for High Stability Silicon Anodes in Lithium-Ion Batteries

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Electrode design from “internal” to “external” for high stability silicon anodes in lithium ion battery Shaowei Qi, Xinghao Zhang, Wei Lv, Yunbo Zhang, Debin Kong, Zhijia Huang, and Quan-Hong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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ACS Applied Materials & Interfaces

Electrode design from “internal” to “external” for high stability silicon anodes in lithium ion battery Shaowei Qi#,†, Xinghao Zhang#,⊥, Wei Lv*,†, Yunbo Zhang∥, Debin Kong*,⊥, Zhijia Huang∥ and Quan-Hong Yang*,†,‡ †.Shenzhen

Geim Graphene Center, Engineering Laboratory for Functionalized Carbon

Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China ⊥.CAS

Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence

in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China ‡.Nanoyang

Group, State Key Laboratory of Chemical Engineering, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 300072, China ∥.Tsinghua-Berkeley

#.These

Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, China.

authors contributed equally.

KEYWORDS: Lithium-ion batteries, Si anode, Conductive polymer coating, Stability, Graphene framework

ABSTRACT: Building a stable electrode structure is an effective way to promote the practical applications of Si anode, which has large volume changes during charge-discharge process, in

ACS Paragon Plus Environment

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lithium-ion batteries. Herein, we fabricated an integrated electrode structure reinforced from “internal” to “external” to boost the performance of Si nanoparticles (NPs). The electrode contains the conductive polymer of poly(3,4-ethylene dioxythiophene): poly(styrenesulphonic acid) (PEDOT:PSS) as the binder, reduced graphene oxide (rGO) and hydroxylated Si NPs which help form the “internal” interaction between them through the hydrogen bonding, while the “external” malleable network built by the flexible polymers and two-dimensional rGO sheets as the framework endows the highly flexible network to accommodate the Si expansion and forms long range conductive network. Thus, the built integrated electrode by the simple casting method shows high capacity, good rate performance and long cycling stability. It is noted such electrode shows high areal capacity of 3.29 mAh cm-2 and high volumetric capacity of 3290 Ah cm-3 at 0.09 mA cm-2. The integrated electrode design is promising to promote the practical use of Si anodes and can be extended to other noncarbon anodes with large volume changes.

1. INTRODUCTION The research of high-performance anode has become one of the most active parts in lithium-ion batteries. Among different anode materials, the most promising one is silicon (Si) because of its low working potential and high gravimetric capacity (3579 mAh g-1 for Li15Si4).1-5 Although it has broad application prospects, the commercialization is still limited due to the poor cycle life of Si anode. This serious disadvantage stems from its inherent poor conductivity and the well-known bulk variations of up to 300% between charge and discharge states3,6-8 which trigger a variety of fading mechanisms, including pulverization of active material, destroy and repeating formation of solid-electrolyte interphase (SEI) layer, loss of electrical connections, and electrode delamination.9 The electrode nanostructure design is an effective way to solve above problems and promote its


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use in LIBs. Additionally, to boost the next-generation flexible electronic devices, designing high performance, integrated and flexible Si based anodes is also highly desired.10-13 In these regards, diverse exciting Si-based electrode paradigms, especially the construction of flexible integrated electrodes, have been extensively exploited.12,13 The skeleton networks that are mostly built by carbon in these structures not only provide electron transport network, but also alleviate the huge expansion of Si and maintain the electrode stability. The binders also play a key role in ensuring the intact of the electrode and stabilize the Si surface during the reactions.14,15 It has been shown the binder-stabilized Si nanoparticles (NPs) have stable SEI film, enhancing the coulombic efficiency and cyclic stability.16 However, Si NPs in the integrated electrodes easily lose electric connection during cycling owing to the failure of binders encountering the large volume changes, which finally destroys the electrode structure. Thus, how to design a stable Si based integrated electrode is still not well solved until now. Herein, we proposed an effective stabilization strategy for the integrated Si NPs-graphene hybrid electrode, which is featured by the synergistic effect of the “internal” strong bonding between the binder, Si NPs and the electrode skeleton (reduced graphene oxide (rGO)) and the “external” malleable network built by the flexible polymers and two-dimensional (2D) rGO nanosheets (NSs), as shown in Figure 1. The Si NPs are functionalized by introducing the hydroxy groups, and conductive polymer, poly(3,4-ethylene dioxythiophene): poly(styrenesulphonic acid) (PEDOT:PSS), is adopted as the ideal binder. Thus, the binder could interact with both Si NPs and rGO that contains hydroxyl/carboxyl groups, resulting in stable interfaces between them to lower the internal resistance and reinforcing the electrode structure to accommodate the volume changes of Si. Moreover, such electrode can be easily prepared by the casting of homogeneously mixed slurry composed of hydroxylated Si NPs, the PEDOT:PSS and rGO. Benefiting from above


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advantages, the fabricated electrodes demonstrated high rate performance, high areal capacity and improved cycling stability. 2. EXPERIMENTAL SECTION 2.1 Preparation of hydroxylated silicon nanoparticles (H-Si NPs): Hydroxylated Si NPs were synthesized by an oxidation reaction in acid. 15 mL concentrated sulfuric acid was mixed with 5 mL 30% H2O2 slowly, and 100 mg Si NPs was added into the above solution and followed by stirring at room temperature for 24 h. The obtained product was denoted as H-Si NPs. 2.2 Preparation of reduced graphene oxide (rGO): Graphite oxide was synthesized by a modified Hummers method as reported.23 10 mg mL-1 GO solution was obtained by strong sonication of graphite oxide for 40 min. The 55% HI solution were added into 5 mL aforementioned GO suspension, followed by stirring in a hermetically sealed chamber for 4 h protected from light. After the reaction was finished, deionized water was added to the product, and the precipitate was washed and centrifuged repeatedly. 2.3 Synthesis of H-Si NPs-PEDOT:PSS hybrids (H-Si-P): The H-Si NPs-PEDOT:PSS hybrids were synthesized by an in situ oxidation reaction in liquid phase. The obtained H-Si NPs (100 mg), 3,4-ethylenedioxythiophene (EDOT, 10 mg) and poly(sodium-p-styrenesulfonate) (PSS, 45mg) were dispersed in 10 mL deionized water with stirring for 20 min to obtain a homogeneous mixture. Then 20 mg sodium persulfate (Na2S2O8) was added into above solution slowly, followed by adding 0.015 mg FeCl3 as initiator and stirring for 12 h. The resultant suspension was further purified by ion-exchange resin and suction filter. The obtained product was denoted as H-Si-P. For the Si NPs without hydroxylation, the product was denoted as N-Si NPs (non-hydroxylation Si NPs).


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2.4 Fabrication of silicon based anode electrode: To prepare hydroxylated Si NPsPEDOT:PSS/rGO (denoted as H-Si-P/rG) electrode, 75 mg H-Si-P and 25 mg rGO were mixed in deionized water. The slurry was coated on copper foil and dried in a vacuum oven for 8 h, H-SiP/rG hybrid electrode was prepared. At the same time, N-Si-P/rG electrode was also prepared. For comparison, conductive carbon black (super-P) and carbon nanotube (CNT) were used to replace rGO, and the H-Si NPs-PEDOT:PSS/super-P (denoted as H-Si-P/SP) and H-Si NPsPEDOT:PSS/CNT (denoted as H-Si-P/CNT) electrodes were fabricated in the same way. In order to further compare the differences between PEDOT:PSS and traditional binders, different binders of styrene-butadiene rubber (SBR) and poly vinylidene fluoride (PVDF) were used in proportion. The obtained electrodes were denoted as H-Si-SBR/rG and H-Si-PVDF/rG, respectively. For all the electrode, the mass ratio of Si NPs, polymer (binder) and carbon materials is 2:1:1. The loading of active materials in different electrodes is around 1 mg cm-2. 2.5 Material characterization: The morphology of the obtained Si based electrodes were observed by a field emission scanning electron microscope (HITACH SU8010) at 5 kV. The microstructure and the elemental distributions of the Si-PEDOT:PSS hybrids were investigated by a high resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30) at an accelerating voltage of 300 kV. For the peeling test, the sample sheets with a dimension of 100 × 20 × 1 mm3 by a casting process were adjusted vertically to two sample clips with a resulted dimension of 80 × 20 × 1 mm3. The rate of peeling test was maintained at 20 mm min-1 till the sample sheet was pulled off into two parts. X-ray diffraction (XRD) tests were conducted on a Bruker D8 Advance diffractometer with Cu K α radiation (k = 0.154 nm). Fourier Transform infrared (FTIR) measurements was performed on a Nicolet iS50. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi (Thermo Fisher) at room temperature


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with a monochromatic Al K α X-ray source. Component analysis of the Si-PEDOT:PSS hybrids was carried out by using a thermogravimetric analyzer (NETZSCH STA 449F3) with a heating rate of 5 ℃ min-1 from room temperature to 800 ℃ in air. 2.6 Electrochemical measurements: The electrochemical performance of the electrodes was tested using CR2032 coin cells assembled in an argon-filled glove box with the as-prepared electrodes as cathodes and Li metal foil as counter electrode. The electrolyte was 1 M LiPF6 in a 1:9 vol/vol mixture of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC). The cycling performance and rate capability of the cells were conducted on a multichannel battery tester (Land 2001A Battery Testing System) with the voltage between 0.001 to 1.5 V versus Li/Li+. Cyclic voltammetry (CV) was obtained at the scan rates of 0.1, 0.2, 0.5 mV s-1 in the voltage range of 0 - 1.5 V on a VMP3 electrochemical working station (Bio Logic Science Instruments), and the electrochemical impedance spectra (EIS) was measured with an AC voltage amplitude of 5 mV with a frequency range of 100 kHz to 10 mHz by an impedance analyzer. 3. RESULTS AND DISCUSSION As shown in Figure 1, the integrated electrode fabrication was realized through (I) manipulating Si NPs by surface hydroxylation (H-Si NPs) to ensure its uniform dispersion and surface hydrophilicity, (II) forming a ultra-thin coating on Si NPs surface via in situ polymerization (Figure S1 a and b, Supporting Information), and (III) incorporating with rGO to prepare the integrate electrode with “plant cells” like structure. Figure 1 and Figure S1 illustrate the schematic of the electrode structure in which the H-Si NPs coated with PEDOT:PSS are imbedded in the space between the rGO NSs.17, 18 The rGO NSs as the “cell walls” form a cavity structure and play as flexible supports to improve the stability of electrode by effectively tolerant the volume changes


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of Si. Conductive polymer of PEDOT:PSS as the ideal binder can not only provide suitable binding interactions but also enhance the electrical conductivity of the whole electrode.19-21 More importantly, strong binding interactions due to the formation of hydrogen bonds between the PEDOT:PSS, Si NPs and rGO enhance the interfacial contact which leads to the improved stability of the electrode and help form a tight polymer coating on Si NPs to isolated them from electrolyte.11 Thus, this tightly bonded and firm structure possesses very high reliability and stability.

Figure 1. A schematic diagram of the structure of the H-Si-P/rG electrode, the enlarged portion in red frame shows the bonding between Si NPs, polymer and rGO. The morphology and “external” structural stability of the as-prepared H-Si-P/rG electrode have been illustrated by optical images and scanning electron microscopy (SEM) images. Figure 2a are photos of the electrode. It can be seen the surface of the electrode is flat and the electrode materials can adhere to the collector after bending (Figure 2b) and tearing (Figure 2c). The reference electrodes using super-P (SP) and CNT as the carbon supports demonstrate the active materials


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falling off (Figure S2 a and b, Supporting Information). The cross-sectional SEM images of H-SiP/rG electrode in Figure 2d and e demonstrated a layered structure, where the Si NPs coated with PEDOT:PSS were uniformly encapsulated in the small “cells” formed by the cross-linked rGO. These cells also provide enough space for the electrolyte storage which shortens the Li-ion diffusion distance. It is noted that the surface hydroxylation of Si NPs played critical role in the formation of “plant cell” structure due to the enhanced dispersing ability in solution. In contrast, aggregation of Si NPs appeared in the N-Si-P/rG electrode which restrains the formation of above structure (Figure S3, Supporting Information). The intimate contact between the electrode materials before and after cycling is confirmed by the cross-sectional SEM images (Figure S4 a-f, Supporting Information), which clearly show the polymer and rGO can well stabilize the electrode structure. Figure 2f shows the strain resistance obtained by the peeling test to further quantitatively evaluate the mechanical stability of the electrodes with different binders (PEDOT:PSS, SBR, PVDF) and carbon frameworks (rGO, CNT and super-P). The maximum force recorded on the forcedisplacement curve is the largest peeling force that can be endured between the electrode materials and between the electrode materials and the collector.15 As shown in Figure2f, the H-Si-P/rG, HSi-P/CNT and H-Si-P/SP electrodes show an initial peeling force of 2.36, 1.12 and 0.85 N, demonstrating the 2D structure of rGO NSs help enhance the mechanical property. Moreover, the peeling forces of H-Si-SBR/rG and H-Si-PVDF/rG electrodes are 1.80 and 0.25 N respectively, further indicating the synergistic effect between PEDOT:PSS and rGO can effectively enhance the anti-deformation capability of the electrode. This is possibly ascribed to the strong interaction between the rGO and PEDPT:PSS which will be discussed later.22 Moreover, as shown in Figure 2g, the H-Si-P/rG electrode demonstrates the highest conductivity of 1.6×105 S m-1 among all the


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samples because the conductive binder and the 2D rGO NSs provide interconnected electron transfer path in a long range, which effectively decrease the internal resistance of the whole electrode. In order to further prove that GO has been properly reduced and is with good electrical conductivity, the conductivities of the films prepared with different carbon materials (GO, rGO, CNT and SP) and PVDF binders were also tested and presented in Figure S5. Above results clear show the “external” malleable network of H-Si-P/rG electrode.

Figure 2． Photos of the H-Si-P/rG electrode with (a) normal, (b) bended and (c) torn states. (d, e) Cross sectional SEM images of the H-Si-P/rG electrode, white arrows, black arrows and dotted lines in (e) indicate the Si NPs, PEDOT:PSS and rGO respectively. (f) The mechanical properties of different electrodes. (g) Conductivities of different electrodes measured by four probe method.


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The combination between the H-Si NPs and PEDOT:PSS was investigated by transmission electron microscopy (TEM). As shown in Figure 3a and b, the H-Si NPs surface are covered by a thin polymer layer. In the high magnification TEM (Figure 3c), the lattice stripe corresponding to Si can be seen from the central region of the particle, while the edge corresponding to PEDOT:PSS is amorphous. In the preparation of TEM samples, the composites undergo intense ultrasonic process. However, Figure 3c shows that PEDOT:PSS still firmly coats on the H-Si NPs, indicating the strong interaction between them.23 In contrast, the surface of Si NP without hydroxylation is clean (Figure 3d), suggesting the PEDOT:PSS was not coated on their surface. The energydispersive X-ray spectroscopy (EDS) elemental mappings in Figure 3e demonstrate homogenously distributed C, O and S elements originated from PEDOT:PSS, further showing the firm anchoring of PEDOT:PSS on Si NPs, which can avoid the direct contact between the Si NPs and the electrolyte, and then reduce the occurrence of side reactions.24

Figure 3 ． TEM images of the (a-c) H-Si-P and (d) N-Si-P. The white dotted line in (c) is a demarcation line of PEDOT:PSS and Si. (e) TEM and EDX element mappings of H-Si-P.


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To characterize the “internal” interaction between Si NPs, PEDOT:PSS and rGO, fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy tests were carried out. The FTIR spectra of PEDOT:PSS mixed with rGO (denoted as P/rG), H-Si-P and N-Si-P are shown in Figure 4a. The peaks at around 660~680 and 1580~1610 cm-1 is respectively related to C=C and C-S stretching in PEDOT:PSS.25 In the P/rGO mixture, a dispersive peak in the range of 2800-3500 cm-1 which is related to hydrogen bond can be found.26 The C=C stretching peak and C-S stretching peak are also weak possibly due to that the –SO3 of PSS combine with the –OH/-COOH of rGO by forming hydrogen bond.20 The vibration peak of hydroxyl group at around 3200-3700 cm-1 shows more obvious shift for H-Si-P than N-Si-P which means the hydrogen bonding effect in H-Si-P is stronger.27 The two peaks around 2800-3000 cm1

also prove the hydrogen bonding contributed by abundant hydroxyl groups on hydroxylated Si

NPs and PEDOT:PSS (Figure S6),28 and thus, PEDOT:PSS can be tightly coated on the surface of H-Si NPs. The XPS spectra of Si 2p (Figure 4b) for the H-Si-P hybrid show two peaks, one is the spin-orbit split Si 2p signals at 99.4 eV (-Si-O-C-) further indicating the interaction between the Si NPs with PEDOT:PSS.29, 30 The Si content in the H-Si-P hybrids is 60 wt% and in the N-Si-P hybrids is 70% according to the TGA curves (Figure S7). Since the surface of Si NPs is rich in hydroxyl groups after hydroxylation, which can bind more PEDOT:PSS, resulting in relatively lower Si content in the H-Si-P compared with N-Si-P.31 XRD patterns of PEDOT:PSS, rGO and H-Si-P/rG are shown in Figure 4c. The PEDOT:PSS is an amorphous polymer with no characteristic peaks,32 and the broad peak distributed at 26° is derived from the partial stacking of rGO.33 The Raman spectra of H-Si-P/rG and H-Si-rG hybrid are shown in Figure 4d and Figure S8. The D and G bands of rGO


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locate at about 1345 and 1590 cm-1..23 The D and G bands shift of the H-Si-P/rG hybrid also suggests the PEDOT:PSS strongly interacts with rGO.34

Figure 4． (a) FTIR spectra of PEDOT:PSS mixed with rGO, H-Si-P, N-Si-P. (b) XPS spectra of Si 2p for the Si in H-Si-P. (c) The XRD spectra of H-Si-P/rG, rGO and PEDOT:PSS. (d) Raman of H-Si-rG, H-Si-P/rG hybrid. The electrochemical properties of H-Si-P/rG have been tested with a coin-type cells (2032). Typical CV curves of the H-Si-P/rG electrode are shown in Figure 5a, and the cathodic peak at 1.2 V only appeared in the first cycle, which should be ascribed to the irreversible reaction of rGO with Li+.35 After the first cycle, the cathodic peak intensity at 0.2 V and anodic peak intensities at 0.33 and 0.51 V slightly increase due to the electrode activation.36 At the same time, compared


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with the other samples, the peak separation of cathodic and anodic peak is smaller with the scan rate increasing from 0.1 to 1mV s-1, suggesting the much weaker polarization of H-Si-P/rG electrode (Figure S9).11 As shown in Figure 5b,, the H-Si-P/rG electrode deliver an outstanding rate performance with a high capacity of about 2850 mA h g-1 (based on the weight of Si) at 2 A g-1. When the current density back to 0.5 A g-1, a reversible capacity of about ~3500 mA h g-1 was recovered, suggesting its good structure stability. In comparison, due to aggregation of Si, the NSi-P/rG electrode delivers lower capacity of 2000 mAh g-1 at 2 A g-1. Moreover, the H-Si-P/CNT and H-Si-P/SP electrode deliver much lower capacity of 1477 mAh g-1 and 957 mAh g-1 respectively because CNT and SP cannot well maintain the structural stability. To show the enhanced structure stability of H-Si-P/rG derived from the internal bonding and external flexible framework, the H-Si-P/rG and the control electrodes were first activated at a current density of 100 mA g-1 for the first two cycles, and then cycled at high current density of 500 mA g-1 (Figure 5c). The N-Si-P/rG electrode shows rapidly capacity fading with the low capacity of 426.1 mAh g-1 after 500 cycles, which attribute to the aggregation of Si NPs and the less of bonded PEDOT:PSS layer on their surface, leading to the pulverization of the NPs and the electrode structure destruction during the cycling. At the same time, the H-Si-P/CNT and H-SiP/SP electrodes not only show low capacity of 1775.5 mAh g-1 and 708.7 mAh g-1 at third cycle but also have low capacity retention of 38.4% and 29% respectively after 500 cycles. In contrast, the H-Si-P/rG electrode exhibits excellent cycling stability and a high reversible capacity of 2928.5 mAh g-1 at third cycle, and remain a reversible capacity of 1337.6 mAh g-1 after 500 cycles. In addition, SBR and PVDF binders that cannot form the hydrogen bonds interaction within the electrode are used to show the important role of internal interaction on improving the electrochemical performance. The H-Si-SBR/SP electrode shows rapidly capacity fading in the


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first 150 cycles with the low capacity of 120.9 mAh g-1, and the H-Si-PVDF/SP electrode failed to work (Figure S10, Supporting Information). This should be ascribed to the following reasons. First, the bonding between the hydroxylated Si NPs with PEDOT:PSS helps enhance the electron transfer and the flexibility of the electrode.37 Second, compared with the one-dimensional CNT and zero-dimensional SP, the rGO NSs can greatly enhance the mechanical properties of electrode due to its 2D structure and the bonding between them with the PEDOT:PSS, which can better tolerate the strain of Si expansion and build more efficient conductive networks.38 Moreover, the large surface area of rGO NSs is also profit to the uniformly dispersing of Si NPs in the electrode. Thus, this “internal to external” stabilization strategy sheds an effective way to improve the electrochemical performance of noncarbon anodes with large volume expansion. The first lithiation and delithiation capacities are 4287 and 3144 mA h g-1 (at 0.1 A g-1, Figure S11 a), corresponding to the initial coulombic efficiency of 73.3%. Figure S11 shows a typical discharge profile of Si with a long plateau at around 0.12 V.39 The charge/discharge curves with the first activation cycle are displayed as Figure S11. It is found that the profiles of first cycle are consistent with the CV profiles in Figure 5a and show a typical charge–discharge plateau of Si. Figure 5d demonstrates the galvanostatic charge/discharge curves of the H-Si-P/rG electrode at 0.5 A g-1. In the different cycles, the charge-discharge profiles show no obvious difference expect for the slight decrease of the capacity, showing the good cyclic stability. Benefiting from the high gravimetric capacity, the H-Si-P/rG electrode also exhibits very high areal capacity. As shown in Figure 5e, the H-Si-P/rG electrode delivers a capacity of 3.29 mAh cm-2 at 0.09 mA cm-2, which is maintained at about 2.64 mAh cm-2 even with a high current density of 1.85 mA cm-2. The areal capacity of the prepared electrode is much better than those of other reported works when the current density is over 1 mA cm-2 (Table S1). Such high rate performance should be ascribed to


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Figure 5. CV profiles of the initial two cycles of (a) H-Si-P/rG electrode. (b) Rate capability of H-Si-P/rG, N-Si-P/rG, H-Si-P/CNT and H-Si-P/SP electrodes. (c) Cycling performance of the HSi-P/rG, N-Si-P/rG, H-Si-P/CNT and H-Si-P/SP electrodes at 0.1 A g-1 after activation at 0.5 A g-1. (d) Galvanostatic charge/discharge profiles of H-Si-P/rG electrode at 0.5 A g-1. (e) Performance comparison of H-Si-P/rG with the typical silicon-based anodes recently reported (Capacities are calculated based on the weight of Si, and the detailed data and materials are described in Table S1 in Supporting Information. SEM images of (f) H-Si-P/rG and (g) N-Si-P/rG after cycling.


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the much lowered internal resistance (Figure S12) and charge transfer resistance due to the highly bonded interfaces between the different components. Based on the above results, the enhanced capacity, cycling life as well as rate performance of the H-Si-P/rG electrode can be attributed to the following three reasons: (I) The stability of electrode structure. The hydrogen bonds between H-Si NPs, conductive PEDOT:PSS and rGO NSs enable the inactive binders in electrode to play a dual-role of flexible buffer layer and conductive network. (II) By using the conductive PEDOT:PSS, the conductivity of the whole electrode has been greatly improved which enhances the rate performance. (III) The good dispersion of H-Si NPs also plays a key role in improving the high utilization of Si NPs. The SEM images of above different electrodes after cycling are shown in Figure 5f and g and Figure S13 and S14. Figure 5f and g show the structure of the H-Si-P/rG and N-Si-P/rG after cycling which demonstrate the well maintained Si NPs in the H-Si-P/G electrode and the cracked Si NPs in the N-Si-P/G electrode, suggesting the PEDOT:PSS layers on Si surface well protect and accommodate their volume expansion during cycling. In addition, the low magnification SEM images of the electrodes with different carbon frameworks (rGO, CNT and super-P) and different Si NPs (H-Si and N-Si) are shown in Figure S13 and S14. It is shown that H-Si NPs are uniformly distributed in the electrode, and after cycling, the H-Si-P/rG electrode shows no cracking compared with the other electrodes. These results suggest the integrated H-Si-P/rG electrode has much better structure stability, which could accommodate the large volume change of active materials and keep the electric contact during cycling. 4. CONCLUSION


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We proposed an effective method to stabilize the Si NPs by building the integrated flexible electrode reinforced from “internal” to “external”. The strong “internal” interaction between the conductive PEDOT:PSS, Si NPs and the rGO simultaneous stabilizes the interfaces between them, facilitates the electron transfer and helps maintain the structure stability of the electrode, while the “external’ malleable network built by the flexible polymers and 2D rGO NSs creates stable and robust framework to accommodate the Si expansion, maintain the electrode integrity and provide the long range conductive network. Thus, the as-fabricated H-Si-P/rG electrode exhibited very low internal resistance and good mechanical properties. Owing to the synergistic effect from “internal” to “external”, the H-Si-P/rG electrode demonstrates enhanced electrochemical performances in terms of high rate capability (2850 mAh g-1 at 2 A g-1), long cycling capability (1309 mAh g-1 after 500 cycles at 500 mA g-1) and high areal capacity (3.29 mAh cm-2 at 0.09 mA cm-2). Moreover, the fabrication process for the H-Si-P/rG electrode is easy to realize and promising for industrial production. Overall, this study offers a simple solution to solve the problems of alloying anode materials with high volume changes (such as Si, Sn etc.) to promote their practical uses. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Q.-H. Yang); [email protected] (W. Lv); [email protected] (D. Kong) ORCID Quan-Hong Yang: 0000-0003-2882-3968 Author Contributions


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This paper was written through contributions of all authors. All authors have given approval to the final version of this paper. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. U1601206), the National Science Fund for Distinguished Young Scholars, China (No. 51525204), the Guangdong Special Support Program (2017TQ04C664), the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N111), the Beijing Natural Science Foundation (2192061), and the Shenzhen Basic Research Project (Grant No. JCYJ20170412171630020 and JCYJ20170412171359175). REFERENCES (1) Kang, S.; Yang, K.; White, S. R.; Sottos, N. R. Silicon Composite Electrodes with Dynamic Ionic Bonding. Adv. Energy Mater. 2017, 7 (17), 1700045. (2) Ko, M.; Chae, S.; Ma, J.; Kim, N.; Lee, H.; Cui, Y.; Cho, J. Scalable Synthesis of SiliconNanolayer-Embedded Graphite for High-Energy Lithium-Ion Batteries. Nat. Energy 2016, 1 (9), 1-8. (3) Choi, S.; Kwon, T. W.; Coskun, A.; Choi, J. W. Highly Elastic Binders Integrating Polyrotaxanes for Silicon Microparticle Anodes in Lithium Ion Batteries. Science 2017, 357 (6348), 279-283. (4) Zong, L.; Jin, Y.; Liu, C.; Zhu, B.; Hu, X.; Lu, Z.; Zhu, J. Precise Perforation and Scalable Production of Si Particles from Low-Grade Sources for High-Performance Lithium Ion Battery Anodes. Nano Lett. 2016, 16 (11), 7210-7215.


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for High Stability Silicon Anodes in Lithium-Ion Batteries

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