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Functional Nanostructured Materials (including low-D carbon)

A Hierarchical Carbon@SnS2 Aerogel with “Skeleton/ Skin” Architectures as a High Capacity, High-rate Capability and Long-cycle Life Anode for Sodium Ion Storage Zhiyuan Yang, Peng Zhang, Jian Wang, Yan Yan, Yang Yu, Qinghong Wang, and Mingkai Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14861 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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ACS Applied Materials & Interfaces

A Hierarchical Carbon@SnS2 Aerogel with “Skeleton/Skin” Architectures as a High Capacity, High-rate Capability and Long-cycle Life Anode for Sodium Ion Storage

Zhiyuan Yang, Peng Zhang, Jian Wang, Yan Yan,* Yang Yu, Qinghong Wang, Mingkai Liu*

School of Chemistry & Materials Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou 221116, China. Email: *[email protected]; *[email protected]

Keywords:

SnS2

nanoflakes,

carbon

nanofiber/graphene

matrix,

skeleton/skin

morphology, sodium ion batteries, outstanding electrochemical performances

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Abstract Developing high performance electrode materials with high-energy and long-term cycling stability is a hot topic and of great importance for sodium ion batteries (SIBs). In this work, a highly porous carbon / tin sulfide aerogel with a “skeleton/skin” morphology (SSC@SnS2) has been developed and further used as a binder free anode for SIB. This SSC@SnS2 electrode delivers a high specific capacity of 612 mAh g-1 at 0.1 A g-1, a good rate capability and a long-term cycling stability up to 1000 times with an average Coulombic efficiency of ~ 99.9%. Meanwhile, this SSC@SnS2 aerogel also achieves a stable cycling performance even at a high current density up to 5.0 A g-1. The fast-yet-stable sodium ions storage performance of the prepared SSC@SnS2 aerogel can be ascribed to the reasons that (i) carbon nanofiber / graphene (CNF/G) skeleton provides unimpeded pathways for the rapid transfer of electrons; (ii) Thin SnS2 skin with non-aggregated morphology can provide a great number of active sites for sodium ion storage; (iii) The porous structure of SSC@SnS2 aerogel ensure rapid penetration of electrolyte and can further accommodate the volume expansion of active SnS2 nanoflakes; (ⅳ) Intermediate product of Na15Sn4 alloy contributes greatly to sodium ions storage performance of SSC@SnS2 aerogel. The excellent electrochemical performances coupling with the unique structural features of this SSC@SnS2 aerogel makes it a promising anode candidate for SIBs.

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1. Introduction Sodium ion batteries (SIBs), as a promising substitution to lithium ion batteries, have become a research hotspot and have attracted great attention in the renewable energy storage fields due to their superior characteristics of low cost and inexhaustible sodium resources.1-11 SIBs have the similar chemical insertion / desertion mechanism of ions with that of lithium ion battery.12-18 However, most used electrodes for lithium ion batteries cannot be utilized in SIBs due to the larger ionic radius of sodium ions (1.02 Å for Na+ vs 0.76 Å for Li+).19-20 Until now, many materials have been investigated as cathode materials with promising performances for SIBs, such as Na0.7MnO2, NaxCoO2, NaNi0.5Mn0.5O2 and so on.21-26 However, only several anode materials with low capacitance have been reported in recent years, including hard carbons, reduced graphene oxide and disordered carbon.27-28 So, the major obstacle in promoting the practical applications of SIBs is the absence of advanced anode electrode materials. Therefore, several non-graphitic carbon materials have been investigated as anodes for SIBs, in which tin and tin-based compounds have drawn much attention due to the high theoretical capacity of tin-sodium alloy (847 mAh g-1) and low redox potential.21,29-33 During all the tin-based anodic materials, tin disulfide (SnS2), with a layered hexagonal CdI2-type crystal structure and an interlayer spacing of 0.59 nm,34 can provides unblocked channels for sodium ions diffusion. More importantly, SnS2 monolithic can deliver a high sodium-storage capacitance based on the reaction of 4 SnS2 + 31 Na+ + 31 e- → Na15Sn4 + 8 Na2S. For example, Meng and coworkers have developed a composite material of SnS2-rGO and achieved a high capacity of 630 mAh g-1 by means of the formation of Na15Sn4 alloy for sodium ions storage.27 Shen group have prepared a graphene foam supported self-branched SnS2 (B-SnS2) nanoarray electrode for sodium ion storage, which exhibited a high capacity of 900 mAh g-1.35 However, the volume change during alloying-dealloying process will lead to the pulverization of SnS2 nanomaterials, and the heavy aggregation of SnS2 can undoubtedly restrict their sodium 3



ions storage capabilities and working life-span.36 Therefore, developing SnS2-based nanomaterials with non-polarized effect, especially with rapid transfer of electrons, no aggregation and excellent expansion space for the volume change of SnS2, will of great importance for the sodium ions storage applications. In order to solve the problem mentioned above, and also achieve the best sodium ions storage performance of SnS2-based nanomaterials, an effective strategy is to design hierarchical architectures that can fully exploit the energy storage potential of SnS2 with the assistance of excellent conductive pathways. To this end, we have developed a hierarchical carbon / SnS2 aerogel with a “skeleton/skin” morphology in this work, in which the carbon nanofibers / graphene sheets (CNF/G) were utilized as the skeleton, and the SnS2 nanoflakes acting as a thin skin were tightly anchored on the surface of CNF/G skeleton. The non-aggregated SnS2 nanoflakes can ensure all their active sites for sodium ions storage, and the electrical conductive CNF/G skeleton can provide rapid pathway for the transfer of electrons. Furthermore, the good porous structures will contribute to the fast penetration of electrolyte and transportation of sodium ions. In this way, the tightly anchored SnS2 skin can be fully utilized for sodium ions storage during the charge / discharge process, and the optimum electrochemical performances of SSC@SnS2 aerogel can be achieved. As a result, a high specific capacity of 612 mAh g-1 at 0.1 A g-1 as well as a good rate capability have been achieved by this SSC@SnS2 anode due to its unique structural features. More interestingly, a long-term cycling stability up to 1000 times with a capacity retention of 86.4% at 0.5 A g-1 and an average Coulombic efficiency of ~ 99.9% were also realized by this SSC@SnS2 anode. The structural features of this SSC@SnS2 aerogel can further provide new sights for the development of advanced electrode materials for other energy storage systems.

2. Experimental section 2.1. Materials synthesis 4


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Carbon nanofibers (CNFs) were derived from a polyacrylonitrile (PAN) nanofiber membrane, which was produced according to an electrospinning method. In detail, PAN powders were dissolved into N, N-dimethylacetamide (DMAc) solution with a weight percent of 12%, which was further used as the electrospinning solution. The voltage charged between the syringe needle and the aluminum foil collector was 13 kV with a distance of about 16 cm. The obtained PAN nanofiber membrane was carbonized at 800 ºC for 3 h in Ar, following by a treatment of ball milling for 4 h. Individual CNF with length of 10 ~ 20 μm can be prepared. Graphene oxide (GO) was prepared based on the oxidation of natural graphite according to a modified Hummers' method.37 CNFs (200 mg) were dispersed into the GO solution (100 mL, 1 mg/mL) under strong sonication with a mass ratio of 2 : 1. The obtained hybrid solution of carbon nanofibers / graphene oxide sheets (CNF/GO) was freeze-dried in a vacuum freeze drying oven. The obtained CNF/GO aerogel was further chemically reduced by hydrazine hydrate in a stainless steel autoclave (100 mL) at 98 ºC for 24 h, resulting in the formation of CNF/G aerogel. 2.2. Synthesis of SSC@SnS2 aerogel 1.403 g SnCl4·5H2O (4 mmol) and 1.203 g thioacetamide (TAA) (16 mmol) were added into deionized water (40 mL) under strong stirring. Then, CNF/G aerogel (270 mg) was immersed into the composite solution. The mixture was sonicated for 10 min before being transferred to a Teflon-lined stainless steel autoclave (50 mL). The reaction was carried out at 160 ºC for 12 h, and the obtained product was washed with deionized water and alcohol for 5 times and further vacuum dried at 80 ºC for 12 h. The dried product was further treated in Ar at 400 ºC for 2 h with a temperature increasing rate of 5 ºC/min, resulting in the preparation of SSC@SnS2 aerogel. Pure SnS2 was prepared according to the same procedure without the addition of CNF/G aerogel. 2.3. Materials characterizations The crystal and phase structures of prepared materials were characterized by X-ray diffraction equipment (XRD, Bruker D8 advanced, Germany) with a Panalytical X-pert 5



diffractometer using Cu Kα radiation (40 kV, 40 mA). The structures and morphologies were observed based on a field emission scanning electron microscopy (SEM, SU8010, Hitachi) with an energy-dispersive X-ray spectroscopy (EDS, EDAX, PW9900), and a transmission electron microscopy (TEM, FEI Tecnai F20). The weight percent of SnS2 in SSC@SnS2 aerogel was evaluated by a thermogravimetric analysis equipment (TGA, TA500). X-ray photoelectron spectroscopy (XPS) measurements were taken on a Thermo escalab 250Xi spectrometer with an Al Kα source (1486.6 eV). Specific surface area and the pore size distribution of the prepared materials were detected based on an accelerated surface area and porosimetry system (ASAP, 2460) at 77 K. Electrical conductivity of prepared materials were measured with a four-probe-point instrument (RST-8). 2.4. Electrochemical evaluation In this work, SSC@SnS2 aerogel (~13.5 mg for each one) was directly used as the working electrode. Sodium metal foil and glass microfiber (Whatman) were utilized as the counter electrode and separator, respectively. 1 M NaPF6 (EC + PC + 5 vol% fluoroethylene carbonate (FEC)) was used as the electrolyte. For the controlled sample of pure SnS2, a SnS2 slurry consisted of pure SnS2 powder, super P and polyvinylidene fluoride (PVDF) with a weight ratio of 8:1:1 has been developed with N-methyl-2-pyrrolidinone (NMP) as the solvent. The slurry was coated on a copper foil and was further cut into disks with a diameter of 12 mm after being dried at 60 ºC under vacuum for 12 h. The weight of each pure SnS2 electrode is about 4.5 mg. SIB cells were assembled in an Ar-filled golvebox (Mikrouna) with H2O and O2 content less than 0.1 ppm. Galvanostatic charge / discharge measurements, coupling with the rate tests and long-term cycling performances, were carried out based on a Land battery system (LAND, CT2001A) between 0.01 and 2.5 V. Cyclic voltammetry (CV) curves ranging from 0.01 to 2.5 V at a scan rate of 0.1 mV s-1 were recorded on a multichannel battery tester (ARBIN, 10 V / 10 mA, 32 channels). The Nyquist plots of assembled SIBs were recorded on a on a system of Princeton and Solartron (SI 1260, SI 1287) from 100 kHz to 6


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0.01 Hz with an AC amplitude of 5 mV. 3. Results and discussion

Figure 1. Schematic illustration for the preparation of SSC@SnS2 aerogel.

The overall synthetic process of SSC@SnS2 aerogel was schematically illustrated in Figure 1. Firstly, CNFs were dispersed into graphene oxide solution to form a mixed carbon matrix consisting of one-dimensional (1 D) CNFs and 2 D graphene oxide sheets. The obtained solution of CNFs / graphene oxide sheets was freeze-dried and further reduced by hydrazine vapor, achieving the preparation of 3D CNF/G aerogel (Figure S1). Here, the chemical reduction of CNFs / graphene oxide can effective remove the oxygen-containing groups on the surface of graphene oxide sheets and CNFs, in which the conjugated structures can be recovered and the electron transfer pathways were repaired. In this work, interfacial loading of SnS2 nanoflakes on the surface of CNF/G aerogel was realized based on a hydrothermal process. Hybridization of SnS2 and 3D CNF/G matrix can solve the drawbacks of each component: (i) SnS2 can storage large number of sodium ions but suffers poor electrical conductivity and large volume expansion; (ii) CNF/G matrix possesses excellent electron transfer ability but lacks sufficient active sites for sodium ions storage. The prepared SSC@SnS2 aerogel can achieve a synergistic effect of the CNF/G matrix and the tightly anchored SnS2 nanoflakes with such a “skeleton/skin” morphology. 7



Figure 2. SEM images of SSC@SnS2 aerogel at (a) low and (b, c) high magnifications; (d) SEM image of selected area of SSC@SnS2 and corresponding EDS mappings of (e) C, (f) Sn and (g) S elements; (h) Energy spectrum of SSC@SnS2 aerogel obtained from the EDS detections.

The CNFs were developed from PAN nanofiber membrane (Figure S2), which was prepared by an elecrospinning method. CNFs (Figure S3) with a diameter of 1 μm and a length of 10 ~ 20 μm were used as supporting skeletons to expand the graphene sheets, resulting in the good mechanically compressible ability of the SSC@SnS2 aerogel (Figure S4). As known, graphene sheets are prone to aggregate based on the interfacial “π-π” stacking and can be easily crumpled due to the lowest energy principle. In this work, the graphene sheets with a large size (Figure S5) can be fully expanded due to the introduced CNF skeletons (Figure S6), resulting in a high specific surface area (581 m2 g-1) for the CNF/G aerogel (Figure S7). In this work, the expanded graphene sheets can be ascribed to the hydrogen bonding interaction between graphene oxide sheets and the CNFs. CNFs 8


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will be tightly wrapped by the edge of a large size graphene sheet, which will further wrap other CNFs on the other edges or the central position, which results in the fully expansion of the graphene sheets in the developed CNF/G aerogel. Figure 2a shows the SEM image of SSC@SnS2 aerogel at low magnification. Note that small SnS2 nanoflakes were homogeneously anchored on the surface of CNF/G aerogel and the CNF skeletons can be clearly observed, forming a SSC@SnS2 aerogel with a “skeleton/skin” morphology. SEM images at high magnifications (Figure 2b and 2c) exhibit the detailed structural information of SSC@SnS2 aerogel. Compared to the pure SnS2 aggregations (Figure S8), a thin SnS2 layer was tightly deposited on the conductive CNF/G matrix without any aggregation. Not only the graphene sheets but also the CNFs were coated by a thin layer of SnS2 with an average lateral size of 300 ~ 400 nm. Here, the thin SnS2 skin can ensure all their active sites for the electrochemical reactions with sodium ions and the CNF/G matrix can offer good transfer pathways for electrons, resulting in the synergistic effect of these two components. In order to confirm the successful deposition of SnS2 nanflakes on the surface of CNF/G matrix, energy dispersive spectroscopy (EDS) analysis of the SSC@SnS2 aerogel was performed. Figure 2d shows the SEM image of selected areal of SSC@SnS2 aerogel, and corresponding energy mappings of carbon (C), Tin (Sn) and sulfur (S) elements were also conducted, as seen in Figure 2e - 2g. Figure 2e exhibits a feeble carbon detection compared with the surrounding matrix because the SSC@SnS2 aerogel was adhered on the surface of conductive carbon adhesive for EDS detection. Also, the homogeneously deposited SnS2 nanoflakes will decrease the detection singles of the CNF/G matrix. Figure 2f and 2g exhibit intense Sn and S detections with integrated outlines with the selected area of SSC@SnS2 aerogel, which confirms that the SnS2 nanoflakes were homogeneously anchored on the surface of CNF/G matrix via the hydrothermal deposition method. These results confirm the successful interfacial hybridization of the active SnS2 with the electrical conductive carbon matrix. Moreover, the energy spectrum 9



(Figure 2h) detected from the EDS analysis show apparent elemental signals of C, S and Sn, indicating the co-existence of the carbon and SnS2 nanomaterials.

Figure 3. (a) XRD patterns and (b) TGA curves of pure SnS2, SSC@SnS2 and CNF/G matrix; (c) XPS survey of SSC@SnS2 aerogel with high resolution spectra of (d) Sn 3d, (e) S 2p and (f) C 1s.

Figure 3a exhibits the XRD patterns of pure SnS2, SSC@SnS2 aerogel and CNF/G matrix. CNF/G matrix shows a broad diffraction peak at about 26.1º, which corresponds to the (002) crystal diffraction of the carbon materials.38-42 SSC@SnS2 aerogel exhibits diffraction peaks at 14.9, 28.5, 32.3 and 50.3º, corresponding to the (001), (100), (101) and (110) phases as that of pure SnS2 (JCPDS card no. 23-0677),35,43-46 respectively. Here, the (001) diffraction peak of SSC@SnS2 that located at 2θ = 14.8º indicates an interlayer spacing of 5.9 Å, which allows the efficient insertion and extraction of sodium ions.47,48 Here, the diffraction peak intensity of SSC@SnS2 is a little weaker than that of the pure SnS2, which is possibly due to the porous morphology of SSC@SnS2 aerogel and the uniform dispersion of SnS2 nanoflakes. In order to determine the mass percentage of SnS2 in this SSC@SnS2 aerogel, thermogravimetric analysis (TGA) of CNF/G matrix, pure SnS2 and SSC@SnS2 aerogel was conducted from 25 to 800 ºC with a heating rate 10


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of 10 ºC min-1 in air atmosphere, as seen in Figure 3b. Compared with the TGA curves of pure SnS2 and CNF/G matrix, it can be known that the weight loss of SSC@SnS2 aerogel can be divided into two major steps: (1) the weight loss occurred between 250 and 420 ºC can be ascribed to the oxidation of SnS2 to SnO2;49 (2) the following weight loss between 450 and 620 ºC is due to the combustion of CNF/G matrix.50 Based on the TGA analysis results, the weight percent of SnS2 in this SSC@SnS2 aerogel is about 73%. The chemical compositions and surface features of SSC@SnS2 aerogel were further investigated by the XPS analysis, as seen in Figure 3c - 3f. Figure 3c exhibits the XPS survey spectrum of SSC@SnS2 aerogel with distinguish peaks of Sn, S and C elements. Here, the detected oxygen (O) signal maybe comes from the carbon dioxide or the water from the air. Sn 3d spectra of SSC@SnS2 aerogel shows two main peaks located at 495.6 and 487.2 eV, which can be ascribed to Sn 3d3/2 and Sn 3d5/2 of Sn4+.51,52 In the S 2p spectrum, there are doublet peaks at 162.2 and 163.5 eV, which can be attributed to the S2- in SnS2.53-55 These results confidently confirm that the Sn and S composites in this prepared aerogel are SnS2 nanomaterials. In addition, XPS result of C 1s spectrum was also conducted, as seen in Figure 3f. The hardly detected oxygen-containing groups, for example C-O bond at 287.1 eV and C=O bond at 289.2 eV, can be ascribed to the effectively chemical reduction by the hydrazine vapor, which further contributes to the electrical conductivity of CNF/G matrix.

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Figure 4. (a, b) TEM and (c, d) HRTEM images of SSC@SnS2 aerogel; (e) N2 adsorption / desorption isotherm of pure SnS2; (f) N2 adsorption / desorption isotherm of SSC@SnS2 aerogel. Insets in (c) and (d) schematically illustrate the (001) and (100) facets of the SnS2 laminar structure; Insets in (e) and (f) indicate the pore size distribution of pure SnS2 and SSC@SnS2 aerogel, respectively.

Figure 4a and 4b provide the TEM images of SSC@SnS2 aerogel at different magnifications. It can be seen that SnS2 nanoflakes were homogeneously deposited on the surface of CNF/G matrix with space voids between each other. Clear lattice fringe with an interplanar spacing of about 0.59 nm, corresponding to the interlayer distance of (001) plane of the hexagonal SnS2,56,57 can be obtained from the high-resolution TEM (HRTEM) image (Figure 4c). This result indicates that the SnS2 nanoflakes are growing with a preferential (001) edge orientation.58 In addition, the lattice fringes of 0.32 and 0.59 nm that corresponded to the (100) and (001) planes of SnS2, as being schematically illustrated by the insets in Figure 4c and 4d, can be clearly observed from the HRTEM image of SSC@SnS2 aerogel (Figure 4d). Compared with the lower specific surface area of pure SnS2 (35 m2/g) (Figure 4e), a much higher value of 182 m2/g was achieved by this SSC@SnS2 aerogel due to its non-aggregation morphology, as being demonstrated by the Barrrett-Joyner-Halenda (BJH) result (Figure 4f). Furthermore, a disordered curve resulted from the pore size distribution of pure SnS2 (inset in Figure 4e) indicates a poor porous structure. However, the pore size distribution derived from the BJH result shows peaks at 9, 19 and 28 nm (Figure 4f), indicating a highly mesoporous feature of this SSC@SnS2 aerogel.

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Figure 5. Electrochemical performances of SSC@SnS2 anode for SIBs. (a) CV curves at 0.1 mV s-1 and (b) discharge / charge curves at 0.1 A g-1 of SSC@SnS2 anode; (c) Rate capabilities of SSC@SnS2 aerogel and pure SnS2 materials tested at different current densities from 0.1 to 5.0 A g-1; (d) Comparative cycling performances of SSC@SnS2 and pure SnS2 anodes at 0.1 A g-1; (e) Nyquist plots and (f) the fitting Z' - ω-1/2 curves in the low-frequency region of SSC@SnS2 and pure SnS2 anodes.

Due to the compressible property, coupling with the porous structures and uniformly dispersed SnS2 nanoflakes, SSC@SnS2 aerogel was directly used as a free-standing anode for SIBs. The electrochemical performances of SIBs with SSC@SnS2 aerogel and pure SnS2 anodes were compared, as seen in Figure 5. Cyclic voltammograms (CV) curves of SSC@SnS2 anode between 0.01 and 2.5 V at a scan rate 0.1 mV s-1 have been present in Figure 5a. In the first cathodic scan, an apparent reduction peak at about 1.48 V was emerged due to the intercalation of sodium ions into the interlayer of SnS2, which results in the formation of NaxSnS2.59-61 A board reduction peak at around 0.4 V can be assigned to the combination of synergistic conversion / alloying reactions and the irreversible formation of the solid electrolyte interphase (SEI).62 In the anodic process, a main oxidation peak at around 1.31 V can be observed due to the desodiation of Na-Sn alloy phase,63-65 which results in the reformation of SnS2.66,67 In addition, the CV curves 13



on the 2nd and the 5th cycles were almost overlapped, which confirms good reversible reactions occurred between SSC@SnS2 aerogel and sodium ions. Figure 5b shows the discharge / charge curves of SSC@SnS2 aerogel on the 1st, 2nd and 5th cycles at 0.1 A g-1. The discharge curve on the 1st cycle exhibits a high-voltage plateau at about 1.5 V, which is consistent with the reduction peak observed in the 1st cathodic curve. The inconspicuous voltage plateaus at 0.75 and 0.4 V observed in the discharge curves on the 2nd cycle are in agreement with the broad reduction peaks that existed in the cathodic curves. The main plateau at around 1.3 V in the charge curves of SSC@SnS2 aerogel agrees well with the oxidation peak in the CV curve due to the reformation of SnS2. SSC@SnS2 aerogel delivers an initial specific capacity of 925 mAh g-1 and a reversible specific capacity of 612 mAh g-1. The nearly overlapped discharge / charge curves on the 2nd and 5th cycles confirm the good cycling performance of this SSC@SnS2 aerogel. Meanwhile, rate capability is also an important property for energy storage systems. Figure 5c exhibits the rate performances of SSC@SnS2 and pure SnS2 anodes at different current densities ranging from 0.1 to 5.0 A g-1. The SSC@SnS2 aerogel delivers a reversible specific capacity of 609 mAh g-1 for the first 10 cycles at 0.1 A g-1. With the current density gradual increasing, SSC@SnS2 aerogel exhibits stable reversible specific capacities of 556, 502, 456 and 411 mAh g-1 at 0.5, 1.0, 2.0 and 5.0 A g-1. When the current densities were returned to 1.0 and 0.1 A g-1, the specific capacity of SSC@SnS2 aerogel was successfully increased to 505 and 612 mAh g-1. Here, the slightly increased capacities of SSC@SnS2 aerogel at recovered 1.0 and 0.1 A g-1 can be ascribed to the activated active sites of SnS2 during the slow and rapid insertion / extraction of sodium ions under the previous low current densities. Contrastively, pure SnS2 with an initial specific capacity of 402 mAh g-1 at 0.1 A g-1 can only possess an ultralow capacity of 35 mAh g-1 when the current density was increased to 5.0 A g-1. Based on the rate performances of these SSC@SnS2 aerogel and pure SnS2 nanomaterials, it can be confirmed that the SSC@SnS2 aerogel achieves good resilient architectures for sodium 14


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ions storage due to the negligible changed capacities at various current densities. These resilient architectures can be attributed to the thin SnS2 “skin” for sodium ions storage, the good conductive CNF/G “skeleton” that allows the rapid transfer of electrons, as well as the good porous structures that ensure quick penetration of electrolyte and further accommodate the volume expansion of active SnS2 nanoflakes. Figure 5d shows the cycling performances of SSC@SnS2 aerogel and pure SnS2 at 0.1 A g-1. CNF/G matrix only contributes a low specific capacity of 113 mAh g-1 for sodium ions storage but possesses a good cycling stability (Figure S9). Pure SnS2 exhibits a high initial specific capacity as that of SSC@SnS2 aerogel (612 mAh g-1), but suffers a severe capacity fading in the following cycles with only a low capacity of 155 mAh g-1 was maintained at the end of 100 cycles. The severe capacity fading of pour SnS2 could be due to the low electronic conductivity of unsupported SnS2 and unrestrained aggregation of Sn / NaxSn during the long-term cycling process. Whereas, due to the unique “skeleton/skin” architectures, SSC@SnS2 aerogel demonstrates a much improved cycling performance with a reversible specific capacity of 564 mAh g-1 maintained after 100 cycles at 0.1 A g-1. The superior electrochemical performance of SSC@SnS2 aerogel compared with pure SnS2 nanomaterials was further demonstrated by the electrochemical impedance spectroscopy (EIS) measurement. Figure 5e exhibits corresponding Nyquist plots of pure SnS2 and SSC@SnS2 aerogel from 0.01 Hz to 100 kHz. Both the EIS curves have a single depressed semicircle in the high-medium frequency region and an inclined line at low frequency. The resistance at the intersection of high frequency represents the electrolyte resistance (Re).68 The diameter value of the semicircle on the Z' axis corresponds to the interfacial impedance, including the SEI impedance (RSEI) and the charge transfer resistance (Rct).69 While, the straight line in the low frequency indicates the diffusion resistance of sodium ions in the SSC@SnS2 anode. Corresponding equivalent circuit used for modeling the EIS spectra was present in Figure S10. And the obtained resistance values were collected in Table S1. As seen, SSC@SnS2 aerogel anode 15



exhibits a lower Rct value of 98 Ω compared with the result of pure SnS2 anode (212 Ω). Here, the good electrical conductivity of CNF/G matrix of 46 S cm-1, that confirmed by the substitution of copper wire with this CNF/G matrix (Figure S11), is responsible for the lower Rct value of SSC@SnS2 aerogel anode. For pure SnS2 nanomaterials, although the resulted SEI layer with an ultrathin morphology can benefit the transportation of sodium ions, the electron transfer ability inside the active anode materials is yet inhibited for the insulation nature of SEI layer. For SSC@SnS2 anode, besides the tight contact between SnS2 and the CNF/G matrix, as well as the homogeneous deposition of SnS2 nanoflakes without any aggregation, the much lower Rct value resulted from the assistance of CNF/G skeleton can greatly contribute to its superior rate performances. Furthermore, sodium ions transfer kinetics both in SSC@SnS2 and pure SnS2 anodes were investigated. As seen in Figure 5f, the relationship between ZW and ω-1/2 in the low frequency region can be present according to Equation 1 and 2, which indicates that the lower slope at low frequency, the faster transportation kinetics of ions in the electrode materials.

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Figure 6. (a) Long-term cycling performances of SSC@SnS2 aerogel at current densities of 0.5 and 5 A g-1; (b) Nyquist plots of SSC@SnS2 anode after being cycled for 10 and 1000 times; (c) The cycling performances of three different SSC@SnS2 anodes at 1.0 A g-1; (d) Discharge / charge curves of the three SSC@SnS2 anodes on the 200th cycle at 1.0 A g-1; (e) A series of LED were lit up by a SIB with SSC@SnS2 aerogel anode.

17



R 2T2 D 2A 2 n 4 F4 C2  2 ZW  R D  R L  1/ 2

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(1) (2)

where R is the gas constant, T is the room temperature as the experiment carried out, n is the number of electrons per molecule during oxidization (for Na+, it is 1), F is the Faraday constant, C is the initial concentration of sodium ions, σ is the Warburg factor which has a great relationship with ZW as seen in Equation 2.70 Here, the value of σ can be obtained from the slope of Zw ∼ ω−1/2, as shown in Figure 5f. Apparently, SSC@SnS2 aerogel shows lower slope than pure SnS2, as indicated by the fitted lines in Figure 5f. As a result, SSC@SnS2 aerogel with a unique “skeleton/skin” morphology shows smaller charge transfer resistance and better sodium ions transportation kinetics than that of pure SnS2 nanomaterials. To further verify the good cycle-span of SSC@SnS2 aerogel, long-term cycling performances at current densities of 0.5 and 5 A g-1 have been evaluated, as seen in Figure 6a. The SIBs were firstly activated at low current density of 0.2 A/g for the first two cycles. SSC@SnS2 aerogel exhibits an initial specific capacity of 500 mAh g-1 at 0.5 A g-1, and further delivers a stable performance in the following cycles. The capacities at the end of 200, 400, 600 and 800 cycles were 481.2, 476.1, 459.5 and 452.4 mAh g-1, corresponding to a capacity retention of 96.2, 95.2, 91.9 and 90.5%, respectively. After 1000 cycles, the capacity of SSC@SnS2 aerogel still remains at 432 mAh g-1, achieving a high retention rate up to 86.4%, which is superior than or comparable with most reported SnS2-related electrode materials.71-73 It should be noted that, SSC@SnS2 aerogel anode also exhibits an excellent cycling stability even at a high rate of 5 A g-1, with a capacity of 245 mAh g-1 maintained after 1000 cycles compared with the initial capacity of 321 mAh g-1. The good cycling performances of SSC@SnS2 aerogel anode can be confirmed by the EIS spectra which were conducted at the 10th and 1000th cycles, as seen in Figure 6b. The approximate EIS curves can evidently demonstrate the cycling stability of this 18


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SSC@SnS2 aerogel anode with “skeleton/skin” morphology. Here, the slightly increased Rct value from 98.4 Ω (on the 10th cycle) to 122.8 Ω (on the 1000th cycle) maybe resulted from the partial pulverization of the SSC@SnS2 electrode.74 In order to confirm the repeatable capabilities of SSC@SnS2 aerogel anode for sodium ions storage, three SIBs with SSC@SnS2 anodes were tested at 1.0 A g-1 for 200 times, as seen in Figure 6c. All these three SIBs exhibit good cycling performances with specific capacities of ~ 500 mAh g-1, as well as high Coulombic efficiencies of ~ 99.9%. The good repeatable sodium ions storage capabilities of SSC@SnS2 aerogel can also be demonstrated by the nearly overlapped discharge / charge curves of the three SIBs at the 200th cycle (Figure 6d). To further confirm the practical applications of SSC@SnS2 aerogel, a SIB cell with SSC@SnS2 aerogel anode was used as a power resource for a closed circuit. Figure 6e shows that a series of red light emitting diodes (LEDs) can be lit up by an assembled SIB cell with SSC@SnS2 anode, which suggests a good potential of this SSC@SnS2 aerogel for practical applications.

Figure 7. Schematic illustration of the high rate capability for sodium ions storage achieved by the SSC@SnS2 aerogel.

Here, the superior electrochemical sodium ions storage capability of SSC@SnS2 19



aerogel can be ascribed to its unique structural features with SnS2 nanoflakes homogeneously anchored on the conductive CNF/G matrix, as being illustrated in Figure 7. Firstly, the chemically bonded SnS2 and carbon matrix provide good structural stability and ensure excellent electronic conductivity throughout the SSC@SnS2 anode. Secondly, the nanoscale dimension of SnS2, especially its thin morphology on the conductive CNF/G matrix, has been emphasized as an important factor for the rate properties. Thirdly, the good porous structures of SSC@SnS2 aerogel can provide sufficient pathways for the rapid penetration of electrolyte and large amount of channels for the transfer of sodium ions.75 Last but not least, the ultrathin “skin” of the SnS2 coupling with its highly porous structures enables an interior sodium ions access into its pores and further have good interfacial interaction with the active sites of SnS2, which ensure both the exterior and interior parts of SnS2 to participate in the electrochemical reactions. All these features resulted from the unique structures of SSC@SnS2 aerogel can greatly contribute to its high-capacity, high-rate and long-term cycle life performances.

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Figure 8. (a) Ex situ XRD patterns of SSC@SnS2 electrode at different discharge / charge states. (b) Schematic illustration of the sodium ions storage mechanism for SSC@SnS2 electrode. (c) In situ EIS spectra and (d) the corresponding resistance values of SSC@SnS2 electrode at different discharge / charge states. (e) TEM and (f) HRTEM images of SSC@SnS2 electrode after being cycled for 1000 times. Inset in (e) shows the TEM image of cycled SSC@SnS2 electrode at high magnification; Inset in (f) is the SAED pattern. Scale bar: (e) 2 μm, (f) 2 nm.

In order to deeply understand the sodium ions storage mechanism of SSC@SnS2 aerogel, the phase transformation of SSC@SnS2 electrode detected by ex situ XRD analysis at different discharge / charge states were performed, as seen in Figure 8a. Compared with the pristine state of SSC@SnS2 aerogel (Figure 3a), the diffraction peaks of SnS2 phase were gradually disappeared in the discharge state. Meanwhile, a broad peak at around 31.4º was emerged when the SIB cell was discharged to 1.2 V, which corresponds to the formation of Sn phase.76 When the cell was discharged to 0.7 V, a new 21



diffraction peak at about 22.6º can be detected in the XRD pattern, indicating the emergence of Na2S phase.77 Also, a broad peak with diminished intensity at about 31.2 º was appeared, which confirms the formation of Na-Sn alloy of Na15Sn4. Later, two distinguish diffraction peaks at about 19.0º and 31.8º, corresponding to amorphous Na-Sn alloys, can be clearly observed in the XRD pattern at 0.2 V, thus the diffraction peaks of crystalline Sn metal were disappeared. After being charged to 0.5 V, the XRD pattern of SSC@SnS2 electrode exhibits the similar diffraction peaks as that at 0.2 V, which means the existence of Na15Sn4 nanomaterials. However, XRD pattern of SSC@SnS2 electrode that was charged to 1.2 V shows a small broad diffraction peak at about 44.7º, which means the emergence of Sn metal although most of the electrode materials were still existed in the form of Na15Sn4 alloy. When being charged to 1.7 V, the SSC@SnS2 electrode exhibits a broad diffraction peak at about 28.9, which means the recovery of SnS2 materials. Here, it is interesting to note that the emergence / disappear of Na2S material occurred simultaneously with the Na15Sn4 alloying reactions, which indicates that the conversion and alloying reactions were happened simultaneously in the lower voltage region. These results were consistent with the results of SnS2-rGO electrode.20 The proposal sodium ions storage mechanism of this SSC@SnS2 aerogel was schematically illustrated in Figure 8b. The synergistic storage of sodium ions via the formation of Na2S and Na15Sn4 intermediates can simultaneously accelerate the kinetics of the sodium ions storage reactions, and further improve the sodium ions storage capability of this SSC@SnS2 electrode. The XRD results can further confirm the reversible feature and superior kinetics of this SSC@SnS2 aerogel for sodium ions because no residual metallic Sn and Na2S were detected in the full charge state. Figure 8c and 8d show the in situ EIS spectra of the sodium ion battery at the selected discharge / charge voltage points and the resistance values, respectively. It can be seen that the EIS spectra of SSC@SnS2 anode were greatly varied with the changes of the internal structures and phase of SnS2 at different discharge / charge states. During the discharge 22


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process, the fitted charge transfer resistance value was gradually increased from 98.4 Ω (D 1.2 V) to 182.4 Ω (D 0.2 V), which can be ascribed to the formation of Na2S and the gradual volume expansion of the SSC@SnS2 electrode, because the insulated Na2S will undoubtedly increase the internal resistance of the electrode. In the following charge process, metallic Sn was separated from the Na15Sn4 alloy in the dealloying reactions. And the resistance value for charge transfer was decreased from 182.4 Ω (D 0.2 V) to 73.6 Ω (C 1.7 V). This variation of the resistance maybe resulted from the removal of metallic Na and the shrinkage of the internal structure of SSC@SnS2. Furthermore, the TEM images of SSC@SnS2 electrode after being cycled for 1000 times indicate that the nanostructure of SSC@SnS2 aerogel was generally maintained, as seen in Figure 8e. SnS2 nanomaterials were stilled anchored on the surface of CNF/G matrix (Figure S12). HRTEM image of the cycled SSC@SnS2 electrode (Figure 8f) shows apparent crystal interlayer d-spacing of 2.0, 2.6 and 2.3 Å, corresponding to the (332) and (011) crystal phases of Na15Sn4, as well as (220) phase of Na2S materials.78 After being cycled for a long-term, corresponding {100} and (001) facets of SnS2 can also be found in the SSC@SnS2 electrode (Figure S13), indicating the recover ability of the SnS2 active materials. These results can also be confirmed by the selected area electron diffraction (SAED) results with apparent diffusive rings of Na15Sn4 (011) and Na2S (220), as inset in Figure 8f. Meanwhile, the synergistic mechanism can be proposed that generated by the formation of Na2S and Na15Sn4, which makes it possible to take both the advantages of conversion and alloying reactions. All these results demonstrate the full utilization of SnS2 and the critical intermediate of Na15Sn4 alloy for the storage of sodium ions. 4. Conclusion In summary, a hierarchical SSC@SnS2 aerogel with a “skeleton/skin” morphology has been synthesized with thin SnS2 skin tightly anchored on the surface of CNF/G skeleton. The electrical conductive CNF/G skeleton can provide unobstructed pathway for the rapid transfer of electrons and the thin SnS2 “skin” can fully exposed their active sites for 23



the storage of sodium ions. Also, the good porous structures of this SSC@SnS2 aerogel ensure rapid penetration of electrolyte and fast transfer of sodium ions. When serving as a binder-free anode for SIBs, this SSC@SnS2 electrode delivers a high specific capacity of 612 mAh g-1 at 0.1 A g-1, and a promising rate capability with a reversible specific capacity of 411 mAh g-1 even at high current density of 5.0 A g-1. Meanwhile, this SSC@SnS2 aerogel also achieves a long-term cycling stability up to 1000 times even at high and low current densities of 0.5 and 5 A g-1, respectively. These promising electrochemical performances of SSC@SnS2 aerogel can be attributed to the 3D porous structures with unique “skeleton/skin” morphology, which benefits the fast sodiation / desodiation kinetics and even accommodates the volume change of active SnS2 materials. It is anticipated that the SSC@SnS2 aerogel developed in this work can provide convenient access to the fast kinetics for sodium ions storage, and further provide new insights for the development of advanced electrode materials for other energy storage systems.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Digital photos, SEM images of the CNFs, pure graphene sheets, CNF/G aerogel, pure SnS2 and SSC@SnS2 aerogel, BET results of CNF/G aerogel, electrochemical performances of pure CNF/G matrix, TEM images of SSC@SnS2 electrode, resistance values are supplied as Supporting Information.

Conflicts of interest There are no conflicts to declare. 24


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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51703087, 21601072, 51702138), PAPD, and Natural Science Foundation of Jiangsu Province (BK20150238, BK20170240).

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Anodes for Lithium/Sodium-Ion Batteries. ChemSusChem 2018, 11 (9), 1549-1557.

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