Hexagonal SnS2 nanosheets crosslinked by bacterial cellulose derived carbon nanofibers for fast sodium ion batteries

Journal of Alloys and Compounds 802 (2019) 269e275

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Hexagonal SnS2 nanosheets crosslinked by bacterial cellulose derived carbon nanofibers for fast sodium ion batteries Zhanying Liu, Jiaxin Xu, Yanzhang Zhao, Yufeng An, Jie Tao, Fang Zhang*, Xiaogang Zhang Jiangsu Key Laboratory of Electrochemistry Energy Storage Technologies, College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 February 2019 Received in revised form 11 June 2019 Accepted 13 June 2019 Available online 14 June 2019

Metal dichalcognide SnS2 have been demonstrated a potential candidate for sodium-ion batteries (SIBs) anode due to its high theoretical specific capacity and large interlayer spacing. However, the issues of sluggish kinetics and large volume change associated with the conversion and alloying reaction need to be addressed prior to the practical application. Herein, mesoporous composite of SnS2 nanosheets crosslinked by bacterial cellulose-derived carbon nanofibers (SnS2/BC-CNFs) has been fabricated by hydrothermal method, which displayed superior rate capability and cycling stability. The nanocomposite of SnS2/BC-CNFs-5 (Specific surface is 144.7 m2 g
1) delivered a high specific capacity of 408 mAh g
1 at 50 mA g
1 after 100 cycles and 196.4 mAh g
1 at a high rate of 2 A g
1. The enhanced Na storage properties of SnS2/BC-CNFs may due to the rational design of SnS2 nanosheets interconnected by conductive carbon nanofibers, which facilitated the ion and electron transport throughout the electrode. © 2019 Elsevier B.V. All rights reserved.

Keywords: SnS2 nanosheet Carbon nanofiber Sodium ion batteries Pseudocapacitance

1. Introduction Electrical energy storage in large-scale plays important role in smart power grids in addressing the energy crisis and climate change by integrating a variety of renewable energies. Among various types of energy storage devices, lithium-ion batteries (LIBs) have gained great success in consumer electrons area due to their relatively high energy densities and long lifespan [1]. However, considering the large-scale application in electrical vehicles (EVs) and stationery energy storage, high material cost resulting from the expensive Li-precursor and Cu current collector will be a great concern [2,3]. Emerging sodium-ion batteries (SIBs) technologies have similar electrochemistry behavior with LIBs [4]. Therefore, SIBs are regarded as low-cost alternatives to LIBs due to widelyavailable Na-containing resource on earth [5,6]. However, the issues of sluggish kinetics and poor stability upon long-term cycling resulted from the large Na-ion (Na ion is z 55% larger and 330% heavier than the Li ion) have to be addressed [7,8]. Thus, it is urgent to discover suitable Na intercalation material with high performance for SIBs. As a potential candidate of SIBs anode material, twodimensional (2D) layered metal dichalcogenides SnS2 has

* Corresponding author. E-mail address: [email protected] (F. Zhang). https://doi.org/10.1016/j.jallcom.2019.06.165 0925-8388/© 2019 Elsevier B.V. All rights reserved.

attracted intensive attention due to a high theoretical specific capacity of 1136 mAh g
1 and large interlayer spacing of 0.59 nm [7]. Moreover, recent papers have demonstrated that some SnS2 nanostructures exhibited pseudocapacitive behavior which originated from the Na adsorption on the surface and ultrafast ion intercalation into the SnS2 layers [9]. Unfortunately, large volume change associated with the formation of alloyed Na3.75Sn (420%) and the unsatisfactory electrical conductivity lead to poor Na storage performance [10]. In order to take advantage to the high capacity and large interlayer spacing of SnS2, an effective strategy is to integrate conductive carbon materials with SnS2 to improve the electrical conductivity and structural stability of hybrid materials [11]. Carbon nanofiber (CNF) aerogels, which is composed of interconnected three-dimensional (3D) networks of nanofibers, are of considerable interest due to their unique physical properties, such as low density, high electrical conductivity, high porosity and specific surface area [12,13]. Particularly, carbon nanofiber aerogels generated by biomass bacterial cellulose (BC), represents an ecofriendly and cost-effective way to obtain 3D networks of nanofibers [14]. Unlike graphene and carbon nanotube, bacterial cellulose-derived carbon nanofibers (BC-CNFs) are intrinsically amorphous carbon materials, which have been demonstrated to own outstanding Na storage material for SIBs [15,16]. The attributes of flexible 3D network framework with distinguishing electrical

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conductivity make BC-CNFs aerogel an excellent carbon support, can be served as scaffold to buffer the volume change and prevent the restacking and aggregation of active materials. In this work, a simple hydrothermal method was used to anchor SnS2 nanosheets onto 3D interpenetrated carbon nanofibers to construct a SnS2/BC-CNFs composite electrode. SnS2/BC-CNFs composite structure not only can effectively increase the specific surface area, but also improved the electrical conductivity of SnS2 nanosheets, which can ensure the rapid electron and ion transport. Meanwhile, the aggregation and stacking of SnS2 nanosheets were suppressed by the interconnected carbon nanofibers, which is beneficial to take advantage of Na storage performance of SnS2 nanosheets. When evaluated as anode material for SIBs, SnS2/BCCNFs composite electrode showed better rate capability and cycling performance than that of pure SnS2. An optimal SnS2/BC-CNFs composite with a SnS2 mass loading of 76.01% delivered a high specific capacity of 408 mAh g
1 at 50 mA g
1 after 100 cycles and 196.4 mAh g
1 at 2 A g
1. The enhanced Na storage performance may due to the improved electrical conductivity and much increased specific surface area of SnS2, which facilitated the electron and ion transport throughout the electrode. In addition, surface capacitance-dominated kinetic process may account for the superior electrochemical property as well. 2. Experimental 2.1. Synthesis of SnS2/BC-CNFs BC-CNFs aerogels were prepared via a high temperature carbonization method in an inert atmosphere. Fresh BC pellicles (Hainan Yide Food Company) were firstly cut into small slices and freeze-dried using a freeze-dryer (LGJ-10 freezer dryer) to obtain BC aerogel. Subsequently, the BC aerogels were carbonized in a tube furnace under a nitrogen atmosphere at 1000  C for 2 h with a heating rate of 2  C min
1. The SnS2/BC-CNFs nanocomposite was synthesized via a facile hydrothermal reaction. Specifically, 0.35 g SnCl4$H2O and 0.3 g thioacetamide (TAA) were added into 30 mL deionized water and stirred for 30 min to form a faint yellow solution. A certain amount of BC-CNFs aerogels (30, 50 and 70 mg) was added into the above solution, respectively, which sealed in a Teflon-lined autoclave at 160  C for 10 h. The products were washed with deionized water for several times and dried at 60  C for 6 h in vacuum, which named as SnS2/BC-CNFs-3, SnS2/BC-CNFs-5 and SnS2/BC-CNFs-7. As comparison, pure SnS2 nanosheets was synthesized using the same method except for the adding of BC-CNFs.

2.3. Electrochemical tests The slurry was prepared by mixing SnS2/BC-CNFs, acetylene black and carboxymethyl cellulose with a weight ratio of 8:1:1 using deionized water as solvent, which was coated on a Cu foil and dried at 60  C for 12 h in vacuum. A mass loading of 1.0 mg and disk of 12 mm in diameter was punched from coated Cu foil. The 2032 coin cells were assembled in an argon-filled glove box (O2/ H2O < 0.1 PPM) using the punched disk as the working electrode, sodium metal foil as the counter electrode and a glass fiber (Whatman, GF/D) as the seperator. The electrolyte was 1 M NaClO4 dissolved in ethylene carbonate (EC) and propylene carbonate (PC) (1:1 by volume) with the addition of 5 wt% fluoroethylene carbonate (FEC). The galvanostatically charged/discharged tests were conducted on a Land CT2001A battery testing system. Cyclic voltammetry (CV) testing were performed on a CHI 760E electrochemical working station in a potential range of 0.01e3.0 V at a scan rate of 0.2 mV s
1. Electrochemical impedance spectroscopy (EIS) tests were carried out on the same electrochemical working station within a frequency range from 0.01 Hz to 100 kHz. All the electrochemical tests were performed at 25  C. 3. Results and discussion The synthetic procedure of SnS2/BC-CNFs can be schematically illustrated in Fig. 1. According to the synthesis condition in our experiment, the formation mechanism of SnS2/BC-CNFs should be as follows: (1) the hydrolysis of thioacetamide (eq (1)) and the in situ metathesis reactions (eq (2)); (2) “self-assembly” and “oriented crystallization” processes. During the hydrothermal reaction, the thioacetamide reacts with water to generate H2S gases. Meanwhile, the Sn4þ ions in situ react with the produced H2S to form sulfide nanocrystals, which grows on the surface of BC-CNF aerogels (eq (2)). Based on the report, the hexagonal shape of 2D layered SnS2 nanosheets can be explained by the self-repair-epitaxial growth mechanism. That is the unique hexagonal CdI2 layered structure character of SnS2 phase offered an oriented growth of nanosheets. To be specific, the crystal growth of SnS2 along the [001] direction was strongly confined due to the weak van der Waals’ force between the layers. Hence, SnS2 crystallized onto the edges of created nanocrystallites to form regular hexagonal shape [17]. Benefited from the synergistic effect of high capacity originated from the conversion and the alloying reaction of SnS2 and the fast ion and electron transport along conductive carbon nanofibers, the SnS2/ BC-CNFs electrode exhibited an enhanced Na storage performance, which will be demonstrated by the electrochemical tests.

2.2. Materials characterization The crystal structures of pure SnS2 and SnS2/CNFs were investigated by X-ray diffraction (XRD) method using a Bruker D8 Advance power X-ray diffractometer with Cu Ka radiation (l ¼ 0.154 nm). The contents of SnS2 in SnS2/CNFs were determined by thermogravimetry analysis (NETZSCH STA 409PC) in air from 25 to 900  C at a heating rate of 10  C min
1. The chemical composition of SnS2/CNFs was identified by photoelectron spectroscopy (XPS) surface analysis (ESCALAB, 250xi, ThermoFisher). The morphologies of SnS2 and SnS2/BC-CNFs were examined by a field emission scanning electron microscope (FESEM, JSM-7800F, Japan) and a transmission electron microcopy (TEM, Tecnai12, Holland). The energy dispersive analysis (EDS) was performed on the FESEM. The nitrogen adsorption/desorption isotherms were tested at 77 K on an adsorption/desorption apparatus (Quantachrome, NOVA2000e, USA). The special surface area was determined by the BrunauerEmmett-Teller (BET) method and the pore size distribution was calculated by density functional theory (DFT).

CH3CSNH2 þ H2O / CH3CONH2 þ H2S

(1)

Sn4þ þ2H2S / SnS2 þ 4Hþ

(2)

The shape of SnS2 nanosheet and the morphology of SnS2/BCCNFs were demonstrated by the SEM and TEM characterization (Fig. 2). SEM image in Fig. 2a reveals a heavily stacked thin sheetlike morphology of pure SnS2 with hexagonal shape. The SnS2 nanosheet shows a lateral size of around 100 nm and a thickness of about 10 nm. SEM image from Fig. 2b presents a 3D highly porous network structure of BC-CNFs consisting of numerous carbon nanofibers with an average diameter of 30e50 nm. As shown in Fig. 2c, SnS2 in the SnS2/BC-CNFs remains the thin sheetlike nanostructure (<100 nm), which are crosslinked by numerous carbon naonfibers. The 3D interpenetrated network of BC-CNFs aerogels effectively reduced the restacking and the aggregation of SnS2 nanosheets. Such hybrid structure is beneficial to the transport of electrons and the infiltration of electrolytes. TEM image in

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271

Fig. 1. Synthesis schematic diagram of SnS2/BC-CNFs.

Fig. 2. (a) SEM image of SnS2, (b) SEM image of BC-CNFs, (c) SEM image of SnS2/BC-CNFs-7, (d) TEM image of SnS2/BC-CNFs-5 (e) HRTEM image (the inset is SAED) of SnS2/BC-CNFs5 (f) HRTEM image of BC-CNFs, (gei) EDS mapping of C, S and Sn elements of SnS2/BC-CNFs-5.

Fig. 3d highlights the hexagonal shape of SnS2 nanosheet. HRTEM image (Fig. 2e) from the nanosheet reveals an interplanar crystal spacing of 0.314 nm, corresponding to the (100) plane of SnS2 [18]. Selected area electron diffraction (SEAD) pattern (inset of Fig. 2e) clearly presents the (100) plane. The HRTEM image in Fig. 2f taken from the carbon nanofibers indicates that the carbon structure are primarily disordered, with some tiny “graphitic” crystal domain being sub-3nm in scale, which agreed well with the typical microstructure of hard carbon [19]. The EDS mappings of SnS2/BCCNFs shown in Fig. 2gei confirm the uniform distribution of element C, S, and Sn in the composite, which corresponded to an atomic percent of 79.67%, 15.6% and 4.7%, respectively. XRD measurements were performed to investigate the crystal structure of pure SnS2 and SnS2/BC-CNFs. A contrast of XRD patterns from pure SnS2 and SnS2/BC-CNFs is shown in Fig. 2a. All the diffraction peaks of the SnS2/BC-CNFs and the pure SnS2 can be indexed to 2T-type hexagonal SnS2 (JCPDS No.23e0677) [20]. Based on Bragg equation, two main peaks at z 15.02 and z28.35 corresponding to the crystal plane of (001) and (100) reveals a lattice fringe of 0.589 nm and 0.314 nm, respectively. The lattice fringe of 0.589 nm verifies the layered structure of SnS2 nanosheets and the interplanar spacing of 0.314 nm agrees well with the TEM observation. No XRD peaks assigned to the impurities were detected in

the XRD pattern. It is noted that XRD peaks belong to carbon support were hardly be found which might due to the relative lower peak intensity compared with the SnS2 crystal. The element composition of SnS2/BC-CNFs were further assessed by X-ray photoelectron spectroscopy (XPS). As can be seen from Fig. 2b, the general spectra of SnS2/BC-CNFs contained some typical peaks ascribed to Sn, S, C and O, in which element C and O are mainly derived from the BC-CNFs. The relative contents of Sn, S, C and O are determined to be 7.28%, 13.65%, 66.35% and 12.72%, respectively. Two peaks at a binding energy of 487.0 eV and 495.5 eV, are corresponding to the Sn 3d3/2 and 3d5/2 of Sn4þ, indicating that the synthesized sulfide are pure SnS2 (Fig. S1) [21]. The C1s high resolution spectra in Fig. S2 can be divided into four type carbons including conspicuous CeC peak (284.8 eV), CeO (285.6 eV), C¼O (287.3 eV) and OeC¼O (289.5 eV) [22]. To determine the mass loading of SnS2 in three composites, thermogravimetric analysis (TGA) was carried out, and the result is shown in Fig. S2. Two obvious weight losses were found between the temperature range of 400e440  C and 440e620  C, respectively, corresponding to the oxidation of SnS2 to SnO2 and the combustion of BC-CNFs [23]. In terms of the chemical reaction happened in TGA measurement, the mass loading of SnS2 in the composite of SnS2/BC-CNF-3, SnS2/BCCNF-5 and SnS2/BC-CNF-7 is estimated to be 85.4%, 76.01% and

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Fig. 3. (a) XRD patterns of SnS2 and SnS2/BC-CNFs, (b) XPS spectrum of SnS2/BC-CNFs-5, (c) N2 absorption/desorption isotherms of pure SnS2, BC-CNFs and SnS2/BC-CNFs-5, (d) Pore size distributions of SnS2, BC-CNFs and SnS2/BC-CNFs-5.

67.15%, respectively. As demonstrated by the SEM observation in Fig. 2c, SnS2 nanosheets crosslinked by the interconnected nanofibers may lead to the formation of nanoscale space between sheets and fibers, which can serve as a buffer zones to accommodate the large volume change induced by the alloying reaction of SnS2. Such structure is very beneficial to the cycling stability of SnS2/BC-CNFs electrode. The porosity of pure SnS2 nanosheet, BC-CNFs and the SnS2/BCCNFs-5 are examined by the N2 adsorption/desorption isotherms. The contrast of N2 adsorption/desorption curves and the pore size distribution plots are shown in Fig. 3c and d, respectively. As shown in Fig. 3c, the isotherms from SnS2 nanosheets can be identified as the type of IV isotherms defined by the international Union of Pure and Applied Chemistry (IUPAC) [24,25]. The sharp increase in the uptake of N2 at higher relative pressure of p/p0 > 0.9 reveals the macropore structure originated from the interparticles space. By contrast, the isotherms of both BC-CNFs and SnS2/BC-CNFs-5 show evident hysteresis loop, indicating the presence of mesopore structures [26]. Pore size distribution plots shown in Fig. 3d suggest that BC-CNFs has a high concentration of micropores which may related to the nanopores structure assigned to hard carbon. Compared to pure SnS2 nanosheets, SnS2/BC-CNFs-5 shows an increased micropores and mesopores, revealing a feature of hierarchical porous structures. The specific surface area and the total pore volume of SnS2, SnS2/BC-CNFs-5 and BC-CNFs are calculated to be 20.8, 144.7 and 787.3 m2 g
1 and 0.259, 0.463 and 0.672 cm3 g
1, respectively. It was clearly evident that the combination of SnS2 with BC-CNFs effectively increased the specific surface area and the total pore volume, which might provide more reactive sites to boost the electrolyte infiltration and ion transport [11,27]. The Na storage performance of SnS2 nanosheets and SnS2/BCCNFs electrode for SIB half-cells were investigated by the electrochemical tests. Fig. 4a and b displays the CV curves of SnS2 and SnS2/BC-CNFs-5 for the initial three cycles. In the first cathodic

scan, the reduction peak at about 1.55 V may belong to the intercalation of Na into the SnS2 layers, corresponding to equation (3). The small reduction peak around 1.16 V could be attributed to the conversion reaction from SnS2 to Sn, according to equation (4). The reduction peak of 0.5 V can be assigned to the alloying reaction (equation (5)) and the formation of irreversible solid electrolyte interface (SEI) film [28]. Due to the activation of first scan, the reduction peaks of 1.16 V and 0.5 V shifted to the potential of about 0.99 V and 0.66 V in the subsequent scans. In the anodic scan, three oxidation peaks appeared at the potential of 0.35, 0.75 and 1.2 V, which may be ascribed to the dealloying of Na3.75Sn, the formation of NaSnS2 and the extraction of sodium from the SnS2 layers [29]. Both pure SnS2 and SnS2/BC-CNFs-5 reveal nearly identical CV curves, indicating a similar electrochemical reaction mechanism. No surprisingly, the composite electrodes of SnS2/BC-CNFs-3 and SnS2/BC-CNFs-7 displayed same CV curves with the SnS2/BC-CNFs5 (Fig. S4). The voltage-capacity plots of SnS2 and SnS2/BC-CNFs-5 tested in a potential range of 0.01e3 V at a current rate of 50 mA g
1 are shown in Fig. 4c and d. Three voltage plateau presented at about 1.75, 1.25 and 0.75 V in the first discharge curves exactly coincided with the redox peaks in the CV curves. The initial discharge and charge capacities of SnS2/BC-CNFs-5 are 1333 and 641 mAh g
1, respectively, which are higher than the 1188 and 617 mAh g
1 of SnS2. The increased active sites originated from the porous BC-CNFs may account for the improved charge and discharge capacity of SnS2/BC-CNFs-5 [30]. The profiles of charge/discharge curves of SnS2/BC-CNFs-3 and SnS2/BC-CNFs-7 (Figure S 4c and d) agreed well with the SnS2/BC-CNFs-5. It is noteworthy that the charge/ discharge curves of SnS2/BC-CNFs-7 on the second and third cycles manifest better overlap, which predicting better cycling performance as compared with SnS2/BC-CNFs-3 and SnS2/BC-CNFs-5. SnS2 þ Naþ þ e
/NaSnS2

(3)

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273

Fig. 4. (a) CV curves of SnS2, (b) CV curves of SnS2/BC-CNFs-5, (c) Galvanostatic charge/discharge curves of SnS2, (d) Galvanostatic charge/discharge curves of SnS2/BC-CNFs-5.

þ 4e
/ 2Na2S þ Sn

(4)


Sn þ 3.þ 75Na þ 3.75e / Na3.75Sn

(5)

4Na þ

SnS2

To evaluate the cycling performance of SnS2 and SnS2/BC-CNFs electrode, the discharge specific capacities as a function of cycle

numbers at 50 mA g
1 for all the electrodes were tested (Fig. 5a). It is obvious that the cycling stability of the three SnS2/BC-CNFs are superior to the pure SnS2. After 100 cycles, the composite electrode of SnS2/BC-CNFs-3, SnS2/BC-CNFs-5 and SnS2/BC-CNFs-7 delivered a discharge specific capacity of 242.6, 408 and 324 mAh g
1, respectively, which are all higher than the 175 mAh g
1 of SnS2.

Fig. 5. (a) Cyclic performance, (b) rate capability, and (c) electrochemical impedance spectroscopy plots of pure SnS2 and SnS2/BC-CNFs.

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Specifically, the SnS2/BC-CNFs-7 with a maximum loading amount of carbon nanofibers shows the best capacity retention. However, it revealed lower discharge specific capacity as compared to SnS2/BCCNFs-5 due to the much reduced amount of SnS2. By contrast, SnS2/ BC-CNFs-3 displays an unsatisfactory cycling stability due to an insufficient adding of BC-CNFs, which might not fully in favor of the formation of buffer zone to accommodate the volume effect during the charge/discharge process [31]. The fact that the specific capacity over 100 cycles of all SnS2/BC-CNFs outperformed SnS2, indicating that BC-CNFs play a crucial role in stabilizing the electrode structure. As a comparison, the cycle life measurement for the individual BC-CNFs electrode has been delivered (Fig. S5). Nearly no decay on the specific capacity of BC-CNFs over 100 cycles suggests an excellent cycling performance. To investigate the rate capability of SnS2 and SnS2/BC-CNFs electrode, the discharge/charge specific capacities as a function of current density were tested (Fig. 5b). It can be seen that the specific capacity of SnS2 fades evidently with the increase of current density. Only a specific capacity value of 38.5 mAh g
1 was maintained when the current rate increased to 2 A g
1. By contrast, all the SnS2/BC-CNFs electrodes exhibited better rate capability, in which specific capacity attenuation with moderate decrease has been demonstrated as current rates increased from 0.05 to 2 A g
1. Electrochemical impedance spectroscopy (EIS) was carried out to obtain an insightful investigation of an enhanced electrode reaction kinetics. As indicated by Fig. 5c, the Nyquist plots possess similar feature of depressed semicircles and sloped lines in the high-medium and low frequency regions. The charge transfer resistance (Rct) of electrode can be acquired by the semicircle diameter. It is evident that the values of Rct for the composite electrodes were smaller than that of the pure SnS2 electrode. Among three composite electrodes, SnS2/BC-CNFs-7 revealed a lowest Rct value due to the much increased contents of carbon nanofibers, which significantly improved the electrical

conductivity of electrode [32,33]. As the sodiation/desodiation processes required a simultaneous charge accumulation and transport, charge transfer ability plays important role in the whole electrochemical reaction. Therefore, the reduced Rct of composite electrode demonstrated a significant kinetic enhancement in the high rate electrode performance [34]. To disclose the Na storage mechanism of SnS2 and SnS2/BCCNFs, kinetics analysis based on CV curves is performed. Fig. 6a shows the CV curves of SnS2/BC-CNFs-5 from the scan rate of 0.2 mV s
1 to 1.0 mV s
1. The shape of CV curves remains almost unchanged with the increase of scan rates. The peak current (i) and the scan rates (v) from the CV curves can be quantitatively analyzed by the relationship: i ¼ avb, where a and b are both adjustable parameters [35]. The value of b is between 0.5 and 1, which can be determined by the slope of log(i) versus log(v) plot. For the diffusion-controlled electrode reaction b is close to 0.5, surface capacitance-dominated process b approaches to 1 [36]. The higher b value (0.93 vs 0.79 of SnS2/BC-CNFs-5 to SnS2) suggests a capacitance preferential kinetics process of SnS2/BC-CNFs-5 compared to SnS2. In general, the current response (i) is always derived from the combination of diffusion-controlled reaction and surface capacitive storage. Thus, the degree of capacitance contribution to the current can be quantitatively evaluated by the equation: i(v) ¼ k1v þ k2v1/2, where k1v represents the surface capacitance contribution, k2v1/2 is corresponding to the current generated by diffusion-controlled reaction [37]. The constants k1 and k2 can be determined from the CV curves at various scan rates. As shown in the red area of Fig. 6c and 61.5% of the total capacity is defined as capacitive contribution for the electrode of SnS2/BC-CNFs-5 at a scan rate of 0.8 mV s
1, which is higher than 58% of SnS2 (the inset in Fig. 6c). According to same method, the capacity contribution from the diffusion-controlled reaction and the surface-capacitive process as a function of scan rates can be determined and the result is shown

Fig. 6. Quantitative capacitive analysis of Na storage mechanism. (a) CV curves of SnS2/BC-CNFs-5 electrode at various of scan rates, (b) relationship of cathodic peak current and scan rates for the electrode of SnS2 and SnS2/BC-CNFs-5, (c) capacitive contribution of SnS2/BC-CNFs-5 (inset for SnS2), (d) charge contributions by the diffusion/capacitivecontrolled process as a function of scan rate.

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in Fig. 6d. As expected, the capacitive contribution gradually improves with an increase of scan rate. The surface capacitance dominated kinetics of SnS2/BC-CNFs-5 originated from the combination of carbon nanofibers may account for the enhanced rate capability and cycling performance. 4. Conclusion In summary, mesoporous composites of SnS2/BC-CNFs have been synthesized via a simple hydrothermal method aiming to boost the Na storage performance of SnS2. Uniform SnS2 nanosheets are anchored onto high conductive carbon nanofibers aerogels, which effectively prevented the restacking and alleviated the volume change of SnS2 nanosheets. The optimization of relative contents for carbon nanofibers and SnS2 in the composite is required to manifest the best synergic effect. Among the SnS2/BCCNFs composites, the SnS2/BC-CNFs-5 with a carbon mass loading of about 24% showed optimal Na storage performance, which delivered a specific capacity of 408 mAh g
1 at 50 mA g
1 after 100 cycles and 196.4 mAh g
1 at a high current rate of 2 A g
1. A surfacecapacitive dominated kinetics process for the SnS2/BC-CNFs electrode is responsible for the enhanced Na storage performance. The facile synthesis method with excellent Na storage performance makes SnS2/BC-CNFs a competitive candidate for anode material of SIBs. Acknowledgments This work was supported by Foundation of Graduate Innovation Center (grant no. kfjj20180617) in Nanjing University of Aeronautics and Astronautics Open Fund of Key Laboratory of Materials Preparation, the Fundamental Research Funds for the Central Universities, Protection for Harsh Environment (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology (grant no. 1006-56XCA1815901r), and the Priority Academic Program Development (PAPD) of the Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.165. References [1] L. Yin, R. Cheng, Q. Song, J. Yang, X. Kong, J. Huang, Y. Lin, H. Ouyang, Electrochim. Acta 293 (2019) 408e418. [2] J. Xu, J. Chen, Z. Shuang, C. Han, M. Xu, Z. Ni, C.P. Wong, Nano Energy 50 (2018) 323e330.

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