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C nanofibers as binder-free anode for lithium–ion and sodium-ion batteries
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Electrospun SnSe/C nanofibers as binder-free anode for lithium–ion and sodium-ion batteries Jing Xia a, c, Yiting Yuan a, Hanxiao Yan a, Junfang Liu a, Yue Zhang a, Li Liu a, *, Shu Zhang a, Wanjun Li a, Xiukang Yang a, Hongbo Shu a, Xianyou Wang a, Guozhong Cao b, ** a
National Base for International Science & Technology Cooperation, Key Laboratory of Environmentally Friendly Chemistry and Application (Ministry of Education), College of Chemistry, Xiangtan University, Xiangtan, 411105, PR China Department of Materials Science and Engineering, University of Washington, Seattle, WA, 98195-2120, United States c Institute of Molecular Plus, Tianjin University, Tianjin, 300072, PR China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� The SnSe/C nanofiber membrane was synthesized by electrospinning technology. � The SnSe/C nanofiber membrane ex hibits superior mechanical flexibility. � The SnSe/C nanofiber membrane shows excellent lithium/sodium storage performance. � The after-cycled SnSe/C nanofiber elec trode still retains its structural integrity.
A R T I C L E I N F O
A B S T R A C T
Keywords: SnSe Binder-free Lithium-ion battery Sodium-ion battery Electrospinning
The design of electrodes with superior mechanical flexibility is the key to developing energy storage devices with mechanical durability and excellent electrochemical performance. Herein, a flexible SnSe/C nanofiber mem brane is successfully synthesized by electrospinning technology and subsequent calcination. From the macro scopic view, the SnSe/C nanofiber membrane can tolerate a bending angle of 180� without any breakage, indicating its superior mechanical flexibility. From the microscopic view, SnSe nanoparticles are uniformly distributed along the carbon nanofiber framework. The carbon nanofiber framework serves not only as a conductive matrix that improves the electrical conductivity of the composite, but also as a buffer material that alleviates the volume expansion during electrochemical reaction. These merits endow excellent electrochemical performance to SnSe/C nanofibers when used as a binder-free and current collector-free anode for lithium/so dium ion batteries. The SnSe/C nanofiber anode delivers a stable discharge capacity of 405 mAh g 1 at 1000 mA g 1 after 500 cycles in lithium ion battery and 290 mAh g 1 at 200 mA g 1 after 200 cycles in sodium ion battery. These results demonstrate that the SnSe/C nanofiber is a promising anode material for flexible lithiumion and sodium-ion batteries.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Liu), [email protected] (G. Cao). https://doi.org/10.1016/j.jpowsour.2019.227559 Received 11 October 2019; Received in revised form 29 November 2019; Accepted 2 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jing Xia, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227559
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1. Introduction
developing, flexible electronics and wearable devices are at the forefront of new trends [27]. Flexible products require flexible batteries to pro vide energy output. The conventional electrodes are prepared by coating active materials, binders and conductive materials on metal current collectors, which are difficult to meet the flexibility needs due to the inherent inflexible electrode structure. Therefore, the development of flexible electrodes has become the key to achieve the applications of flexible products. As we all know, electrospinning is a simple, inex pensive and scalable strategy for in-situ compounding with carbon and producing membrane-based electrode materials [14,28–30]. To our knowledge, there have been no reports on the electrospinning synthesis of SnSe. Thus, it is meaningful to synthesize SnSe with effective carbon coating by electrospinning and investigate its lithium/sodium storage performance. Herein, we first synthesized the SnSe/C nanofiber (denoted as SnSe/ CNF) membrane via electrospinning technology. The SnSe/CNF has superior mechanical flexibility and 1D nanofiber morphology with a diameter of around 180 nm. When the SnSe/CNF was directly used as a flexible anode for LIBs and SIBs without the use of binders, conductive additives and metal current collectors, excellent electrochemical prop erties including high capacity and stable cycling performance were ob tained. The microstructure characterization reveals that the after-cycled electrodes still retain initial electrode structure and fibrous morphology even after cycling at high current densities in LIBs and SIBs. These characteristics make it possible for the binder-free SnSe/CNF anode to achieve the practical application of flexible LIBs and SIBs.
Lithium-ion batteries (LIBs) have been extensively used in portable devices and, more recently, electric vehicles [1–10]. However, the increasing use of LIBs and unevenly distributed lithium resources on the earth, are expected to strongly impact the availability as well as the price of lithium salts in the future [11–13]. Even though this problem can be alleviated to some extent by recycling the lithium, the development of sodium-ion batteries (SIBs) in parallel with developing LIBs may be a more effective strategy in the long term due to greater abundance and lower cost of sodium salts [14,15]. Tin(II)-chalcogenides (such as SnS and SnSe), which have high theoretical capacity as anode material in both LIBs and SIBs, have received important research attention in the past decade. By contrast, SnSe has a higher electrical conductivity than SnS owing to the narrower band gap of SnSe than SnS (0.9 eV for SnSe vs. 1.3 eV for SnS) [16–18]. Besides, during the electrochemical reaction process, SnSe experiences a smaller volume expansion and has better electrochemical reversibility than SnS [19]. These advantages make SnSe more suited for repeated cycling in LIBs and SIBs. Despite these advantages, however, the high theoretical capacity of SnSe is also based on conversion reaction com bined with alloying reaction. The huge volume expansion of the Sn–Li alloying reaction (�260%) and the Sn–Na alloying reaction (�400%) causes electrode pulverization and aggregation of active materials, resulting in deterioration of electrochemical performance [20]. In order to improve the lithium/sodium storage performance of SnSe, some strategies, including designing the morphology with different sizes, alien ions doping and coating with conductive materials have been re ported [21–24]. Although these strategies improved the electrochemical performance of SnSe to a certain extent, the capacity and cycle life of the reported SnSe are far below the application requirements in LIBs and SIBs. The underlying cause for the poor electrochemical performance is that traditional SnSe composites synthetic route is mainly based on two-step process, namely the fabrication of pure SnSe (including solid-state reaction, hydrothermal/solvothermal reaction) and subse quent compounding with conductive materials. This two-step synthetic process often produces micrometer-sized or bulk SnSe with insufficient carbon coating, resulting in rapid capacity fading due to the fact that the large-sized bare SnSe particles are not conducive to electrolyte perme ation and electron transfer [23,25,26]. Since the 21st century, economics and technology are rapidly
2. Results and discussion 2.1. Synthesis and characterization of SnSe/CNF The SnSe/CNF was synthesized by electrospinning and subsequent heat treatment. Fig. 1 schematically illustrates the synthetic process for SnSe/CNF. First of all, the precursor solution of N, N-Dimethylforma mide (DMF), tin (II) chloride dihydrate (SnCl2⋅2H2O), selenium powder (Se) and polyvinylpyrrolidone (PVP) was first electrospun into a white precursor nanofiber membrane. In this case, DMF is a volatile solvent that is removed during electrospinning process and PVP is carbon source to produce carbon matrix. Afterward, the white precursor nanofiber membrane was heat-treated in an Ar/H2 atmosphere to obtain SnSe/ CNF membrane. In this process, SnCl2 reacts with Se to form SnSe, the
Fig. 1. Schematic illustration of the preparation process for SnSe/CNF. 2
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Fig. 2. (a, b) FE-SEM images of precursor nanofibers. (c, d) FE-SEM images of SnSe/CNF. Inset: digital photograph of SnSe/CNF electrode.
Fig. 3. (a) XRD pattern, (b) Raman spectrum and (c) TG-DSC curves of SnSe/CNF. (d) Pore size distribution curve and N2 adsorption–desorption isotherm (inset) of the SnSe/CNF. High-resolution (e) N1s and (f) C1s XPS spectra of the SnSe/CNF.
color of the nanofiber membrane turned from white to black owing to the carbonization of PVP. The obtained SnSe/CNF membrane was then cut into disks as a binder-free and current collector-free electrode, and these electrodes were directly assembled into Li/Na coin cells. Neither mechanical milling nor slurry casting is needed, which not only sim plifies the manufacturing process of the cells, but also reduces the pro duction cost of the cells. Detailed experimental procedures can be found in the Experimental Section (Supplementary Materials). Fig. 2 shows the field emission scanning electron microscopy (FE-
SEM) images for the precursor nanofibers (Fig. 2 a, b) and SnSe/CNF (Fig. 2 c, d). As shown in Fig. 2 panels a and b, the precursor nanofibers are interwoven and have a uniform diameter of about 300 nm. After heat treatment, the SnSe/CNF still maintain their fiber morphology, but the fiber diameter shrank to around 180 nm. In addition, the 1D nanofibers are woven into a 3D conductive network, shortening the ionic transport path. The bending test is the most common method to evaluate the us ability of flexible electrodes. The inset in Fig. 2d shows a digital photograph of the SnSe/CNF electrode, and the SnSe/CNF electrode can 3
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Fig. 4. (a) TEM image of SnSe/CNF. (b) HR-TEM images and corresponding interplanar crystal spacing statistical table (the inset) of SnSe/CNF. (c) HAADF image and (d–i) corresponding elemental mapping images of SnSe/CNF.
tolerate a bending angle of 180� without any breakage, indicating that the SnSe/CNF electrode has superior mechanical flexibility. The crystal structure of the SnSe/CNF was analyzed by the powder Xray diffraction (XRD) (Fig. 3a). All diffraction peaks of SnSe/CNF match perfectly with orthorhombic SnSe (JCPDS card No. 48–1224) without any detectable impurity. Raman spectroscopy was also used to study the SnSe/CNF (Fig. 3b). There are two peaks at 1350 and 1580 cm 1 cor responding to the disordered structure (D-band) and the graphitic structure (G-band) of carbon material, respectively [31,32]. The G-band has a higher peak intensity than the D-band (IG/ID ¼ 1.07), indicating that the carbon material has a high graphitization and good electrical conductivity. The content of SnSe in the SnSe/CNF was investigated by the ther mogravimetric analysis and differential scanning calorimetry (TG-DSC) (Fig. 3c). The slight weight loss below 300 � C can be attributed to the evaporation of adsorbed water. The second weight loss around 500 � C is ascribed to the oxidation of carbon, and the subsequent weight increase between 550 � C and 630 � C may be attributed to the oxidation of SnSe (2SnSe þ O2 → 2SnOSe) [33]. The weight loss after 630 � C might be related to the further oxidation of SnOSe (2SnOSe þ O2 → 2SnO2 þ 2Se ↑). Based on the final remaining 61.9% of SnO2 (the XRD pattern in Fig. S1 indicates that the final combustion product of SnSe/CNF is SnO2), the weight percentage of SnSe in SnSe/CNF can be calculated to be about 81.2%. Fig. 3d shows the pore size distribution curve and N2 adsorption–desorption isotherm (inset) of the SnSe/CNF. The pore size distribution curve indicates that there are many micropores below the 2.0 nm inside the nanofibers. Based on the Brunauer–Emmett–Teller (BET) method, the specific surface area of the SnSe/CNF was calculated to be 76.9 m2 g 1. The large specific surface area and porous structure are beneficial for enlarging the electrode/electrolyte contact area and shortening the diffusion distance of the Liþ/Naþ ions. Moreover, X-ray photoelectron spectroscopy (XPS) was used to research the nitrogen-doping configurations in carbon nanofibers. Fig. 3e shows the high-resolution N1s spectra, the four peaks at 401.9, 400.8, 399.8 and 398.4 eV are attributed to graphitic-N, pyrrolic-N, pyridine-N and pyridinic-N, respectively [34,35]. The high-resolution C1s spectra in Fig. 3 f further illuminates the existence of C–N bond (285.9 eV). It is worth noting that the existence of C–N bond can be assigned to the self-doping of nitrogen from PVP. The doped N can provide abundant electrons for the π-conjugated system of carbon, thereby improving the
electronic conductivity of the composite [36]. Furthermore, the pyrrolic-N and pyridinic-N can cause some defects in the carbon skel eton, providing more active sites for the storage of Liþ and Naþ [37]. In Fig. 3f, the peaks at 286.7 and 284.8 eV correspond to the C–O and C–C bonds, respectively. The C–O bonds may come from the PVP that is not fully carbonized. The transmission electron microscopy (TEM) image of SnSe/CNF in Fig. 4a exhibits smooth fiber surface, which is consistent with FE-SEM images. Fig. 4b shows the high-resolution TEM (HR-TEM) image and corresponding interplanar crystal spacing statistical table (inset). The HR-TEM image clearly illustrates that the SnSe nanoparticles are very small and less than 2 nm in diameter. The obvious lattice fringes and the corresponding interplanar crystal spacing statistical table present that the interplanar spacing of 3.51 Å corresponds to the (201) plane of the orthorhombic SnSe (JCPDS card No. 48–1224). Fig. 4d–i shows the elemental mapping images of C, N, Sn and Se elements corresponding to the rectangular region of Fig. 4c. The elemental mapping images reveal the uniform distributions of C, N, Sn and Se elements along the SnSe/ CNF fiber framework, meaning that the SnSe nanoparticles are evenly distributed in the carbon nanofibers. The uniform distribution of SnSe nanoparticles in the carbon nanofibers can buffer the volume expansion caused by the alloying reactions to some extent. 2.2. Electrochemical evaluations in LIBs The lithium storage performance of SnSe/CNF was evaluated by using SnSe/CNF as a binder-free working electrode, and the lithium metal as the counter/reference electrode. Fig. 5a shows the cyclic vol tammetry (CV) curves for the first three cycles. During the first cathodic scan, the reduction peak at 0.96 V, which shifts to 1.20 V in the subse quent cycles, can be assigned to the conversion of SnSe (SnSe þ 2Liþ þ 2e ↔ Sn þ Li2Se) [38,39]. The peak shift of the subsequent cycles compared to the first cycle may be owing to the structural variations of materials or/and the formation of solid electrolyte interface (SEI) film [40,41]. The reduction peaks at 0.6 V and below can be attributed to the formation of SEI film and LixSn (Sn þ xLiþ þ xe ↔ LixSn (0 < x � 4.4)) [42,43]. In the subsequent scan, a weak reduction peak appeared at around 2.06 V, which is generally considered to be the formation of poly-selenides [43,44]. During anodic scan, several peaks at around 0.69 V in all cycles are attributed to the multi-step de-alloying process 4
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Fig. 5. Electrochemical performance of SnSe/CNF in LIBs: (a) Cyclic voltammetry curve at a scan rate of 0.1 mV s 1; (b) discharge/charge profiles at 50 mA g 1; (c) rate performance and corresponding coulombic efficiency; (d) discharge/charge profiles at various current densities from 50 to 4000 mA g 1; cycling performance and corresponding coulombic efficiency for 500 cycles at 1000 mA g 1.
at 50, 100 and 200 mA g 1, respectively. Not only that, the average discharge capacities are as high as 429 and 384 mAh g 1 even at 3000 and 4000 mA g 1, respectively, and could rapidly recover to about 630 mAh g 1 when the current density turned back to 50 mA g 1. In addi tion, the CE remained close to 100% after the first cycle. The excellent rate performance results from the fact that the carbon matrix improves the electrical conductivity of the composite, and the 1D fiber structure provides fast transport path for ions and electrons. Fig. 5d exhibits the discharge/charge curves corresponding to the rate performance test above. Although the specific capacity decreases with the increase of current density, the discharge/charge voltage plateaus are still stable and obvious, indicating that the SnSe/CNF electrode has high stability. Fig. 5e shows the cycling performance of SnSe/CNF electrode at a current density of 1000 mA g 1. The SnSe/CNF electrode delivers a high first discharge capacity of 659 mAh g 1 and charge capacity of 525 mAh g 1 with the initial CE of 80%. In the subsequent cycles, the capacity remained relatively stable and the CE kept at a level of close to 100%. After 500 cycles, a high discharge capacity of 405 mAh g 1 is still
[45,46]. The oxidation peaks at 1.83 V and above may be attributed to the reversible formation of SnSe combined with the decomposition of poly-selenides. It is worth mentioning that the CV curve of the third scan overlaps nearly the second scan, indicating a good reversibility for the lithium storage reaction of SnSe. Fig. 5b exhibits the galvanostatic dis charge/charge curves of SnSe/CNF at a current density of 50 mA g 1 in the 0.01–2.50 V voltage range. All discharge/charge curves show obvious voltage plateau. The SnSe/CNF deliver a high first discharge capacity of 796 mAh g 1 and charge capacity of 685 mAh g 1 with an initial coulombic efficiency (CE) of 86%. The irreversible capacity loss of 14% is mainly due to the electrolyte decomposition and the inevitable formation of SEI film. In the subsequent cycle, all discharge/charge curves are nearly completely overlapped, which are consistent with the CV test result. The rate performance of SnSe/CNF electrode was investigated by the galvanostatic discharge/charge tests at various current densities of be tween 50 and 4000 mA g 1. As shown in Fig. 5c, the SnSe/CNF electrode delivers high average discharge capacities of 675, 649 and 624 mAh g 1 5
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Fig. 6. Characterization of SnSe/CNF electrode after the rate performance test in LIBs: (a) digital photograph; (b) TEM and (c) HAADF images; (d–i) elemental mapping images.
obtained, and the capacity retention is approaches 77.6% as compared with the second discharge. The cycling performance of SnSe/CNF is preferable to many previously reported SnSe-based anodes in LIBs [25,
26,39,43,47]. Table S1 enumerates some of the previously reported works for transition metal selenide anodes in LIBs and makes a comprehensive comparison with the SnSe/CNF electrode. Obviously,
Fig. 7. Electrochemical performance of SnSe/CNF in SIBs: (a) Cyclic voltammetry curve at a scan rate of 0.1 mV s 1; (b) discharge/charge profiles at 50 mA g 1; (c) rate performance and corresponding coulombic efficiency; (d) discharge/charge profiles at various current densities from 50 to 2000 mA g 1; cycling performance and corresponding coulombic efficiency for 200 cycles at 200 mA g 1. 6
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mixing SnSe with carbon-based materials is a common strategy to improve the lithium storage performance of transition metal selenides. Some SnSe-based composites such as SnSe@CNFs [43], SnSe–C [26], SnSe/C [25] have achieved improved electrochemical performance in LIBs, but their cycle life is hard to over 100 cycles. Lee et al. [39] syn thesized a composite of SnSe and C using a solid-state reaction method and obtained a high capacity in LIBs. However, the high capacity can be obtained only at small current densities (100 mA g 1 or lower). Guo’s group [47] reported a 2D ultra-thin layered SnSe nanoplates prepared by colloid chemistry method and achieved a high capacity at an ultrahigh current density of 5000 mA g 1, but its cycle life is also only 300 cycles. Most importantly, all of the listed electrodes are non-flexible and contain a heavy metal current collector as well as more than 10% of non-conductive binder, which not only makes them difficult to meet the flexibility needs, but also increases the production cost of the battery. Although some flexible transition metal selenide anodes such as Sb2Se3 nanowires [48], Co0.85Se NSs/G [49], and Bi2Se3/graphene [50] have been developed, their cycle life does not exceed 300 cycles. Therefore, the SnSe/CNF electrode obtained in this work indisputably demon strates its superiority compared to the other transition metal selenide anodes listed above. To investigate the mechanism for the excellent Li-storage perfor mance of the SnSe/CNF electrode, the charge transfer kinetics of SnSe/ CNF was studied by electrochemical impedance spectroscopy (EIS) analysis. Fig. S2 presents the Nyquist plots and corresponding simula tion curves of the SnSe/CNF electrode after 1 cycle and rate performance test (corresponding to Fig. 5c) in LIBs. Each Nyquist plot consists of a semicircle and a sloping line, suggesting that the electrochemical reac tion involves both charge transfer and Li-ion diffusion [51–54]. The SnSe/CNF electrode shows a small charge transfer resistance (Rct) of 186.2 Ω after 1cycle (the simulation results are displayed in Table S3), and the Rct value increased only to 285.7 Ω even after cycling at various high current densities, indicating that the SnSe/CNF electrode has a fast charge transfer kinetics and stable electrode structure. Furthermore, the lithium cell after the rate performance test (cor responding to Fig. 5c) was disassembled, and the micromorphology of the SnSe/CNF electrode was observed using both FE-SEM and TEM. Fig. 6a shows the digital photograph of the after-cycled SnSe/CNF electrode. The after-cycled SnSe/CNF electrode still maintained its intact electrode structure without breakage. Fig. S4 shows the FE-SEM image of the after-cycled SnSe/CNF with rough fiber surface due to the coverage of the compact SEI film. The formation of SEI film is the main reason accounting for the large first irreversible capacity loss [55]. Fortunately, the SnSe/CNF still maintained its fibrous morphology without being destroyed, indicating that the strong mechanical flexi bility allows the SnSe/CNF electrode to endure repeated cycles by buffering the volume expansion even at ultrahigh current densities. In the TEM image (Fig. 6b), the compact SEI film on the fiber surface can also be clearly observed. Moreover, Fig. 6d–i shows the elemental mapping images of C, N, Sn and Se elements corresponding to the rectangular region of Fig. 6c. All elements including C, N, Sn and Se are still uniformly distributed along the fiber framework, and no obvious Sn/Se bright specks are observed, indicating that the carbon matrix effectively inhibits the agglomeration of SnSe nanoparticles.
the first scan can be assigned to the conversion from SnSe to Sn and Na2S (SnSe þ 2Naþ þ 2e ↔ Sn þ Na2Se)), the formation of SEI film as well as Na–Sn alloying reaction process (Sn þ xNaþ þ xe ↔ NaxSn (0 < x � 3.75) [24,56]. There are several small cathodic peaks below 0.3 V ascribed to the gradual Na–Sn alloying reaction process [18,21]. The oxidation peaks at 0.01–0.9 V during all anodic scan process arise from the step-by-step de-alloying reaction from NaxSn to Na–Sn intermediate phases and final Sn [57]. The anodic peaks located at 1.12 V and above in all cycles are attributed to the conversion of Sn to SnSe. In the sub sequent cathodic scan, the two cathodic peaks at 1.00 and 0.69 V may be derived from the conversion and alloying reactions, respectively. Fig. 7b shows the galvanostatic discharge/charge profiles of the SnSe/CNF electrode at a current density of 50 mA g 1. The voltage profiles present clear voltage platforms, which were consistent with the current peaks observed in the CV curves. The discharge and charge curves of the third cycle nearly overlap with the second cycle, suggesting that the SnSe/CNF electrode has good reversibility. Fig. 7c demonstrates the rate performance of the SnSe/CNF electrode in the current density range of 50–2000 mA g 1. The SnSe/CNF elec trode delivers high first discharge capacity of 604 mAh g 1 with the initial CE of 63%. The large irreversible capacity loss in the first cycle is mainly due to the irreversible formation of SEI film, which has been confirmed by the above CV analysis. Except for the first cycle, the SnSe/ CNF electrode provides high average discharge capacities of 379, 352, 333, 311, 277 and 241 mAh g 1 at current densities of 50, 100, 200, 500, 1000 and 2000 mA g 1, respectively. More importantly, the discharge capacity can quickly recover to about 370 mAh g 1 and remain stable when the current density returns to 50 mA g 1. Moreover, the CE is close to 100% after the first cycle. The discharge/charge curves in Fig. 7d also exhibit obvious voltage platform even at 2000 mA g 1. These results reveal that SnSe/CNF with large specific surface area and high electrical conductivity can accelerate the transport of Naþ ions and electrons even at higher current density without irreversible changes of 1D nano structures [58]. The cycling performance (Fig. 7e) of SnSe/CNF electrode was also performed by the galvanostatic discharge and charge tests at a current density of 200 mA g 1. The SnSe/CNF electrode delivers a high first discharge capacity of 543 mAh g 1 with an initial CE of 62%. After ongoing 200 cycles, 87.8% discharge capacity (290 mAh g 1) was still retained compared to the second discharge. The cycling performance of SnSe/CNF electrode was compared favorably with previously reported transition metal selenide anodes for SIBs (Table S2) [18,21,59]. For example, Yuan et al. [21] developed a single-crystalline SnSe nanosheet clusters and achieved a discharge capacity of 271 mAh g 1 at 100 mA g 1 after 100 cycles. Ren et al. [18] synthesized the composite of SnSe nanosheets and nitrogen-doped carbon using a cation-exchange strategy and obtained a discharge capacity of 258 mAh g 1 after 200 cycles. Recently, Zhang’s group [59] reported a yolk–shell structured SnSe that achieved a cycle life of 150 cycles when used as an anode material for SIBs. Moreover, the SnSe nanoplates fabricated by Kang’s group [24] and Guo’s group [47] exhibit high capacity and long cycle life, respec tively. However, the capacity output of the above-mentioned SnSe-based materials requires a heavy metal current collector as a support and the electrode contains more than 25% of the inactive ingredients, which greatly reduces the energy density of the battery. In addition, these electrodes are not suitable for flexible batteries. Some transition metal selenide anodes such as MoSe2/CF [60], Sb2Se3 nanowires [48], and WSe2/C [61] nanofibers have been developed and used in flexible SIBs, but their electrochemical performance is still unsatisfactory. In conse quence, the SnSe/CNF anode demonstrates distinct superiorities compared to the above-listed transition metal selenide anode materials. Fig. S3 shows the Nyquist plots of the SnSe/CNF electrode after 1 cycle and rate performance (corresponding to Fig. 7c) in the SIBs. The simulation results in Table S4 reveal that the Rct of the SnSe/CNF electrode after 1 cycle is 287.6 Ω. After repeated discharge/charge at various current densities from 50 to 2000 mA g 1, Rct value increase rate
2.3. Electrochemical evaluations in SIBs The extraordinary electrochemical performance of SnSe/CNF elec trode in LIBs encourages us to further research its performance in SIBs. The SnSe/CNF was also cut into disks and directly used as a binder-free working electrode, and sodium metal was used as the counter/reference electrode to assemble Na cell. Fig. 7a shows the CV curves of the SnSe/ CNF electrode for the initial three cycles. The weak reduction peak at around 1.09 V in the first scan disappearing in the subsequent cycles is generally considered as the Naþ intercalating into the interlayers of SnSe without phase transition [22]. The wide peak between 0.3 and 1.0 V in 7
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Fig. 8. Characterization of SnSe/CNF electrode after the rate performance test in SIBs: (a) digital photograph; (b) TEM and (c) HAADF images; (d–i) elemental mapping images.
was 154% (442.8 Ω), which is comparable to 153% in LIBs. Therefore, SnSe/CNF electrode also has fast charge transfer kinetics and a stable electrode structure in SIBs, thereby providing stable sodium storage capacity. The micromorphology of after-cycled SnSe/CNF electrode in SIBs was also investigated by both FE-SEM and TEM. The sodium cell after the rate performance test (corresponding to Fig. 7c) was disassembled, and the digital photograph of the after-cycled SnSe/CNF electrode was shown in Fig. 8a. The after-cycled SnSe/CNF electrode still maintains its original disk appearance. Both the FE-SEM image in Fig. S5 and the TEM image in Fig. 8b exhibit unbroken fibrous morphology. The unsmooth fiber surface is attributable to the formation of the SEI film on the fiber surface. In addition, Fig. 8d–i displays the elemental mapping images of C, N, Sn and Se corresponding to the rectangular region of HAADF image in Fig. 8c. After the rate performance test, the C, N, Sn and Se elements are still uniformly distributed along the fiber framework. These phe nomena are similar to the corresponding test results in LIBs. Therefore, excellent electrochemical performance is also achieved in SIBs.
Acknowledgments This work was supported financially by the National Natural Science Foundation of China (Grant No. 51672234), the Research Foundation for Hunan Youth Outstanding People from Hunan Provincial Science and Technology Department (2015RS4030), Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Envi ronmental Benignity and Effective Resource Utilization, Program for Innovative Research Cultivation Team in University of Ministry of Ed ucation of China (1337304). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227559. References [1] C.-Y. Wang, G. Zhang, S. Ge, T. Xu, Y. Ji, X.-G. Yang, Y. Leng, Nature 529 (2016) 515. [2] X. Feng, M. Ouyang, X. Liu, L. Lu, Y. Xia, X. He, Energy Storag. Mater. 10 (2018) 246–267. [3] B. Diouf, R. Pode, Renew. Energy 76 (2015) 375–380. [4] V. Ruiz, A. Pfrang, A. Kriston, N. Omar, P. Van den Bossche, L. Boon-Brett, Renew. Sustain. Energy Rev. 81 (2018) 1427–1452. [5] L. Wang, Q. Zhang, J. Zhu, X. Duan, Z. Xu, Y. Liu, H. Yang, B. Lu, Energy Storag. Mater. 16 (2019) 37–45. [6] X. Lu, Q. Zhang, J. Wang, S. Chen, J. Ge, Z. Liu, L. Wang, H. Ding, D. Gong, H. Yang, X. Yu, J. Zhu, B. Lu, Chem. Eng. J. 358 (2019) 955–961. [7] J. Llanos, C. Contreras-Ortega, J. P� aez, M. Guzm� an, C. Mujica, J. Alloy. Comp. 201 (1993) 103–104. [8] C. Han, Z. Li, W.-j. Li, S.-l. Chou, S.-x. Dou, J. Mater. Chem. A 2 (2014) 11683–11690. [9] M. Mao, L. Jiang, L. Wu, M. Zhang, T. Wang, J. Mater. Chem. A 3 (2015) 13384–13389. [10] A. Paolella, C. George, M. Povia, Y. Zhang, R. Krahne, M. Gich, A. Genovese, A. Falqui, M. Longobardi, P. Guardia, T. Pellegrino, L. Manna, Chem. Mater. 23 (2011) 3762–3768. [11] J.-M. Tarascon, Nat. Chem. 2 (2010) 510. [12] A. Darwiche, C. Marino, M.T. Sougrati, B. Fraisse, L. Stievano, L. Monconduit, J. Am. Chem. Soc. 134 (2012) 20805–20811. [13] X. Xu, S. Ji, M. Gu, J. Liu, ACS Appl. Mater. Interfaces 7 (2015) 20957–20964. [14] S. Nie, L. Liu, J. Liu, J. Xie, Y. Zhang, J. Xia, H. Yan, Y. Yuan, X. Wang, Nano-Micro Lett. 10 (2018) 71. [15] B. Qu, C. Ma, G. Ji, C. Xu, J. Xu, Y.S. Meng, T. Wang, J.Y. Lee, Adv. Mater. 26 (2014) 3854–3859. [16] N. Koteswara Reddy, K.T. Ramakrishna Reddy, Thin Solid Films 325 (1998) 4–6. [17] M. Sharon, K. Basavaswaran, Sol. Cells 25 (1988) 97–107. [18] X. Ren, J. Wang, D. Zhu, Q. Li, W. Tian, L. Wang, J. Zhang, L. Miao, P.K. Chu, K. Huo, Nano Energy 54 (2018) 322–330. [19] W. Wang, P. Li, H. Zheng, Q. Liu, F. Lv, J. Wu, H. Wang, S. Guo, Small 13 (2017) 1702228. [20] Q. Tang, Y. Cui, J. Wu, D. Qu, A.P. Baker, Y. Ma, X. Song, Y. Liu, Nano Energy 41 (2017) 377–386. [21] S. Yuan, Y.-H. Zhu, W. Li, S. Wang, D. Xu, L. Li, Y. Zhang, X.-B. Zhang, Adv. Mater. 29 (2017) 1602469. [22] R. Chen, S. Li, J. Liu, Y. Li, F. Ma, J. Liang, X. Chen, Z. Miao, J. Han, T. Wang, Q. Li, Electrochim. Acta 282 (2018) 973–980. [23] X. Yang, R. Zhang, N. Chen, X. Meng, P. Yang, C. Wang, Y. Zhang, Y. Wei, G. Chen, F. Du, Chem. Eur J. 22 (2016) 1445–1451. [24] G.D. Park, J.H. Lee, Y.C. Kang, Nanoscale 8 (2016) 11889–11896.
3. Conclusions In summary, a novel SnSe/CNF has been successfully synthesized by a simple electrospinning strategy and subsequent heat treatment. When the SnSe/CNF membrane was cut into disks as the binder-free and cur rent collector-free anode for LIBs and SIBs, excellent electrochemical properties including high capacity and stable cycling performance were obtained. In LIBs, the discharge capacity can reach as high as 405 mAh g 1 after 500 cycles when a high current density of 1000 mA g 1 was applied. Even at an ultrahigh current density of 4000 mA g 1, the average discharge capacity can still retain 384 mAh g 1. In SIBs, the SnSe/CNF electrode can also retain a discharge capacity of 290 mAh g 1 after 200 cycles at 200 mA g 1. The excellent electrochemical perfor mance of SnSe/CNF electrode is ascribed to the following advantages. Firstly, the 1D nanofibers are woven into a 3D conductive network, shortening the ionic transport path. Secondly, the SnSe/CNF composites with large specific surface area and high electrical conductivity accel erate the ionic and electronic transport. Thirdly, the SnSe nanoparticles are uniformly distributed and entirely encapsulated in carbon nano fibers, thus, the carbon matrix not only prevents the aggregation of SnSe, but also buffers the volume expansion during electrochemical reaction. Finally, and most importantly, the superior mechanical flexibility of the SnSe/CNF stabilizes the electrode structure, resulting in stable cycling performance. These merits indicate that the SnSe/CNF is expected to be employed as a flexible anode material for LIBs and SIBs. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 8
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electrospun SnSe/C nanofibers as binder-free anode for lithium–ion and sodium-ion batteries Jing Xia a, c, Yiting Yuan a, Hanxiao Yan a, Junfang Liu a, Yue Zhang a, Li Liu a, *, Shu Zhang a, Wanjun Li a, Xiukang Yang a, Hongbo Shu a, Xianyou Wang a, Guozhong Cao b, ** a
National Base for International Science & Technology Cooperation, Key Laboratory of Environmentally Friendly Chemistry and Application (Ministry of Education), College of Chemistry, Xiangtan University, Xiangtan, 411105, PR China Department of Materials Science and Engineering, University of Washington, Seattle, WA, 98195-2120, United States c Institute of Molecular Plus, Tianjin University, Tianjin, 300072, PR China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� The SnSe/C nanofiber membrane was synthesized by electrospinning technology. � The SnSe/C nanofiber membrane ex hibits superior mechanical flexibility. � The SnSe/C nanofiber membrane shows excellent lithium/sodium storage performance. � The after-cycled SnSe/C nanofiber elec trode still retains its structural integrity.
A R T I C L E I N F O
A B S T R A C T
Keywords: SnSe Binder-free Lithium-ion battery Sodium-ion battery Electrospinning
The design of electrodes with superior mechanical flexibility is the key to developing energy storage devices with mechanical durability and excellent electrochemical performance. Herein, a flexible SnSe/C nanofiber mem brane is successfully synthesized by electrospinning technology and subsequent calcination. From the macro scopic view, the SnSe/C nanofiber membrane can tolerate a bending angle of 180� without any breakage, indicating its superior mechanical flexibility. From the microscopic view, SnSe nanoparticles are uniformly distributed along the carbon nanofiber framework. The carbon nanofiber framework serves not only as a conductive matrix that improves the electrical conductivity of the composite, but also as a buffer material that alleviates the volume expansion during electrochemical reaction. These merits endow excellent electrochemical performance to SnSe/C nanofibers when used as a binder-free and current collector-free anode for lithium/so dium ion batteries. The SnSe/C nanofiber anode delivers a stable discharge capacity of 405 mAh g 1 at 1000 mA g 1 after 500 cycles in lithium ion battery and 290 mAh g 1 at 200 mA g 1 after 200 cycles in sodium ion battery. These results demonstrate that the SnSe/C nanofiber is a promising anode material for flexible lithiumion and sodium-ion batteries.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Liu), [email protected] (G. Cao). https://doi.org/10.1016/j.jpowsour.2019.227559 Received 11 October 2019; Received in revised form 29 November 2019; Accepted 2 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jing Xia, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227559
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1. Introduction
developing, flexible electronics and wearable devices are at the forefront of new trends [27]. Flexible products require flexible batteries to pro vide energy output. The conventional electrodes are prepared by coating active materials, binders and conductive materials on metal current collectors, which are difficult to meet the flexibility needs due to the inherent inflexible electrode structure. Therefore, the development of flexible electrodes has become the key to achieve the applications of flexible products. As we all know, electrospinning is a simple, inex pensive and scalable strategy for in-situ compounding with carbon and producing membrane-based electrode materials [14,28–30]. To our knowledge, there have been no reports on the electrospinning synthesis of SnSe. Thus, it is meaningful to synthesize SnSe with effective carbon coating by electrospinning and investigate its lithium/sodium storage performance. Herein, we first synthesized the SnSe/C nanofiber (denoted as SnSe/ CNF) membrane via electrospinning technology. The SnSe/CNF has superior mechanical flexibility and 1D nanofiber morphology with a diameter of around 180 nm. When the SnSe/CNF was directly used as a flexible anode for LIBs and SIBs without the use of binders, conductive additives and metal current collectors, excellent electrochemical prop erties including high capacity and stable cycling performance were ob tained. The microstructure characterization reveals that the after-cycled electrodes still retain initial electrode structure and fibrous morphology even after cycling at high current densities in LIBs and SIBs. These characteristics make it possible for the binder-free SnSe/CNF anode to achieve the practical application of flexible LIBs and SIBs.
Lithium-ion batteries (LIBs) have been extensively used in portable devices and, more recently, electric vehicles [1–10]. However, the increasing use of LIBs and unevenly distributed lithium resources on the earth, are expected to strongly impact the availability as well as the price of lithium salts in the future [11–13]. Even though this problem can be alleviated to some extent by recycling the lithium, the development of sodium-ion batteries (SIBs) in parallel with developing LIBs may be a more effective strategy in the long term due to greater abundance and lower cost of sodium salts [14,15]. Tin(II)-chalcogenides (such as SnS and SnSe), which have high theoretical capacity as anode material in both LIBs and SIBs, have received important research attention in the past decade. By contrast, SnSe has a higher electrical conductivity than SnS owing to the narrower band gap of SnSe than SnS (0.9 eV for SnSe vs. 1.3 eV for SnS) [16–18]. Besides, during the electrochemical reaction process, SnSe experiences a smaller volume expansion and has better electrochemical reversibility than SnS [19]. These advantages make SnSe more suited for repeated cycling in LIBs and SIBs. Despite these advantages, however, the high theoretical capacity of SnSe is also based on conversion reaction com bined with alloying reaction. The huge volume expansion of the Sn–Li alloying reaction (�260%) and the Sn–Na alloying reaction (�400%) causes electrode pulverization and aggregation of active materials, resulting in deterioration of electrochemical performance [20]. In order to improve the lithium/sodium storage performance of SnSe, some strategies, including designing the morphology with different sizes, alien ions doping and coating with conductive materials have been re ported [21–24]. Although these strategies improved the electrochemical performance of SnSe to a certain extent, the capacity and cycle life of the reported SnSe are far below the application requirements in LIBs and SIBs. The underlying cause for the poor electrochemical performance is that traditional SnSe composites synthetic route is mainly based on two-step process, namely the fabrication of pure SnSe (including solid-state reaction, hydrothermal/solvothermal reaction) and subse quent compounding with conductive materials. This two-step synthetic process often produces micrometer-sized or bulk SnSe with insufficient carbon coating, resulting in rapid capacity fading due to the fact that the large-sized bare SnSe particles are not conducive to electrolyte perme ation and electron transfer [23,25,26]. Since the 21st century, economics and technology are rapidly
2. Results and discussion 2.1. Synthesis and characterization of SnSe/CNF The SnSe/CNF was synthesized by electrospinning and subsequent heat treatment. Fig. 1 schematically illustrates the synthetic process for SnSe/CNF. First of all, the precursor solution of N, N-Dimethylforma mide (DMF), tin (II) chloride dihydrate (SnCl2⋅2H2O), selenium powder (Se) and polyvinylpyrrolidone (PVP) was first electrospun into a white precursor nanofiber membrane. In this case, DMF is a volatile solvent that is removed during electrospinning process and PVP is carbon source to produce carbon matrix. Afterward, the white precursor nanofiber membrane was heat-treated in an Ar/H2 atmosphere to obtain SnSe/ CNF membrane. In this process, SnCl2 reacts with Se to form SnSe, the
Fig. 1. Schematic illustration of the preparation process for SnSe/CNF. 2
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Fig. 2. (a, b) FE-SEM images of precursor nanofibers. (c, d) FE-SEM images of SnSe/CNF. Inset: digital photograph of SnSe/CNF electrode.
Fig. 3. (a) XRD pattern, (b) Raman spectrum and (c) TG-DSC curves of SnSe/CNF. (d) Pore size distribution curve and N2 adsorption–desorption isotherm (inset) of the SnSe/CNF. High-resolution (e) N1s and (f) C1s XPS spectra of the SnSe/CNF.
color of the nanofiber membrane turned from white to black owing to the carbonization of PVP. The obtained SnSe/CNF membrane was then cut into disks as a binder-free and current collector-free electrode, and these electrodes were directly assembled into Li/Na coin cells. Neither mechanical milling nor slurry casting is needed, which not only sim plifies the manufacturing process of the cells, but also reduces the pro duction cost of the cells. Detailed experimental procedures can be found in the Experimental Section (Supplementary Materials). Fig. 2 shows the field emission scanning electron microscopy (FE-
SEM) images for the precursor nanofibers (Fig. 2 a, b) and SnSe/CNF (Fig. 2 c, d). As shown in Fig. 2 panels a and b, the precursor nanofibers are interwoven and have a uniform diameter of about 300 nm. After heat treatment, the SnSe/CNF still maintain their fiber morphology, but the fiber diameter shrank to around 180 nm. In addition, the 1D nanofibers are woven into a 3D conductive network, shortening the ionic transport path. The bending test is the most common method to evaluate the us ability of flexible electrodes. The inset in Fig. 2d shows a digital photograph of the SnSe/CNF electrode, and the SnSe/CNF electrode can 3
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Fig. 4. (a) TEM image of SnSe/CNF. (b) HR-TEM images and corresponding interplanar crystal spacing statistical table (the inset) of SnSe/CNF. (c) HAADF image and (d–i) corresponding elemental mapping images of SnSe/CNF.
tolerate a bending angle of 180� without any breakage, indicating that the SnSe/CNF electrode has superior mechanical flexibility. The crystal structure of the SnSe/CNF was analyzed by the powder Xray diffraction (XRD) (Fig. 3a). All diffraction peaks of SnSe/CNF match perfectly with orthorhombic SnSe (JCPDS card No. 48–1224) without any detectable impurity. Raman spectroscopy was also used to study the SnSe/CNF (Fig. 3b). There are two peaks at 1350 and 1580 cm 1 cor responding to the disordered structure (D-band) and the graphitic structure (G-band) of carbon material, respectively [31,32]. The G-band has a higher peak intensity than the D-band (IG/ID ¼ 1.07), indicating that the carbon material has a high graphitization and good electrical conductivity. The content of SnSe in the SnSe/CNF was investigated by the ther mogravimetric analysis and differential scanning calorimetry (TG-DSC) (Fig. 3c). The slight weight loss below 300 � C can be attributed to the evaporation of adsorbed water. The second weight loss around 500 � C is ascribed to the oxidation of carbon, and the subsequent weight increase between 550 � C and 630 � C may be attributed to the oxidation of SnSe (2SnSe þ O2 → 2SnOSe) [33]. The weight loss after 630 � C might be related to the further oxidation of SnOSe (2SnOSe þ O2 → 2SnO2 þ 2Se ↑). Based on the final remaining 61.9% of SnO2 (the XRD pattern in Fig. S1 indicates that the final combustion product of SnSe/CNF is SnO2), the weight percentage of SnSe in SnSe/CNF can be calculated to be about 81.2%. Fig. 3d shows the pore size distribution curve and N2 adsorption–desorption isotherm (inset) of the SnSe/CNF. The pore size distribution curve indicates that there are many micropores below the 2.0 nm inside the nanofibers. Based on the Brunauer–Emmett–Teller (BET) method, the specific surface area of the SnSe/CNF was calculated to be 76.9 m2 g 1. The large specific surface area and porous structure are beneficial for enlarging the electrode/electrolyte contact area and shortening the diffusion distance of the Liþ/Naþ ions. Moreover, X-ray photoelectron spectroscopy (XPS) was used to research the nitrogen-doping configurations in carbon nanofibers. Fig. 3e shows the high-resolution N1s spectra, the four peaks at 401.9, 400.8, 399.8 and 398.4 eV are attributed to graphitic-N, pyrrolic-N, pyridine-N and pyridinic-N, respectively [34,35]. The high-resolution C1s spectra in Fig. 3 f further illuminates the existence of C–N bond (285.9 eV). It is worth noting that the existence of C–N bond can be assigned to the self-doping of nitrogen from PVP. The doped N can provide abundant electrons for the π-conjugated system of carbon, thereby improving the
electronic conductivity of the composite [36]. Furthermore, the pyrrolic-N and pyridinic-N can cause some defects in the carbon skel eton, providing more active sites for the storage of Liþ and Naþ [37]. In Fig. 3f, the peaks at 286.7 and 284.8 eV correspond to the C–O and C–C bonds, respectively. The C–O bonds may come from the PVP that is not fully carbonized. The transmission electron microscopy (TEM) image of SnSe/CNF in Fig. 4a exhibits smooth fiber surface, which is consistent with FE-SEM images. Fig. 4b shows the high-resolution TEM (HR-TEM) image and corresponding interplanar crystal spacing statistical table (inset). The HR-TEM image clearly illustrates that the SnSe nanoparticles are very small and less than 2 nm in diameter. The obvious lattice fringes and the corresponding interplanar crystal spacing statistical table present that the interplanar spacing of 3.51 Å corresponds to the (201) plane of the orthorhombic SnSe (JCPDS card No. 48–1224). Fig. 4d–i shows the elemental mapping images of C, N, Sn and Se elements corresponding to the rectangular region of Fig. 4c. The elemental mapping images reveal the uniform distributions of C, N, Sn and Se elements along the SnSe/ CNF fiber framework, meaning that the SnSe nanoparticles are evenly distributed in the carbon nanofibers. The uniform distribution of SnSe nanoparticles in the carbon nanofibers can buffer the volume expansion caused by the alloying reactions to some extent. 2.2. Electrochemical evaluations in LIBs The lithium storage performance of SnSe/CNF was evaluated by using SnSe/CNF as a binder-free working electrode, and the lithium metal as the counter/reference electrode. Fig. 5a shows the cyclic vol tammetry (CV) curves for the first three cycles. During the first cathodic scan, the reduction peak at 0.96 V, which shifts to 1.20 V in the subse quent cycles, can be assigned to the conversion of SnSe (SnSe þ 2Liþ þ 2e ↔ Sn þ Li2Se) [38,39]. The peak shift of the subsequent cycles compared to the first cycle may be owing to the structural variations of materials or/and the formation of solid electrolyte interface (SEI) film [40,41]. The reduction peaks at 0.6 V and below can be attributed to the formation of SEI film and LixSn (Sn þ xLiþ þ xe ↔ LixSn (0 < x � 4.4)) [42,43]. In the subsequent scan, a weak reduction peak appeared at around 2.06 V, which is generally considered to be the formation of poly-selenides [43,44]. During anodic scan, several peaks at around 0.69 V in all cycles are attributed to the multi-step de-alloying process 4
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Fig. 5. Electrochemical performance of SnSe/CNF in LIBs: (a) Cyclic voltammetry curve at a scan rate of 0.1 mV s 1; (b) discharge/charge profiles at 50 mA g 1; (c) rate performance and corresponding coulombic efficiency; (d) discharge/charge profiles at various current densities from 50 to 4000 mA g 1; cycling performance and corresponding coulombic efficiency for 500 cycles at 1000 mA g 1.
at 50, 100 and 200 mA g 1, respectively. Not only that, the average discharge capacities are as high as 429 and 384 mAh g 1 even at 3000 and 4000 mA g 1, respectively, and could rapidly recover to about 630 mAh g 1 when the current density turned back to 50 mA g 1. In addi tion, the CE remained close to 100% after the first cycle. The excellent rate performance results from the fact that the carbon matrix improves the electrical conductivity of the composite, and the 1D fiber structure provides fast transport path for ions and electrons. Fig. 5d exhibits the discharge/charge curves corresponding to the rate performance test above. Although the specific capacity decreases with the increase of current density, the discharge/charge voltage plateaus are still stable and obvious, indicating that the SnSe/CNF electrode has high stability. Fig. 5e shows the cycling performance of SnSe/CNF electrode at a current density of 1000 mA g 1. The SnSe/CNF electrode delivers a high first discharge capacity of 659 mAh g 1 and charge capacity of 525 mAh g 1 with the initial CE of 80%. In the subsequent cycles, the capacity remained relatively stable and the CE kept at a level of close to 100%. After 500 cycles, a high discharge capacity of 405 mAh g 1 is still
[45,46]. The oxidation peaks at 1.83 V and above may be attributed to the reversible formation of SnSe combined with the decomposition of poly-selenides. It is worth mentioning that the CV curve of the third scan overlaps nearly the second scan, indicating a good reversibility for the lithium storage reaction of SnSe. Fig. 5b exhibits the galvanostatic dis charge/charge curves of SnSe/CNF at a current density of 50 mA g 1 in the 0.01–2.50 V voltage range. All discharge/charge curves show obvious voltage plateau. The SnSe/CNF deliver a high first discharge capacity of 796 mAh g 1 and charge capacity of 685 mAh g 1 with an initial coulombic efficiency (CE) of 86%. The irreversible capacity loss of 14% is mainly due to the electrolyte decomposition and the inevitable formation of SEI film. In the subsequent cycle, all discharge/charge curves are nearly completely overlapped, which are consistent with the CV test result. The rate performance of SnSe/CNF electrode was investigated by the galvanostatic discharge/charge tests at various current densities of be tween 50 and 4000 mA g 1. As shown in Fig. 5c, the SnSe/CNF electrode delivers high average discharge capacities of 675, 649 and 624 mAh g 1 5
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Fig. 6. Characterization of SnSe/CNF electrode after the rate performance test in LIBs: (a) digital photograph; (b) TEM and (c) HAADF images; (d–i) elemental mapping images.
obtained, and the capacity retention is approaches 77.6% as compared with the second discharge. The cycling performance of SnSe/CNF is preferable to many previously reported SnSe-based anodes in LIBs [25,
26,39,43,47]. Table S1 enumerates some of the previously reported works for transition metal selenide anodes in LIBs and makes a comprehensive comparison with the SnSe/CNF electrode. Obviously,
Fig. 7. Electrochemical performance of SnSe/CNF in SIBs: (a) Cyclic voltammetry curve at a scan rate of 0.1 mV s 1; (b) discharge/charge profiles at 50 mA g 1; (c) rate performance and corresponding coulombic efficiency; (d) discharge/charge profiles at various current densities from 50 to 2000 mA g 1; cycling performance and corresponding coulombic efficiency for 200 cycles at 200 mA g 1. 6
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mixing SnSe with carbon-based materials is a common strategy to improve the lithium storage performance of transition metal selenides. Some SnSe-based composites such as SnSe@CNFs [43], SnSe–C [26], SnSe/C [25] have achieved improved electrochemical performance in LIBs, but their cycle life is hard to over 100 cycles. Lee et al. [39] syn thesized a composite of SnSe and C using a solid-state reaction method and obtained a high capacity in LIBs. However, the high capacity can be obtained only at small current densities (100 mA g 1 or lower). Guo’s group [47] reported a 2D ultra-thin layered SnSe nanoplates prepared by colloid chemistry method and achieved a high capacity at an ultrahigh current density of 5000 mA g 1, but its cycle life is also only 300 cycles. Most importantly, all of the listed electrodes are non-flexible and contain a heavy metal current collector as well as more than 10% of non-conductive binder, which not only makes them difficult to meet the flexibility needs, but also increases the production cost of the battery. Although some flexible transition metal selenide anodes such as Sb2Se3 nanowires [48], Co0.85Se NSs/G [49], and Bi2Se3/graphene [50] have been developed, their cycle life does not exceed 300 cycles. Therefore, the SnSe/CNF electrode obtained in this work indisputably demon strates its superiority compared to the other transition metal selenide anodes listed above. To investigate the mechanism for the excellent Li-storage perfor mance of the SnSe/CNF electrode, the charge transfer kinetics of SnSe/ CNF was studied by electrochemical impedance spectroscopy (EIS) analysis. Fig. S2 presents the Nyquist plots and corresponding simula tion curves of the SnSe/CNF electrode after 1 cycle and rate performance test (corresponding to Fig. 5c) in LIBs. Each Nyquist plot consists of a semicircle and a sloping line, suggesting that the electrochemical reac tion involves both charge transfer and Li-ion diffusion [51–54]. The SnSe/CNF electrode shows a small charge transfer resistance (Rct) of 186.2 Ω after 1cycle (the simulation results are displayed in Table S3), and the Rct value increased only to 285.7 Ω even after cycling at various high current densities, indicating that the SnSe/CNF electrode has a fast charge transfer kinetics and stable electrode structure. Furthermore, the lithium cell after the rate performance test (cor responding to Fig. 5c) was disassembled, and the micromorphology of the SnSe/CNF electrode was observed using both FE-SEM and TEM. Fig. 6a shows the digital photograph of the after-cycled SnSe/CNF electrode. The after-cycled SnSe/CNF electrode still maintained its intact electrode structure without breakage. Fig. S4 shows the FE-SEM image of the after-cycled SnSe/CNF with rough fiber surface due to the coverage of the compact SEI film. The formation of SEI film is the main reason accounting for the large first irreversible capacity loss [55]. Fortunately, the SnSe/CNF still maintained its fibrous morphology without being destroyed, indicating that the strong mechanical flexi bility allows the SnSe/CNF electrode to endure repeated cycles by buffering the volume expansion even at ultrahigh current densities. In the TEM image (Fig. 6b), the compact SEI film on the fiber surface can also be clearly observed. Moreover, Fig. 6d–i shows the elemental mapping images of C, N, Sn and Se elements corresponding to the rectangular region of Fig. 6c. All elements including C, N, Sn and Se are still uniformly distributed along the fiber framework, and no obvious Sn/Se bright specks are observed, indicating that the carbon matrix effectively inhibits the agglomeration of SnSe nanoparticles.
the first scan can be assigned to the conversion from SnSe to Sn and Na2S (SnSe þ 2Naþ þ 2e ↔ Sn þ Na2Se)), the formation of SEI film as well as Na–Sn alloying reaction process (Sn þ xNaþ þ xe ↔ NaxSn (0 < x � 3.75) [24,56]. There are several small cathodic peaks below 0.3 V ascribed to the gradual Na–Sn alloying reaction process [18,21]. The oxidation peaks at 0.01–0.9 V during all anodic scan process arise from the step-by-step de-alloying reaction from NaxSn to Na–Sn intermediate phases and final Sn [57]. The anodic peaks located at 1.12 V and above in all cycles are attributed to the conversion of Sn to SnSe. In the sub sequent cathodic scan, the two cathodic peaks at 1.00 and 0.69 V may be derived from the conversion and alloying reactions, respectively. Fig. 7b shows the galvanostatic discharge/charge profiles of the SnSe/CNF electrode at a current density of 50 mA g 1. The voltage profiles present clear voltage platforms, which were consistent with the current peaks observed in the CV curves. The discharge and charge curves of the third cycle nearly overlap with the second cycle, suggesting that the SnSe/CNF electrode has good reversibility. Fig. 7c demonstrates the rate performance of the SnSe/CNF electrode in the current density range of 50–2000 mA g 1. The SnSe/CNF elec trode delivers high first discharge capacity of 604 mAh g 1 with the initial CE of 63%. The large irreversible capacity loss in the first cycle is mainly due to the irreversible formation of SEI film, which has been confirmed by the above CV analysis. Except for the first cycle, the SnSe/ CNF electrode provides high average discharge capacities of 379, 352, 333, 311, 277 and 241 mAh g 1 at current densities of 50, 100, 200, 500, 1000 and 2000 mA g 1, respectively. More importantly, the discharge capacity can quickly recover to about 370 mAh g 1 and remain stable when the current density returns to 50 mA g 1. Moreover, the CE is close to 100% after the first cycle. The discharge/charge curves in Fig. 7d also exhibit obvious voltage platform even at 2000 mA g 1. These results reveal that SnSe/CNF with large specific surface area and high electrical conductivity can accelerate the transport of Naþ ions and electrons even at higher current density without irreversible changes of 1D nano structures [58]. The cycling performance (Fig. 7e) of SnSe/CNF electrode was also performed by the galvanostatic discharge and charge tests at a current density of 200 mA g 1. The SnSe/CNF electrode delivers a high first discharge capacity of 543 mAh g 1 with an initial CE of 62%. After ongoing 200 cycles, 87.8% discharge capacity (290 mAh g 1) was still retained compared to the second discharge. The cycling performance of SnSe/CNF electrode was compared favorably with previously reported transition metal selenide anodes for SIBs (Table S2) [18,21,59]. For example, Yuan et al. [21] developed a single-crystalline SnSe nanosheet clusters and achieved a discharge capacity of 271 mAh g 1 at 100 mA g 1 after 100 cycles. Ren et al. [18] synthesized the composite of SnSe nanosheets and nitrogen-doped carbon using a cation-exchange strategy and obtained a discharge capacity of 258 mAh g 1 after 200 cycles. Recently, Zhang’s group [59] reported a yolk–shell structured SnSe that achieved a cycle life of 150 cycles when used as an anode material for SIBs. Moreover, the SnSe nanoplates fabricated by Kang’s group [24] and Guo’s group [47] exhibit high capacity and long cycle life, respec tively. However, the capacity output of the above-mentioned SnSe-based materials requires a heavy metal current collector as a support and the electrode contains more than 25% of the inactive ingredients, which greatly reduces the energy density of the battery. In addition, these electrodes are not suitable for flexible batteries. Some transition metal selenide anodes such as MoSe2/CF [60], Sb2Se3 nanowires [48], and WSe2/C [61] nanofibers have been developed and used in flexible SIBs, but their electrochemical performance is still unsatisfactory. In conse quence, the SnSe/CNF anode demonstrates distinct superiorities compared to the above-listed transition metal selenide anode materials. Fig. S3 shows the Nyquist plots of the SnSe/CNF electrode after 1 cycle and rate performance (corresponding to Fig. 7c) in the SIBs. The simulation results in Table S4 reveal that the Rct of the SnSe/CNF electrode after 1 cycle is 287.6 Ω. After repeated discharge/charge at various current densities from 50 to 2000 mA g 1, Rct value increase rate
2.3. Electrochemical evaluations in SIBs The extraordinary electrochemical performance of SnSe/CNF elec trode in LIBs encourages us to further research its performance in SIBs. The SnSe/CNF was also cut into disks and directly used as a binder-free working electrode, and sodium metal was used as the counter/reference electrode to assemble Na cell. Fig. 7a shows the CV curves of the SnSe/ CNF electrode for the initial three cycles. The weak reduction peak at around 1.09 V in the first scan disappearing in the subsequent cycles is generally considered as the Naþ intercalating into the interlayers of SnSe without phase transition [22]. The wide peak between 0.3 and 1.0 V in 7
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Fig. 8. Characterization of SnSe/CNF electrode after the rate performance test in SIBs: (a) digital photograph; (b) TEM and (c) HAADF images; (d–i) elemental mapping images.
was 154% (442.8 Ω), which is comparable to 153% in LIBs. Therefore, SnSe/CNF electrode also has fast charge transfer kinetics and a stable electrode structure in SIBs, thereby providing stable sodium storage capacity. The micromorphology of after-cycled SnSe/CNF electrode in SIBs was also investigated by both FE-SEM and TEM. The sodium cell after the rate performance test (corresponding to Fig. 7c) was disassembled, and the digital photograph of the after-cycled SnSe/CNF electrode was shown in Fig. 8a. The after-cycled SnSe/CNF electrode still maintains its original disk appearance. Both the FE-SEM image in Fig. S5 and the TEM image in Fig. 8b exhibit unbroken fibrous morphology. The unsmooth fiber surface is attributable to the formation of the SEI film on the fiber surface. In addition, Fig. 8d–i displays the elemental mapping images of C, N, Sn and Se corresponding to the rectangular region of HAADF image in Fig. 8c. After the rate performance test, the C, N, Sn and Se elements are still uniformly distributed along the fiber framework. These phe nomena are similar to the corresponding test results in LIBs. Therefore, excellent electrochemical performance is also achieved in SIBs.
Acknowledgments This work was supported financially by the National Natural Science Foundation of China (Grant No. 51672234), the Research Foundation for Hunan Youth Outstanding People from Hunan Provincial Science and Technology Department (2015RS4030), Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Envi ronmental Benignity and Effective Resource Utilization, Program for Innovative Research Cultivation Team in University of Ministry of Ed ucation of China (1337304). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227559. References [1] C.-Y. Wang, G. Zhang, S. Ge, T. Xu, Y. Ji, X.-G. Yang, Y. Leng, Nature 529 (2016) 515. [2] X. Feng, M. Ouyang, X. Liu, L. Lu, Y. Xia, X. He, Energy Storag. Mater. 10 (2018) 246–267. [3] B. Diouf, R. Pode, Renew. Energy 76 (2015) 375–380. [4] V. Ruiz, A. Pfrang, A. Kriston, N. Omar, P. Van den Bossche, L. Boon-Brett, Renew. Sustain. Energy Rev. 81 (2018) 1427–1452. [5] L. Wang, Q. Zhang, J. Zhu, X. Duan, Z. Xu, Y. Liu, H. Yang, B. Lu, Energy Storag. Mater. 16 (2019) 37–45. [6] X. Lu, Q. Zhang, J. Wang, S. Chen, J. Ge, Z. Liu, L. Wang, H. Ding, D. Gong, H. Yang, X. Yu, J. Zhu, B. Lu, Chem. Eng. J. 358 (2019) 955–961. [7] J. Llanos, C. Contreras-Ortega, J. P� aez, M. Guzm� an, C. Mujica, J. Alloy. Comp. 201 (1993) 103–104. [8] C. Han, Z. Li, W.-j. Li, S.-l. Chou, S.-x. Dou, J. Mater. Chem. A 2 (2014) 11683–11690. [9] M. Mao, L. Jiang, L. Wu, M. Zhang, T. Wang, J. Mater. Chem. A 3 (2015) 13384–13389. [10] A. Paolella, C. George, M. Povia, Y. Zhang, R. Krahne, M. Gich, A. Genovese, A. Falqui, M. Longobardi, P. Guardia, T. Pellegrino, L. Manna, Chem. Mater. 23 (2011) 3762–3768. [11] J.-M. Tarascon, Nat. Chem. 2 (2010) 510. [12] A. Darwiche, C. Marino, M.T. Sougrati, B. Fraisse, L. Stievano, L. Monconduit, J. Am. Chem. Soc. 134 (2012) 20805–20811. [13] X. Xu, S. Ji, M. Gu, J. Liu, ACS Appl. Mater. Interfaces 7 (2015) 20957–20964. [14] S. Nie, L. Liu, J. Liu, J. Xie, Y. Zhang, J. Xia, H. Yan, Y. Yuan, X. Wang, Nano-Micro Lett. 10 (2018) 71. [15] B. Qu, C. Ma, G. Ji, C. Xu, J. Xu, Y.S. Meng, T. Wang, J.Y. Lee, Adv. Mater. 26 (2014) 3854–3859. [16] N. Koteswara Reddy, K.T. Ramakrishna Reddy, Thin Solid Films 325 (1998) 4–6. [17] M. Sharon, K. Basavaswaran, Sol. Cells 25 (1988) 97–107. [18] X. Ren, J. Wang, D. Zhu, Q. Li, W. Tian, L. Wang, J. Zhang, L. Miao, P.K. Chu, K. Huo, Nano Energy 54 (2018) 322–330. [19] W. Wang, P. Li, H. Zheng, Q. Liu, F. Lv, J. Wu, H. Wang, S. Guo, Small 13 (2017) 1702228. [20] Q. Tang, Y. Cui, J. Wu, D. Qu, A.P. Baker, Y. Ma, X. Song, Y. Liu, Nano Energy 41 (2017) 377–386. [21] S. Yuan, Y.-H. Zhu, W. Li, S. Wang, D. Xu, L. Li, Y. Zhang, X.-B. Zhang, Adv. Mater. 29 (2017) 1602469. [22] R. Chen, S. Li, J. Liu, Y. Li, F. Ma, J. Liang, X. Chen, Z. Miao, J. Han, T. Wang, Q. Li, Electrochim. Acta 282 (2018) 973–980. [23] X. Yang, R. Zhang, N. Chen, X. Meng, P. Yang, C. Wang, Y. Zhang, Y. Wei, G. Chen, F. Du, Chem. Eur J. 22 (2016) 1445–1451. [24] G.D. Park, J.H. Lee, Y.C. Kang, Nanoscale 8 (2016) 11889–11896.
3. Conclusions In summary, a novel SnSe/CNF has been successfully synthesized by a simple electrospinning strategy and subsequent heat treatment. When the SnSe/CNF membrane was cut into disks as the binder-free and cur rent collector-free anode for LIBs and SIBs, excellent electrochemical properties including high capacity and stable cycling performance were obtained. In LIBs, the discharge capacity can reach as high as 405 mAh g 1 after 500 cycles when a high current density of 1000 mA g 1 was applied. Even at an ultrahigh current density of 4000 mA g 1, the average discharge capacity can still retain 384 mAh g 1. In SIBs, the SnSe/CNF electrode can also retain a discharge capacity of 290 mAh g 1 after 200 cycles at 200 mA g 1. The excellent electrochemical perfor mance of SnSe/CNF electrode is ascribed to the following advantages. Firstly, the 1D nanofibers are woven into a 3D conductive network, shortening the ionic transport path. Secondly, the SnSe/CNF composites with large specific surface area and high electrical conductivity accel erate the ionic and electronic transport. Thirdly, the SnSe nanoparticles are uniformly distributed and entirely encapsulated in carbon nano fibers, thus, the carbon matrix not only prevents the aggregation of SnSe, but also buffers the volume expansion during electrochemical reaction. Finally, and most importantly, the superior mechanical flexibility of the SnSe/CNF stabilizes the electrode structure, resulting in stable cycling performance. These merits indicate that the SnSe/CNF is expected to be employed as a flexible anode material for LIBs and SIBs. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 8
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