SnOx Frameworks within Polyaniline

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Confined Porous Graphene/SnOx Frameworks within PolyanilineDerived Carbon as Highly Stable Lithium-Ion Battery Anodes Dan Zhou, Weili Song, Xiaogang Li, and Li-Zhen Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01875 • Publication Date (Web): 12 May 2016 Downloaded from http://pubs.acs.org on May 12, 2016

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ACS Applied Materials & Interfaces

Confined Porous Graphene/SnOx Frameworks within Polyaniline-Derived Carbon as Highly Stable Lithium-Ion Battery Anodes Dan Zhou, Wei-Li Song, Xiaogang Li, Li-Zhen Fan* Key Laboratory of New Energy Materials and Technologies, Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing, 100083, China

1 Environment ACS Paragon Plus


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ABSTRACT. Tin oxides are promising anode materials for their high theoretical capacities in rechargeable lithiumion batteries (LIBs). However, poor stability usually limits the practical application owing to the large volume variation during the cycling process. Herein, a novel carbon confined porous graphene/SnOx framework was designed using a silica template assisted nanocasting method followed by a polyanilinederived carbon coating process. In this process, silica served as template to anchor SnOx nanoparticles on porous framework and polyaniline was used as the carbon source for coating on the porous graphene/SnOx framework. The synthesized carbon confined porous graphene/SnOx frameworks demonstrate substantially improved rate capacities and enhanced cycling stability as the anode materials in LIBs, showing a high reversible capacity of 907 mAh g−1 after 100 cycles at 100 mA g−1 and 555 mAh g−1 after 400 cycles at 1000 mA g−1. The remarkably improved electrochemical performance could be assigned to the unique porous architecture, which effectively solves the drawbacks of SnOx including poor electrical conductivity and undesirable volume expansion during cycling process. Consequently, such design concept for promoting SnOx performance could provide a novel stage for improving anode stability in LIBs. KEYWORDS. Lithium-ion batteries, anode, Tin oxides, graphene, carbon coating, cycling stability


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INTRODUCTION Rechargeable lithium-ion batteries (LIBs) have attracted increasing interest as the high-performance energy storage devices for portable electronics, hybrid electric vehicles (HEVs) and electric vehicles (EVs) due to their high energy density and long cycle life.1–5 Currently, commercial graphite anodes appear insufficient to satisfy the capacity demand for HEVs and EVs owing to the limited theoretical capacity (372 mAh g−1).6,7 As the promising candidates, Sn-based materials reveal huge advantages such as high theoretical capacity (875 mAh g−1 for SnO and 782 mAh g−1 for SnO2), abundant resources, inexpensive cost and environmental friendliness.8–11 However, inherent drawbacks such as poor electrical conductivity and large volume expansion during Li+ insertion process often lead to serve pulverization in the anode and formation of unstable solid electrolyte interface (SEI) layer, which results in limited cycling capacity and unexpected rate performance.12–16 In order to achieve excellent electrochemical performance, design of nanosized SnOx/carbonaceous materials with porous structure is considered to be one of the most promising strategies.17–19 Noticeably, Sn-based anodes that integrate nanosized SnOx with three dimensional (3D) graphene present enhanced cycling stability and improved rate capacities in term of the synergistic effects.20–23 First, nanosized SnOx can effectively reduce the volume expansion and thus a partial improvement of the electrochemical performance is achieved.24,25 On the other hand, 3D porous graphene offers strong mechanical strengths, high specific surface area and high electrical conductivity, which can provide efficient electrolyte penetration, short diffusion length and rapid transport kinetics for Li+ in the electrodes.26,27 Consequently, enhanced cycling stability and improved rate capacities would be realized. Actually, a variety of studies have been done in the design of 3D nanosized SnOx/carbonaceous materials to improve the anode electrochemical performance. Yao et al.28 designed a 3D anisotropic SnO2/graphene aerogel anode, which delivered an improved stable cycling stability of 872 mAh g−1



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upon 50 cycles at 100 mA g−1. Shen et al.29 prepared SnO2/graphene composite with cambered nanowalls structure and the assembled anode offered a reversible capacity of 855 mAh g−1 after 100 cycles at 100 mA g−1. Li et al.30 anchored nano-sized SnO2 vertically on the aligned graphene, which exhibited a specific capacity of 210 mAh g−1 with an ultra-long cycle life over 5000 cycles at 9 A g−1. In general, the excellent electrochemical performance of these anode materials could be ascribed to the synergistic roles between nano-sized SnO2 and graphene, which provide the composite electrodes with stable structural integrity, fast charge transfer, improved electrical conductivity and efficient accommodation of volume expansion.31,32 Nevertheless, some critical issues still remain and the concern is focused on the formation of thick solid electrolyte interphase (SEI) layer that is difficult to prevent due to the direct contact of SnOx with electrolytes. Meanwhile, the aggregation of SnOx nanoparticles on the surface of graphene substrate is still difficult to be avoided.33,34 To address these problems, design of a 3D porous SnOx/graphene composite with additional carbon coating seems a promising prospect, where SnOx is expected to be confined by the introduced carbon coating and thus direct contact between SnOx and electrolyte could be well avoided. Herein, a novel carbon confined porous graphene/SnOx framework was stepwise conducted by a silica template assisted nanocasting method and a subsequent polyaniline-derived carbon coating process, in which the 3D porous graphene substrate anchored with SnOx could be coated with polyaniline (PANI)derived carbon. The unique 3D porous architecture can not only prevent the direct contact of SnOx with electrolyte, but also provide sufficient space to accommodate the volume expansion during cycling, which is favorable for the structural integrity in the electrode. As a consequence, such carbon confined porous graphene/SnOx framework presents excellent electrochemical performance with high initial specific capacity, excellent rate ability and highly stable cyclability. EXPERIMENTAL SECTION


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Synthesis of graphene/SiO2 composite. Briefly, graphene oxide (GO) powders were first prepared using natural graphite flakes as raw materials via the modified Hummers method.35 Then, 0.2 g of cetyltrimethyl ammonium bromide (CTAB) was mixed with a homogeneous GO solution (0.4 g GO powders dispersed in a mixture containing 160 mL ethyl alcohol and 160 mL deionized water) under mild stirring for 20 min. Subsequently, 3 mL concentrated ammonia solution (NH3·H2O, 28 wt%) and 2.5 mL tetraethylorthosilicate (TEOS) were added in the above solution. After strong stirring for 10 h, black GO/SiO2 powders were collected by centrifugation and washing with deionized water, and then dried at 80 oC for 12 h. Finally, GO/SiO2 powders were converted into graphene/SiO2 composite by annealing at 800 oC for 3 h under N2 atmosphere. Fabrication of graphene/SiO2/SnO2 composite. For the synthesis of graphene/SiO2/SnO2 composite, as-synthesized graphene/SiO2 composite (0.48 g) was added to 2 g SnCl4·H2O containing 40 mL ethyl alcohol, and magnetically stirred at 50 oC for several hours. After ethyl alcohol was vaporized, the collected precipitate was treated at 500 oC for 3 h under N2 atmosphere and the resultant powders were assigned as the graphene/SiO2/SnO2 composite. Preparation of carbon confined porous graphene/SnOx frameworks. Carbon confined porous graphene/SnOx frameworks were prepared as follows. Initially, 0.15 g aniline monomers were mixed in 40 mL deionized water under constant stirring. Then, diluted hydrochloric acid (HCl, 2 mol L−1) was added into the solution to adjust the pH to 1. Next, 0.15 g graphene/SiO2/SnO2 composite was added and the mixture was stirred for another 15 min. Subsequently, 0.24 g ammonium persulfate (APS) was used as oxidant for the reaction solution and the mixture was stirred for 12 h in ice bath. Upon the completion of polymerization, the color of reaction mixture turned to dark green, suggesting the formation of PANI coating

on

the

surface

of

graphene/SiO2/SnO2

composite.

The

resulting

reactants

(graphene/SiO2/SnO2/PANI) were obtained via centrifugation and washing with deionized water,



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followed by drying at 80 oC for 12 h. Graphene/SiO2/SnO2/PANI was then treated at 600 oC for 2 h under N2 atmosphere and immersed into 10% HF solution for 24 h at room temperature. The carbon confined porous graphene/SnOx framework was finally obtained after washing (deionized water) and drying (80 oC for 12 h). As control samples, porous graphene/SnOx framework without carbon coating (denoted pG/SnO2) was synthesized by immersing the sintered graphene/SiO2/SnO2 composite in 10% HF solution for 24 h at room temperature, followed by washing and drying at 80 oC for 12 h. Pure SnO2 was prepared by sintering pG/SnO2 at 700 oC in air for 3 h with a heating rate of 3 oC min−1. Characterizations. The phases and crystal structures of the synthesized samples were recorded by Xray diffraction (XRD, Rigaku D/max-Rb) using Cu Kα irradiation at λ=1.5406 Å. The morphologies and microstructures were determined by field emission scanning electron microscopic (FE-SEM, JEOL JSM-6330) and transmission electron microscopy (TEM, JEM-2010F). The SnOx loading mass was investigated by thermogravimetric analysis (TGA) using a simultaneous thermalanalyzer (NETZSCH STA449F3) at a raising rate of 10 oC min−1 with air atmosphere from ambient temperature to 900 oC. Xray photoelectron spectroscopy (XPS) spectrum was performed by an X-ray electron spectrometer (EscaLab 250Xi) with Al Kα irradiation. The elemental mapping was collected on JEM-2010F TEM equipped with X-Max EDS detector. N2 adsorption-desorption isotherms and pore size distribution were conducted using a gas Quantachrome autosorb-IQ gas adsorption analyzer with a desorption temperature of 300 oC. Electrochemical measurements. Electrochemical measurements were conducted with the assembled 2032 coin-type half-cells. For the preparation of working electrodes, the synthesized active materials, super P and polyvinylidene fluoride (PVDF) were homogeneously mixed in N-methylpyrrolidinone (NMP) with a mass ration of 8:1:1, and then the resultant slurry was coated on a fresh copper foil. Subsequently, the electrodes were treated at 80 oC for 12 h in a vacuum oven with subsequent assembled


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into the coin cells in a glove box filled with argon. The mass loading of active material is calculated about 0.8 mg cm−2. Lithium foil was chosen as the counter electrode, and the separator is polypropylene film (Celgard 2400). The electrolyte solution was employed with 1 M LiPF6 in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) (1:1:1 by volume). Galvanostatic charge/discharge performances were measured using a LAND battery test instrument (CT2001A, Wuhan Jinnuo Co., China) within a cutoff voltage range of 0.01–3.0 V at various current densities. Cyclic voltammetry (CV) curves were collected using an electrochemical workstation (CHI 660C, Shanghai Chenhua Co., China) at a scan rate of 0.2 mV s−1 between 0.01 and 3.0 V. Electrochemical impedence spectroscopy (EIS) tests were also conducted by the CHI 660C electrochemical workstation over the frequency range from 100 kHz to 0.01 Hz at the amplitude of 5 mV. RESULTS AND DISCUSSION The formation of carbon confined porous graphene/SnOx frameworks is described in Scheme 1. Firstly, porous SiO2 was coated on the surface of graphene with a solution reaction (the hydrolysis of TEOS and CTAB) and subsequent high temperature reduction (800 oC in N2) (Scheme 1a). Two major roles are considered for the deposition of SiO2 on the surface of graphene. It could not only inhibit the graphene restacking and forming the 3D framework, but also served as the template to generate porous SnO2 on the graphene substrate.36 Then, Sn resources were pulled into porous SiO2 via a nanocasting process with ethyl alcohol, resulting in the formation of graphene/SiO2/SnO2 (Scheme 1b). Subsequently, PANI was coated on the surface of graphene/SiO2/SnO2 through in-situ polymerization reaction of anilne monomers (Scheme 1c). PANI was carbonized and converted to carbon by annealing at 600 oC in N2, and SnO2 was simultaneously partially reduced to SnOx. After etching for removing SiO2, the carbon confined porous graphene/SnOx framework (graphene/SnOx/C) was finally obtained (Scheme



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1d). The unique structure can offer the graphene/SnOx/C (denoted pG/SnOx/C) electrode with enhanced electron conductivity, improved kinetics, stable SEI layers and large space for the accommodation of volume expansion during cycling process, which play a substantial role in promoting Li+ storage performance. The crystalline structures of pG/SnOx/C, pG/SnO2 and pure SnO2 were investigated by XRD, as shown in Figure 1a. No obvious peaks were observed in pG/SnOx/C, which indicates the amorphous feature. One possible reason is considered to the fact that SnO2 was partially reduced during the PANI carbonization. In contrast, the peaks of pG/SnO2 and pure SnO2 are well assigned to tetragonal SnO2 (JCPDS 99-0024). The chemical components of the pG/SnOx/C composite were analyzed by XPS. As shown in Figure 1b, the pG/SnOx/C composite mainly contains four elements of C, N, O and Sn, and the N species were largely introduced by PANI. In addition, two main states of Sn (Sn2+ and Sn4+) were observed in the high resolution Sn3d XPS spectra (Figure 1c),37 which would provide a further evidence for the partial reduction of SnO2, resulting in the formation of amorphous pG/SnOx/C. TGA was carried out to calculate the SnOx loading in pG/SnOx/C (Figure S1a). According to the TGA curve, graphene, PANI-derived carbon coating and the partial reduced SnO2 below 575 oC were estimated to be ~81.8 wt% in the composite, while the SnOx loading is roughly calculated around 18.2%. Note that the slight error from the reduction process was ignored. The relatively low SnOx content in the pG/SnOx/C is linked to the presence of carbon coating on pG/SnO2 (Figure S1b). Since high loading of SnOx is responsible for improving the capacity of the electrodes, several studies would be used to further improve the SnOx content. Moreover, increased deposition of SnO2 in the nanocasting process could decrease the content of the carbon coating. Figure 1d shows the N2 adsorption-desorption isotherm of pG/SnOx/C. The composite possess a large Brunauer-Emmett-Teller (BET) surface area of 531 m2 g−1 with a total pore volume of 0.8 cc g−1 and a centered pore diameter of 1.7 nm, according to the


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calculation by Barrett-Joyner-Halenda (BJH) method (Figure S2). The uniform pore size distribution with high BET surface area can provide the electrode with rapid mass transport, efficient Li+ storage as well as large space for volume expansion.38 The morphology and microstructure of pG/SnOx/C were observed by FE-SEM and TEM. As shown in Figure 2a-2b, pG/SnOx/C reveals 3D porous architecture without agglomerated skeleton owing to the restacking-inhibited role of SiO2 on the graphene surface. Compared to pG/SnO2, the rougher surface observed in pG/SnOx/C indicates that the presence of carbon coating derived from the PANI carbonization (Figure 2c and Figure S3). In addition, pG/SnOx/C possesses porous structure and mainly contains interconnected SnOx nanoclusters (Figure S4) with a diameter of about 5-10 nm, which were uniformly anchored on the graphene substrate. Meanwhile, sufficient space originated from the etching of SiO2 inside the interconnected SnOx clusters was found in Figure 2d-2e. Figure 2f-2i demonstrates the TEM-element mapping of the selected area in Figure 2d. C represents the graphene substrate and PANIderived carbon coating is observed everywhere, while Sn could be partially observed in the image due to the existence of space inside the interconnected SnOx nanoclusters. The distribution of N (mainly from carbon coating) is similar to the observation of Sn mapping, revealing the well deposition of carbon on the surface of porous graphene/SnOx frameworks. Thus, the element distribution of pG/SnOx/C is in good agreement with the porous graphene/SnOx frameworks that were confined with PANI-derived carbon coating. The unique 3D porous architecture is expected to enhance the electrochemical performance in the LIB electrode. The electrochemical performance of the synthesized pG/SnOx/C, pG/SnO2 and pure SnO2 was evaluated by assembling them into 2032 coin-type half-cells with Li foils as counter electrodes. Without specific notification, the specific capacities of the pG/SnOx/C and pG/SnO2 electrodes were calculated based on the total mass of the pG/SnOx/C and pG/SnO2 composites. Each component contributes to the



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capacities of the composites. Figure 3a gives the CV curves of pG/SnOx/C electrode tested for three cycles at a scan rate of 0.2 mV s−1 in the potential range from 0.01–3 V. Weak peaks are observed in the CV curves since the SnOx is amorphous in the pG/SnOx/C. The suppressed peak located at around 0.75 V in the initial cathodic scan is assigned to the formation of SEI film. Two redox peaks observed at about 0.01 V (cathodic scan) and 0.52 V (anodic scan) represent Li+ insertion/extraction into/from carbonaceous matrix and alloying/dealloying with Sn, respectively. The peak around 1.2 V in the first anodic scan is mostly ascribed to partial reversible reaction of SEI and slight oxidation of Sn during the charge-discharge processes.39 The curve of the second cycle is similar to that of the third one, indicating the highly reversible performance.40,41 Figure 3b describes the selected galvanostatic charge/discharge voltage profiles of pG/SnOx/C electrode at a current density of 100 mA g−1 between 0.01 V and 3 V. The electrode delivers high initial discharge/charge capacities of 1787/946 mAh g−1, corresponding to a Coulombic efficiency (CE) of about 53%. The irreversible capacities during the initial cycle could be ascribed to the formation of SEI layer.42,43 In the next two cycles, the charge/discharge curves are well identical, suggesting the high reversibility and stability of the pG/SnOx/C electrode. In addition, the pG/SnOx/C electrode exhibits more stable performance than pG/SnO2 and pure SnO2 (Figure 3c). At the current density of 100 mA g−1, the capacity suffers from a severe loss in the initial 10 cycles because of the formation of SEI layers, and remains stable upon the rest cycles. After 100 cycles, the discharge capacity stays around 907 mAh g−1 without significant fading, which is much higher than that in pG/SnO2 and pure SnO2. It is suggested that the carbon coating on the surface of porous graphene/SnOx prevents the direct contact of SnOx with electrolytes, which forms stable SEI layers for delivering stable cycling ability. In addition, the pG/SnOx/C electrode also shows enhanced electrochemical properties in comparison with the similar anodes including the carbon coating without silicon-induced pores, demonstrating the important role of the porous features of pG/SnOx/C for LIBs (Table S1). Figure 3d


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displays a comparison of the rate performance of the pG/SnOx/C, pG/SnO2 and pure SnO2 electrodes at different current densities. The pG/SnOx/C electrode shows a high rate capacity of 835 mAh g−1 at the current density of 100 mA g−1. Even at the current density of 3000 mA g−1, a rate capacity of 384 mAh g−1 can still be retained. More importantly, the capacity can recover to 886 mAh g−1 upon the current density back to 100 mA g−1, indicating excellent rate performance. In comparison, the pG/SnO2 and pure SnO2 electrodes deliver much lower rate capacities at all the current densities. To evaluate the long cycling performance of pG/SnOx/C, the electrode was also tested at higher current density of 600 mA g−1 and 1000 mA g−1 (Figure 4). After 300 cycles, the discharge capacity of the electrode retained at 587 mAh g−1, which shows 93% retention of the capacity in the 2nd cycle at 600 mA g−1 (Figure 4a), implying improved reversible capacity after long-term cycles. Similarly, the electrode delivers a highly reversible capacity of 555 mAh g−1 after 400 cycles at a higher current density of 1000 mA g−1 (Figure 4b), corresponding to a capacity retention of 86% in comparison with the value at the 2nd cycle. It should be noted that the capacity of the two electrodes both show a slight increase after the initial decaying, which is probably associated with the gradual activation of the active materials. In general, repeated volume variation of the carbonaceous matrix and SnOx nanoparticles caused by the Li+ insertion/extraction upon the cycles would lead to cracks in the carbonaceous matrix, which facilities the fast transport of the electrolyte to the active materials. As a result, the embedded active materials are able to be fully activated and thus would increase capacity in pG/SnOx/C.44–46 In addition, the electrochemical impedance spectroscopy was measured to further investigate the mechanism for the enhanced electrochemical performance in pG/SnOx/C. Figure 5 displays the nyquist plots of the delithiated pG/SnOx/C electrode before and after 30th and 100th cycles at 100 mA g−1 (the inset is the equivalent circuit). All these three curves demonstrate identical shapes, with a semicircle appearing in the high frequency region and a straight line in the low frequency region. The charge



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transfer resistance (Rct) presents a decreasing trend along with the cycles. The reduction of Rct indicates the activation and improved kinetics of the electrochemical reaction, which is attributed to the existence of high conductive graphene substrate and carbon coating. According to the results above, the pG/SnOx/C electrode presents excellent electrochemical performance with high initial specific capacity, excellent rate ability and highly stable cycling capability. The mechanism has been illustrated in Figure 6. Firstly, 3D frameworks with the high conductive graphene substrate and carbon coating can improve the electrical conductivity of SnOx, leading to enhanced kinetics of the electrochemical reaction. Second, the PANI-derived carbon coating can inhibit the direct contact between SnOx and electrolyte, promoting the formation of stable SEI layers on the surface of pG/SnOx/C. Third, porous SnOx was anchored on the robust and flexible graphene substrate, which is beneficial to endure the stress and strain induced by SnOx. Finally, the porous 3D architecture of pG/SnOx/C allows the composite to accommodate the volume expansion of SnOx during the cycling. CONCLUSION In summary, novel carbon confined porous graphene/SnOx frameworks were fabricated using a silica template assisted nanocasting method followed by a PANI-derived carbon coating process. Owing to the unique 3D porous architecture, the composite can provide enhanced electrical conductivity, improved kinetics, stable SEI layers as well as large space for the accommodation of volume expansion during the cycling process. As a consequence, the resultant pG/SnOx/C electrode has shown improved electrochemical performance with high initial specific capacity, excellent rate ability and highly stable cycling capability. This strategy employed in our study gives a novel platform to design highperformance Sn-based anode materials.

ASSOCIATED CONTENT


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Supporting Information. TGA analysis for the pG/SnOx/C and pG/SnO2 composites. Pore size distribution curve of the pG/SnOx/C composite. FE-SEM image of pG/SnO2. High-Resolution TEM image of pG/SnOx/C. Comparison of the electrochemical properties of the porous pG/SnOx/C composite with the similar anodes including the carbon coating without silicon-induced pores for LIBs. The Supporting Information is available free of charge via the Internet at http: // pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel./fax: +86 10 62334311. E-mail: [email protected] (L.-Z. Fan). Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial supports from 973 Project (2015CB932500), NSF of China (51532002, 51372022, 51575030) and State Key Laboratory of New Ceramic and Fine Processing Tsinghua University are gratefully acknowledged.

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Figure captions: Scheme 1. Synthetic process of the pG/SnOx/C composite through a silica template assisted nanocasting method along with polyaniline-derived carbon coating process: (a) the deposition of porous SiO2 on the surface of graphene; (b) the deposition of SnO2 nanoparticles into porous SiO2 via a nanocasting process; (c) PANI was coated on the surface of graphene/SiO2/SnO2; (d) PANI carbonization and SiO2 etching to form the carbon confined porous graphene/SnOx framework. Figure 1. Physical characterization. (a) XRD patterns of pG/SnOx/C, pG/SnO2 and pure SnO2; (b) XPS spectra of the pG/SnOx/C composite; (c) High-resolution Sn3d XPS spectra of the pG/SnOx/C composite; (d) N2 adsorption-desorption isotherms of the pG/SnOx/C composite. Figure 2. Morphology and microstructure characterization. (a-c) FE-SEM images of the pG/SnOx/C composite; (d-e) TEM images of the pG/SnOx/C composite; (f-i) TEM-EDS mapping of the pG/SnOx/C composite. Figure 3. Electrochemical performances. (a) CV profiles of the pG/SnOx/C composite for the initial three cycles with a scan rate of 0.2 mV s−1, (b) Galvanostatic charge-discharge curves of the pG/SnOx/C composite at a current density of 100 mA g−1 in the voltage range of 0.01–3 V vs Li+/Li; (c) Cycling performance of the pG/SnOx/C, pG/SnO2 and pure SnO2 electrodes at 100 mA g−1; (d) Rate behavior of the pG/SnOx/C, pG/SnO2 and pure SnO2 electrodes at various current densities. Figure 4. Long cycling performance of the pG/SnOx/C electrode at the current densities of 600 mA g−1 (a) and 1000 mA g−1 (b). Figure 5. Nyquist plots of the delithiated pG/SnOx/C electrode tested before and after 30th and 100th cycles at 100 mA g−1 (the inset is the equivalent circuit, where Re represents the ohmic resistance of electrodes, Rct represents the charge transfer resistance, Rf represents the SEI film resistance, CPE1


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represents the surface film capacitance, CPE2 represents the double-layer capacitance, and Zw represents the Warburg impedance). Figure 6. The related mechanism of pG/SnOx/C electrode for excellent Li+ storage performance: (a) the lithiation process; (b) the delithiation process.



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Scheme 1


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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Figure 6


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Table of Contents Graphic


SnOx Frameworks within Polyaniline

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