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polyvinylidene fluoride composite membranes as interlayers in high-performance LithiumSulfur batteries
Journal of Power Sources 329 (2016) 305e313
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Flexible carbon nanofiber/polyvinylidene fluoride composite membranes as interlayers in high-performance LithiumeSulfur batteries Zhenhua Wang a, b, Jing Zhang a, Yuxiang Yang a, Xinyang Yue a, Xiaoming Hao a, Wang Sun a, David Rooney c, Kening Sun a, b, * a Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environmental, Beijing Institute of Technology, Beijing 100081, People's Republic of China b Collaborative Innovation Center of Electric Vehicles in Beijing, No. 5 Zhongguancun South Avenue, Haidian District, Beijing 100081, People's Republic of China c School of Chemistry and Chemical Engineering, Queen's University, Belfast, Northern Ireland BT9 5AG, United Kingdom
h i g h l i g h t s CNF/PVDF composite membrane can effectively solve polysulfide permeation problem. CNF/PVDF composite membrane exhibits an excellent cycling stability. The simple preparation process of CNF/PVDF is amenable for industrial production.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2016 Received in revised form 14 July 2016 Accepted 19 August 2016
Traditionally polyvinylidene fluoride membranes have been used in applications such as membrane distillation, wastewater treatment, desalination and separator fabrication. Within this work we demonstrate that a novel carbon nanofiber/polyvinylidene fluoride (CNF/PVDF) composite membrane can be used as an interlayer for LithiumeSulfur (LieS) batteries yielding both high capacity and long cycling life. This PVDF membrane is shown to effectively separate dissolved lithium polysulfide with the high electronic conductivity CNF not only reducing the internal resistance in the sulfur cathode but also helping immobilize the polysulfide through its abundant nanospaces. The resulting LieS battery assembled with the CNF/PVDF composite membrane effectively solves the polysulfide permeation problem and exhibits excellent electrochemical performance. It is further shown that the CNF/PVDF electrode has an excellent cycling stability and retains a capacity of 768.6 mAh g1 with a coulombic efficiency above 99% over 200 cycles at 0.5C, which is more than twice that of a cell without CNF/PVDF (374 mAh g1). In addition, the low-cost raw materials and the simple preparation process of CNF/PVDF composite membrane is also amenable for industrial production. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite membrane Polysulfide separating ability High capacity Lithiumesulfur batteries
1. Introduction To support the development of electric vehicles and smart utility grids, rechargeable Lithium-ion (Li-ion) batteries have attracted much attention over the past two decades [1]. While this attraction
* Corresponding author. Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environmental, Beijing Institute of Technology, Beijing 100081, People's Republic of China. E-mail address: [email protected] (K. Sun). http://dx.doi.org/10.1016/j.jpowsour.2016.08.087 0378-7753/© 2016 Elsevier B.V. All rights reserved.
is due to their relative high energy density compared to other common battery systems, it is considered insufficient to meet the increasing demand of high power devices. Other options include Lithium-sulfur (LieS) batteries, which have also attracted particular attention in energy storage applications as the sulfur cathode possess a high theoretical specific capacity of 1675 mAh g1 and energy density of 2600 W h kg1 through the formation of Li2S. In addition, sulfur exhibits many other impressive characteristics such as relative abundance, environmental friendliness, and low cost [2,3]. However, there are several critical challenges in the
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development of advanced LieS batteries, including poor cycling performance, low active material utilization, the large volumetric expansion, and the shuttle phenomenon of dissolved polysulfide species [4e6]. In order to overcome these challenges, many efforts and improvements have been made in the past decades, including sulfurcarbon nanocomposites [7e9], sulfur-polymer composites [10e13], and sulfur-inorganic compound composites [14,15]. These various ways all increase active material utilization and suppress the loss of the active material. However, such methods involve complicated and elaborate synthesis processes, which limit the application of LieS batteries [16]. Recently, a cell configuration consisting of a functional membrane as an interlayer inserted between the sulfur cathode and the separator has been determined to be a crucial factor in improving LieS batteries [16e24]. During the cell discharge, driven by the chemical potential and the concentration differences, the dissolved polysulfide will move towards the anode. As such, the interlayer is considered to be a polysulfide trap in between the sulfur cathode and the separator, which can localize the polysulfide species at the cathode side and enhance the utilization of active materials. Many approaches have been explored, such as carbon interlayers [16e21], polymer interlayers [22,23], and oxide interlayers [24]. For example, a free-standing carbon nanofiber interlayer had been developed by electrospinning techniques, and this highly conductive interlayer significantly enhanced the utilization of active materials by trapping the soluble polysulfide [20]. A polypyrrole nanotube film has also been prepared by a self-assembly method, which can decrease the polarization of the sulfur cathode and suppress the shuttle effect [23]. These methods are all effective at improving the electrochemical performance of LieS batteries, but not very propitious to the demands of wholesale industrialization. Polyvinylidene fluoride (PVDF) membranes have also received attention in battery applications due to their good chemical stability, low thickness, appropriate porosity and good mechanical strength [25,26]. Recent research has shown that PVDF membranes could also help confine polysulfide shuttling through a size sieving effect [27]. However, the PVDF membrane has a negative effect on the electron conduction, which results in low specific capacity and poor cycling stability of the LieS batteries. Herein, we designed and tested a carbon nanofiber/polyvinylidene fluoride (CNF/PVDF) composite membrane for use as an interlayer in LieS batteries between the sulfur cathode and the
separator (Fig. 1). CNF is a type of carbonaceous material produced from a typical biomass material, bacterial cellulose (BC), which is composed of interconnected cellulose nanofibers and can be constructed into a natural three-dimensional (3D) network structure. The CNF with this unique 3D interconnected nanofiber network structure attracted much attentions and showed promising properties in energy storage systems, and it can effectively improve the electronic conductivity of sulfur cathode and store the shuttling polysulfide intermediates during cycling [28]. Furthermore, different from other traditional nanostructured carbonaceous materials (e.g. carbon nanofibers, carbon nanotubes, carbon spheres, and graphene), the production of CNF from natural substances (biomass materials) is a cheaper and easier way to produce carbonaceous materials; these biomass materials are abundant in nature, easy to obtain, environmentally friendly and could generally be suitable for industrial scale-up. Within this work, the electrochemical performance of LieS batteries assembled with a CNF/ PVDF composite membrane has been studied. From this it was observed that the CNF/PVDF composite membrane can significantly improve the discharge capacity and the cycling stability of LieS batteries. Furthermore, the simplified preparation method for CNF/ PVDF composite membrane can avoid complicated synthesis processes and the damage caused by general carbon-based interlayer peeling processes. 2. Experimental 2.1. Materials Sulfur (S, 99.5%, Aladdin), Ethanol absolute (99.5%, Aladdin), super P carbon black (C-65, TIMCAL Graphite & Carbon Ltd.), polyvinylidene fluoride (Solef-5130), N-methyl-2-pyrrolidinone (NMP, 99.9%, Aladdin), 1,3-dioxolane (DOL, Sigma-Aldrich) and 1,2dimethoxyethane (DME, Sigma-Aldrich). Bacterial Cellulose hydrogel (BC hydrogel), kindly provided by Hainan Yeguo Foods Co., Ltd., China, were used in this work. PVDF membranes (thickness: ~80 mm) were provided by Ande Membrane Separation Technology & Engineering Co., Ltd., Beijing, China. 2.2. Preparation of CNF/PVDF membrane The BC hydrogel was cut into suitable pieces, purified by soaking in DI water for 30 h and washed till pH 7. The purified BC hydrogels
Fig. 1. A schematic cell configuration modification of LieS battery.
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were then immersed into liquid nitrogen and freeze-dried to obtain BC aerogels. Subsequently, the BC aerogels were pyrolyzed at 900 C for 2 h under flowing argon to produce CNF. 25 mg of CNF was dispersed in 100 mL of alcohol and bar-sonicated for 6 h to make a well-dispersed solution. The CNF/PVDF composite membrane was prepared by simply filtrating the CNF dispersion through a PVDF membrane using a Millipore Cell (Amicon 8400), and then drying at 50 C for 48 h under vacuum. Finally, the obtained CNF/ PVDF composite membrane was roll-pressed and punched into circular disks with a diameter of 12 mm and where the thickness of CNF layer was ~25 mm. 2.3. Preparation of the sulfur cathode The sulfur cathode was prepared by mixing 70 wt % sulfur and 20 wt% Super P together with 10 wt % PVDF as a binder in NMP to form a homogeneous slurry. Then, the slurry was carefully coated onto an aluminum foil and dried at 50 C for 20 h under vacuum. Finally, the sulfur cathode was cut into circular disks again with a dimeter of 12 mm (sulfur weight: ~1.18 mg cm1).
2.4. Fabrication and evaluation of the battery The electrodes were assembled into 2025-type coin cells in an argon-filled glove box (MBRAUN, H2O < 0.5 ppm, O2 < 0.5 ppm) with lithium metal as anode and the Celgard 2400 as the separator. The electrolyte was 1.0 M lithium bis(trifluoromethanesulfonyl) imide in DOL and DME (1:1 by volume) with a 0.4 M LiNO3 additive. Between the Celgard 2400 separator and sulfur-based working electrode lies the CNF/PVDF composite membrane. The galvanostatic charge-discharge tests were conducted on a LAND CT-2001A galvanostat between 1.5 and 2.8 V (vs. Liþ/Li0). The cyclic voltammetry (CV) measurements were performed at a scan rate of 0.1 mV s1 with a voltage window of 1.6e2.8 V vs. Liþ/Li0 on a CHI660D (Shanghai Chenhua Instrument). Electrochemical impedance spectroscopy (EIS) was performed on a PARSTAT 2273 at a frequency range from 100 kHz to 50 mHz with an AC voltage amplitude of 5 mV.
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3. Results and discussion 3.1. Morphology The SEM image of the BC aerogel shows highly porous networks of numerous interconnected nanofibers with a diameter of 30e70 nm (Fig. 2a). After carbonization, the diameter of the BC nanofibers (CNF) decreased to 20e30 nm (Fig. 2c), probably caused by the evaporation of volatile species such as CO2, CO, methanol and acetic acid during the carbonization process [29]. Besides, CNF still maintained the 3D interconnected networks with a randomly open mesoporous and macroporous structure (Fig. 2b). In addition, N2 adsorption methods were used to further characterize the porous structure of the CNF. The pore size distribution of the CNF (Fig. S1) presents a narrow maxima positioned at around 0.6 nm, and a clear observation of mesopores located in the range of 2e10 nm, indicating the presence of both micropores and mesopores. The calculated BET surface area and the total pore volume of the CNF are 286.2 m2 g1 and 2.52 cm3 g1, respectively. Such a porous structure with good surface area is expected to be favorable to the electrochemical performance of the LieS battery. The morphology of the PVDF membrane (Fig. 2d) shows a dense microstructure on the surface, and the pore size is hardly distinguished. In Fig. 2e, it can be seen that the thickness of the CNF layer of CNF/PVDF composite membrane is ~25 mm. The SEM image of the composite without roll-pressing is provided in Fig. S2. It can be seen that after roll-pressing, the structure of the CNF/PVDF membrane is still complete, and the gap between the interfaces disappeared, indicating the good quality contact between interfaces after rollpressing. The image of the folded CNF/PVDF composite membrane is shown in Fig. 2f, indicating the excellent flexible property of the CNF/PVDF composite membrane, which can provide smooth contact with the top surface of the sulfur cathode. Fig. 2g shows the EDS data and elemental mapping of CNF/PVDF composite before battery tests, the elemental mapping shows carbon and oxygen signals uniformly distributed. The oxygen signals are from oxygencontaining functional groups of the CNF, which can improve the electrolyte wettability of the CNF/PVDF interlayer and increase the hydrophilicity. 3.2. Polysulfide permeability performance
2.5. Characterization Scanning electron microscope (SEM) images and energy disperse spectroscopy (EDS) results were collected by a scanning electron microscope (FEI Quanta FEG 250). The contact angle was measured using a Data-Physics OCA-15E contact angle analyzer (DataPhysics Instruments GmbH, Filderstadt, Germany). Liquid electrolyte uptake (EU) was measured by soaking weighed membrane samples in the liquid electrolyte for 2 h at room temperature. The EU was calculated based on the weight difference between the dried and swollen membranes by:
EU ¼
Wwet Wdry 100% Wdry
(1)
The X-ray diffraction (XRD, Rigaku Ultima IV, Cu Ka radiation, 40 kV, 40 mA) patterns were recorded at a scanning rate of 10 min1 in a 2q range of 5 e80 . The Raman spectra was recorded on a Renishaw RM 2000 using a 633 nm laser. The Fourier transformed infrared (FTIR) spectra of the sample was collected on a Nicolet iS10. The surface area and pore structure were characterized using a Micrometrics ASAP 2020 physisorption analyzer. The X-ray photoelectron spectroscopy (XPS) data was collected using a Physical Electronics 5400 ESCA.
As shown in Fig. 3, the polysulfide permeability of the membrane samples was tested. The left glass tube was filled with a 15 mL solution of 0.43 M Li2S6 and 15 mL of the DME/DOL (1:1, by volume) mixed solvent. 30 mL electrolyte was filled in the right glass tube. With time, the polysulfide all diffused gradually under the pressure/concentration gradient. The Celgard 2400 separator showed a fast diffusion of polysulfide with the color changing fast in the right vial, turning to dark brown after 6 h. As for the PVDF membrane, the polysulfide permeation rate was much slower, indicating a good polysulfide separating ability of the PVDF membrane. Interestingly, the CNF/PVDF composite membrane exhibited a better performance. Here the permeation rate was further decreased with only a little change in color found after 6 h. This phenomenon indicates that the CNF can help immobilize the polysulfide, and that the CNF with a 3D interconnected structure acts as a carbonfiber repository for polysulfide, thereby storing the polysulfide within its abundant nanospaces. 3.3. Electrolyte wettability and uptake The contact angles of the membrane samples are shown in Fig. 4. The PVDF membrane exhibited a smaller contact angle (23.4 ) with the electrolyte than celgard 2400 (40.9 ), indicating
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Fig. 2. The morphologies of (a) BC aerogel, (bec) CNF, (d) the surface of PVDF membrane, (e) cross-section of CNF/PVDF composite membrane, (f) the folded CNF/PVDF composite membrane, and (g) the EDS data and elemental mapping of CNF/PVDF composite before battery tests.
the good wetting properties of the PVDF membrane. In addition, the contact angle of the CNF/PVDF composite membrane is 14.1, which confirms the good electrolyte wettability of the CNF/PVDF
composite membrane. The electrolyte uptake capacity of the membrane samples was also measured. The electrolyte uptake capacity of CNF/PVDF membrane could reach 119% (by weight),
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Fig. 3. Diffusion test of polysulfide (aec) with Celgard 2400 separator, (def) with PVDF membrane, (gei) with CNF/PVDF composite membrane at different permeation times.
which is much higher than that of the celgard 2400 (63%). Generally, high electrolyte uptake is beneficial for improving the electrochemical performance of the lithium batteries [30]. 3.4. Electrochemical performance To analyze the impact of the CNF/PVDF composite membrane as an interlayer in LieS batteries, the battery performance was studied by using a galvanostatic charge-discharge process. All capacity values were calculated based on the mass of sulfur. The CV curves (at 0.1 mV s1 scan rate) of the LieS battery with the CNF/PVDF composite membrane for first five cycles is shown in Fig. 5a. Two cathodic peaks at around 2.3 and 1.95 V, correspond to a solid-to-liquid (from elemental S8 to soluble polysulfide) phase transition and a liquid-to-solid (from the dissolved polysulfide to solid Li2S2/Li2S) phase transition, respectively [31]. The anodic peak at around 2.5 V was the coupled conversion from Li2S4 to a higher order of polysulfide. The overpotential of the cathodic peak disappears gradually after first cycle, possibly due to the rearrangement of the migrating active material from its original positions to electrochemically favorable positions with lower resistance. In the last three cycles, the CV curves nearly overlap without any obvious change in peak intensities or locations, demonstrating the good cycling stability and reaction reversibility of the LieS battery. Fig. 5b shows the galvanostatic discharge/charge voltage profiles of the battery with the CNF/PVDF electrode measured at 0.5C. Two discharge plateaus and one charge plateau can be observed, which are basically consistent with the CV plots. The upper discharge plateau represents the transformation of elemental sulfur into soluble polysulfide and the lower discharge plateau represents the conversion of solid polysulfide (Li2S2/Li2S). During the reverse charging process, the charge plateau is matched with the conversion from Li2S4 to a higher order of polysulfide, and finally
Fig. 4. Contact angle photographs of (a) Celgard 2400, (b) PVDF membrane, (c) CNF/ PVDF composite membrane using liquid electrolyte.
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Fig. 5. (a) CV curves of the LieS battery with the CNF/PVDF composite membrane (first 5 cycles). (b) Galvanostatic discharge/charge voltage profiles of the LieS battery with CNF/ PVDF at 0.5C. (c) Long-term cycling performance of the LieS battery with CNF/PVDF at 0.5C. (d) Cycling stability comparison of the LieS batteries with and without CNF/PVDF at 1C. (e) Rate performance of the LieS batteries with and without CNF/PVDF.
oxidized to S8. In addition, it appears that the plateaus are becoming flat and stable after a few cycles, which means that the active materials need a time to reach a steady state in the cathode. This stable electrochemical performance is due to the solubility of the intermediate polysulfide in the electrolyte [32]. As shown in Fig. 5c, the long-term cycling performance of the LieS battery with the CNF/PVDF composite membrane was
investigated at 0.5C. The CNF/PVDF electrode demonstrates an excellent cyclic stability and retains a capacity of 768.6 mAh g1 with a coulombic efficiency above 99% (post 10th cycle) after 200 cycles, demonstrating a suppression of the polysulfide shuttle and an enhanced cycling stability. Fig. 5d shows the cycling stability comparison and coulombic efficiency of LieS batteries with and without CNF/PVDF the composite membrane at a constant rate of
Fig. 6. (a) Nyquist plots of the LieS batteries with different membranes before cycling. (b) EIS spectra of the LieS batteries with CNF/PVDF before and after the initial cycle.
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Fig. 7. (a) SEM image and corresponding EDS elemental mapping of the cycled CNF/PVDF composite membrane. (b) XPS spectra of the cycled CNF/PVDF composite membrane.
1C up to 100 cycles. There is a significant improvement in the battery performance while using the CNF/PVDF composite membrane. The initial discharge capacity of the battery using a conventional sulfur cathode increases from 824.7 to 1739.2 mAh g1 after adding the CNF/PVDF between the cathode and the separator. The initial specific capacity (CNF/PVDF electrode) is as high as 1739.2 mAh g1, much higher than the theoretical capacity of the LieS battery (1675 mAh g1), and the irreversible capacity is 292.8 mAh g1. The XRD patterns (Fig. S3) and Raman spectra (Fig. S4) indicate a disordered carbon structure of the CNF, so that the CNF may cause some additional lithium storage intercalation [33,34]. The capacity of the battery with CNF/PVDF composite membrane becomes stable and reversible after the first cycle, and an obvious improvement can also be observed in cyclability. After 100 cycles, the capacity was still above 680 mAh g1, however, the battery without CNF/PVDF displays poor cycling stability with a low capacity of 309.8 mAh g1 after 100 cycles. In addition, the cell with PVDF membrane has also been tested (Fig. S5), the initial capacity is 664.7 mAh g1. After 100 cycles, the capacity is only 174.5 mAh g1. The negative effect on the electron conduction of the PVDF results in this poor cycling performance. Such an improvement of CNF/
PVDF electrode in cycling performance is attributed to the CNF/ PVDF composite membrane, both in terms of providing efficient electron pathways and as the traps for migrating polysulfide species. The micropores and mesopores of the CNF are believed to be beneficial for facilitating electrolyte infiltration [35] and Liþ-ion transport [36], and the micropores could also enhance the polysulfide-trapping capacity because of the size effect [18]. Here, the 3D interconnected structure of CNF generates a strong tortuosity that suppresses the diffusion of large polysulfide species to the anode side and the PVDF membrane also helps immobilize the polysulfide, thereby improving the re-utilization of the active material and the discharge capacity. Furthermore, the cycling performance (Fig. S6a) and the EIS of CNF/PVDF electrode (Fig. S6b) with different amount of CNF have been tested to study the effect of CNF amount in the CNF/PVDF composite. EIS data illustrates that the CNF is effective to decrease the resistance of the LieS cells, thus enhancing the conductivity of the cathode. After 100 cycles at 0.5C, the remaining capacity of the 15 mg-CNF/PVDF and 50 mg-CNF/ PVDF is only 615 mAh g1 and 433.5 mAh g1. The low capacity of the 15 mg-CNF/PVDF may be because of the conductivity, and the poor cycling performance of the 50 mg-CNF/PVDF probably caused
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by the high thickness of the membrane, which have a negative effect on electrolyte diffusion and Liþ-ion transport. The results indicates that the EIS and the cycling performance are all affected by the amount of CNF and 25 mg is the most appropriate amount for CNF/PVDF composite membrane. The rate capability of the LieS batteries with and without CNF/ PVDF composite membrane was further evaluated by increasing the discharge/charge current density from 0.2 C to 2C. As shown in Fig. 5e, The first specific discharge capacity at a low current rate of 0.2 C is 2101.4 mAh g1, as the current density increases, a specific capacity of 1047.3 mAh g1, 868.3 mAh g1, 797.3 mAh g1, 500.5 mAh g1 is obtained at 0.4 C, 0.8 C, 1 C, and 2 C, respectively. Furthermore, an excellent reversible capacity of 1150.2 mAh g1 is recovered when the current density is reverted to 0.2 C, indicating the good stability of the cathode with CNF/PVDF. As expected, the LieS battery with CNF/PVDF exhibits better capacity retention upon cycling. The battery without CNF/PVDF shows fast capacity fading as the charge-discharge rate increases, and its rate performance is much lower compared to the LieS battery with CNF/PVDF. These results demonstrate the assistance of the effective CNF/PVDF composite membrane as an interlayer in enhancing the conductivity and active material utilization. And this significant improved performance may be attributed to the synergistic effect of the CNF and the PVDF membrane, both of which have an effective inhibition of polysulfide diffusion. In addition, the CNF can effectively improve the electronic conductivity of the cathode by creating a superior electrically conductive network. To further analyze the impact of the CNF/PVDF composite membrane in LieS batteries, the EIS measurements were performed. Fig. 6a shows the comparison of the batteries with different membranes before cycling. Batteries with a CNF/PVDF composite membrane, a PVDF membrane and that without membrane all exhibit typical semicircles in the high-to-medium frequency region, which is regarded as the charge transfer resistance, while the leaning lines in the low frequency region is related with mass transfer process [37]. One can observe from the EIS that there has been an increase after inserting a PVDF membrane in the LieS battery, which is caused by the electronic non-conductivity of PVDF. However, the impedance semicircle shrinks after the combination of the CNF and PVDF membrane (inserting a CNF/PVDF composite membrane), which is attributed to the good conductivity of the CNF. This decrease of the charge transfer resistance predicts a better performance of the LieS battery. Here, the CNF works as a current collector for the low-conductivity sulfur cathode, enhancing the active material utilization and thereby improving the electrochemical performance of the LieS batteries [18]. As shown in Fig. 6b, the EIS spectra of the LieS battery with CNF/PVDF composite membrane exhibits a significant decrease after the initial cycle. This reduction may be attributed to the rearrangement of the active materials and deposition at more electrochemical sites through enlargement of the contact area between the active material and the membrane. The SEM images and the elemental mapping of the CNF/PVDF composite membrane surface towards the cathode confirmed the entrapment of the active material. Here the LieS cell with CNF/ PVDF composite membrane was disassembled after 200 cycles, and both the morphology and the physical properties were characterized. The SEM and elemental mapping of the in-plane are shown in Fig. 7a. These SEM image reveal a complete structure with no damage evident, and the elemental mapping shows carbon and sulfur signals uniformly distributed with no evidence of agglomerated active material, which confirms the trapping of the migrating polysulfide within the CNF/PVDF composite membrane. To further confirm the presence of the migrating polysulfide into the cycled CNF/PVDF composite membrane XPS measurements
were performed (Fig. 7b). The presence of polysulfide is evidenced by S2p3/2 contributions at 161.7 and 163.1 eV, as expected for the terminal and bridging sulfur [38]. Elemental sulfur is detected at 164.8 eV. In addition, one strong peak at 167.1 eV is in accord with the binding energy of thiosulfate and another at 169 eV is assigned to the sulfate [39,40]. Thus, we believe that, after long-term cycling, the migrating polysulfide species gradually diffuses through the CNF/PVDF composite membrane and is trapped before it can escape from the cathode. 4. Conclusions In this work, CNF/PVDF composite membranes were produced by a facile method and used as an interlayer in high-performance LieS battery fabrication. When used as the interlayer between the sulfur cathode and the separator, the CNF/PVDF composite membrane demonstrated its key role in providing efficient electron pathways and as a trap for migrating polysulfide species. The resulting LieS battery containing a CNF/PVDF interlayer exhibits an excellent cycling stability which retained a capacity of 768.6 mAh g1 at a coulombic efficiency above 99% (post 10th cycle) for 200 cycles. It is therefore concluded that the use of CNF/PVDF composite membranes as interlayers for LieS batteries is suitable for low-cost and high-capacity LieS batteries with long-term cycling performance. Acknowledgments This work was supported by the National Natural Science Foundation of China (21376001 and 21506012) and the Beijing Higher Education Young Elite Teacher Project (YETP1205). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.08.087. References [1] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19e29. [2] B. Dunn, H. Kamath, J.M. Tarascon, Mater. Grid Energy Grid A Battery Choices 334 (2011) 928e935. [3] R.V. Noorden, Nature 507 (2014) 26e28. [4] A. Manthiram, Y. Fu, S.H. Chung, C. Zu, Y.S. S, Chem. Rev. 114 (2014) 11751e11787. [5] G. Xu, B. Ding, J. Pan, P. Nie, L. Shen, X. Zhang, J. Mater. Chem. A 2 (2014) 12662. [6] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Angew. Chem. Int. Ed. Engl. 52 (2013) 13186e13200. [7] S. Choudhury, M. Agrawal, P. Formanek, D. Jehnichen, D. Fischer, B. Krause, V. Albrecht, M. Stamm, L. Ionov, ACS Nano 9 (2015) 6147e6157. [8] Y. Yang, G. Zheng, Y. Cui, Chem. Soc. Rev. 42 (2013) 3018e3032. [9] G. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Nano Lett. 11 (2011) 4462e4467. [10] H. Chen, W. Dong, J. Ge, C. Wang, X. Wu, W. Lu, L. Chen, Sci. Rep. 3 (2013) 1910. [11] C. Wang, W. Wan, J.-T. Chen, H.-H. Zhou, X.-X. Zhang, L.-X. Yuan, Y.-H. Huang, J. Mater. Chem. A 1 (2013) 1716e1723. [12] Y. Yang, G. Yu, J.J. Cha, H. Wu, M. Vosgueritchian, Y. Yao, Z. Bao, Y. Cui, ACS Nano 5 (2011) 9187e9193. [13] W. Zhou, Y. Yu, H. Chen, F.J. DiSalvo, H.D. Abruna, J. Am. Chem. Soc. 135 (2013) 16736e16743. [14] Z. Zhang, Q. Li, S. Jiang, K. Zhang, Y. Lai, J. Li, Chemistry 21 (2015) 1343e1349. [15] Z. Wei Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.C. Hsu, Y. Cui, Nat. Commun. 4 (2013) 1331. [16] Y.S. Su, A. Manthiram, Chem. Commun. 48 (2012) 8817e8819. [17] C.L. Lee, I.D. Kim, Nanoscale 7 (2015) 10362e10367. [18] Y.S. Su, A. Manthiram, Nat. Commun. 3 (2012) 1166. [19] S.H. Chung, A. Manthiram, Chem. Commun. 50 (2014) 4184e4187. [20] R. Singhal, S.-H. Chung, A. Manthiram, V. Kalra, J. Mater. Chem. A 3 (2015) 4530e4538. [21] X. Gu, C. Lai, F. Liu, W. Yang, Y. Hou, S. Zhang, J. Mater. Chem. A 3 (2015) 9502e9509.
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Flexible carbon nanofiber/polyvinylidene fluoride composite membranes as interlayers in high-performance LithiumeSulfur batteries Zhenhua Wang a, b, Jing Zhang a, Yuxiang Yang a, Xinyang Yue a, Xiaoming Hao a, Wang Sun a, David Rooney c, Kening Sun a, b, * a Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environmental, Beijing Institute of Technology, Beijing 100081, People's Republic of China b Collaborative Innovation Center of Electric Vehicles in Beijing, No. 5 Zhongguancun South Avenue, Haidian District, Beijing 100081, People's Republic of China c School of Chemistry and Chemical Engineering, Queen's University, Belfast, Northern Ireland BT9 5AG, United Kingdom
h i g h l i g h t s CNF/PVDF composite membrane can effectively solve polysulfide permeation problem. CNF/PVDF composite membrane exhibits an excellent cycling stability. The simple preparation process of CNF/PVDF is amenable for industrial production.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2016 Received in revised form 14 July 2016 Accepted 19 August 2016
Traditionally polyvinylidene fluoride membranes have been used in applications such as membrane distillation, wastewater treatment, desalination and separator fabrication. Within this work we demonstrate that a novel carbon nanofiber/polyvinylidene fluoride (CNF/PVDF) composite membrane can be used as an interlayer for LithiumeSulfur (LieS) batteries yielding both high capacity and long cycling life. This PVDF membrane is shown to effectively separate dissolved lithium polysulfide with the high electronic conductivity CNF not only reducing the internal resistance in the sulfur cathode but also helping immobilize the polysulfide through its abundant nanospaces. The resulting LieS battery assembled with the CNF/PVDF composite membrane effectively solves the polysulfide permeation problem and exhibits excellent electrochemical performance. It is further shown that the CNF/PVDF electrode has an excellent cycling stability and retains a capacity of 768.6 mAh g1 with a coulombic efficiency above 99% over 200 cycles at 0.5C, which is more than twice that of a cell without CNF/PVDF (374 mAh g1). In addition, the low-cost raw materials and the simple preparation process of CNF/PVDF composite membrane is also amenable for industrial production. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite membrane Polysulfide separating ability High capacity Lithiumesulfur batteries
1. Introduction To support the development of electric vehicles and smart utility grids, rechargeable Lithium-ion (Li-ion) batteries have attracted much attention over the past two decades [1]. While this attraction
* Corresponding author. Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environmental, Beijing Institute of Technology, Beijing 100081, People's Republic of China. E-mail address: [email protected] (K. Sun). http://dx.doi.org/10.1016/j.jpowsour.2016.08.087 0378-7753/© 2016 Elsevier B.V. All rights reserved.
is due to their relative high energy density compared to other common battery systems, it is considered insufficient to meet the increasing demand of high power devices. Other options include Lithium-sulfur (LieS) batteries, which have also attracted particular attention in energy storage applications as the sulfur cathode possess a high theoretical specific capacity of 1675 mAh g1 and energy density of 2600 W h kg1 through the formation of Li2S. In addition, sulfur exhibits many other impressive characteristics such as relative abundance, environmental friendliness, and low cost [2,3]. However, there are several critical challenges in the
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development of advanced LieS batteries, including poor cycling performance, low active material utilization, the large volumetric expansion, and the shuttle phenomenon of dissolved polysulfide species [4e6]. In order to overcome these challenges, many efforts and improvements have been made in the past decades, including sulfurcarbon nanocomposites [7e9], sulfur-polymer composites [10e13], and sulfur-inorganic compound composites [14,15]. These various ways all increase active material utilization and suppress the loss of the active material. However, such methods involve complicated and elaborate synthesis processes, which limit the application of LieS batteries [16]. Recently, a cell configuration consisting of a functional membrane as an interlayer inserted between the sulfur cathode and the separator has been determined to be a crucial factor in improving LieS batteries [16e24]. During the cell discharge, driven by the chemical potential and the concentration differences, the dissolved polysulfide will move towards the anode. As such, the interlayer is considered to be a polysulfide trap in between the sulfur cathode and the separator, which can localize the polysulfide species at the cathode side and enhance the utilization of active materials. Many approaches have been explored, such as carbon interlayers [16e21], polymer interlayers [22,23], and oxide interlayers [24]. For example, a free-standing carbon nanofiber interlayer had been developed by electrospinning techniques, and this highly conductive interlayer significantly enhanced the utilization of active materials by trapping the soluble polysulfide [20]. A polypyrrole nanotube film has also been prepared by a self-assembly method, which can decrease the polarization of the sulfur cathode and suppress the shuttle effect [23]. These methods are all effective at improving the electrochemical performance of LieS batteries, but not very propitious to the demands of wholesale industrialization. Polyvinylidene fluoride (PVDF) membranes have also received attention in battery applications due to their good chemical stability, low thickness, appropriate porosity and good mechanical strength [25,26]. Recent research has shown that PVDF membranes could also help confine polysulfide shuttling through a size sieving effect [27]. However, the PVDF membrane has a negative effect on the electron conduction, which results in low specific capacity and poor cycling stability of the LieS batteries. Herein, we designed and tested a carbon nanofiber/polyvinylidene fluoride (CNF/PVDF) composite membrane for use as an interlayer in LieS batteries between the sulfur cathode and the
separator (Fig. 1). CNF is a type of carbonaceous material produced from a typical biomass material, bacterial cellulose (BC), which is composed of interconnected cellulose nanofibers and can be constructed into a natural three-dimensional (3D) network structure. The CNF with this unique 3D interconnected nanofiber network structure attracted much attentions and showed promising properties in energy storage systems, and it can effectively improve the electronic conductivity of sulfur cathode and store the shuttling polysulfide intermediates during cycling [28]. Furthermore, different from other traditional nanostructured carbonaceous materials (e.g. carbon nanofibers, carbon nanotubes, carbon spheres, and graphene), the production of CNF from natural substances (biomass materials) is a cheaper and easier way to produce carbonaceous materials; these biomass materials are abundant in nature, easy to obtain, environmentally friendly and could generally be suitable for industrial scale-up. Within this work, the electrochemical performance of LieS batteries assembled with a CNF/ PVDF composite membrane has been studied. From this it was observed that the CNF/PVDF composite membrane can significantly improve the discharge capacity and the cycling stability of LieS batteries. Furthermore, the simplified preparation method for CNF/ PVDF composite membrane can avoid complicated synthesis processes and the damage caused by general carbon-based interlayer peeling processes. 2. Experimental 2.1. Materials Sulfur (S, 99.5%, Aladdin), Ethanol absolute (99.5%, Aladdin), super P carbon black (C-65, TIMCAL Graphite & Carbon Ltd.), polyvinylidene fluoride (Solef-5130), N-methyl-2-pyrrolidinone (NMP, 99.9%, Aladdin), 1,3-dioxolane (DOL, Sigma-Aldrich) and 1,2dimethoxyethane (DME, Sigma-Aldrich). Bacterial Cellulose hydrogel (BC hydrogel), kindly provided by Hainan Yeguo Foods Co., Ltd., China, were used in this work. PVDF membranes (thickness: ~80 mm) were provided by Ande Membrane Separation Technology & Engineering Co., Ltd., Beijing, China. 2.2. Preparation of CNF/PVDF membrane The BC hydrogel was cut into suitable pieces, purified by soaking in DI water for 30 h and washed till pH 7. The purified BC hydrogels
Fig. 1. A schematic cell configuration modification of LieS battery.
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were then immersed into liquid nitrogen and freeze-dried to obtain BC aerogels. Subsequently, the BC aerogels were pyrolyzed at 900 C for 2 h under flowing argon to produce CNF. 25 mg of CNF was dispersed in 100 mL of alcohol and bar-sonicated for 6 h to make a well-dispersed solution. The CNF/PVDF composite membrane was prepared by simply filtrating the CNF dispersion through a PVDF membrane using a Millipore Cell (Amicon 8400), and then drying at 50 C for 48 h under vacuum. Finally, the obtained CNF/ PVDF composite membrane was roll-pressed and punched into circular disks with a diameter of 12 mm and where the thickness of CNF layer was ~25 mm. 2.3. Preparation of the sulfur cathode The sulfur cathode was prepared by mixing 70 wt % sulfur and 20 wt% Super P together with 10 wt % PVDF as a binder in NMP to form a homogeneous slurry. Then, the slurry was carefully coated onto an aluminum foil and dried at 50 C for 20 h under vacuum. Finally, the sulfur cathode was cut into circular disks again with a dimeter of 12 mm (sulfur weight: ~1.18 mg cm1).
2.4. Fabrication and evaluation of the battery The electrodes were assembled into 2025-type coin cells in an argon-filled glove box (MBRAUN, H2O < 0.5 ppm, O2 < 0.5 ppm) with lithium metal as anode and the Celgard 2400 as the separator. The electrolyte was 1.0 M lithium bis(trifluoromethanesulfonyl) imide in DOL and DME (1:1 by volume) with a 0.4 M LiNO3 additive. Between the Celgard 2400 separator and sulfur-based working electrode lies the CNF/PVDF composite membrane. The galvanostatic charge-discharge tests were conducted on a LAND CT-2001A galvanostat between 1.5 and 2.8 V (vs. Liþ/Li0). The cyclic voltammetry (CV) measurements were performed at a scan rate of 0.1 mV s1 with a voltage window of 1.6e2.8 V vs. Liþ/Li0 on a CHI660D (Shanghai Chenhua Instrument). Electrochemical impedance spectroscopy (EIS) was performed on a PARSTAT 2273 at a frequency range from 100 kHz to 50 mHz with an AC voltage amplitude of 5 mV.
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3. Results and discussion 3.1. Morphology The SEM image of the BC aerogel shows highly porous networks of numerous interconnected nanofibers with a diameter of 30e70 nm (Fig. 2a). After carbonization, the diameter of the BC nanofibers (CNF) decreased to 20e30 nm (Fig. 2c), probably caused by the evaporation of volatile species such as CO2, CO, methanol and acetic acid during the carbonization process [29]. Besides, CNF still maintained the 3D interconnected networks with a randomly open mesoporous and macroporous structure (Fig. 2b). In addition, N2 adsorption methods were used to further characterize the porous structure of the CNF. The pore size distribution of the CNF (Fig. S1) presents a narrow maxima positioned at around 0.6 nm, and a clear observation of mesopores located in the range of 2e10 nm, indicating the presence of both micropores and mesopores. The calculated BET surface area and the total pore volume of the CNF are 286.2 m2 g1 and 2.52 cm3 g1, respectively. Such a porous structure with good surface area is expected to be favorable to the electrochemical performance of the LieS battery. The morphology of the PVDF membrane (Fig. 2d) shows a dense microstructure on the surface, and the pore size is hardly distinguished. In Fig. 2e, it can be seen that the thickness of the CNF layer of CNF/PVDF composite membrane is ~25 mm. The SEM image of the composite without roll-pressing is provided in Fig. S2. It can be seen that after roll-pressing, the structure of the CNF/PVDF membrane is still complete, and the gap between the interfaces disappeared, indicating the good quality contact between interfaces after rollpressing. The image of the folded CNF/PVDF composite membrane is shown in Fig. 2f, indicating the excellent flexible property of the CNF/PVDF composite membrane, which can provide smooth contact with the top surface of the sulfur cathode. Fig. 2g shows the EDS data and elemental mapping of CNF/PVDF composite before battery tests, the elemental mapping shows carbon and oxygen signals uniformly distributed. The oxygen signals are from oxygencontaining functional groups of the CNF, which can improve the electrolyte wettability of the CNF/PVDF interlayer and increase the hydrophilicity. 3.2. Polysulfide permeability performance
2.5. Characterization Scanning electron microscope (SEM) images and energy disperse spectroscopy (EDS) results were collected by a scanning electron microscope (FEI Quanta FEG 250). The contact angle was measured using a Data-Physics OCA-15E contact angle analyzer (DataPhysics Instruments GmbH, Filderstadt, Germany). Liquid electrolyte uptake (EU) was measured by soaking weighed membrane samples in the liquid electrolyte for 2 h at room temperature. The EU was calculated based on the weight difference between the dried and swollen membranes by:
EU ¼
Wwet Wdry 100% Wdry
(1)
The X-ray diffraction (XRD, Rigaku Ultima IV, Cu Ka radiation, 40 kV, 40 mA) patterns were recorded at a scanning rate of 10 min1 in a 2q range of 5 e80 . The Raman spectra was recorded on a Renishaw RM 2000 using a 633 nm laser. The Fourier transformed infrared (FTIR) spectra of the sample was collected on a Nicolet iS10. The surface area and pore structure were characterized using a Micrometrics ASAP 2020 physisorption analyzer. The X-ray photoelectron spectroscopy (XPS) data was collected using a Physical Electronics 5400 ESCA.
As shown in Fig. 3, the polysulfide permeability of the membrane samples was tested. The left glass tube was filled with a 15 mL solution of 0.43 M Li2S6 and 15 mL of the DME/DOL (1:1, by volume) mixed solvent. 30 mL electrolyte was filled in the right glass tube. With time, the polysulfide all diffused gradually under the pressure/concentration gradient. The Celgard 2400 separator showed a fast diffusion of polysulfide with the color changing fast in the right vial, turning to dark brown after 6 h. As for the PVDF membrane, the polysulfide permeation rate was much slower, indicating a good polysulfide separating ability of the PVDF membrane. Interestingly, the CNF/PVDF composite membrane exhibited a better performance. Here the permeation rate was further decreased with only a little change in color found after 6 h. This phenomenon indicates that the CNF can help immobilize the polysulfide, and that the CNF with a 3D interconnected structure acts as a carbonfiber repository for polysulfide, thereby storing the polysulfide within its abundant nanospaces. 3.3. Electrolyte wettability and uptake The contact angles of the membrane samples are shown in Fig. 4. The PVDF membrane exhibited a smaller contact angle (23.4 ) with the electrolyte than celgard 2400 (40.9 ), indicating
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Fig. 2. The morphologies of (a) BC aerogel, (bec) CNF, (d) the surface of PVDF membrane, (e) cross-section of CNF/PVDF composite membrane, (f) the folded CNF/PVDF composite membrane, and (g) the EDS data and elemental mapping of CNF/PVDF composite before battery tests.
the good wetting properties of the PVDF membrane. In addition, the contact angle of the CNF/PVDF composite membrane is 14.1, which confirms the good electrolyte wettability of the CNF/PVDF
composite membrane. The electrolyte uptake capacity of the membrane samples was also measured. The electrolyte uptake capacity of CNF/PVDF membrane could reach 119% (by weight),
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Fig. 3. Diffusion test of polysulfide (aec) with Celgard 2400 separator, (def) with PVDF membrane, (gei) with CNF/PVDF composite membrane at different permeation times.
which is much higher than that of the celgard 2400 (63%). Generally, high electrolyte uptake is beneficial for improving the electrochemical performance of the lithium batteries [30]. 3.4. Electrochemical performance To analyze the impact of the CNF/PVDF composite membrane as an interlayer in LieS batteries, the battery performance was studied by using a galvanostatic charge-discharge process. All capacity values were calculated based on the mass of sulfur. The CV curves (at 0.1 mV s1 scan rate) of the LieS battery with the CNF/PVDF composite membrane for first five cycles is shown in Fig. 5a. Two cathodic peaks at around 2.3 and 1.95 V, correspond to a solid-to-liquid (from elemental S8 to soluble polysulfide) phase transition and a liquid-to-solid (from the dissolved polysulfide to solid Li2S2/Li2S) phase transition, respectively [31]. The anodic peak at around 2.5 V was the coupled conversion from Li2S4 to a higher order of polysulfide. The overpotential of the cathodic peak disappears gradually after first cycle, possibly due to the rearrangement of the migrating active material from its original positions to electrochemically favorable positions with lower resistance. In the last three cycles, the CV curves nearly overlap without any obvious change in peak intensities or locations, demonstrating the good cycling stability and reaction reversibility of the LieS battery. Fig. 5b shows the galvanostatic discharge/charge voltage profiles of the battery with the CNF/PVDF electrode measured at 0.5C. Two discharge plateaus and one charge plateau can be observed, which are basically consistent with the CV plots. The upper discharge plateau represents the transformation of elemental sulfur into soluble polysulfide and the lower discharge plateau represents the conversion of solid polysulfide (Li2S2/Li2S). During the reverse charging process, the charge plateau is matched with the conversion from Li2S4 to a higher order of polysulfide, and finally
Fig. 4. Contact angle photographs of (a) Celgard 2400, (b) PVDF membrane, (c) CNF/ PVDF composite membrane using liquid electrolyte.
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Fig. 5. (a) CV curves of the LieS battery with the CNF/PVDF composite membrane (first 5 cycles). (b) Galvanostatic discharge/charge voltage profiles of the LieS battery with CNF/ PVDF at 0.5C. (c) Long-term cycling performance of the LieS battery with CNF/PVDF at 0.5C. (d) Cycling stability comparison of the LieS batteries with and without CNF/PVDF at 1C. (e) Rate performance of the LieS batteries with and without CNF/PVDF.
oxidized to S8. In addition, it appears that the plateaus are becoming flat and stable after a few cycles, which means that the active materials need a time to reach a steady state in the cathode. This stable electrochemical performance is due to the solubility of the intermediate polysulfide in the electrolyte [32]. As shown in Fig. 5c, the long-term cycling performance of the LieS battery with the CNF/PVDF composite membrane was
investigated at 0.5C. The CNF/PVDF electrode demonstrates an excellent cyclic stability and retains a capacity of 768.6 mAh g1 with a coulombic efficiency above 99% (post 10th cycle) after 200 cycles, demonstrating a suppression of the polysulfide shuttle and an enhanced cycling stability. Fig. 5d shows the cycling stability comparison and coulombic efficiency of LieS batteries with and without CNF/PVDF the composite membrane at a constant rate of
Fig. 6. (a) Nyquist plots of the LieS batteries with different membranes before cycling. (b) EIS spectra of the LieS batteries with CNF/PVDF before and after the initial cycle.
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Fig. 7. (a) SEM image and corresponding EDS elemental mapping of the cycled CNF/PVDF composite membrane. (b) XPS spectra of the cycled CNF/PVDF composite membrane.
1C up to 100 cycles. There is a significant improvement in the battery performance while using the CNF/PVDF composite membrane. The initial discharge capacity of the battery using a conventional sulfur cathode increases from 824.7 to 1739.2 mAh g1 after adding the CNF/PVDF between the cathode and the separator. The initial specific capacity (CNF/PVDF electrode) is as high as 1739.2 mAh g1, much higher than the theoretical capacity of the LieS battery (1675 mAh g1), and the irreversible capacity is 292.8 mAh g1. The XRD patterns (Fig. S3) and Raman spectra (Fig. S4) indicate a disordered carbon structure of the CNF, so that the CNF may cause some additional lithium storage intercalation [33,34]. The capacity of the battery with CNF/PVDF composite membrane becomes stable and reversible after the first cycle, and an obvious improvement can also be observed in cyclability. After 100 cycles, the capacity was still above 680 mAh g1, however, the battery without CNF/PVDF displays poor cycling stability with a low capacity of 309.8 mAh g1 after 100 cycles. In addition, the cell with PVDF membrane has also been tested (Fig. S5), the initial capacity is 664.7 mAh g1. After 100 cycles, the capacity is only 174.5 mAh g1. The negative effect on the electron conduction of the PVDF results in this poor cycling performance. Such an improvement of CNF/
PVDF electrode in cycling performance is attributed to the CNF/ PVDF composite membrane, both in terms of providing efficient electron pathways and as the traps for migrating polysulfide species. The micropores and mesopores of the CNF are believed to be beneficial for facilitating electrolyte infiltration [35] and Liþ-ion transport [36], and the micropores could also enhance the polysulfide-trapping capacity because of the size effect [18]. Here, the 3D interconnected structure of CNF generates a strong tortuosity that suppresses the diffusion of large polysulfide species to the anode side and the PVDF membrane also helps immobilize the polysulfide, thereby improving the re-utilization of the active material and the discharge capacity. Furthermore, the cycling performance (Fig. S6a) and the EIS of CNF/PVDF electrode (Fig. S6b) with different amount of CNF have been tested to study the effect of CNF amount in the CNF/PVDF composite. EIS data illustrates that the CNF is effective to decrease the resistance of the LieS cells, thus enhancing the conductivity of the cathode. After 100 cycles at 0.5C, the remaining capacity of the 15 mg-CNF/PVDF and 50 mg-CNF/ PVDF is only 615 mAh g1 and 433.5 mAh g1. The low capacity of the 15 mg-CNF/PVDF may be because of the conductivity, and the poor cycling performance of the 50 mg-CNF/PVDF probably caused
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by the high thickness of the membrane, which have a negative effect on electrolyte diffusion and Liþ-ion transport. The results indicates that the EIS and the cycling performance are all affected by the amount of CNF and 25 mg is the most appropriate amount for CNF/PVDF composite membrane. The rate capability of the LieS batteries with and without CNF/ PVDF composite membrane was further evaluated by increasing the discharge/charge current density from 0.2 C to 2C. As shown in Fig. 5e, The first specific discharge capacity at a low current rate of 0.2 C is 2101.4 mAh g1, as the current density increases, a specific capacity of 1047.3 mAh g1, 868.3 mAh g1, 797.3 mAh g1, 500.5 mAh g1 is obtained at 0.4 C, 0.8 C, 1 C, and 2 C, respectively. Furthermore, an excellent reversible capacity of 1150.2 mAh g1 is recovered when the current density is reverted to 0.2 C, indicating the good stability of the cathode with CNF/PVDF. As expected, the LieS battery with CNF/PVDF exhibits better capacity retention upon cycling. The battery without CNF/PVDF shows fast capacity fading as the charge-discharge rate increases, and its rate performance is much lower compared to the LieS battery with CNF/PVDF. These results demonstrate the assistance of the effective CNF/PVDF composite membrane as an interlayer in enhancing the conductivity and active material utilization. And this significant improved performance may be attributed to the synergistic effect of the CNF and the PVDF membrane, both of which have an effective inhibition of polysulfide diffusion. In addition, the CNF can effectively improve the electronic conductivity of the cathode by creating a superior electrically conductive network. To further analyze the impact of the CNF/PVDF composite membrane in LieS batteries, the EIS measurements were performed. Fig. 6a shows the comparison of the batteries with different membranes before cycling. Batteries with a CNF/PVDF composite membrane, a PVDF membrane and that without membrane all exhibit typical semicircles in the high-to-medium frequency region, which is regarded as the charge transfer resistance, while the leaning lines in the low frequency region is related with mass transfer process [37]. One can observe from the EIS that there has been an increase after inserting a PVDF membrane in the LieS battery, which is caused by the electronic non-conductivity of PVDF. However, the impedance semicircle shrinks after the combination of the CNF and PVDF membrane (inserting a CNF/PVDF composite membrane), which is attributed to the good conductivity of the CNF. This decrease of the charge transfer resistance predicts a better performance of the LieS battery. Here, the CNF works as a current collector for the low-conductivity sulfur cathode, enhancing the active material utilization and thereby improving the electrochemical performance of the LieS batteries [18]. As shown in Fig. 6b, the EIS spectra of the LieS battery with CNF/PVDF composite membrane exhibits a significant decrease after the initial cycle. This reduction may be attributed to the rearrangement of the active materials and deposition at more electrochemical sites through enlargement of the contact area between the active material and the membrane. The SEM images and the elemental mapping of the CNF/PVDF composite membrane surface towards the cathode confirmed the entrapment of the active material. Here the LieS cell with CNF/ PVDF composite membrane was disassembled after 200 cycles, and both the morphology and the physical properties were characterized. The SEM and elemental mapping of the in-plane are shown in Fig. 7a. These SEM image reveal a complete structure with no damage evident, and the elemental mapping shows carbon and sulfur signals uniformly distributed with no evidence of agglomerated active material, which confirms the trapping of the migrating polysulfide within the CNF/PVDF composite membrane. To further confirm the presence of the migrating polysulfide into the cycled CNF/PVDF composite membrane XPS measurements
were performed (Fig. 7b). The presence of polysulfide is evidenced by S2p3/2 contributions at 161.7 and 163.1 eV, as expected for the terminal and bridging sulfur [38]. Elemental sulfur is detected at 164.8 eV. In addition, one strong peak at 167.1 eV is in accord with the binding energy of thiosulfate and another at 169 eV is assigned to the sulfate [39,40]. Thus, we believe that, after long-term cycling, the migrating polysulfide species gradually diffuses through the CNF/PVDF composite membrane and is trapped before it can escape from the cathode. 4. Conclusions In this work, CNF/PVDF composite membranes were produced by a facile method and used as an interlayer in high-performance LieS battery fabrication. When used as the interlayer between the sulfur cathode and the separator, the CNF/PVDF composite membrane demonstrated its key role in providing efficient electron pathways and as a trap for migrating polysulfide species. The resulting LieS battery containing a CNF/PVDF interlayer exhibits an excellent cycling stability which retained a capacity of 768.6 mAh g1 at a coulombic efficiency above 99% (post 10th cycle) for 200 cycles. It is therefore concluded that the use of CNF/PVDF composite membranes as interlayers for LieS batteries is suitable for low-cost and high-capacity LieS batteries with long-term cycling performance. Acknowledgments This work was supported by the National Natural Science Foundation of China (21376001 and 21506012) and the Beijing Higher Education Young Elite Teacher Project (YETP1205). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.08.087. References [1] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19e29. [2] B. Dunn, H. Kamath, J.M. Tarascon, Mater. Grid Energy Grid A Battery Choices 334 (2011) 928e935. [3] R.V. Noorden, Nature 507 (2014) 26e28. [4] A. Manthiram, Y. Fu, S.H. Chung, C. Zu, Y.S. S, Chem. Rev. 114 (2014) 11751e11787. [5] G. Xu, B. Ding, J. Pan, P. Nie, L. Shen, X. Zhang, J. Mater. Chem. A 2 (2014) 12662. [6] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Angew. Chem. Int. Ed. Engl. 52 (2013) 13186e13200. [7] S. Choudhury, M. Agrawal, P. Formanek, D. Jehnichen, D. Fischer, B. Krause, V. Albrecht, M. Stamm, L. Ionov, ACS Nano 9 (2015) 6147e6157. [8] Y. Yang, G. Zheng, Y. Cui, Chem. Soc. Rev. 42 (2013) 3018e3032. [9] G. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Nano Lett. 11 (2011) 4462e4467. [10] H. Chen, W. Dong, J. Ge, C. Wang, X. Wu, W. Lu, L. Chen, Sci. Rep. 3 (2013) 1910. [11] C. Wang, W. Wan, J.-T. Chen, H.-H. Zhou, X.-X. Zhang, L.-X. Yuan, Y.-H. Huang, J. Mater. Chem. A 1 (2013) 1716e1723. [12] Y. Yang, G. Yu, J.J. Cha, H. Wu, M. Vosgueritchian, Y. Yao, Z. Bao, Y. Cui, ACS Nano 5 (2011) 9187e9193. [13] W. Zhou, Y. Yu, H. Chen, F.J. DiSalvo, H.D. Abruna, J. Am. Chem. Soc. 135 (2013) 16736e16743. [14] Z. Zhang, Q. Li, S. Jiang, K. Zhang, Y. Lai, J. Li, Chemistry 21 (2015) 1343e1349. [15] Z. Wei Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.C. Hsu, Y. Cui, Nat. Commun. 4 (2013) 1331. [16] Y.S. Su, A. Manthiram, Chem. Commun. 48 (2012) 8817e8819. [17] C.L. Lee, I.D. Kim, Nanoscale 7 (2015) 10362e10367. [18] Y.S. Su, A. Manthiram, Nat. Commun. 3 (2012) 1166. [19] S.H. Chung, A. Manthiram, Chem. Commun. 50 (2014) 4184e4187. [20] R. Singhal, S.-H. Chung, A. Manthiram, V. Kalra, J. Mater. Chem. A 3 (2015) 4530e4538. [21] X. Gu, C. Lai, F. Liu, W. Yang, Y. Hou, S. Zhang, J. Mater. Chem. A 3 (2015) 9502e9509.
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