthen the overall electronic conductivity and prohibit the volume variation of Si active materials to some extent, benefiting the improvement of rate capability and cycling stability.38,39 In addition, the rough substrate (the predeposited Cu-nanoparticle-assembled film) also benefits the effective anchoring of the subsequently deposited Si active layer. Third, the amorphous feature of the Si active layer can release mechanical strain more effectively than ordered crystalline Si because of the more homogeneous volume change associated with the inherent isotropic feature of amorphous materials, which plays a positive role in the improvement of structure stability and cycling performance.40,41 Finally, the absence of any binders that are always electrical insulators due to the binder-free film-based anodes benefits the electronic conductivity and thus the performance of batteries.42,43 Increasing the mass loading of active materials is of great importance to achieve high-energy lithium ion batteries. In this context, the multilayer design of the Si/Cu composite film is implemented through layer-by-layer sputtering (···Cu−Si−Cu−

Si···) in order to increase the mass loading of Si active materials and preserve the porous feature simultaneously (Figure 2). On the basis of the above electrochemical results, the sputtering time of each Si active layer in multilayer Si/Cu composite films is 263 s, coinciding with that of sample a2. Considering the close lithium storage performance of sample a2 and sample a3, the deposition time of each Cu-nanoparticle-assembled film in the multilayer porous Si/Cu composite films increases from 171 s (sample a2) to 214 s (sample a3) in order to further enhance their electronic conductivity. The SEM micrographs of the obtained multilayer Si/Cu composite films with a controllable layer from two to four layers are shown in Figure 6 and Figure S12. Taking the fourlayer Si/Cu composite film as an example, the top-view SEM image (Figure 6a) suggests the rough surface of the multilayer composite film, which is analogous to that of the single-layer counterpart (sample a2, Figure 3c). The multilayer structure is clearly evidenced from the side-view observation revealed in Figure 6b, wherein the regions with different contrast in turn are explicit. The layers with bright contrast are the Cunanoparticle-assembled layers, and the layers with dark contrast are the Si layers, demonstrating that a fine and linear layer-bylayer structure has been built up successfully. The overall thickness of the four-layer composite film is about 340 nm. The pores or void spaces of the multilayer composite film can be discerned distinctly from the high-resolution SEM image in Figure 6b, inset. A Pt layer is coated on the surface of the four6899


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Figure 7. Cycling performances at 1 A g−1 of the multilayer porous Si/Cu composite films with different layer numbers and the plane Si film (a), CV curves at 0.1 mV s−1 between 0.01 and 3.0 V (b), and galvanostatic discharge−charge profiles (c) of the four-layer porous Si/Cu composite film.

Figure 8. Schematic illustrations of electron transportation and stress release processes in the four-layer porous Si/Cu composite film electrode.

layer porous Si/Cu composite film in order to protect the composite film during the TEM sample preparation process through focused ion beam technology. As demonstrated in Figure 6c, the dark layer indicated by an arrow is the Pt coating, the layers with bright contrast are the Si layers, and the layers with dark contrast are the Cu-nanoparticle-assembled films, further evidencing their layer-by-layer structure. The HRTEM image collected from the contact interface between the Cunanoparticle-assembled film and the Si active layer (the circle region in Figure 6c) clearly demonstrates the lattice fringes of Cu nanoparticles with an interplanar spacings of 0.209 nm, and the Cu nanoparticles and Si active layer contact each other tightly at the interface region. The STEM image and the corresponding element mappings are shown in Figure 6e−h, from which the layer-by-layer structure of the Si/Cu composite film can be confirmed again, being in good agreement with the above SEM results (Figure 6b). The X-ray diffraction (XRD) pattern of the four-layer porous Si/Cu composite film is displayed in Figure S13, and the amorphous feature of the four-layer porous Si/Cu composite film can be observed clearly, being in good agreement with the result of the single-layer porous Si/Cu composite film (Figure 3d). N2 adsorption−desorption isotherm measurement of the four-layer Si/Cu composite film is performed in order to test its Brunauer−Emmett−Teller specific surface area and pore size distribution. As manifested in Figure S14, the specific surface area of the four-layer Si/Cu composite film is calculated to be 197.6 m2 g−1, and the pore size distribution on the basis of the Barrett−Joyner−Halenda method suggests that the pores mainly center near 6 nm, evidencing the mesoporous feature of the as-produced four-layer Si/Cu composite film. These results suggest that the multilayer construction is an effective approach to increase the mass loading of Si active materials while keeping the porous nature unchanged. Figure 7a depicts the cycling performances of the synthesized multilayer porous Si/Cu composite films with different layer numbers at 1 A g−1. One can find that all the multilayer Si/Cu composite films illustrate a superior cyclability, although the

specific capacity decreases as the layer number increases from one to two, three, and four layers. Interestingly, the fabricated multilayer composite films (two, three, or four layers) exhibit a close reversible capacity in spite of the layer number. This behavior is due to the porous configuration of multilayer Si/Cu composite films, which can provide plenty of access for the diffusion of Li+ into the inner Si active layer. As a result, lithium ions can easily and fully insert into and be extracted from the whole multilayer Si/Cu composite films during the lithiation and delithiation process. The four-layer Si/Cu composite films (4L, sample a3) deliver a relative high specific capacity of 1763 mA h g−1 after 100 cycles. For comparison, the plane Si film with the same total sputtering time as Si in four-layer composite films is prepared and employed as the binder-free anode in lithium ion batteries. The electrochemical measurement suggests that the specific capacity of the plane Si film deteriorates quickly with cycle number and decreases to below 500 mA h g−1 after 100 cycles. Undoubtedly, the multilayer porous Si/Cu composite films show greatly enhanced electrochemical properties in comparison with the plane Si film, well verifying the advantages of the multilayer porous configuration and the modification of three-dimensional Cu conductive network. The first-three cycle voltammograms of the four-layer porous Si/Cu composite film are shown in Figure 7b, from which an obvious peak at 0.38 V during the first cathodic sweep can also be found, which derives from the formation of an SEI layer. In addition, two cathodic peaks around 0.01 and 0.12 V, as well as two anodic peaks at 0.34 and 0.55 V, can be observed during the first cycle, which relate to the reversible alloying−dealloying reactions between Li+ and Si. The CV results of the multilayer porous Si/Cu composite films are in good accordance with those of the single-layer counterpart; the slight differences in peak position and shape can be possibly ascribed to the change in layer number and microstructure of the composite film. As shown in Figure 7c, the voltage versus discharge and charge capacity profiles of the four-layer porous Si/Cu composite film for the first two cycles have similar profiles in shape to the 6900


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ACS Nano single-layer Si/Cu composite film, representing the same electrochemical reaction mechanism during lithium insertion−desertion even after the multilayer construction. With the purpose of literally illustrating the structural advantages of multilayer porous Si/Cu composite films on conductivity and cycling stability, the charge carrier transportation and stress release processes during cycling are schematically shown in Figure 8. The porous framework throughout the whole multilayer composite films favors the release of the large structural strain resulting from the drastic volume variation of the Si active materials during cycling by offering extra cushion space and provides plenty of accesses (white solid lines) for the diffusion of Li+ into the inner Si active layer, strengthening the specific capacity and cycling stability. The large specific surface area of the four-layer Si/Cu composite film is capable of offering more electrochemical active sites for lithium storage, which is beneficial for the improvement of reversible capacity. Moreover, the incorporation of three-dimensional Cu nanoparticle-assembled films can not only ensure the rapid electron transport pathways (black dotted line) but also prohibit the pulverization of electrodes to some extent, which is helpful to the improvement of cyclability. In addition, it is well accepted that the amorphous feature of composite films could accommodate a volume variation more homogeneously than the crystal one, facilitating the enhancement of cycling stability. Consequently, the fascinating structural characteristics of the obtained multilayer porous Si/Cu composite films are responsible for their remarkable lithium storage properties.

vaporization (work powers: pCu = 400 w of copper targets with a diameter of 3 in.) and collisions (with Ar gas, Ar gas flow rate: RAr = 350 sccm) of copper atoms in the sputtering chamber as well as the cluster generation in the cluster growth room, were equitably deposited onto the substrates in the deposition chamber. The transportation of particles from one chamber to another is executed by differential pumping, with a small nozzle to sift out small-size clusters and a skimmer to intercept part of the cluster beam. Uniform and fine Cu nanoparticles were finally deposited on silicon wafers for morphology characterization, glass slides for component analysis, and copper foils for electrochemical measurements in the deposition chamber. Then, Si layers were deposited on the predeposited Cunanoparticle-assembled films (growth direction template) to fabricate the single-layer Si/Cu composite films. The multilayer Si/Cu composite films were also prepared by Cu-nanoparticle-assembled layers and Si layers deposited alternately. The deposition rate (mass per unit area and per time, g cm−2 s−1) of Cu nanoparticles can be measured by a quartz-crystal microbalance. The mass loading of Si can be calculated by multiplying the deposition rate by the time and substrate area. Material Characterization. The morphology and structure of the harvested predeposited Cu-nanoparticle-assembled films and Si/Cu composite films were analyzed in detail by SEM (SU-70) and TEM (JEM-2100, 200 kV), and the phase structure was investigated by a PANalytical X’pert PRO X-ray diffractometer (Cu Kα radiation 40 kV, 30 mA). Electrochemical Measurements. Coin-type 2025 cells, including lithium metal as the counter/reference electrode, a 1 M LiPF6 mixed solution (in EC:DEC with a v/v of 1:1) as the electrolyte, and Celgard 2300 as the separator, were assembled to characterize the electrochemical performance of the as-produced products. All the cells were assembled in an argon-filled glovebox with the various synthesized Sibased composite films as the binder-free working electrodes directly. Galvanostatic charge and discharge cycling was conducted on a multichannel battery testing system (Neware, China) between 3.0 and 0.01 V vs Li+/Li. CV was obtained using an Autolab electrochemical workstation (NOVA 1.8) at a scanning rate of 0.1 mV s−1 at room temperature. The specific capacity of the electrode is achieved referring to the mass of Si films but not Cu nanoparticle-assembled films, because Cu is inactive to lithium ions. The mass loading of the Si layer in the Si/Cu composite films could be calculated by multiplying the deposition rate by the time, the density of Si, and the substrate area. In order to ensure the accuracy of the deposition rate, a thick Si film with a thickness of more than 1 μm was deposited on copper foil under the same deposition parameters, and the deposition rate can be obtained by the film thickness divided by the deposition time. The thickness of the Si film was carefully determined by a surface profiler (Alpha-StepD-100) and verified by a cross-sectional SEM image.

CONCLUSIONS In summary, we have successfully put forward an effective strategy to prepare porous Si/Cu composite amorphous films through a plasma-gas-aggregation-type cluster beam deposition technique, aiming at offsetting two major imperfections of Sibased electrode materials, namely, the large volume expansion during Li+ uptake−removal and the poor electronic conductivity. The predeposited Cu-nanoparticle-assembled film on the current collector functions as a growth direction template to subsequently grow the porous Si active layer. The electrochemical measurements suggest that the achieved single-layer porous Si/Cu composite film with optical sputtering times of Cu and a subsequent Si layer demonstrates excellent electrochemical properties when used as the binderfree anode for LIBs. Furthermore, the multilayer porous Si/Cu composite films with a controllable layer number from two to four layers are successfully produced through the layer-by-layer strategy, which still exhibit a relatively high reversible capacity and good cyclability. The fascinating structural merits of multilayer Si/Cu composite films including porous construction, amorphous feature, stable 3D Cu conductive network, superior mechanical stability, and the absence of any binder are responsible for their high specific capacity, superior cycling stability, and good rate performance.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02030. Additional figures including SEM/TEM images, XPS spectra, EIS spectra, and N2 adsorption−desorption isotherms of the various obtained samples (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (Q. Xie). *E-mail: [email protected] (D.-L. Peng).

METHODS Synthesis. The samples were prepared by a plasma-gasaggregation-type cluster beam deposition apparatus with two copper metal targets, which had been carefully cleaned with acetone and ethanol. This apparatus, comprising a sputtering chamber, a cluster growth room, and a deposition chamber, exploits simultaneously both the plasma-glow-discharge vaporization and the inert gas condensation technique (Figure 1).44 The copper clusters, which were formed by the

ORCID

Liang Lin: 0000-0003-2868-9199 Yating Ma: 0000-0001-7358-7998 Qingshui Xie: 0000-0003-2105-6962 Laisen Wang: 0000-0001-9531-4480 6901


Article

ACS Nano

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Qinfu Zhang: 0000-0001-5810-429X Dong-Liang Peng: 0000-0003-4155-4766 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors express heartfelt thanks for financial support for the work, which was provided by the National Key R&D Program of China (Grant No. 2016YFA020602), the National Natural Science Foundation of China (Grant Nos. 51371154 and 51571167), and the Fundamental Research Funds for the Central Universities of China (Xiamen University: No. 20720160082). REFERENCES (1) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; Schalkwijk, V. W. Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4, 366−377. (2) Kim, M. G.; Cho, J. Reversible and High-Capacity Nanostructured Electrode Materials for Li-Ion Batteries. Adv. Funct. Mater. 2009, 19, 1497−1514. (3) Kasavajjula, U.; Wang, C.; Appleby, A. J. Nano-and Bulk-SiliconBased Insertion Anodes for Lithium-Ion Secondary Cells. J. Power Sources 2007, 163, 1003−1039. (4) Wu, H.; Cui, Y. Designing Nanostructured Si Anodes for High Energy Lithium Ion Batteries. Nano Today 2012, 7, 414−429. (5) Gonzalez, J.; Sun, K.; Huang, M.; Lambros, J.; Dillon, S.; Chasiotis, I. Three Dimensional Studies of Particle Failure in Silicon Based Composite Electrodes for Lithium Ion Batteries. J. Power Sources 2014, 269, 334−343. (6) Zhang, W. J. A Review of The Electrochemical Performance of Alloy Anodes for Lithium-Ion Batteries. J. Power Sources 2011, 196, 13−24. (7) Nguyen, C. C.; Song, S.-W. Characterization of SEI Layer Formed on High Performance Si-Cu Anode in Ionic Liquid Battery Electrolyte. Electrochem. Commun. 2010, 12, 1593−1595. (8) Maroni, F.; Raccichini, R.; Birrozzi, A.; Carbonari, G.; Tossici, R.; Croce, F.; Marassi, R.; Nobili, F. Graphene/Silicon Nanocomposite Anode with Enhanced Electrochemical Stability for Lithium-Ion Battery Aapplications. J. Power Sources 2014, 269, 873−882. (9) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. High-Performance Lithium Battery Anodes Using Silicon Nanowires. Nat. Nanotechnol. 2008, 3, 31−35. (10) Hwang, T. H.; Lee, Y. M.; Kong, B. S.; Seo, J.-S.; Choi, J. W. Electrospun Core-Shell Fibers for Robust Silicon Nanoparticle-Based Lithium Ion Battery Anodes. Nano Lett. 2012, 12, 802−807. (11) Yu, Y.; Gu, L.; Zhu, C.; Tsukimoto, S.; Aken, P. A.; Maier, J. Reversible Storage of Lithium in Silver-Coated Three-Dimensional Macroporous Silicon. Adv. Mater. 2010, 22, 2247−50. (12) Yao, Y.; McDowell, M. T.; Ryu, I.; Wu, H.; Liu, N.; Hu, L.; Nix, W. D.; Cui, Y. Interconnected Silicon Hollow Nanospheres for Lithium-Ion Battery Anodes with Long Cycle Life. Nano Lett. 2011, 11, 2949−2954. (13) Song, H.; Wang, H. X.; Lin, Z.; Jiang, X.; Yu, L.; Xu, J.; Yu, Z.; Zhang, X.; Liu, Y.; He, P.; et al. Highly Connected Silicon-Copper Alloy Mixture Nanotubes as High-Rate and Durable Anode Materials for Lithium-Ion Batteries. Adv. Funct. Mater. 2016, 26, 524−531. (14) Li, Y.; Yan, K.; Lee, H.-W.; Lu, Z.; Liu, N.; Cui, Y. Growth of Conformal Graphene Cages on Micrometre-Sized Silicon Particles as Stable Battery Anodes. Nat. Energy 2016, 1, 15029. (15) Yu, B.-C.; Hwa, Y.; Kim, J.-H.; Sohn, H.-J. Carbon Coating for Si Nanomaterials as High-Capacity Lithium Battery Electrodes. Electrochem. Commun. 2014, 46, 144−147. (16) Usui, H.; Nomura, M.; Nishino, H.; Kusatsu, M.; Murota, T.; Sakaguchi, H. Gadolinium Suicide/Silicon Composite with Excellent High-Rate Performance as Lithium-Ion Battery Anode. Mater. Lett. 2014, 130, 61−64. 6902


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