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Covalent Organic Frameworks as the Coating Layer of Ceramic Separator for High Efficiency Lithium-Sulfur Batteries Jianyi Wang, Liping Si, Qin Wei, Xu-Jia Hong, Songliang Cai, and Yuepeng Cai ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00057 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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ACS Applied Nano Materials

Covalent Organic Frameworks as the Coating Layer of Ceramic Separator for High Efficiency Lithium-Sulfur Batteries Jianyi Wang, Liping Si*, Qin Wei, Xujia Hong, Songliang Cai, Yuepeng Cai* School of Chemistry and Environment, South China Normal University, 510006, P.R. China Corresponding Authors: Liping Si (*E-mail: [email protected]) Yuepeng Cai (*E-mail: [email protected]) KEYWORDS: nanoporous; covalent organic frameworks; lithium-sulfur batteries; separator；high electrochemical performance；

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Abstract Covalent organic frameworks (COFs) have been proven to be an efficient host material for trapping sulfur in lithium-sulfur batteries. However, the potential application as the coating layer of the separator has not been well addressed yet. Here, we synthesized an imine-based COF, DMTA-COF, which exhibited an AB-stacking mode and had a pore size of 0.56 nm. For the first time, we applied this nanoporous COF as the coating layer of the ceramic separator and the corresponding cell gave an initial discharge capacity up to 1415mAh/g and 1000mAh/g remained after 100 cycles at 0.5 C. The performance is much better than that of the pristine ceramic separator and the super-P coated ceramic separator, demonstrating that the nanopores in the composite separator can effectively block the polysulfide across the separator, thus reducing the “shuttle” effect and the loss of active materials. This study provides a new design strategy for separators in lithium-sulfur batteries.

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Introduction With the rapid development of consumer electronics and electric vehicles, the current energy density of electrochemical power source cannot meet the demand. Exploration and development new energy system with high energy density, low cost and environmental friendly has been becoming the research hotspot1. Because of its high theoretical energy density, abundance in nature and being environment-friendly, lithium-sulfur batteries have been regarded as the most promising candidate for the next generation energy storage system2-4. However, the lithium-sulfur battery is impeded by several problems5-7, such as: low sulfur utilization, fast capacity fade, and short cycle life. These problems mainly arise from the insulating nature of the S8 and Li2S, ready dissolution of the intermediate of polysulfide, severe consumption of electrolytes, and corrosion of the lithium-metal during cycling. To suppress the polysulfide dissolution, the main strategy is to confine sulfur in the mesoporous materials8, such as: porous carbon9-11, conduct polymers12-14, metal-organic frameworks15-19, porous aromatic polymers20, etc. All these porous materials can prevent polysulfide dissolution to some extent. However, to realize regular and suitable porous structures, appropriate templates or multistep synthesis procedures are required. Covalent organic frameworks (COFs)21 are a class of crystalline porous materials, connected by covalent bonds and possessing ordered pore size and controllable structure22-23.COFs have high potential for applications in gas storage and separation24-26, photoelectric27-29, catalysis30-32, and other fields. Due to the regular 3



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nanoporous structure, the channels of the framework can adsorb polysulfide and reduce the dissolution erosion process. In addition, many COFs with different structures and pore sizes could be the host for sulfur loading in lithium-sulfur batteries. In literature, COF based hosts can improve the cycle performance of lithium-sulfur batteries33-36. It was reported that the substituent at theterephthalaldehyde had a great influence on

the

topology

of

COFs

derived

from

4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetraaniline(ETTA)

the and

condensation

of

terephthalaldehyde37.

Intrigued by their results, we examined the topology of the condensation result of methoxy

substituted

terephthalaldehyde

and

the

ETTA

monomer.

The

adsorption-desorption, PXRD results suggested that the 2, 5-Dimethoxy-1, 4-Dicarboxaldehyde

(DMTA)-4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)

tetraaniline-COF

(DMTA-COF) has a single pore at 0.56 nm with AB staggered mode. We further applied the prepared COF as the host material and the coating layer of the commercially ceramic separator for lithium-sulfur batteries (the ceramic separator is more safe than polymer separator). The experiment results showed that when the DMTA-COF was implemented only as the host material, the cycling stability and the capacity were not satisfied, but when used together with the DMTA-COF modified separator (DMTA-COF/ceramic separator), the capacity and cycling stability were significantly improved. The cells constructed by the DMTA-COF-sulfur cathode and DMTA-COF/ceramic separator with 0.6 mg/cm2 sulfur loading gave an initial discharge specific capacity up to 1415mAh/g and 1000mAh/g remained after 100 4


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cycles, corresponding to only 0.24% of capacity decay per cycle under 0.5 C current density. Results and discussion Figure 1a shows the synthesis route of the DMTA-COF. The DMTA-COF is a two-dimension framework connected by imine-linkage, and prepared by the Schiff-base reaction of the DMTA and ETTA monomers. It is well known that Schiff-based COFs usually exhibit high thermal and chemical stability38-40, which makes it possible to apply this material in Li-S batteries. The as-prepared DMTA-COF was first characterized by Fourier-transform infrared (FT-IR).As shown in Figure S1 (supplementary Information), a new peak at 1616.75 cm-1 appeared in DMTA-COF corresponding to the C=N stretching vibration. The solid state

13

C CP-MASNMR spectrum (Figure S2, Supporting Information),

showed a resonance peak at 153.93 ppm, further confirming the presence of C=N bonds. The porosity measurement of DMTA-COF was carried out through nitrogen adsorption-desorption measurement under 77K, as shown in Figures 1b and 1c. The Brunauer-Emmett-Teller (BET) surface area of DMTA-COF was 305 m2/g and the pore size was 0.56 nm from the density function theory (DFT) calculation. The pore size of this COF is smaller than that of the polysulfides (Li2Sn, 4 ≤ n ≤ 8). Thus，we predict that the DMTA-COF modified separator can effectively block the polysulfide from migrate across the separator. The powder X-ray diffraction (PXRD) pattern of DMTA-COF displayed 5



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characteristic patterns with an intense peak at 2θ=10°(Figure2), which differed from the two starting materials (Figure S3, Supporting Information), indicating the crystalline COF had been successfully synthesized. In order to determine the exact periodical structure of the as-prepared COF, the simulation of PXRD was carried out by using Reflex in Accelrys Material Studio 8.0 software package to predict which kind of unit cell the COF would generate (see details in Figure S4). In the simulation, the eclipsed structure (AA-stacking) and staggered structure (AB-stacking) were constructed with unit cell parameters of a=28.6698, b=28.5347, c=3.4935 and a=28.7363, b=28.463, c=6.8667, respectively. After the geometry optimization by semiempirical calculations at PM3 level, the PXRD pattern generated from the AA and AB stacking were compared with the experimental data. The results showed this DMTA-COF preferred AB-stacking mode. The thermogravimetric analysis (TGA) result showed that the DMTA-COF was stable up to 450 ℃ and the sulfur loading was about 50 wt% (Figure S5, Supporting Information) for the S@DMTA-COF composite prepared by the melt diffusion method. The XPS spectra showed that there was no obvious chemical shift of the S2p spectra in the DMTA-COF/S composite (164.0 and 165.2 eV) and sublimed sulfur (164.0 and 165.2 eV), indicating there is no chemical interaction between the DMTA-COF and sublimed sulfur (Figure S6, Supporting Information). Elemental mapping within the DMTA-COF/S are shown in supplementary Figure S7. The sulfur mapping clearly demonstrates that sulfur is uniformly distributed into the DMTA-COF channel. Figure S8 (Supporting Information) shows the PXRD patterns 6


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before and after sulfur loading. It shows that the intensity of the sulfur element is very strong after embody sulfur, suggesting that the DMTA-COF as the host might not effectively enhance the performance of the lithium-sulfur battery. Schematic of the preparation process to produce DMTA-COF/ceramic separator Shows in Figure S12 (Supplementary Information).The scanning electron microscope (SEM) images of DMTA-COF and the DMTA-COF/ceramic separator are shown in Figure 2b-d. Pure DMTA-COF is uniformed nano-circular particle. A rough surface was observed at lower magnification, after coating super-P and DMTA-COF mixed slurry on the separator surface. When the increase in magnitude, we can see that super-P and DMTA-COF formed a porous layer, which could serve as the trap site for the dissolved polysulfide. Further increasing the magnification, we found that the conductive additive super-P was uniformly distributed on DMTA-COF surface, which could increase the conductivity of the DMTA-COF/ceramic separator. On the contrary, the untreated ceramic surface was smoother than the coated side at the same magnitude, and a higher resolution image couldn't be obtained because increasing the magnitude would burn the pristine separator surface. The electrochemical performances of the as-prepared DMTA-COF/sulfur composite were evaluated as a cathode for lithium-sulfur batteries in coin cells at a sulfur load of 0.6 and 1.5 mg/cm2. All of the capacity values of the cells were calculated on the basis of the sulfur mass. To eliminate the contribution of super-P to the cell's performance, cells based on the super-P modified separator were also assembled and tested in the same condition. The photographic pictures of the pristine 7



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separator, super-P coated separator and DMTA-COF coated separator are shown in Figure S9 (Supplementary Information).Figure 3a shows the cycling performance of lithium-sulfur battery based on the above three separators at 0.5C.The pristine ceramic separator based cells delivered a low initial capacity of 964mAh/g and exhibited a poor cycling stability with the discharge capacity decreased to 452mAh/g after 50 cycles. This specific capacity was from the sulfur contribution as the pure COF has negligible values (Figure S10, Supplementary Information).While for the cells based on the super-P coated ceramic separator the initial discharge was 1093 mAh/g, and after 50 cycles, 780 mAh/g remained. The capacity and cycling stability both increased after coating super-P on the pristine ceramic separator. Meanwhile, the addition of super-P can improve the coating transmission Li+ and reduce the battery resistance, improving the discharge capacity under different rate41. However, for the cells based on the DMTA-COF/ceramic separator, the capacity and cycling stability were improved more dramatically. The initial discharge capacity was increased to 1415 mAh/g and at the 2nd cycle this value decreased to 1252 mAh/g, and 1030 mAh/g after 50 cycles at 0.5 C. The capacity decay was 0.34% from the first cycle to the 100 cycles. The results suggested that DMTA-COF serving as the host material alone couldn't effectively anchor the sulfur and polysulfide from dissolute into the electrolyte. The better performance of DMTA-COF modified separator than that of the Super-P could be due to DMTA-COF having a regular pore size able to mitigate polysulfide across the separator to reduce the “shuttle” effect and active material loss and achieve a higher capacity and increase cycle life. The EIS plots and the equivalent 8


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circuit are shown in Figure S14. The elements in the equivalent circuit include ohmic resistance of the electrolyte and cell components (Rs) and charge-transfer resistance (Rct) which represents the kinetics of electrode reaction42. It can be seen that the value of Rct for the cell with a DMTA-COF/ceramic separator (100 Ohm) is an order of magnitude lower than that with a pristine ceramic separator (350 Ohm), revealing a dramatic decrease in the cathode resistance. This could be another reason for the excellent

electrochemical

performance

of

the

cells

constructed

with

the

of

the

DMTA-COF/ceramic separator. The

first

cycle

galvanostatic

charge–discharge

profiles

DMTA-COF/ceramic separator at a current density of 0.5C in the voltage range of 1.8-2.8V is given in Figure 3b.All the three types of cells showed typical two-stage discharge and charge profiles, which is also demonstrated on the cyclic voltammetry (CV) curve (Figure S11, Supporting Information). However, the overpotentials between charge and discharge in the DMTA-COF system were the smallest among the three cells. This might be due to the DMTA-COF coating layer on the separator providing a reaction surface, thereby reducing the transmission distance of polysulfide to the cathode. The rate performances of the three types of cells are shown in Figures 3c and d. The 0.5C galvanostatic charge/discharge profiles shown in Figures 3a and 3c are not in full agreement due to the load difference of the cathode. Cells assembled by the DMTA-COF coated ceramic separator gave the best performance at every current rate (925 mAh/g at 0.5 C, 859 mAg/h at 1 C, 710 mAh/g at 2 C). Even when the current 9



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density was increased to 5 C, the battery still delivered an impressive capacity of 455 mAg/h. Once the current density returned to 0.5 C, a capacity of 970 mAh/g was obtained, and after 60 cycles, a high capacity of 890 mAh/g remained. Furthermore, we increased the sulfur loading from 0.6 mg/cm2to 1.5 mg/cm2in the cathode by coating it with a thick layer of the DMTA-COF/sulfur composite slurry. The cell based on the DMTA-COF modified ceramic separator delivered a high initial discharge capacity of 1300 mAh/g at 0.5 C, with 797 mAh/g remaining after 100 cycles (Figure 3e). When the current density was increased to 2 C, the battery delivered a reversible capacity of 1015 mAh/g, and a relatively large polarization was observed at long-cycle, due to the sluggish reaction kinetics of electrodes with high sulfur loadings at high current densities43. However, the specific capacity remained as high as 457 mAh/g after 500 cycles (Figure 3f). The corresponding capacity decay rate is 0.11% per cycle. Significantly, the battery with the DMTA-COF/ceramic separator shows a remarkably high Coulombic efficiency (>99.5%), indicating the DMTA-COF/ceramic separator’s efficient blocking of polysulfide in high sulfur load and high current. To illustrate the nanopores advantages of DMTA-COF, we prepared a separator coated with TAPB-PDA-COF, which has pore size of 2.52 nm44. The cell based on the TAPB-PDA-COF separator delivered a high initial capacity of 1065 mA h g−1 at 0.5 C. However, the capacity reduced to only 525 mA h g−1 after 100 cycles (Figure S16, Supporting Information), The capacity and cycling stability is much lower than that of DMTA-COF based battery. The results suggest that the nanopores in the DMTA-COF 10


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have a special advantage in trapping the polysulfides. Which is ascribed to the pores in the TAPB-PDA-COF being too large to effectively hold polysulfides inside. In order to more intuitively prove membrane coating can effectively inhibit polysulfide shutting, we examined the separator surface at the anode side by disassembling the cells after an electrochemical test of 100 cycles at 2C. As shown in Figure S13(Supplementary Information), the DMTA-COF coated surface was still white, while the super-P coated separator and pristine ceramic separator were already showing a yellow color, indicating sulfur. And the diffusion measurement, as shown in Figure S15，also suggest that the DMTA-COF/ceramic separator can inhibition polysulfide shuttle to some extent. The coating side of the separator after cycling for 100 and 300 cycles at 2 C was also evaluated to see whether the coating layer was destroyed during the charge and discharge process. From Figures 4a and b, we can see that the coating layer was still present, however there was a viscous paste deposited on top of the porous layer, which is the amorphous sulfur layer, as confirmed by the EDX mapping (Figure 4c). The sulfur layer was reduced from the dissolved polysulfide in the electrolyte, and become thicker as the cycle number increased. The phenomenon suggests that the coating layer not only restricts the transmission of polysulfide but also serves as a reservoir for sulfur which could be reused during the next discharge process. The PXRD

measurements

of

the

DMTA-COF

powder

scraped

from

the

DMTA-COF/ceramic separators (washed by alcohol) after 100 and 300 cycles at 2 C were performed to check whether the crystalline structure was destroyed during the 11



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test. Figure 4d shows that there is a strong exclusive peak at 10° for both samples, which is the same as that of the pure COFs spectrum, indicating the framework remained intact. In addition, the crystalline sulfur peaks in the pattern were not observed, further proving that the sulfur reduced from the dissolved polysulfide in the electrolyte was amorphous. The SEM, PXRD and EDX results all suggest that the DMTA-COF coated separator has high stability. Conclusion In summary, we synthesized a two-dimensional DMTA-COF, which had a pore size of 0.56 nm with an AB-stacking mode. Solid-state NMR, FT-IR, PXRD were used to characterize the obtained COF. When applying this COF as the host material for encapsulating sulfur, it could not effectively enhance the cycling performance of the lithium-sulfur battery. However, when the DMTA-COF immobilized sulfur cathode was applied in conjunction with a ceramic separator coated by a slurry of DMTA-COF, super-P and LA132, using the doctor-blade technique, the cell performance dramatically improved. This delivered an initial capacity of 1415 mAh/g and an impressive capacity of 1000 mAh/g after 100 cycles, corresponding to only 0.24% of capacity decay per cycle (at a current density of 0.5 C). In addition, the Coulombic efficiency remained more than 99% during the 100 cycles. Even when increasingthe current density to 2 C with high sulfur loading of 1.5 mg/cm2, cells still showed good electrochemical performance, with a reversible capacity of 1015 mAh/g, and after 500 cycles,457 mAh/g still remained. The experimental results demonstrate that coating COFs onto the ceramic separator is an effective way to suppress the 12


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shuttling effect and improve the specific capacity and cycling stability. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acsapplied-nano.xxxxxxx. Additional synthesis methods, measurement conditions, FT-IR spectra, Solid-state 13C NMR spectrum, simulated PXRD patterns, TGA curves, XPS S2p spectra, SEM image. Acknowledgements The authors are grateful for financial aid from the National Natural Science Foundation

of

Guangdong

Province

(Grant

No.

2016A030310435

and

2014A030311001), the National Natural Science Foundation of P. R. China (Grant No.21471061 and 21671071), Youth Scholars Foundation of South China Normal University (Grant No. 15KJ01), Applied Science and Technology Planning Project of Guangdong Province (No. 2015B010135009), Innovation team project of Guangdong Ordinary University (No. 2015KCXTD005), the great scientific research project of Guangdong

Ordinary

University

(No.

2016KZDXM023).Scientific

Research

Foundation of Graduate School of South China Normal University (Grant No. 2016lkxm06) References (1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M., Li-O2 and Li-S Batteries with High Energy Storage. Nat Mater 2012, 11,19-29. (2) Evers, S.; Nazar, L. F., New Approaches for High Energy Density Lithium–Sulfur 13



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Figure captions

Figure 1.(a) Schematic representation of the synthesis of the DMTA-COF. (b) N2 absorption/desorption isotherms of the DMTA-COF and DMTA-COF/S composite and pore size distribution curve of the DMTA-COF.(c) Pore size distribution curve of the DMTA-COF.

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Figure 2. Characterization of DMTA-COF/ceramic separator. (a) PXRD patterns of DMTA-COF/ceramic separator and DMTA-COF. The DMTA-COF structure still remains after coating. (b) SEM image of DMTA-COF. It shows that DMTA-COF with rules of the sample morphology and particle uniformity (2 µm). (c) SEM image of DMTA-COF/ceramic separator cathode-facing side shows that DMTA-COF and super-P distribution uniformity (50 µm). (d) SEM image of DMTA-COF/ceramic separator anode facing side (50 µm).(e) SEM images of DMTA-COF/ceramic separator cathode-facing side (2 µm). (f) SEM images of DMTA-COF/ceramic separator cathode-facing side (500 nm).

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Figure 3. Electrochemical performance of different ceramic separator. (a) Cycling performance of the DMTA-COF/ceramic separator, super-P/ceramic separator and pristine ceramic separator at 0.5 C (0.6 mg/cm2 sulfur-loaded). (b) Galvanostatic charge–discharge profiles of the DMTA-COF/ceramic separator, super-P/ceramic separator and pristine ceramic separator at 0.5 C. (c) discharge capacity for the DMTA-COF/ceramic separator at different rates (0.5 C, 1 C, 2 C, 5 C and 0.5 C). (d) Column diagram of DMTA-COF/ceramic separator, super-P/ceramic separator and pristine ceramic separator at various rates. (e) Cycling performance of the 1.5 22


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mg/cm2sulfur-loaded cathode with DMTA-COF/ceramic separator at 0.5 C. (f) Cycling

performance

of

the

1.5

mg/cm2

sulfur-loaded

cathode

with

DMTA-COF/ceramic separator at 2 C.

Figure 4. Characterization of DMTA-COF/ceramic separator after long cycles. SEM image of DMTA-COF/ceramic separator coated surface 100 cycles (a) and 300 cycles (b). (c) DMTA-COF/ceramic separator after cycled (insert is EDX mapping of sulfur). (d) PXRD patterns of DMTA-COF/ceramic separator after 100 cycles and 300 cycles and DMTA-COF.

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Covalent Organic Frameworks as the Coating ... - ACS Publications

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