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A synergistic modification of polypropylene separator toward stable lithium–sulfur battery
Journal Pre-proof A synergistic modification of polypropylene separator toward stable lithium–sulfur battery Wenyi Zhu, Zhijia Zhang, Jiankun Wei, Yidan Jing, Wei Guo, Zhizhong Xie, Deyu Qu, Dan Liu, Haolin Tang, Junsheng Li PII:
S0376-7388(19)32682-1
DOI:
https://doi.org/10.1016/j.memsci.2019.117646
Reference:
MEMSCI 117646
To appear in:
Journal of Membrane Science
Received Date: 27 August 2019 Revised Date:
3 November 2019
Accepted Date: 5 November 2019
Please cite this article as: W. Zhu, Z. Zhang, J. Wei, Y. Jing, W. Guo, Z. Xie, D. Qu, D. Liu, H. Tang, J. Li, A synergistic modification of polypropylene separator toward stable lithium–sulfur battery, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/j.memsci.2019.117646. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
A Synergistic Modification of Polypropylene Separator Toward Stable Lithium-Sulfur Battery Wenyi Zhu1#, Zhijia Zhang1#, Jiankun Wei1, Yidan Jing1, Wei Guo2*, Zhizhong Xie1, Deyu Qu1, Dan Liu1, Haolin Tang2 and Junsheng Li1,3* 1
School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of
Technology, Wuhan 430070, P. R. China. 2
State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, P. R. China. 3
Hubei provincial key laboratory of fuel cell, Wuhan University of Technology,
Wuhan 430070, P. R. China. *
Wei Guo ([email protected]); Junsheng Li ([email protected])
#
Wenyi Zhu and Zhijia Zhang contribute equally to this work.
Abstract: Lithium sulfur batteries have widespread applications in many different fields ranging from electric vehicles to portable electronic devices. Design and development of advanced separators that can inhibit the polysulfide shuttling is critical for the development of lithium sulfur battery. Herein, we report a synergistic modification of polypropylene (PP) separator using multiwall carbon nanotubes loaded with CeO2 nanoparticles (MWCNTs/CeO2). Due to the high polysulfide affinity of CeO2, such a modification enables effective prevention of polysulfide transport through the separator. Besides, MWCNTs in the modification layer also block polysulfides shuttling by physical adsorption and regulate the charge transfer by function as a secondary current collector. As a result of this functional synergy, lithium sulfur battery assembled with PP- MWCNTs/CeO2 separator shows stable discharge performance with a capacity of 520.7 mAh g-1 retained after 300 cycles.
Keywords: lithium sulfur battery; separator modification; synergy, CeO2; MWCNT.
1
1.Introduction Lithium-Sulfur (Li-S) battery is expected to be a promising energy storage solution due to its high energy density and abundant reserves of sulfur in nature. However, Li-S battery is still faced with several technological challenges which hinders its practical application. Among these challenges, the continuous migration of soluble polysulfide through the separator (denoted as the shuttling effect) is the key limiting process for stable cycling of Li-S battery.[1, 2] The shuttle effect leads to rapid loss of active sulfur materials that sacrifices the energy density of the battery.[3] More importantly, the uneven deposition of shuttled sulfur species on Li electrode causes rapid degradation of the battery.[4] Various approaches have been proposed to alleviate the shuttling effect in Li-S battery. Among these approaches, separator modification is proved to be a cost-effective method.[5-7] Generally, separator modification endows the separator with additional barrier function that inhibits the migration of soluble polysulfides through the separator.[2, 8-11] The barrier performance of the modification layer is closely related to the intrinsic affinity between the material used for modification and soluble polysulfides.
Previous studies have shown that coating of carbonaceous materials, such as porous carbon[12-15], graphene oxide[16], carbon nanofibers[17, 18] on to the separator could enhance the long-term discharge performance of Li-S battery by suppression of shuttling. However, these carbonaceous materials normally have a low binding affinity with soluble polysulfides and effective inhibition of polysulfide shuttling relies on delicate design of their morphological features.[19] From the perspective of binding of polysulfides, sulfiphilic material that have a strong interaction with polysulfide is more favorable for separator modification. So far, a series of sulfiphilic materials, such as TiO2[20], SiO2[21], MoS2[22], TiN[23] and NbN[24], have been successfully employed for separator modification. Another attractive feature of sulfiphilic material is their high polarity, which enhances their electrolyte affinity.[25] Furthermore, some of the sulfiphilic materials were shown to catalyze the conversion of soluble polysulfides and improve the discharge performance of the battery 2
recently.[26-28] Despite these advantages, sulfiphilic materials suffer from their low electronic conductivity, which may pose additional resistance for charge transfer in the battery.
Herein, we propose a synergistic modification of commercially available polypropylene separator. In this modification, CeO2 was stabilized onto multiwall carbon nanotubes (MWCNTs) and the composite was then coated onto commercial PP separator. CeO2, as a highly sulfiphilic material, provides a strong anchor for soluble polysulfides and thus suppress the shuttling effect. MWCNT component mainly acts as a conductive matrix to decrease the charge transfer resistance of the battery. Such a functional synergy of MWCNT/CeO2 modification layer enables high utilization of active sulfur species and efficient charge transport at the electrode-electrolyte interfaces, which altogether leads to a stable cycling of Li-S battery. 2.Experiment section Fabrication of PP-MWCNTs/CeO2 composite separator MWCNTs (Suzhou Hengqiu nano reagent Co. Ltd., China) with a douter of 30-50 nm and dinner of 5-12 nm were firstly activated in hot concentrated nitric acid (68%), followed by multiple washing procedures until the pH of washing solution reached 7. To synthesize MWCNTs/CeO2, 100 mg of pretreated MWCNTs was add to 40 mL of absolute ethanol, and sonicate for 2 h. After the solution is evenly dispersed, 3.7 mL of 0.15 mol·L-1 Ce(NO3)3·6H2O was homogeneously mixed into the solution above by magnetical stirring for 20 min. Finally, 0.125 mol·L-1 NaOH was added to the mixture to adjust the pH to 10. After repeated filtration and washing steps, the collected MWCNTs/CeO2 was heated in an oven at 60 °C for 12 h and then kept in a desiccator for use. To prepare the modified separator, appropriate amount of the MWCNTs/CeO2 was dispersed in an isopropyl alcohol solution, and then ultrasonicated for 1 h to obtain a homogeneous mixed solution, which was vacuum-filtered onto the PP separator. Finally, the MWCNTs/CeO2 modified separator was vacuum dried at 50 °C for 24 h. For comparison, MWCNTs-modified PP separator was also prepared in the same way. To 3
prepare Li2S6, Li2S (0.095 g) and S (0.4 g) were added into 30 mL of steamed Tetrahydrofuran (THF) in a glove box filled with Argon. Then the mixture was stirred for 96 h at room temperature to obtain a 0.1 M polysulfide (Li2S6) solution. Materials Characterizations The X-ray diffraction (XRD) pattern of the sample was collected by a Bruker D8 Advance diffractometer (Cu Kα, λ = 1.5406 Å). Scanning microscope (SEM) measurements were conducted using a Hitachi S-4800 electron microscope at an accelerating voltage of 5 kV. TEM measurements were performed on a JEM-2100F electron microscope (200 kV). Fourier infrared (FT-IR) characterizations were performed with a Nicolet AVATAR 370 infrared spectrometer (4000-400 cm-1). X-ray photoelectron spectroscopy (XPS) was tested with Al-Kα X-ray source on an ESCALAB 250Xi spectrometer and elemental content was determined by an Elementar Vario Micro cube elemental analyzer. The thermogravimetric (TGA) curve was recorded on a SDT Q600 thermal analyzer in an air atmosphere at a flow rate of 60 mL min-1 and a ramping rate of 5 °C min-1. The contact angle of the separator to water and electrolyte was measured by a contact angle apparatus (JC 2000C), and the amount of water for one measurement was 2 mL. Electrochemical measurements The sulfur electrode was prepared by mixing sulfur (S), Super P and LA133 in a ratio of 6:3:1. Next, the uniformly stirred slurry of the mixture was applied to a carbon coated aluminum foil, which was then dried at room temperature and kept under vacuum at 50 ° C for 12 h. Finally, the dried film was cut into 10 mm diameter electrode pieces by a microtome and weighed, and the load of each slice of the active material (S) was controlled to be 1.8-2.0 mg·cm-2. The composition of electrolyte was 1 M LiTFSI in a mixture solvent of 1,2-dimethoxyethane and1,3-dioxacyclopentane (1:1 (v/v)), which contained LiNO3 dopant (0.1 M). The amount of electrolyte used for battery assembling was 25 µL·mg−1 sulfur. To test the ionic conductivity of the separator, stainless steel/separator (with electrolyte)/stainless steel cell was assembled. The ionic conductivity of the separator was measured using an Autolab (PG302N) electrochemical workstation with the frequency set to be 10-5–10-2 Hz. Linear sweep voltammetry (LSV) characterization was performed from 1-6 V (5 mV s−1) with a lithium 4
foil/separator/stainless steel cell to examine the stability of separator. The charge-discharge performance of the assembled battery was evaluated by a battery test system (LAND CT2001A) from 1.5–2.8 V at different rates (1 C = 1675 mA·g−1). Cyclic voltammetry (CV) test was also conducted from 1.5 to 2.8 V (scanning rate 0.1 mV s-1). Lithium ion transfer number (t) of the electrolyte-soaked separator was quantified with a combined chronoamperometry and impedance analysis as described elsewhere.[5] Density functional theory (DFT) investigation was used to determine the binding strengthen (defined with E ) between MWCNT (or CeO2) and the polysulfides, which is calculated by E
=E
−E
− E , where E
, E , and E
are
the energy of the polysulfides-MWCNT (or CeO2), polysulfides, and MWCNT (or CeO2), respectively. DFT implemented in the VASP code was use to optimize geometry structures. The exchange–correlation interactions were calculated using the generalized gradient approximation (GGA) in the form of the Perdew–Burke– Ernzerhof functional (PBE). The cut-off energy of the plain-wave basis sets was 400 eV. The convergence criterion for the energy was set to be 10 eV and the convergence criterion for the force was set to be 0.002 eV Å , respectively. To model the CeO -Li S structures, a four-layer 2×2×1 super cell with a 1.5 nm vacuum layer was considered and only top three layer atoms were set to be full relaxed. The Brillouin zone was sampled with a Monkhorst-Pack 2 × 2 × 1 k-point grid. To model the C-Li S structures, 5x5x4 carbon nanotube with a 1.5 nm vacuum layer was considered. The DFT-U (U stands for Hubbard U parameter) approach with the vdW correction was employed to describe the vdW interactions precisely.
3.Results and Discussion The schematic diagram of the synthesis of PP-MWCNTs/CeO2 separators are shown in Fig. 1a. In the first step, CeO2 nanoparticles were grown onto pre-activated MWCNTs through a facile solution process. Successful grafting of uniform CeO2 nanoparticles onto the walls of MWCNTs is demonstrated with TEM measurements 5
(Fig. S1) and XPS measurement (Fig. S2). In addition, the grafting process didn’t lead to agglomeration of MWCNTs, as observed from the SEM images of MWCNTs/CeO2 (Fig. S3). The high uniformity of CeO2 on MWCNTs was further evidenced with the EDX mapping measurement (Fig. S4). TG measurements were conducted to investigate the relative content of CeO2 in MWCNTs/CeO2 composite. It can be seen from the curve that the composite exhibit a two-stage weight loss (Fig. S5). The first weight loss of 4.1% below 100 °C can be ascribed to the physically adsorbed moisture in the sample. The weight loss (78.1%) in the second period is the oxidative weight loss of MWCNTs. Thus, the content of CeO2 was determined to be 17.8% in the composite.
MWCNTs/CeO2 were dispersed in isopropanol and then applied onto the separator through filtering through the separator. The surface SEM images of MWCNTs/CeO2 modified separator (PP-MWCNTs/CeO2) proved that a homogeneous and compact coating layer was formed on PP. (Fig. 1b) Cross-sectional SEM images showed that the thickness of the coating was ~10.5 μm (Fig. 1c). The loading mass of the modification layer on the separator is ~0.15 mg cm-2. The presence of CeO2 on the modified separator is evidenced using XRD and FTIR measurements with PP-MWCNTs/CeO2. Characteristic peaks assigned to CeO2 (111), (200), (220), (311), (222), (400), (331), (420) and (422) peak (at 2θ = 28.6°, 32.9°, 42.4°, 56.4°, 59.9°, 69.4°, 76.3° and 88.4°, respectively) were found in the XRD spectra of MWCNTs/CeO2 (Fig.2a). In addition, diffraction peaks from MWCNTs (002), (101) and (004) peak (at 2θ = 26.3°, 43.2° and 57.3°, respectively) were also identified in the XRD spectra. From the FTIR spectra (Fig. 2b), strong stretching vibration (3100 cm-1) and bending vibration (1550 cm-1) of -OH are observed due to the oxidative activation of MWCNTs. In addition, the bending vibration of Ce-O at 476 cm-1 was also seen in the spectra of the sample, further proving successful modification of CeO2 onto the separator.
6
Figure 1. (a) Schematic diagram for the preparation of PP-MWCNTs/CeO2 separators. (b) Surface SEM image of PP-MWCNTs/CeO2. (c) Cross sectional SEM image of PP-MWCNTs/CeO2.
Figure 2. (a) XRD pattern of MWCNTs/CeO2. (b) FTIR spectra of modified separator.
DFT investigations were performed to investigate the affinity of soluble polysulfides with carbon nanotube and CeO2. The optimized binding configuration of polysulfide species on model carbon nanotube and the dominant CeO2 facet (CeO2 (111)) is shown in Fig. 3. Polysulfide species could only physically adsorb onto the model carbon nanotube, with low adsorption energy of -0.292, -0.574 and -0.418 eV for Li2S4, Li2S6 and Li2S8, respectively. In contrast, Li in polysulfide species could 7
chemically bind to O in CeO2, forming strong chemical binding. The binding energy of Li2S4, Li2S6 and Li2S8 on CeO2 (111) was calculated to be -2.584, -1.966 and -1.970 eV, respectively. These results suggest that the PP- MWCNTs/CeO2 may provide a strong anchor to soluble polysulfides that is beneficial for the prevention of the shuttling process. The high affinity of CeO2 was also verified with experimental studies. MWCNTs (20 mg) incubated in the Li2S6 solution (4 mM) could quickly adsorb a large portion of Li2S6 through physical interactions (Fig. S6, inset). When the same amount of MWCNTs/CeO2 was placed into the Li2S6 solution, more Li2S6 was adsorbed by the MWCNTs/CeO2 powder, as reflected by the UV-Vis spectra of the supernatants (Fig. S6). This result further confirms the improved polysulfide affinity of MWCNTs/CeO2. Considering the fact that the fraction of CeO2 in MWCNTs/CeO2 composite is low, the enhanced polysulfide adsorption of MWCNTs/CeO2 suggests a strong chemical binding of CeO2 with polysulfides, which is in good agreement with the DFT results. These results also indicate that PP- MWCNTs/CeO2 separator could improve the performance of the Li-S battery.
Figure 3. Optimized configuration of Li2S4 (left), Li2S6 (middle) and Li2S8 (right) on (A) MWCNT and (B) CeO2 (111) facet.
8
Since electrolyte wettability of the separator is closely related to its battery performance, the wettability of the separator toward both water and electrolyte is quantified. As shown in Fig. 4a, PP separator is highly hydrophobic. After modification with either MWCNTs or MWCNTs/CeO2, the separator’s water contact angle changes from ~112.6° to ~23°, and their electrolyte contact angle drops rapidly from 41.0° to 0°. The enhanced wettability of the modified separator is originated from the polar functional groups (such as hydroxyl groups and carboxyl groups) on MWCNTs and CeO2. A high electrolyte affinity is indicative of a high Li+ conductivity of the electrolyte-soaked separator. In order to demonstrate this point, the Li+ conductivity of the electrolyte-soaked separators was measured using EIS (Fig. 4b). The quantified ionic conductivity of PP, PP-MWCNTs and PP-MWCNTs/CeO2 was 0.85, 1.25 and 1.38 S cm-1, respectively. It should be pointed out that the apparent ionic conductivity measured is the sum of the conductivity of both the Li+ and anions in the electrolyte. To identify the conductivity from Li+, the lithium ion transfer number was characterized by a combined analysis of chronoamperometry and EIS (Fig. 4c). The lithium transfer number of pristine PP separator was 0.43. After modification of MWCNTs/CeO2, the lithium ion transfer number of the separator
9
increased to 0.52. The increase of the lithium transfer number of PP-MWCNTs/CeO2
could be possibly resulted from the Lewis acid-base interaction between the anions and Ce that trap the anions. The high ionic conductivity and improved lithium ion transfer number is expected to improve the electrochemical performance of the lithium-sulfur battery. To examine if the modification affects the electrochemical stability of the separator, LSV measurements were conducted (Fig. S7). The PP-MWCNTs and PP-MWCNTs/CeO2 exhibited an oxidative peak with an onset potential of ~3.1 V, which could be ascribed to the oxidation of MWCNT on the separator. Since the charge-discharge potential of Li-S battery is much lower than 3.1 V, the modified separator is stable enough to sustain a stable cycling of the battery.
Figure 4. (a) Water and electrolyte contact angles of PP, PP-MWCNTs and PP-MWCNTs/CeO2 separators. (b) EIS curves of stainless steel/electrolyte-soaked separator/stainless
steel
cells
assembled
with
PP,
PP-MWCNTs
or
PP-MWCNTs/CeO2 separators. (c) Chronoamperometry of Li/electrolyte-soaked separator/Li cells assembled with PP, PP-MWCNTs or PP-MWCNTs/CeO2 separators (insets: corresponding EIS curves of the cells before and after 10
polarization).
To characterize the charge-discharge performance of the separator, CR 2032 type coin cell was assembled and characterized. CV characterizations were first performed. All the batteries exhibited typical redox peaks of sulfur species: reduction peaks at ~ 2.2 V and 1.95 V assigned to reduction of elemental sulfur (S8) to soluble polysulfide and reduction of soluble polysulfide to insoluble Li2S and Li2S2, respectively; oxidation peak at ~2.5 V corresponding to oxidation of insoluble Li2S and Li2S2 to soluble lithium polysulfide and their further oxidation to sulfur (Fig. S8). The battery with PP-MWCNTs/CeO2 separator showed a slightly lower polarization compared to their counterparts with PP or MWCNTs separator, suggesting a better electrochemical reversibility
of
the
battery assembled
with
PP-MWCNTs/CeO2
separator.
Charge-discharge performance of the batteries were firstly evaluated at 0.2C (Fig. 5a). All the batteries showed a high first cycle discharge performance, with the 1st cycle discharge capacity being 717.7, 787.8 and 898.3 mAh g-1, for PP, PP-MWCNTs and PP-MWCNTs/CeO2, respectively. The battery with PP-MWCNTs/CeO2 separator retained a relatively stable discharge performance during the whole battery cycling. A high capacity of 520.7 mAh g-1 was retained after 300 charge-discharge cycles for PP-MWCNTs/CeO2. The discharge capacity at 300th cycle for battery assembled using PP and PP-MWCNTs separator was 345.0, 436.7 mAh g-1, respectively. The rate performance of batteries with pristine PP and modified PP separators were also investigated (Fig. 5b and Fig. S9). The battery with PP-MWCNTs/CeO2 separator has the highest discharge capacity at each discharge rate. In addition, when the discharge rate returns to 0.2C, a high discharge capacity of 649.3 mAh g-1 was retained. From the discharge performance of the batteries, it could be clearly seen that the coating of the separator with MWCNTs enhances the discharge performance of the batteries. The key to the improvement of the MWCNTs modification is its capability for the physically adsorption of soluble polysulfide and function of the secondary current collector. When MWCNTs/CeO2 was introduced onto the separator, the battery performance was further enhanced because of the improvement of the polysulfide anchoring performance as demonstrated above. The interaction of sulfur species with 11
the separator was also demonstrated with fine XPS analysis of the separator (on the cathode side) disassembled from the discharged batteries (Fig. 6a, b). Generally, the S 2p peaks of the two separators show multiple peaks corresponding to different chemical states of S species[29], of which are polysulfide (Li2Sx, ~163.0 eV), elemental sulfur (~164.4 eV), thiosulfate(~167.4 eV) and sulfate (~169.2 eV). Clearly, the polysulfide content was much lower on PP- MWCNTs/CeO2. This observation infers that the CeO2 containing modification layer facilitates the conversion of adsorbed polysulfides and thus improves the utilization of S species. The XPS characterizations with the separator disassembled from the a charged battery also confirms this point (Fig. 6c, d).
Figure 5 (a) Cycling and (b) rate performance of lithium sulfur battery assembled with PP, PP-MWCNTs or PP-MWCNTs/CeO2 separator. (c) EIS curves of fresh lithium sulfur battery assembled with PP, PP-MWCNTs or PP-MWCNTs/CeO2 separator. (d) EIS curves of cycled lithium sulfur battery assembled with PP, PP-MWCNTs or PP-MWCNTs/CeO2 separator.
To gain insight into the improved performance of the modified separator, EIS of the fresh battery and cycled battery were recorded (Fig. 5c). Before cycling, 12
PP-MWCNTs/CeO2 has a lowest charge transfer resistance (Rct=3.04 Ω). In contrast, the charge resistance of battery with PP, PP-MWCNTs separator was 73.52 Ω and 36.18 Ω, respectively. The reduction in Rct is due to that the separator modification layer enhances the affinity of the separator to the electrolyte and thus improves the transport of lithium ions at the electrode-electrolyte interface. After cycling, the Rct of all batteries reduced significantly due to the formation of SEI layers. The battery with PP-MWCNTs/CeO2 still showed the lowest Rct (10.61 Ω) value compared to other batteries. Besides, the battery also had a lowest RSEI (15.59 Ω). The above results show that PP-MWCNTs/CeO2 can improve lithium ion transport and alleviate the shuttle effect, which contribute to a conformal electrode-electrolyte interface and battery performance.
Figure 6. High resolution S 2p XPS spectra of the separator disassembled from a discharged lithium sulfur battery assembled with (a) PP or (b) PP-MWCNTs/CeO2 separator. High resolution S 2p XPS spectra of the separator disassembled from a charged lithium sulfur battery assembled with (c) PP or (d) PP-MWCNTs/CeO2 separator.
13
4.Conclusion We propose a new strategy of separator modification by using MWCNTs/CeO2 that provides functional synergies. The MWCNTs/CeO2 modified separator not only restricts polysulfide shuttling by the chemical and physical interactions but also improves the electrolyte affinity and charge transfer at the electrode-electrolyte interface. Due to the favorable features introduced by the synergistic modification, the lithium sulfur battery exhibits a more stable charge-discharge performance compared to the battery with pristine PP separator. The new separator developed in this study is promising for practical battery applications. In addition, our results may also inspire further exploration of the development of functional separators for advanced battery applications.
Acknowledgement: This work was supported by Nation Natural Science Foundation of China (Grant No. 51972254, 21706200), Fundamental Research Funds for the Central Universities (WUT: 2018IB026, 2019IB003), China Postdoctoral Science Foundation funded project (2018T110813), State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology, 2019-KF-10) and Fundamental Research Funds for the Central Universities for financial support (2019-HS-B1-15,
2019III048GX).
We
thank
the
Material
Research
and
Characterization Center at WUT for their assistance with the characterizations.
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Mesoporous TiN microspheres as an efficient polysulfide barrier for lithium-sulfur batteries, J. Mater. Chem. A, 6 (2018) 14359-14366. [24] W. Qiu, C. An, Y. Yan, J. Xu, Z. Zhang, W. Guo, Z. Wang, Z. Zheng, Z. Wang, Q. Deng, J. Li, Suppressed polysulfide shuttling and improved Li+ transport in LiS batteries enabled by NbN modified PP separator, J. Power Sources 423 (2019) 98-105. [25] Y.Y. Xiang, Z. Wang, W.J. Qiu, Z.R. Guo, D. Liu, D.Y. Qu, Z.Z. Xie, H.L. Tang, J.S. Li, Interfacing soluble polysulfides with a SnO2 functionalized separator: An efficient approach for improving performance of Li-S battery, J Membrane Sci, 563 (2018) 380-387. [26] Z. Zhang, X. Li, Y. Yan, W. Zhu, L.-H. Shao, J. Li, A Bioinspired Functionalization of Polypropylene Separator for Lithium-Sulfur Battery, Polymers, 11 (2019) 728. [27] D. Liu, C. Zhang, G. Zhou, W. Lv, G. Ling, L. Zhi, Q. Yang, Catalytic effects in lithium-sulfur batteries: promoted sulfur transformation and reduced shuttle effect, Adv. Sci., 5 (2018) 1700270. [28] H. Yuan, H.-J. Peng, B.-Q. Li, J. Xie, L. Kong, M. Zhao, X. Chen, J.-Q. Huang, Q. Zhang, Conductive and Catalytic Triple-Phase Interfaces Enabling Uniform Nucleation in High-Rate Lithium-Sulfur Batteries, Adv. Energy Mater., 9 (2019) 1802768. [29] H. Al Salem, G. Babu, C.V. Rao, L.M.R. Arava, Electrocatalytic Polysulfide Traps for Controlling Redox Shuttle Process of Li-S Batteries, J. Am. Chem. Soc., 137 (2015) 11542-11545.
17
A synergistic modification is used for separator modification in Li-S battery. The separator is modified with MWCNTs/CeO2. The modification enables improved barrier property and charge transfer. Li-S battery assembled with the separator shows excellent performance.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0376-7388(19)32682-1
DOI:
https://doi.org/10.1016/j.memsci.2019.117646
Reference:
MEMSCI 117646
To appear in:
Journal of Membrane Science
Received Date: 27 August 2019 Revised Date:
3 November 2019
Accepted Date: 5 November 2019
Please cite this article as: W. Zhu, Z. Zhang, J. Wei, Y. Jing, W. Guo, Z. Xie, D. Qu, D. Liu, H. Tang, J. Li, A synergistic modification of polypropylene separator toward stable lithium–sulfur battery, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/j.memsci.2019.117646. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
A Synergistic Modification of Polypropylene Separator Toward Stable Lithium-Sulfur Battery Wenyi Zhu1#, Zhijia Zhang1#, Jiankun Wei1, Yidan Jing1, Wei Guo2*, Zhizhong Xie1, Deyu Qu1, Dan Liu1, Haolin Tang2 and Junsheng Li1,3* 1
School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of
Technology, Wuhan 430070, P. R. China. 2
State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, P. R. China. 3
Hubei provincial key laboratory of fuel cell, Wuhan University of Technology,
Wuhan 430070, P. R. China. *
Wei Guo ([email protected]); Junsheng Li ([email protected])
#
Wenyi Zhu and Zhijia Zhang contribute equally to this work.
Abstract: Lithium sulfur batteries have widespread applications in many different fields ranging from electric vehicles to portable electronic devices. Design and development of advanced separators that can inhibit the polysulfide shuttling is critical for the development of lithium sulfur battery. Herein, we report a synergistic modification of polypropylene (PP) separator using multiwall carbon nanotubes loaded with CeO2 nanoparticles (MWCNTs/CeO2). Due to the high polysulfide affinity of CeO2, such a modification enables effective prevention of polysulfide transport through the separator. Besides, MWCNTs in the modification layer also block polysulfides shuttling by physical adsorption and regulate the charge transfer by function as a secondary current collector. As a result of this functional synergy, lithium sulfur battery assembled with PP- MWCNTs/CeO2 separator shows stable discharge performance with a capacity of 520.7 mAh g-1 retained after 300 cycles.
Keywords: lithium sulfur battery; separator modification; synergy, CeO2; MWCNT.
1
1.Introduction Lithium-Sulfur (Li-S) battery is expected to be a promising energy storage solution due to its high energy density and abundant reserves of sulfur in nature. However, Li-S battery is still faced with several technological challenges which hinders its practical application. Among these challenges, the continuous migration of soluble polysulfide through the separator (denoted as the shuttling effect) is the key limiting process for stable cycling of Li-S battery.[1, 2] The shuttle effect leads to rapid loss of active sulfur materials that sacrifices the energy density of the battery.[3] More importantly, the uneven deposition of shuttled sulfur species on Li electrode causes rapid degradation of the battery.[4] Various approaches have been proposed to alleviate the shuttling effect in Li-S battery. Among these approaches, separator modification is proved to be a cost-effective method.[5-7] Generally, separator modification endows the separator with additional barrier function that inhibits the migration of soluble polysulfides through the separator.[2, 8-11] The barrier performance of the modification layer is closely related to the intrinsic affinity between the material used for modification and soluble polysulfides.
Previous studies have shown that coating of carbonaceous materials, such as porous carbon[12-15], graphene oxide[16], carbon nanofibers[17, 18] on to the separator could enhance the long-term discharge performance of Li-S battery by suppression of shuttling. However, these carbonaceous materials normally have a low binding affinity with soluble polysulfides and effective inhibition of polysulfide shuttling relies on delicate design of their morphological features.[19] From the perspective of binding of polysulfides, sulfiphilic material that have a strong interaction with polysulfide is more favorable for separator modification. So far, a series of sulfiphilic materials, such as TiO2[20], SiO2[21], MoS2[22], TiN[23] and NbN[24], have been successfully employed for separator modification. Another attractive feature of sulfiphilic material is their high polarity, which enhances their electrolyte affinity.[25] Furthermore, some of the sulfiphilic materials were shown to catalyze the conversion of soluble polysulfides and improve the discharge performance of the battery 2
recently.[26-28] Despite these advantages, sulfiphilic materials suffer from their low electronic conductivity, which may pose additional resistance for charge transfer in the battery.
Herein, we propose a synergistic modification of commercially available polypropylene separator. In this modification, CeO2 was stabilized onto multiwall carbon nanotubes (MWCNTs) and the composite was then coated onto commercial PP separator. CeO2, as a highly sulfiphilic material, provides a strong anchor for soluble polysulfides and thus suppress the shuttling effect. MWCNT component mainly acts as a conductive matrix to decrease the charge transfer resistance of the battery. Such a functional synergy of MWCNT/CeO2 modification layer enables high utilization of active sulfur species and efficient charge transport at the electrode-electrolyte interfaces, which altogether leads to a stable cycling of Li-S battery. 2.Experiment section Fabrication of PP-MWCNTs/CeO2 composite separator MWCNTs (Suzhou Hengqiu nano reagent Co. Ltd., China) with a douter of 30-50 nm and dinner of 5-12 nm were firstly activated in hot concentrated nitric acid (68%), followed by multiple washing procedures until the pH of washing solution reached 7. To synthesize MWCNTs/CeO2, 100 mg of pretreated MWCNTs was add to 40 mL of absolute ethanol, and sonicate for 2 h. After the solution is evenly dispersed, 3.7 mL of 0.15 mol·L-1 Ce(NO3)3·6H2O was homogeneously mixed into the solution above by magnetical stirring for 20 min. Finally, 0.125 mol·L-1 NaOH was added to the mixture to adjust the pH to 10. After repeated filtration and washing steps, the collected MWCNTs/CeO2 was heated in an oven at 60 °C for 12 h and then kept in a desiccator for use. To prepare the modified separator, appropriate amount of the MWCNTs/CeO2 was dispersed in an isopropyl alcohol solution, and then ultrasonicated for 1 h to obtain a homogeneous mixed solution, which was vacuum-filtered onto the PP separator. Finally, the MWCNTs/CeO2 modified separator was vacuum dried at 50 °C for 24 h. For comparison, MWCNTs-modified PP separator was also prepared in the same way. To 3
prepare Li2S6, Li2S (0.095 g) and S (0.4 g) were added into 30 mL of steamed Tetrahydrofuran (THF) in a glove box filled with Argon. Then the mixture was stirred for 96 h at room temperature to obtain a 0.1 M polysulfide (Li2S6) solution. Materials Characterizations The X-ray diffraction (XRD) pattern of the sample was collected by a Bruker D8 Advance diffractometer (Cu Kα, λ = 1.5406 Å). Scanning microscope (SEM) measurements were conducted using a Hitachi S-4800 electron microscope at an accelerating voltage of 5 kV. TEM measurements were performed on a JEM-2100F electron microscope (200 kV). Fourier infrared (FT-IR) characterizations were performed with a Nicolet AVATAR 370 infrared spectrometer (4000-400 cm-1). X-ray photoelectron spectroscopy (XPS) was tested with Al-Kα X-ray source on an ESCALAB 250Xi spectrometer and elemental content was determined by an Elementar Vario Micro cube elemental analyzer. The thermogravimetric (TGA) curve was recorded on a SDT Q600 thermal analyzer in an air atmosphere at a flow rate of 60 mL min-1 and a ramping rate of 5 °C min-1. The contact angle of the separator to water and electrolyte was measured by a contact angle apparatus (JC 2000C), and the amount of water for one measurement was 2 mL. Electrochemical measurements The sulfur electrode was prepared by mixing sulfur (S), Super P and LA133 in a ratio of 6:3:1. Next, the uniformly stirred slurry of the mixture was applied to a carbon coated aluminum foil, which was then dried at room temperature and kept under vacuum at 50 ° C for 12 h. Finally, the dried film was cut into 10 mm diameter electrode pieces by a microtome and weighed, and the load of each slice of the active material (S) was controlled to be 1.8-2.0 mg·cm-2. The composition of electrolyte was 1 M LiTFSI in a mixture solvent of 1,2-dimethoxyethane and1,3-dioxacyclopentane (1:1 (v/v)), which contained LiNO3 dopant (0.1 M). The amount of electrolyte used for battery assembling was 25 µL·mg−1 sulfur. To test the ionic conductivity of the separator, stainless steel/separator (with electrolyte)/stainless steel cell was assembled. The ionic conductivity of the separator was measured using an Autolab (PG302N) electrochemical workstation with the frequency set to be 10-5–10-2 Hz. Linear sweep voltammetry (LSV) characterization was performed from 1-6 V (5 mV s−1) with a lithium 4
foil/separator/stainless steel cell to examine the stability of separator. The charge-discharge performance of the assembled battery was evaluated by a battery test system (LAND CT2001A) from 1.5–2.8 V at different rates (1 C = 1675 mA·g−1). Cyclic voltammetry (CV) test was also conducted from 1.5 to 2.8 V (scanning rate 0.1 mV s-1). Lithium ion transfer number (t) of the electrolyte-soaked separator was quantified with a combined chronoamperometry and impedance analysis as described elsewhere.[5] Density functional theory (DFT) investigation was used to determine the binding strengthen (defined with E ) between MWCNT (or CeO2) and the polysulfides, which is calculated by E
=E
−E
− E , where E
, E , and E
are
the energy of the polysulfides-MWCNT (or CeO2), polysulfides, and MWCNT (or CeO2), respectively. DFT implemented in the VASP code was use to optimize geometry structures. The exchange–correlation interactions were calculated using the generalized gradient approximation (GGA) in the form of the Perdew–Burke– Ernzerhof functional (PBE). The cut-off energy of the plain-wave basis sets was 400 eV. The convergence criterion for the energy was set to be 10 eV and the convergence criterion for the force was set to be 0.002 eV Å , respectively. To model the CeO -Li S structures, a four-layer 2×2×1 super cell with a 1.5 nm vacuum layer was considered and only top three layer atoms were set to be full relaxed. The Brillouin zone was sampled with a Monkhorst-Pack 2 × 2 × 1 k-point grid. To model the C-Li S structures, 5x5x4 carbon nanotube with a 1.5 nm vacuum layer was considered. The DFT-U (U stands for Hubbard U parameter) approach with the vdW correction was employed to describe the vdW interactions precisely.
3.Results and Discussion The schematic diagram of the synthesis of PP-MWCNTs/CeO2 separators are shown in Fig. 1a. In the first step, CeO2 nanoparticles were grown onto pre-activated MWCNTs through a facile solution process. Successful grafting of uniform CeO2 nanoparticles onto the walls of MWCNTs is demonstrated with TEM measurements 5
(Fig. S1) and XPS measurement (Fig. S2). In addition, the grafting process didn’t lead to agglomeration of MWCNTs, as observed from the SEM images of MWCNTs/CeO2 (Fig. S3). The high uniformity of CeO2 on MWCNTs was further evidenced with the EDX mapping measurement (Fig. S4). TG measurements were conducted to investigate the relative content of CeO2 in MWCNTs/CeO2 composite. It can be seen from the curve that the composite exhibit a two-stage weight loss (Fig. S5). The first weight loss of 4.1% below 100 °C can be ascribed to the physically adsorbed moisture in the sample. The weight loss (78.1%) in the second period is the oxidative weight loss of MWCNTs. Thus, the content of CeO2 was determined to be 17.8% in the composite.
MWCNTs/CeO2 were dispersed in isopropanol and then applied onto the separator through filtering through the separator. The surface SEM images of MWCNTs/CeO2 modified separator (PP-MWCNTs/CeO2) proved that a homogeneous and compact coating layer was formed on PP. (Fig. 1b) Cross-sectional SEM images showed that the thickness of the coating was ~10.5 μm (Fig. 1c). The loading mass of the modification layer on the separator is ~0.15 mg cm-2. The presence of CeO2 on the modified separator is evidenced using XRD and FTIR measurements with PP-MWCNTs/CeO2. Characteristic peaks assigned to CeO2 (111), (200), (220), (311), (222), (400), (331), (420) and (422) peak (at 2θ = 28.6°, 32.9°, 42.4°, 56.4°, 59.9°, 69.4°, 76.3° and 88.4°, respectively) were found in the XRD spectra of MWCNTs/CeO2 (Fig.2a). In addition, diffraction peaks from MWCNTs (002), (101) and (004) peak (at 2θ = 26.3°, 43.2° and 57.3°, respectively) were also identified in the XRD spectra. From the FTIR spectra (Fig. 2b), strong stretching vibration (3100 cm-1) and bending vibration (1550 cm-1) of -OH are observed due to the oxidative activation of MWCNTs. In addition, the bending vibration of Ce-O at 476 cm-1 was also seen in the spectra of the sample, further proving successful modification of CeO2 onto the separator.
6
Figure 1. (a) Schematic diagram for the preparation of PP-MWCNTs/CeO2 separators. (b) Surface SEM image of PP-MWCNTs/CeO2. (c) Cross sectional SEM image of PP-MWCNTs/CeO2.
Figure 2. (a) XRD pattern of MWCNTs/CeO2. (b) FTIR spectra of modified separator.
DFT investigations were performed to investigate the affinity of soluble polysulfides with carbon nanotube and CeO2. The optimized binding configuration of polysulfide species on model carbon nanotube and the dominant CeO2 facet (CeO2 (111)) is shown in Fig. 3. Polysulfide species could only physically adsorb onto the model carbon nanotube, with low adsorption energy of -0.292, -0.574 and -0.418 eV for Li2S4, Li2S6 and Li2S8, respectively. In contrast, Li in polysulfide species could 7
chemically bind to O in CeO2, forming strong chemical binding. The binding energy of Li2S4, Li2S6 and Li2S8 on CeO2 (111) was calculated to be -2.584, -1.966 and -1.970 eV, respectively. These results suggest that the PP- MWCNTs/CeO2 may provide a strong anchor to soluble polysulfides that is beneficial for the prevention of the shuttling process. The high affinity of CeO2 was also verified with experimental studies. MWCNTs (20 mg) incubated in the Li2S6 solution (4 mM) could quickly adsorb a large portion of Li2S6 through physical interactions (Fig. S6, inset). When the same amount of MWCNTs/CeO2 was placed into the Li2S6 solution, more Li2S6 was adsorbed by the MWCNTs/CeO2 powder, as reflected by the UV-Vis spectra of the supernatants (Fig. S6). This result further confirms the improved polysulfide affinity of MWCNTs/CeO2. Considering the fact that the fraction of CeO2 in MWCNTs/CeO2 composite is low, the enhanced polysulfide adsorption of MWCNTs/CeO2 suggests a strong chemical binding of CeO2 with polysulfides, which is in good agreement with the DFT results. These results also indicate that PP- MWCNTs/CeO2 separator could improve the performance of the Li-S battery.
Figure 3. Optimized configuration of Li2S4 (left), Li2S6 (middle) and Li2S8 (right) on (A) MWCNT and (B) CeO2 (111) facet.
8
Since electrolyte wettability of the separator is closely related to its battery performance, the wettability of the separator toward both water and electrolyte is quantified. As shown in Fig. 4a, PP separator is highly hydrophobic. After modification with either MWCNTs or MWCNTs/CeO2, the separator’s water contact angle changes from ~112.6° to ~23°, and their electrolyte contact angle drops rapidly from 41.0° to 0°. The enhanced wettability of the modified separator is originated from the polar functional groups (such as hydroxyl groups and carboxyl groups) on MWCNTs and CeO2. A high electrolyte affinity is indicative of a high Li+ conductivity of the electrolyte-soaked separator. In order to demonstrate this point, the Li+ conductivity of the electrolyte-soaked separators was measured using EIS (Fig. 4b). The quantified ionic conductivity of PP, PP-MWCNTs and PP-MWCNTs/CeO2 was 0.85, 1.25 and 1.38 S cm-1, respectively. It should be pointed out that the apparent ionic conductivity measured is the sum of the conductivity of both the Li+ and anions in the electrolyte. To identify the conductivity from Li+, the lithium ion transfer number was characterized by a combined analysis of chronoamperometry and EIS (Fig. 4c). The lithium transfer number of pristine PP separator was 0.43. After modification of MWCNTs/CeO2, the lithium ion transfer number of the separator
9
increased to 0.52. The increase of the lithium transfer number of PP-MWCNTs/CeO2
could be possibly resulted from the Lewis acid-base interaction between the anions and Ce that trap the anions. The high ionic conductivity and improved lithium ion transfer number is expected to improve the electrochemical performance of the lithium-sulfur battery. To examine if the modification affects the electrochemical stability of the separator, LSV measurements were conducted (Fig. S7). The PP-MWCNTs and PP-MWCNTs/CeO2 exhibited an oxidative peak with an onset potential of ~3.1 V, which could be ascribed to the oxidation of MWCNT on the separator. Since the charge-discharge potential of Li-S battery is much lower than 3.1 V, the modified separator is stable enough to sustain a stable cycling of the battery.
Figure 4. (a) Water and electrolyte contact angles of PP, PP-MWCNTs and PP-MWCNTs/CeO2 separators. (b) EIS curves of stainless steel/electrolyte-soaked separator/stainless
steel
cells
assembled
with
PP,
PP-MWCNTs
or
PP-MWCNTs/CeO2 separators. (c) Chronoamperometry of Li/electrolyte-soaked separator/Li cells assembled with PP, PP-MWCNTs or PP-MWCNTs/CeO2 separators (insets: corresponding EIS curves of the cells before and after 10
polarization).
To characterize the charge-discharge performance of the separator, CR 2032 type coin cell was assembled and characterized. CV characterizations were first performed. All the batteries exhibited typical redox peaks of sulfur species: reduction peaks at ~ 2.2 V and 1.95 V assigned to reduction of elemental sulfur (S8) to soluble polysulfide and reduction of soluble polysulfide to insoluble Li2S and Li2S2, respectively; oxidation peak at ~2.5 V corresponding to oxidation of insoluble Li2S and Li2S2 to soluble lithium polysulfide and their further oxidation to sulfur (Fig. S8). The battery with PP-MWCNTs/CeO2 separator showed a slightly lower polarization compared to their counterparts with PP or MWCNTs separator, suggesting a better electrochemical reversibility
of
the
battery assembled
with
PP-MWCNTs/CeO2
separator.
Charge-discharge performance of the batteries were firstly evaluated at 0.2C (Fig. 5a). All the batteries showed a high first cycle discharge performance, with the 1st cycle discharge capacity being 717.7, 787.8 and 898.3 mAh g-1, for PP, PP-MWCNTs and PP-MWCNTs/CeO2, respectively. The battery with PP-MWCNTs/CeO2 separator retained a relatively stable discharge performance during the whole battery cycling. A high capacity of 520.7 mAh g-1 was retained after 300 charge-discharge cycles for PP-MWCNTs/CeO2. The discharge capacity at 300th cycle for battery assembled using PP and PP-MWCNTs separator was 345.0, 436.7 mAh g-1, respectively. The rate performance of batteries with pristine PP and modified PP separators were also investigated (Fig. 5b and Fig. S9). The battery with PP-MWCNTs/CeO2 separator has the highest discharge capacity at each discharge rate. In addition, when the discharge rate returns to 0.2C, a high discharge capacity of 649.3 mAh g-1 was retained. From the discharge performance of the batteries, it could be clearly seen that the coating of the separator with MWCNTs enhances the discharge performance of the batteries. The key to the improvement of the MWCNTs modification is its capability for the physically adsorption of soluble polysulfide and function of the secondary current collector. When MWCNTs/CeO2 was introduced onto the separator, the battery performance was further enhanced because of the improvement of the polysulfide anchoring performance as demonstrated above. The interaction of sulfur species with 11
the separator was also demonstrated with fine XPS analysis of the separator (on the cathode side) disassembled from the discharged batteries (Fig. 6a, b). Generally, the S 2p peaks of the two separators show multiple peaks corresponding to different chemical states of S species[29], of which are polysulfide (Li2Sx, ~163.0 eV), elemental sulfur (~164.4 eV), thiosulfate(~167.4 eV) and sulfate (~169.2 eV). Clearly, the polysulfide content was much lower on PP- MWCNTs/CeO2. This observation infers that the CeO2 containing modification layer facilitates the conversion of adsorbed polysulfides and thus improves the utilization of S species. The XPS characterizations with the separator disassembled from the a charged battery also confirms this point (Fig. 6c, d).
Figure 5 (a) Cycling and (b) rate performance of lithium sulfur battery assembled with PP, PP-MWCNTs or PP-MWCNTs/CeO2 separator. (c) EIS curves of fresh lithium sulfur battery assembled with PP, PP-MWCNTs or PP-MWCNTs/CeO2 separator. (d) EIS curves of cycled lithium sulfur battery assembled with PP, PP-MWCNTs or PP-MWCNTs/CeO2 separator.
To gain insight into the improved performance of the modified separator, EIS of the fresh battery and cycled battery were recorded (Fig. 5c). Before cycling, 12
PP-MWCNTs/CeO2 has a lowest charge transfer resistance (Rct=3.04 Ω). In contrast, the charge resistance of battery with PP, PP-MWCNTs separator was 73.52 Ω and 36.18 Ω, respectively. The reduction in Rct is due to that the separator modification layer enhances the affinity of the separator to the electrolyte and thus improves the transport of lithium ions at the electrode-electrolyte interface. After cycling, the Rct of all batteries reduced significantly due to the formation of SEI layers. The battery with PP-MWCNTs/CeO2 still showed the lowest Rct (10.61 Ω) value compared to other batteries. Besides, the battery also had a lowest RSEI (15.59 Ω). The above results show that PP-MWCNTs/CeO2 can improve lithium ion transport and alleviate the shuttle effect, which contribute to a conformal electrode-electrolyte interface and battery performance.
Figure 6. High resolution S 2p XPS spectra of the separator disassembled from a discharged lithium sulfur battery assembled with (a) PP or (b) PP-MWCNTs/CeO2 separator. High resolution S 2p XPS spectra of the separator disassembled from a charged lithium sulfur battery assembled with (c) PP or (d) PP-MWCNTs/CeO2 separator.
13
4.Conclusion We propose a new strategy of separator modification by using MWCNTs/CeO2 that provides functional synergies. The MWCNTs/CeO2 modified separator not only restricts polysulfide shuttling by the chemical and physical interactions but also improves the electrolyte affinity and charge transfer at the electrode-electrolyte interface. Due to the favorable features introduced by the synergistic modification, the lithium sulfur battery exhibits a more stable charge-discharge performance compared to the battery with pristine PP separator. The new separator developed in this study is promising for practical battery applications. In addition, our results may also inspire further exploration of the development of functional separators for advanced battery applications.
Acknowledgement: This work was supported by Nation Natural Science Foundation of China (Grant No. 51972254, 21706200), Fundamental Research Funds for the Central Universities (WUT: 2018IB026, 2019IB003), China Postdoctoral Science Foundation funded project (2018T110813), State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology, 2019-KF-10) and Fundamental Research Funds for the Central Universities for financial support (2019-HS-B1-15,
2019III048GX).
We
thank
the
Material
Research
and
Characterization Center at WUT for their assistance with the characterizations.
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A synergistic modification is used for separator modification in Li-S battery. The separator is modified with MWCNTs/CeO2. The modification enables improved barrier property and charge transfer. Li-S battery assembled with the separator shows excellent performance.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: