Simultaneous Encapsulation of Nano-Si in Redox Assembled rGO

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Simultaneous Encapsulation of Nano-Si in Redox Assembled rGO Film as Binder-free Anode for Flexible/bendable Lithium-ion Batteries Xin Cai, Wen Liu, Zhongqiang Zhao, Simeng Li, Siyuan Yang, Shengsen Zhang, Qiongzhi Gao, Xiaoyuan Yu, Hongqiang Wang, and Yueping Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18134 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Simultaneous Encapsulation of Nano-Si in Redox Assembled rGO Film as Binder-free Anode for Flexible/bendable Lithium-ion Batteries Xin Cai a*, Wen Liu a, Zhongqiang Zhao a, Simeng Li a, Siyuan Yang a, Shengsen Zhang a, Qiongzhi Gao a, Xiaoyuan Yu a, Hongqiang Wang b*, Yueping Fang a* aCollege

of Materials and Energy, South China Agricultural University, Guangzhou, Guangdong

510642, China. E-mail: [email protected]. [email protected] bGuangxi

Key Laboratory of Low Carbon Energy Materials, School of Chemistry & Pharmaceutical

Sciences, Guangxi Normal University, Guilin 541004, P. R. China. [email protected] KEYWORDS: Flexible/wearable energy storage; Lithium-ion batteries; Silicon; Binder-free film anode; Flexible full cells

ABSTRACT The emerging ubiquitous flexible/wearable electronics highly demand for compatible flexible/highenergy rechargeable batteries, which set a collaborative goal to promote the electrochemical performance and the mechanical strength of the fundamental flexible electrodes involved. Herein, free-standing flexible electrode of Si/graphene films is proposed, which is fabricated through a scalable, zinc-driven redox layer-by-layer assembly process. In the hybrid films, silicon nanoparticles are intimately encapsulated and confined in multilayered reduced graphene oxide (rGO) nanosheet films. The designed monolithic rGO/Si film possesses several structural benefits such as high mechanical integrity and 3D conductive framework for accessible charge transport and Li+ diffusion upon cycling. When adopted as binder-free electrode in half-cells, the optimized hybrid rGO/Si film 1

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delivers high gravimetric capacity (981 mA h g-1 at 200 mA g-1 with respect to the total weight of the electrode) and exceptional cycling stability (0.057% decay per cycle over 1000 cycles at 1000 mA g-1). Besides, the binder-free rGO/Si film anode is further combined with a commercial LiCoO2 foil cathode for completely flexible full-cell/battery, which exhibits excellent cycling performance and high capacity retention of over 95% after 30 cycles under continuous bending. This solution processable, elaborately engineered and robust Si/graphene films will further harness the potential of silicon-carbon composites for advanced flexible/wearable energy storage. 1. INTRODUCTION The upcoming ubiquitous smart and wearable electronics have witnessed the emerging flexible/bendable electronic devices, which urgently desire for compatible power sources such as advanced rechargeable lithium-ion batteries (LIBs) with higher energy density, longer life-span and exceptional mechanical deformability.1-3 Indeed, flexible/wearable LIBs are expected to be lightweight, thin and durable without compromising the basic electrochemical performance toward portable consumer electronics and arbitrary applications.4 However, commercial LIBs are always rigid and bulky, restricted by the conventional slurry-casting electrodes which contain the weakly bonded electroactive powders, polymeric binders and conductive additives on metallic foil current collectors (e.g. Cu, Al foils) .5 The insufficient interfacial contacts between the electroactive particles and the metallic foil conductor can impede the uniform charge transport throughout the electrode, but also adversely impact the multidimensional flexibility of the whole electrode especially upon severe deformation.6 Moreover, both of the mechanical strength and gravimetric capacities of the electrodes/batteries are largely limited by the heavy metallic current collectors and the electrochemically inert components (i.e. metallic current collectors, binders and conductive carbon 2


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additives), which account for a dominant weight fraction of the total electrode.7, 8 The pursuit of flexible LIBs highly relies on the elaborate structural design of the core functional electrodes. To fulfill efficient flexible LIBs, one viable approach is to develop binder-free or free-standing electrodes combing electroactive materials with electrically conductive materials in an integrated manner.9 Most good examples of the binder-free electrodes are based on a structural prototype of electroactive materials/conductive carbonaceous scaffolds (e.g. carbon cloth, carbon nanofibers, carbon nanotubes and graphenes, etc.).10-13 Along with superior mechanical flexibility, the specific performance, the mass loading of the active materials, processing scalability and the long-term stability of these binderfree electrodes hold equal challenges. Primarily, to further enhance the electrochemical performance of the flexible electrodes, significant efforts have been made by utilizing highly efficient electroactive nanomaterials.14 Silicon, a natural abundant and non-toxic source with extremely high theoretical capacity of 3579 mA h g-1 (for the stable Li15Si4 phase at room temperature), has intrigued tremendous interest to replace the commercial graphite anode materials with limited Li+ uptake capacity (372 mA h g-1).15 However, the poor intrinsic conductivity and the drastic volume expansion (~300%) of silicon upon lithiationdelithiation cycling are detrimental, which often give rise to unstable solid electrolyte interface (SEI) films and electrode pulverization, leading to electrical/physical isolations and capacity fading of the active electrodes.16 To date, various attempts have been sought to improve the reversible capacities and the cycle life of the silicon-based anodes. Main strategies include nanoengineering and the hierarchical structuration of the silicon electrode to reduce the fracture size, as well as to introduce enough internal voids/space for accelerated Li+ diffusion and buffer the volume change of silicon particles during Li+ insertion/extraction cycles.17-19 Another effective route is to hybridize silicon with 3



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a second elastic/protective/conducting phase (e.g. carbon), so as to establish more stable SEI films and facilitated electrolyte percolation.20,

21

Graphene, a 2D nanomaterial of hexagonal structured

carbon, possesses remarkable features such as outstanding mechanical strength, high electronic conductivity, and chemical/thermal stability, etc.22 The incorporation of moderate graphene substances into silicon has been proved to be quite efficient to promote the cyclability and high-rate performance of the Si-based anode in virtue of the favorable electron/ionic transfer and suppressed structural degradation within the nanocomposites.23-25 In order to get rid of inert binders and conductive additives, self-supporting Si/graphene films/membranes are developed as binder-free anodes for LIBs.26-28 Despite the impressive lithium storage performance of the designed hybrid films, some high-capacity binder-free Si/graphene electrodes are still built on macro-porous metallic foam substrates (e.g. copper foam, nickel foam), which will greatly dilute the actual specific performance of the final hybrid electrodes.29, 30 Notably, many attractive binder-free Si/graphene films are widely prepared through vacuum filtration.31-33 Nevertheless, the easy restacking of the edges of aligned graphene nanosheets during filtration renders the worse cross-plane Li+ diffusivity and inferior electrolyte penetration in the fabricated Si/graphene films, which hinders the performance enhancement of the film electrodes especially under high rates.27 Also, it is difficult to achieve areascalable manufacturing of the hybrid Si/graphene films by vacuum filtration. Here, we propose a simple and scalable solution route to construct novel 3D, free-standing Si/graphene films, where silicon nanoparticles (Si NPs) are intimately embedded and confined in multilayered reduced graphene oxide (rGO) nanosheet films. Starting from nano-Si and graphene oxides (GOs) suspension, Si NPs are simultaneously encapsulated into GO nanosheets which further assembled into multilayered rGO films by a zinc-driven redox layer-by-layer (LBL) process. The 4


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designed rGO/Si films are endowed with several unique features: (1) Upon a combined redox assembly and free drying process, rGO nanosheets are integrated into a multilayer-architectured monolithic film with substantially interconnected and opened channels for electrolyte penetration. (2) Benefiting from the simultaneous adsorption of nano-Si during the redox LBL assembly of GOs, Si NPs are adhered and bonded to rGO nanosheets without using any surfactants, forming close interfaces between Si NPs and the rGO scaffolds that afford fast electron/ion transport without the presence of binders and conductive additives. Both of the silicon ratio and the engineered interfaces of the hybrid rGO/Si films can be conveniently adjusted by pre-functionalization of the nanoassemblies. (3) The monolithic rGO films act as the mechanical strong, conductive and protective framework to alleviate the excessive growth of SEI films and the volume expansion of the active Si NPs during cycles. (4) the overall preparation process of the hybrid films is solution processable and industrially scalable without any toxic reagents and expensive materials/equipment. This synergetic strategy of LBL redox assembly process is also applicable to construct other multilayered graphenebased nanocomposites. When used as binder-free electrodes in LIBs, the optimized rGO/Si film exhibits high reversible capacities and a robust cycle life (713 mA h g-1 after 200 cycles at 200 mA g-1) as well as attractive high-rate capabilities. By further combined with a commercial LiCoO2 foil cathode for flexible full-cells, the binder-free rGO/Si film anode contributes to high capacity performance and excellent cyclability of the flexible full batteries even under bent states. 2. EXPERIMENTAL SECTION Materials Synthesis Preparation of the nano-Si suspension: Si NPs were firstly pretreated by mixed acids or KH550 (3aminopropyltriethoxysilane) before use. In a typical synthesis, 1.0 g silicon NPs (average diameter 5



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of less than 70 nm, 99%, HWNANO) are added into a mixed acid solution containing 4.0 mL hydrofluoric acid (~40wt%), 20.0 mL nitric acid (~65wt%) and 80.0 mL deionized water. The dispersion was continuously stirred for 12 h and then filtered, followed by washing with deionized water and anhydrous ethanol for several times. Later, a specific amount of the collected powder was re-dispersed into anhydrous ethanol with ultrasonic to obtain a stable nano-Si suspension (the HF/HNO3 treated, marked as S1). Meanwhile, The KH550-treated nano-Si suspension (marked as S2) was also prepared according to the procedures in our previous report.34 Preparation of the GO suspension: GO was synthesized based on a modified Hummer’s method. Firstly, 160 mL sulfuric acid was placed in a three-necked flask with the addition of 4.0 g graphite flakes (~0.4 μm, XFNANO) under stirring for 0.5 h. The reaction system was then slowly added by 16.0 g KMnO4 and kept at 45℃ for 24 h. Subsequently, the resultant mixture was slowly diluted in prearranged ice cubes under ice bath. When cooled to room temperature, the mixture was introduced by a suitable amount of H2O2 until it turned into brown. Afterwards, the brown mixture was stirred for 2 h and centrifuged. The received precipitates were fully centrifuged and rinsed with 0.1 M hydrochloric acid and deionized water, respectively. The as-synthesized GO dispersion was further purified by dialysis to remove any acid impurities and produce the final aqueous GO suspension. Preparation of the rGO/Si film: 60 mL GO suspension (5 mg mL-1) was ultrasonically mixed with different amount (1.6 mL, 3.4 mL, 5.4 mL and 10.0 mL) of nano-Si suspension (S1, 9.8 mg mL-1) under vigorous stirring to generate the GO/Si dispersion, which was designated as GO/Si-1, GO/Si2, GO/Si-3 and GO/Si-4, respectively. Noting that the formed GO/Si dispersion was in a mixed solvent including deionized water and anhydrous ethanol (vH2O/vC2H5OH=1‫׃‬1). To assemble the hybrid film, a piece of zinc foil (2 cm×3 cm) was vertically put into the GO/Si dispersion and undisturbedly 6


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reacted for 12 h. The resultant foil with black film was taken out, rinsed with deionized water and immersed in deionized water for another 3 h before freeze drying (Lab-1A-50E, BIOCOOL). After freeze drying, the produced film was peeled from the zinc foil, successively immersed in 0.1 M hydrochloric acid and deionized water to remove the impurities. The as-synthesized rGO/Si film further experienced a second freeze drying step, and was eventually annealed at 800℃ for 3 h under mixed argon/hydrogen (Ar/H2; 95:5) atmosphere. The final rGO/Si film with different silicon content mentioned-above was correspondingly designated as rGO/Si-1, rGO/Si-2, rGO/Si-3 and rGO/Si-4, respectively. Also, rGO/Si-5 was fabricated using 5.4 mL nano-Si suspension (S2, 9.8 mg mL-1) under the same conditions. Material characterizations The microstructure and morphology of the samples were pictured by field-emission scanning electron microscope (SEM, Merlin, Zeiss) equipped with an energy-dispersive spectroscopy (EDS, Oxford). High-resolution transmission electron microscope (HR-TEM, JEOL-2010) of the rGO/Si film was performed by an accelerating voltage of 200 kV. The crystalline patterns of the as-prepared rGO/Si films were characterized by an X-ray diffractometer (XRD, Rigaku) using the Cu Kα radiation (λ=1.54 Å). Thermogravimetric (TG) analyses were applied by a STA449C thermogravimetric analyzer (NETZSCH) between 40 °C and 900 °C at a rate of 10 °C min-1 under air condition. X-ray photoelectron spectroscopy (XPS, VG ESCALAB250) characterization was implemented by a 300 W monochromatized X-ray source using the Al Kα radiation under a high voltage of 15 kV. Raman spectra of the rGO/Si film was derived through a Micro-Raman Spectrometer (LabRAM Aramis, France) equipped with a 532 nm laser source. Device assembly 7



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Device assembly of half-cells: The electrochemical performances of the rGO/Si film were firstly examined by CR2025 half-cells. Unless specified, all the cells were assembled in a glove box filled with high-purity argon. The rGO/Si films were initially cut into small round pieces with a defined diameter. To assemble a half-cell, a round piece of rGO/Si film (rGO/Si-1, rGO/Si-2, rGO/Si-3, rGO/Si-4 and rGO/Si-5) was directly used as the working electrode without any binders and conductive additives, while a lithium foil was adopted as the counter electrode and a piece of Celgard 2400 membrane (thickness: ~20 μm) as the separator between them. The composition of the liquid electrolyte was 1 M LiPF6 dissolving in mixed ethylene carbonate (EC) and dimethyl carbonate (DMC) (vEC/vDMC =1:1) with additive fluoroethylene carbonate (FEC). Device assembly of flexible full cells: To assemble a full cell, a piece of rGO/Si film (2.2 cm×2.2 cm) was directly stacked on copper foil and used as the binder-free anode without any other additives and adhesives, while a commercial lithium cobalt oxide (LiCoO2 on aluminum foil, areal density of the active material: 12.8 mg cm-2; thickness of the active film: 40 ± 6 μm; Shenzhen MTI) was selected as the cathode. The electrolyte and the separator were the same as those in the hall-cells. Both of the injection of electrolyte and the encapsulation were conducted in the argon-filled glove box after carefully stacking the anode, the polymeric separator and the cathode. The electrochemical measurements were applied between a voltage window of 2.75~4.2 V at 287 mA g-1 at ambient temperature for the anode-limited flexible full cells. The specific capacities of the full cells are calculated on the basis of the rGO/Si film anode. Electrochemical measurements The galvanostatic charge/discharge profiles, rate capabilities and cycling tests of the rGO/Si films were all carried out at room temperature by a Land Battery System (LAND CT-2001A) using the same 8


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charge/discharge current. To test the coil-type cells, the cut-off voltage was set from 0.05 to 1.2 V (vs Li/Li+). Unless specified, all the specific capacities in this study were calculated by the total weight of the whole rGO/Si film electrode. Cyclic voltammetry (CV) tests were recorded on CHI660B between 0.01~1.2 V (vs Li/Li+) with a scanning rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) was conducted on an electrochemical workstation (Zahner Im6/6ex, German). The perturbation voltage was 5 mV and the frequency started from 0.01 Hz to 100 kHz. 3. RESULTS AND DISCUSSION

Figure 1 Schematic preparation process of the rGO/Si film. Before mixing with the GO suspension to form the homogeneous GO/Si dispersion, Si NPs were pre-modified by HF/HNO3 solution or KH550. The as-prepared GO nanosheets are negatively charged because of the ionization of the carboxylic and phenolic hydroxyl groups on their surfaces. On the other side, Si NPs are commonly positively charged through the surface grafting of KH550 molecules resulting from the ionization of the terminal amino groups on KH550.35 While the HF/HNO3 mixture can etch the native oxides on silicon particles and then to form a thin uniform SiO2 layer with sufficient silicon hydroxyl groups on the nano-Si surfaces.36 The established SiO2 layer 9



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will help to prevent the aggregation of Si NPs in the suspension, which can be inferred from the stabilized nano-Si dispersion for days (Fig. S1). The electrostatic interactions between GOs and the modified Si NPs is significant to construct desirable rGO/Si films. Typically, the rGO/Si film is prepared from a zinc-driven redox assembly of GOs combined with the synchronous deposition of silicon NPs, as demonstrated in Fig. 1. When firstly immersed in the GO/Si dispersion, zinc metal can spontaneously convert to Zn2+ in the acid GO dispersion by reacting with the abundant oxygencontaining functional groups on the surfaces of the GO nanosheets, leading to the interfacial assembly of rGO and the co-deposition of Si NPs through electrostatic adsorption at zinc surface. The interfacial reaction between the zinc foil and the GO/Si dispersion mainly occurs according to following equation. 37 GO + Zn + H+ → rGO + Zn2+ + H2O

(1)

Owing to the driving force from bottom Zn and the strong interlayer π–π stacking between the rGO nanosheets, multilayered rGO/Si film was layer-by-layer formed on the surfaces of the zinc foil. The zinc foil thus plays a dual function as the substrate but also the reductive agent to facilitate the redox assembly of rGO/Si. During the synthesis of the rGO-based film, the in situ formed large amount of ZnO nanocrystals in the rGO multilayer film can be observed (Fig. S2). Once endured the first freeze drying, the produced black films can be easily peeled from the Zn plate. After the removal of the impurities (e.g. ZnO) by HCl solution and a second freeze drying, rGO/Si film was finally obtained by thermal annealing at 800℃ under Ar/H2 atmosphere.

10


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Figure 2 Thermogravimetric curves (a) and XRD patterns (b) of all the rGO/Si samples. To measure the silicon content in the hybrid rGO/Si films, thermogravimetric (TG) analysis was performed under air condition and the TG curves are presented in Fig. 2a. The steep weight loss between 550 ℃ and 680 ℃ is mainly ascribed to the oxidation and combustion of the graphene components in the composite. Noting that the slight rise of the weight after the turning point at ca. 700℃ is often caused by the mild oxidation of silicon to SiO2.38 On the basis of the residual weight of the rGO/Si films after 700℃, the remained silicon ratio in rGO/Si-1, rGO/Si-2, rGO/Si-3, rGO/Si-4 and rGO/Si-5 is estimated to be 5.5wt%, 20.7wt%, 30.3wt%, 35.5wt% and 26.3wt%, respectively. It is reasonable that the final silicon content of the rGO/Si film increases when higher amount of nano-Si suspension was fed initially. Specially, the silicon content of rGO/Si-3 is higher than that of rGO/Si-5. During the redox assembly process of GO/Si, the generated Zn2+ could possibly inhibit the coadsorption of amino group-terminated Si NPs due to electrostatic repulsion. It suggests that Si NPs by HF/HNO3 treatment rather than by KH550 is more favorable for achieving designed rGO/Si film with a higher silicon content. Besides, the XRD patterns of the Si NPs, GO raw material and asprepared rGO film are provided in Fig. S3. while the XRD patterns of the rGO/Si films are demonstrated in Fig. 2b. All the major diffraction peaks of the five rGO/Si films are basically 11



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identical. The sharp diffraction peak centered at 2θ of ~25o corresponds to the (002) plane of the crystalline graphite. There is a distinct downshift of the peaks, which reflects the decreased van der Waals attraction between the assembled graphene sheets due to the Si NPs intercalated in the graphene layers.39 The characteristic peak appears at 2θ of ~28.2o, 47.1o, 55.9o, 68.9o and 76.3o indexes well to the (111), (220), (311), (400) and (331) plane of the crystalline silicon phase (JCPDS 27-1402), respectively. It is apparent that the intensities of these peaks gradually increase with increased silicon loading amount. No other peaks are detected confirming the absence of silicon carbide phase or any other impurities in the hybrid rGO/Si films.

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Figure 3 (a) Optical photos of as-prepared free-standing and flexible rGO/Si film. (b)-(d) SEM images of the rGO/Si film. (e) EDS mapping images of the rGO/Si-3 film including the Si, C and O element, respectively. (f) TEM image of rGO/Si. (g) HR-TEM image of as-prepared rGO/Si film. Inset: amplified HR-TEM image of a single silicon particle.

Fig. 3a displays the optical photos of the obtained monolithic and paper-like rGO/Si films. The produced rGO/Si films are free-standing, mechanically flexible and highly bendable by wrapping around glass rods (Fig. S4). It is attractive that the basic size of the rGO/Si films can be easily adjusted by the original zinc foil and thus is not limited to a large scale. Fig. 3b-d shows the SEM images of the rGO/Si film with a moderate Si content. The entire monolithic rGO/Si film is composed of multilayer stacked and parallel interconnected graphene-silicon sheets to shape many opened macropores between the layers (Fig. S5a, b). Tiny Si NPs and nanoclusters are almost encapsulated by the rGO layers and embedded in the ultrathin rGO sheets. Due to these structural benefits, the obtained rGO/Si films are quite flexible and soft with superior mechanical strength even though no additional surfactants were involved in the fabrication process. According to the EDS mapping images of the rGO/Si film shown in Fig. 3e, it further proves silicon particles are homogeneously dispersed in the graphene matrix of the hybrid rGO/Si film. The good encapsulation of nano-Si by multilayered rGO films is expected to favor the electrochemical durability of the active Si NPs. Also, the intercalated Si NPs can largely restrict the restacking of the rGO sheets to reserve more interspace for electrolyte percolation. Additionally, we have investigated the morphology and corresponding EDS mapping of the rGO/Si films with different Si content, which are supplied in Fig. S6~S9 and Table S1, respectively. As the initially added nano-Si dispersion increases, more Si NPs are co13



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deposited and aggregated on the surface of the rGO films, resulting in higher loading amount of silicon in the final hybrid rGO/Si film. TEM characterization is further carried out to identify the fine structure of the rGO/Si film. As shown in Fig. 3f, Si NPs (diameter of 50~80 nm) appearing in small clusters are well embedded in the transparent and ultrathin rGO nanosheets with an undisturbed honeycomb-like structure (Fig. S10a). Viewed from the HR-TEM images of the rGO/Si film (Fig. 3g and Fig. S10b), a clear lattice fringe of 0.33 nm and 0.31 nm belongs to the (002) plane of the ordered graphene sheets and the (111) plane of the crystalline silicon phase, respectively.40 The result further verifies the co-existence of Si NPs and rGO nanosheets in the hybrid film. At the same time, a layer of amorphous SiO2 with a thickness of 4~5 nm is distinguished at the surface of the silicon nanoparticle (Fig. S10c), which is likely to serve as a barrier to reduce the agglomeration of Si NPs and alleviate the volume expansion of the active nano-Si during electrochemical cycling.

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Figure 4 (a)-(c) XPS spectra derived from the rGO/Si composite film. (a) Survey spectrum. (b) Si 2p. (c) C 1s. (d) Raman spectrum of the rGO/Si film. To gain more information about the chemical composition of the hybrid rGO/Si film, X-ray photoelectron spectroscopy (XPS) analysis was further conducted using the representative rGO/Si-3 sample. The full spectrum including Si, C and O element is depicted in Fig. 4a. Observing from the derived Si 2p spectrum in Fig. 4b, the major peak at 104.5 eV is assigned to the silicon dioxides (Si4+), which is accordant with aforementioned results. While two weak peaks centered at 99.5 eV and 99.7 eV is assigned to the Si-Si bond (i.e. the monatomic Si0) and the Si-C bond, respectively.30 The low intensity of the former peak should be caused by the shielding effect of the rGO covering and the SiO2 layer outside the Si NPs.41 The latter indicates the benign adhesion force of nano-Si to 15



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the rGO matrix in the hybrid film. In Fig. 4c, the C 1s spectrum is composed of three obvious peaks. The strong peak at 284.5 eV corresponds to the graphitic sp2 C-C bond and the broad peak centered at 285.4 eV corresponds to the C-OH bond, respectively.30 Besides, the distinct peak at 284.8 eV can be attributed to the carbon atoms that are interfaced with the SiO2 species, suggesting the good adhesion between Si NPs and rGO .42 Fig. 4d demonstrates the Raman spectrum of the rGO/Si film. The weak peak occurs at around 520 cm-1 is ascribed to the silicon component, where the low intensity of the peak is largely due to the coverage of nano-Si by the dominant and multilayered rGO framework. The strong peak at 1352 cm-1 and 1597 cm-1 represents the D band (the breathing modes of the sp2 rings, disordered carbonaceous species) and the G band (the E2g phonon mode, ordered carbonaceous species) of typical graphene materials, respectively.43 The sharpness of the D peak infers the substantial edges rather than structural defects within the rGO nanosheets. By calculating through the empirical Tuinstra-Koenig relation, the intensity ratio (ID/IG) corresponds to a 3.3 nmsized ordered graphitic domain in the composite.39 Simultaneously, the featured 2D band of the graphene-based material is blue shifted and split into two peaks at 2693 cm-1 and 2941 cm-1, respectively, which reveal the multilayer-graphene nature of the rGO nanosheets in the hybrid rGO/Si film.44 The electrochemical performances of the obtained composites were firstly examined in coil-type half-cells using a piece of rGO/Si film as the binder-free electrode without any binders and conductive additives involved. Fig. 5a presents the initial galvanostatic charge/discharge profiles of the five different rGO/Si film electrodes at a current density of 50 mA g-1. For the initial discharge, the sharp slope ranging from 1.2 to 0.6 V and an accompanied short plateau are relevant to the decomposition of the electrolytes and the formation of the SEI film.23 Then a long voltage plateau occurs between 16


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0.14 V and 0.05 V because of the alloying of the active crystalline silicon with Li+. During charge, a voltage plateau at around 0.4 V agrees well with the dealloying process of the amorphous LixSi phase.28 The initial discharge capacity of the rGO/Si-1, rGO/Si-2, rGO/Si-3, rGO/Si-4 and rGO/Si-5 film electrode is 1151, 1806, 2729, 981 and 1809 mA h g-1. While the corresponding initial columbic efficiency (CE) of the five film electrode is calculated to be 84.5%, 49.4%, 50.4%, 46.9% and 58.1%, respectively. The capacity loss is largely caused by the severe Li+ consumption by rGOs with considerable specific area and the side reactions of the active silicon NPs with the electrolyte. As the silicon content in the hybrid rGO/Si film rises to over 20wt%, the initial efficiency of the electrode significantly decreases, which is probably due to the increased bare Si NPs/clusters anchoring on the surfaces rather than embedded in the graphene layers with good protection (Fig. S6 and Fig. S11). According to the selected galvanostatic charge/discharge profiles (from the 1st cycle to the 200th cycle) of the rGO/Si-3 and the rGO/Si-5 film electrode (Fig. S12, S13), the hybrid rGO/Si films show greatly suppressed capacity loss and high CE for the subsequent cycles.

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Figure 5 (a) Initial galvanostatic charge/discharge profiles of the rGO/Si film electrode at a current density of 50 mA g-1. (b) Cycling capacities and corresponding CE of the five rGO/Si film electrode upon different cycles at 200 mA g-1 (the first two cycles are measured at 50 mA g-1). (c) Cycling capacities and CE of the rGO/Si-3 film electrode at 1000 mA g-1 for 1000 cycles. The first ten cycles are measured at 200 mA g-1. (d) Rate performances of the five rGO/Si film electrode at different current densities. The discharge capacities and CEs of the five rGO/Si film electrodes for different cycle number are shown in Fig. 5b. Typically, the CE of the rGO/Si-3 electrode increased to 89.3% for the 2nd cycle and gradually approached to 98.0% at the 9th cycle. After 60 cycles, the CE values are basically over 99%, which implies the improved cyclic stability and capacity retention of the hybrid rGO/Si electrode with stable SEI films. 45 The specific capacities of the film electrodes endure a significant 18


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drop at the first few cycles and then tend to be stable, showing good cycling durability for the following cycles. It is mainly attributed to the well-established SEI films on the surfaces of the active electrodes during the initial cycles that is similar to many reported silicon/carbon-based composite anode materials.21, 34, 46 After 200 cycles, the specific capacity of the rGO/Si-1, rGO/Si-2, rGO/Si-3, rGO/Si-4 and rGO/Si-5 film electrode is maintained at 358, 483, 713, 326 and 535 mA h g-1, respectively. Particularly, the average capacity drop per cycle of the optimal rGO/Si-3 film is 0.22%. It is rational that the cycling capacities increase with higher silicon content in the hybrid rGO/Si film on account of the dominant capacitive contribution from the highly active silicon component. However, an even higher silicon content has adverse effect on the cycling capacities of the rGO/Si film electrode (rGO/Si-4), which results from the agglomerated Si clusters lacking of effective encapsulation and protection by the rGO framework in the hybrid film as mentioned above. Also, the variation of cycling capacities under a high charge/discharge current of 1000 mA g-1 is investigated and demonstrated in Fig. 5c. The hybrid rGO/Si film electrode can still retain 43% of its initial discharge capacities (~445 mA h g-1) after 1000 continuous cycles, giving rise to a capacity loss of 0.057% per cycle and attractive high-rate capability. In comparison with previously reported silicongraphene composites, the reversible capacities and cycling stability of our rGO/Si film anode are comparable and even superior to those of many well-designed Si/graphene anode materials, as demonstrated in Table S2.33, 47-50 The rate performances of the five rGO/Si film electrode at different current densities were measured, which are illustrated in Fig. 5d. The variation trends of the rate capabilities for the different rGO/Si film electrode are similar to those of the cycling capacities discussed above. Undoubtedly, the rGO/Si-3 film electrode achieves the highest capacities and superior rate performance compared 19



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to its counterparts due to the suitable amount of silicon incorporated in the hybrid. Specifically, as the current density varies from 100 to 2000 mA g-1, the specific capacity of the rGO/Si-3 electrode is 1210 (at 100 mA g-1), 981 (at 200 mA g-1), 791 (at 400 mA g-1), 613 (at 600 mA g-1), 504 (at 800 mA g-1), 433 (at 1000 mA g-1) and 253 mA h g-1 (at 2000 mA g-1), respectively. When the current returns to 200 mA g-1, the hybrid rGO/Si-3 film electrode restores 96.6% of its initial capacity value, exhibiting prominent reversible capacity and excellent rate capabilities. To further visualize the morphology variation of the binder-free rGO/Si electrode upon cycling, we have collected the topview and cross-sectional SEM images of the hybrid rGO/Si film electrodes after 200 intense charge/discharge cycles at 200 mA g-1 (Fig. S14). After prolonged cycles, the binder-free rGO/Si electrodes are highly lithiated with plenty of alloying products forming on the surface of the hybrid film as well as in the gaps of the multilayered rGO/Si film electrodes. Especially, the optimal rGO/Si electrode (rGO/Si-3) can still maintain its basic multilayered structure and compact morphology without any obvious deterioration and film cracks under intensive cycling. Also, the corresponding TEM images of the rGO/Si-3 after cycling are provided in Fig. S15, which further confirms the microstructural evolution of the film electrode. The above impressive reversible capacities and outstanding cycling tolerance of the binder-free rGO/Si film electrode are benefited from the well-embedded and tightly attached Si NPs in the strong rGO framework. It verifies that the multilayer-structured rGO sheets can not only act as 3D conductive supports to facilitate the charge transport and Li+ diffusion across the hybrid film, but also are quite effective to accommodate the volume expansion of silicon during extensive cycles.

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Figure 6 (a) CV curves of the rGO/Si film electrode at a scan rate of 0.1 mV s-1. (b) Nyquist plots of rGO/Si electrode after the 1st, 10th, 20th, 50th and 100th cycle, respectively. Inset: Enlarged view of the high-mid frequency region in the red dashed box. For a better understanding of the electrochemical lithiation/delithiation process of the binder-free rGO/Si film electrode, cyclic voltammetry (CV) test was applied at a scan rate of 0.1 mV s-1. The CV curves of the rGO/Si-3 film electrode for the first few cycles are shown in Fig. 6a. During the first cathodic curve, a broad protrusion occurs between 1.0~0.6 V and centers at 0.72 V, which is absent 21



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during the following cycles and corresponds to the electrolyte reduction and the subsequent formation of the SEI films on the surfaces of the silicon-based electrode during the initial cycle.51 For the second scan, a reduction peak centered at 0.18 V and two satellite oxidation peaks at 0.32 V and 0.5 V become more distinct. The former is ascribed to the insertion of Li+ into the crystalline silicon NPs and the latter peaks belong to the extraction of Li+ from the amorphous LixSi alloys, which accord well with aforementioned galvanostatic charge/discharge results. The intensities of these characteristic peaks increase during the subsequent scans due to the gradual activation and electrolyte penetration of the hybrid rGO/Si film electrode.49 The good overlapping and repeatability of the curves further prove the high reversibility and cyclability of the rGO/Si film electrode during the electrochemical Li+ storage process. In addition, electrochemical impedance spectroscopy (EIS) was performed to interpret the interfacial electrochemical process of the hybrid electrode. Fig. 6b presents the Nyquist plots of the rGO/Si electrode after the 1st, 10th, 20th, 50th and 100th cycle, respectively. The adopted equivalent circuit for fitting and corresponding fitted EIS parameters are provided in Fig. S16 and Table S3, respectively. The derived series resistance (Rs) of the rGO/Si electrode is relatively low, which implies the excellent electrical conductivity of the hybrid rGO/Si film rooting from the highly conductive matrix of rGO nanosheets. The hybrid rGO/Si electrode still exhibits a smaller Rs (2.4 Ω) even after 100 cycles, suggesting the high cycling stability and enhanced channels for electron transport of the binder-free rGO/Si film electrode. Also, the charge transfer resistance (Rct) of the rGO/Si film electrode is significantly reduced after the first cycle and further drops to ca. 40 Ω during continuous cycles. The decreased Rct reveals the well-established stable SEI films at the electrode/electrolyte interfaces upon cycling, but also relates to the improved charge transfer kinetic 22


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and the accelerated ion diffusion across the 3D porous rGO/Si film electrode.52, 53 At the same time, the hybrid rGO/Si film electrode shows favorable diffusion properties after 20 cycles, which is associated with the gradual activation of the rGO/Si film electrode allowing for rapid Li+ diffusion.20 The good interfacial properties of the rGO/Si film electrode for both electron transport and Li+ diffusion upon cycling are consistent with its excellent storage capacities and high cycling stability during lithiation/delithiation process.

Figure 7 (a) Schematic of the rGO/Si film anode-based flexible full LIB. (b) Optical photo of the flexible full LIB. (c) Optical photo of the highly flexible LIB bending into cylindrical shape. (d) Initial galvanostatic charge/discharge profiles of the flexible full LIB. (e) Cycling capacities and CE of the flexible full LIB. Moreover, to fully evaluate the practical potential of the hybrid rGO/Si film electrodes, we have further assembled flexible full LIBs by coupling the binder-free rGO/Si film anode with commercial LiCoO2 foil cathode. The full cells use the LiPF6-based electrolyte and are encapsulated by aluminum 23



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plastic films, as illustrated in Fig. 7a. Fig. 7b and c show the optical photos of the fabricated rGO/Si film anode-based flexible full LIBs. The entire flexible full battery is very thin and bendable, which can be easily curled to a cylinder shape. In consideration of the lower total capacity of the as-prepared rGO/Si film than the LiCoO2 cathode, the final flexible full cells are anode-limited and thus the corresponding specific capacities of the full batteries are calculated on the basis of the weight of the rGO/Si film anode.10, 33, 54 The electrochemical measurements of the flexible full cells were carried out across the voltage window of 2.75~4.2 V at a current density of 287 mA g-1. Fig. 7d demonstrates the initial charge/discharge profiles of the flexible full LIB under flat state. The voltage-capacity profiles present two long plateaus at an average discharge voltage of ~3.9 V, which corresponds to the stepwise Li+ insertion/extraction behavior of the active electrodes.55 The flexible full LIB owns an initial CE of 87.8% and endures an apparent capacity drop during the first ten cycles (Fig. S17), which largely result from the inevitable capacity loss of the electrodes. It is anticipated that the irreversible capacity loss of the flexible full cells could be reduced by a further prelithiation step on the rGO/Si film anode.43 After the first few cycles, the full LIB can gain high CE that are around 97~100%, as shown in Fig. 7e. Owing to the high reversibility and high stability of the active electrodes during following cycles, the flexible full cell obtains acceptable capacity retentions and still delivers a high reversible capacity of 667 mA h g-1 even after 200 charge/discharge cycles. The SEM characterization of the electrodes of the flexible full-cell and the elemental analyses of the rGO/Si film anode after cycling are displayed in Fig. S18~S20 and Table. S4, respectively. The results indicate that our free-standing rGO/Si film can play as efficient and robust binder-free anode for flexible batteries.

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Figure 8 (a) The rGO/Si film anode-based flexible full LIB is used for lighting a series of “LIB” LED arrays under flat or bended states. (b) Galvanostatic charge/discharge profiles of the flexible full LIB for selected cycles at 287 mA g-1 under different states. (c) Cycling performance and CE of the flexible full battery under flat state (no bending) or bending states (90o bending) at 287 mA g-1. Inset: Schematic of the flat state (0o) and 90o bending state (left). The bending radius for testing the flexible LIB is 2 cm (right). To further demonstrate the mechanical flexibility of the flexible full LIB toward practical application, we have measured its bending tolerance under external pressure. As shown in Fig. 8a, our rGO/Si film anode-based flexible full LIB is used to drive a LED array consisting of a series of LEDs (the capital “LIB” is composed of 32 commercial red LEDs) under different states. Encouragingly, the flexible battery can easily lighten and power the LED array even under highly bent states (See Video S1 in Supporting Information). Fig. 8b and c present the voltage-capacity profiles and the cycling performance of the flexible full LIB under different states (flat state: 0o 25



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without bending; bent state: 90o bending state), respectively. The flexible battery shows a high reversible capacity of ~680 mA h g-1 at 287 mA g-1 under the flat state. When bent to the 90o state with a bending radius of 2 cm (Fig. S21), the flexible full battery does not show any apparent capacity degradation and delivers an impressive discharge capacity of ~660 mA h g-1. Also, the flexible battery keeps more than 95% of its initial capacity after 30 cycles under continuous bending, exhibiting excellent cycling stability and high bending resistance. The results reveal that both of the electrical properties and the capacity performance of the flexible full LIB can be well reserved under external mechanical/bending stress. The binder-free rGO/Si anode is thus quite effective and promising to realize high capacity, durable and completely flexible LIBs. Worth noting that the electrochemical performance of the hybrid rGO/Si film anode can be further promoted by reducing the inter-sheet resistance between the rGO nanosheets. Also, the increase of the effective Si loading in the binderfree anode is necessary to further enhance its actual capacity performance and cycling tolerance toward practical application. 56 It is expected that the rGO/Si film anode-based flexible full batteries will be more practical if the liquid electrolyte is replaced by an efficient solid electrolyte membrane offering better flexibility.57 4. CONCLULSION In summary, silicon NPs embedded monolithic multilayered rGO films, have been proposed and fabricated via a zinc-driven redox LBL assembly followed by freeze drying process. The designed hybrid rGO/Si films own unique structural benefits including the mechanical flexibility and 3D interconnected conductive framework brought by the elastic rGO nanosheets, the intimate interfaces between the encapsulated Si NPs and the rGO clamping layers in conjunction with an integrated architecture. When developed as binder-free electrodes in half-cells and flexible rGO/Si-LiCoO2 full 26


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cells, the hybrid rGO/Si film electrodes exhibit prominent Li+ storage performance, such as high reversible capacity (981 mA h g-1 at 200 mA g-1 with respect to the total weight of the electrode), high cycling capacity (713 mA h g-1 after 200 cycles at 200 mA g-1), excellent high-rate stability (0.057% decay per cycle over 1000 cycles at 1000 mA g-1) and high capacity retention (over 95% after 30 cycles under continuous bending) even at highly bent state. The combined good electrochemical and mechanical performance are contributed by the synergistic interplay between the silicon and the rGO nano-assemblies. The multilayered rGO nanosheet films not only establish 3D accessible channels for electron transport and Li+ diffusion, but also serve as elastically robust matrix to buffer the volume variation of silicon NPs and ensure the structural integrity of the hybrid films during cycles. In light of the simple solution-based and practically scalable fabrication process, the binder-free rGO/Si film will further expand the potential of versatile silicon-based flexible electrodes and the emerging monolithic electrode design toward flexible/wearable energy storage applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Optical photo of the nano-Si suspension (Fig. S1), SEM image of the rGO-ZnO film (Fig. S2), XRD patterns of Si NPs, GO and rGO film (Fig. S3), Optical photo of the bendable rGO/Si film (Fig. S4), SEM images and EDS mapping of rGO/Si film with different Si content (Fig. S5~S9, Table S1), TEM images of the rGO/Si film (Fig. S10), SEM images of rGO/Si-5 (Fig. S11), Selected galvanostatic charge/discharge profiles of the rGO/Si film electrode (Fig. S12, 13), Comparison of the preparation and the electrochemical performance of reported Si/graphene anode materials (Table S2), SEM 27



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images and TEM images of the rGO/Si film electrode after cycling (Fig. S14, 15), Equivalent circuit and fitted EIS parameters (Fig. S16, Table S3), Selected galvanostatic charge/discharge profiles of the flexible full LIB (Fig. S17), SEM images and EDS mapping of the rGO/Si film anode after cycling (Fig. S18~S20, Table S4), Optical photo of the rGO/Si film anode-based flexible full battery (Fig. S21). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] Notes The authors declare no conflict of interest. ACKNOWLEDGEMENTS This work was jointly supported by the Guangdong Provincial Natural Science Foundation (No. 2017A030313283), the National Natural Science Foundation of China (NSFC 51602109, 21802046), and the Guangzhou Science and Technology Planning Project (No. 201704030022). REFERENCES (1) Nathan, A.; Ahnood, A.; Cole, M. T.; Lee, S.; Suzuki, Y.; Hiralal, P.; Bonaccorso, F.; Hasan, T.; Garcia-Gancedo, L.; Dyadyusha, A.; Haque, S.; Andrew, P.; Hofmann, S.; Moultrie, J.; Chu, D. P.; Flewitt, A. J.; Ferrari, A. C.; Kelly, M. J.; Robertson, J.; Amaratunga, G. A. J.; Milne, W. I.: Flexible Electronics: The Next Ubiquitous Platform. P. IEEE 2012, 100, 1486-1517. (2) Cai, X.; Peng, M.; Yu, X.; Fu, Y. P.; Zou, D. C.: Flexible Planar/fiber-architectured Supercapacitors for Wearable Energy Storage. J. Mater. Chem. C 2014, 2, 1184-1200. (3) Fu, K. K.; Cheng, J.; Li, T.; Hu, L. B.: Flexible Batteries: From Mechanics to Devices. Acs 28


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Table of Content 133x85mm (144 x 144 DPI)


Simultaneous Encapsulation of Nano-Si in Redox Assembled rGO

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