Modified Separator Using Thin Carbon Layer Obtained from Its

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Modified Separator Using Thin Carbon Layer Obtained from Its Cathode for Advanced Lithium Sulfur Batteries Naiqiang Liu, Bicheng Huang, Weikun Wang, Hongyuan Shao, Chengming Li, Hao Zhang, Anbang Wang, Keguo Yuan, and Yaqin Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04418 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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ACS Applied Materials & Interfaces

Modified Separator Using Thin Carbon Layer Obtained from Its Cathode for Advanced Lithium Sulfur Batteries Naiqiang Liu,1 Bicheng Huang,1 Weikun Wang,2 Hongyuan Shao,1 Chengming Li,1 Hao Zhang,2 Anbang Wang,2 Keguo Yuan2 and Yaqin Huang*1 1

State Key Laboratory of Chemical Resource Engineering, The Key Laboratory of Beijing City

on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Beijing, 100029, China. 2

Military Power Sources Research and Development Center, Research Institute of Chemical

Defense, 35 Huayuan North Road, Beijing 100191, China KEYWORDS: modified separator; cathode; gelatin; carbon layer; lithium sulfur batteries

ABSTRACT：：The realization of a practical lithium sulfur battery system, despite its high theoretical specific capacity, is severely limited by fast capacity decay, which is mainly attributed to polysulfide dissolution and shuttle effect. To address this issue, we designed a thin cathode inactive material interlayer modified separator to block polysulfides. There are two advantages for this strategy. Firstly, the coating material totally comes from the cathode, thus

1 Environment ACS Paragon Plus


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avoids the additional weights involved. Secondly, the cathode inactive material modified separator improve the reversible capacity and cycle performance by combining gelatin to chemically bond polysulfides and the carbon layer to physically block polysulfides. The research results confirm that with the cathode inactive material modified separator, the batteries retain a reversible capacity of 644 mAh g-1 after 150 cycles, showing a low capacity decay of about 0.11% per circle at the rate of 0.5 C.

Introduction: The lithium sulfur (Li-S) batteries have become one of the most promising alternatives to satisfy the demand for high-energy density rechargeable battery due to advantages of high theoretical energy density of 2600 wh kg-1, natural abundance of sulfur, and environment friendly.1 Despite these advantages, one of the biggest challenges for Li-S batteries are the shuttle of dissolved lithium polysulfide intermediates between the cathode and anode, which results in poor cyclability, short cycle life and low coulombic efficiency.

2-3

During the discharge/charge

process, reaction intermediates of long chain polysulfides dissolved into the electrolyte, and then migrated to the Li anode where they react with Li in a parasitic fashion to generate shorter chain polysulfides. Subsequently, the shorter chain polysulfides diffused back to the sulfur cathode and re-formed the long chain polysulfides. This process takes place repeatedly and creates an internal shuttling mechanism during the electrochemical reaction.4-5 So far, a variety of approaches to address this issue in Li-S batteries have been reported.6-8 Employing conductive carbon interlayer modified separator to restrain the dissolution and shuttling of polysulfides during electrochemical reaction has been regarded as a promising way to improve the cycle life of lithium sulfur battery.9-11 Usually, the carbon interlayer modified separator works as the physical


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barriers to prevent sulfur species migrating to the negative electrode.12 At the same time, the carbon interlayer modified separator can function as absorption agent for localizing the electrolyte containing the dissolved polysulfide within the cathode region. Thus, various carbon materials,13 such as super P,14 nanofibers,15,16 graphene,17 porous carbon,18,19 have been reported to prepare modified separator and showed efficiently suppress the shuttle of polysulfides and decrease the loss of active material. However, the additional carbon interlayer introduced to modify the separator will bring an unavoidable weight to the Li-S batteries and lead to a more or less decrease of the energy density. The weight in the batteries, which is directly related to the energy density of the Li-S batteries, has become the sensitive parameter for commercial applications of high energy battery.20, 21 In order to decrease the influence of the additional materials, some work focus on the development of thin and lightweight carbon layer has been reported.22, 23

To overcome the added weight from the additional components by utilizing common functional interlayers, we present here the feasibility and practicality of thin carbon layer modified separator by using the inactive ingredient from the cathode (scheme 1). This strategy can offer rational distribution of the electrode material and optimize the structure of the cathode. The carbon (AB) from cathode can physically trap the migration of polysulfides and realize the reutilization of dissolved polysulfides. The use of gelatin binder aims to improve the capability of the carbon layer to trap polysulfides and provide adhesive force to bind the carbon material and separator.24 In addition, this designed thin cathode inactive material layer modified separator exhibits four advantages to improve the electrochemical performance of Li-S batteries: ⅰ) The thin carbon layer can block the polysulfides and further reutilize the dissolved active material; ⅱ) The modified material that totally isolated from the cathode could avoid the additional



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weight involved; ⅲ) The water soluble gelatin that can improve the surface hydrophilicity of the separator that will be beneficial in the infiltration of electrolyte; iv) The gelatin binder containing –COOH and –NH3 that can chemically bond with polysulfides and the carbon layer could work as physical barrier. Therefore, this cathode inactive material modified separator effectively anchored polysuldies and enhanced cycle performance and reversible capacity. The experimental results confirmed that batteries assembled with the as-prepared thin cathode inactive material layer modified separator delivered improved reversible capacity of 660 mAh g-1 at 0.2C after 100 cycles and low the capacity decay (0.11% per cycle).

Scheme 1. Schematic illustration of thin carbon layer modified separator obtained from its cathode

Experimental Preparation of cathodes and modified separator


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Elemental sulfur (99.5%, analytically grade, Beijing Yili. Corp., China), acetylene black (AB, Jinpu. Corp., China) and gelatin (160 Bloom g, type B, derived from bovine bones) at the ratio of 58:34:8 were ball-milled. Then, the well mixed slurry was coated onto an Al foil by using a doctor blade to prepare the pristine sulfur cathode. To prepare the modified separator using carbon layer obtained from its cathode (IM modified separator), about 8wt. % of AB and gelatin in the cathode at the ratio of 80:20 were taken out and coated onto one side of the pristine Celgard PP separator (2400). The rest of sulfur, AB and gelatin was mixed and coated onto the Al foil to prepare sulfur cathode with a ratio of 63:30:7. All these sulfur cathodes and separators were dried under vacuum at 60 °C for 24 h, and then cut into circular disks before used. The average sulfur loading in the cathodes is about 2.0 mg cm-2, and the average loading of the cathode inactive material on the separator is about 0.3 mg cm-2.

Electrochemical and structural analysis These sulfur cathodes were assembled into CR2025 coin-type test cells in an argon-filled glove box with lithium as the anode (Aldrich). The electrolyte contains 1 M bis-trifluoromethanesulfonylimide (LiTFSI) salt (Acros Organics) and 0.4 M LiNO3 co-salt (Acros Organics) in a 1:1 volume ratio of 1, 2-dimethoxyethane (DME, Acros Organics) and 1,3-dioxolane (DOL, Acros Organics). All cells were activated at 0.05C before the discharge/charge tests at current densities of 0.2C. The discharge/charge test was performed by Ct2001At battery test with instrument (LAND Electronic Co.) in the voltage range of 1.7–2.8V. Electrochemical impedance spectroscopy (EIS) measurements were performed on CHI660C electrochemical workstation from 1 MHz to 100 mHz with an AC voltage amplitude of 5 mV. The morphologies were observed by using a scanning electron microscopy (SEM,



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HITACHIS-4700) operated at 20 kV. The porosimetry data of the pristine separator and IM modified separator was examined by using mercury intrusion method (AutoPore IV 9500 V1.09).

Results and Discussion

Characterization of the IM modified separator

Figure 1.(a) Photographs of the pristine separator and IM modified separator. (b) The surface SEM of the pristine separator before cycling. (c) The surface SEM of IM modified separator before cycling. (d) Cross-sectional SEM of IM modified separator. (e) The pore size distribution of the pristine separator and IM modified separator. (f) Contact angle of water on the surface of pristine separator and IM modified separator.


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The photographs (Figure 1a) show that the IM modified separator has been prepared after coating with inactive cathode material (parts of AB and gelatin), the color of the pristine separator changes from white to black, which means the surface of the separator has been covered by the inactive material（AB and gelatin） from the cathode. As shown in Figure 1b, the SEM images of pristine separator show a lot of nanopores (around 100 nm) on the smooth surface, which will provide route for Li+ and the polysulfide to migrate between the cathode and the anode. After coating with the cathode inactive material, a dense carbon layer with uniform distribution of carbon particles formed on the surface of the separator (Figure 1c). All the nanopores of the pristine separator were covered by the carbon particles with small particle size around 50 nm. From the stacking of the carbon layer, the IM modified separator is benefit to physically suppress the shuttle of polysulfide. The cross-sectional SEM shows that the cathode inactive material layer with a thickness about 2.5 um was stacked well on the surface of the PP separator. The mercury intrusion characterization has been adopted to investigate the pore structure change after the pristine separator was coated by the cathode inactive material (Figure 1e). The pristine separator shows a porosity of 73.10%, but after the cathode inactive material was coated, pororsity decreased to 0.02%, which indicated that the pristine separator has been filled with gelatin and AB particles. And the disappeared pore size distribution of the IM modified separator in Figure 1e has demonstrated the dense inactive material layer, which is more important to physically break the route of polysulfide diffuse to anode.25 At the same time, the redistributed carbon layer avoids the polysulfides immediately contact with the PP layer, which may lead to the polysulfides accommodate in the separator. The cathode inactive material layer was benefit to collect sulfur species and increased the utilization of the polysulifdes.18 In addition, gelatin as hydrophilic binder in the cathode was also coated onto the surface of the



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pristine separator. As shown in Figure 1f, the surface hydrophilicity of separator increased after the thin carbon layer coating. The contact angle at the water/IM modified separator interface is 24°, a smaller surface tension than the water/pristine separator interface (90°), indicating that the thin carbon layer was more hydophilic than surface of the pristine separator. According to the compatibility principle, the presence of hydophilic gelatin in modified layer improves their surface wettability, which enhances polar interactions with the electrolyte solution.26 Due to polysulfides was affinitive with hydrophilic functional groups and the hydrophilicity of electrolyte, the IM modified separator will be beneficial in the infiltration of electrolyte and restriction of the polysulfides. 27

Electrochemical performance of Li-S cells with IM modified separator

Figure 2.(a) The initial galvanostatic discharge/charge voltage profiles of the pristine separator and IM modified separator cycled at 0.05C.(b) Cycling performance of the pristine separator and the IM modified separator at the rate of 0.2C.

The initial galvanostatic discharge/charge voltage profiles of the Li-S cells with pristine separator and IM modified separator were cycled at 0.05C (Figure 2a). The cell with IM


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modified separator exhibits initial discharge capacity of 1140 mAh g-1, which is higher than the discharge capacity (1073 mAh g-1) of the cell with pristine separator. The discharge profiles of the cell with IM modified separator and pristine separator show two plateaus at 2.2-2.4V and 2.0-2.1V, corresponding to two different reduction reactions: the upper plateau indicates the reduction reaction from sulfur to long-chain polysulfide; the lower discharge plateau represents the transformation of long-chain polysulfide to Li2S2/Li2S.28 The cell with IM modified separator exhibits higher voltage of discharge plateaus and lower charge plateaus than the cell with pristine separator, which may result from reduction of the cells polarization. It can also be seen from Figure 2a that the unclosed discharge/charge profiles of the cell with pristine separator demonstrate a significant loss of active material. In the case of the cell with IM modified separator, the gap between the discharge and charge capacity decreases from 152.7 mAh g-1 to 123.5 mAh g-1, which indicates thin carbon layer IM modified separator can suppress the shuttle of polysulfides and low the loss of the active material. The cycling performance of the cell with IM modified separator was evaluated by galvanostatic discharge/charge cycling at rate of 0.2C (Figure 2b). As shown in Figure 2b, the reversible capacity of the cell with IM modified separator is superior to that with the pristine separator. The cell with IM modified separator exhibited stable cycling performance with a high reversible capacity of 660 mAh g-1 after 100 cycles. The average coulombic efficiency over 100 cycles was about 95%, and the cells capacity retained 87% at the end of 100 cycles, which corresponds to a capacity decay of 0.11% per cycle. For comparison, the pristine cathode showed lower reversible capacity of 463 mAh g-1 after 100 cycles. These results indicate that the thin cathode inactive material layer enhance cyclability and improve the reversible capacity.



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Figure 3.Voltage profiles of the Li-S cells with (a) pristine separator and (b) IM modified separator at different discharge/charge rates. (c) Rate capabilities of Li-S cells with pristine separator and IM modified separator. (d) Long cycle performance of Li-S cells with IM modified separator and pristine separator at the rate of 0.5C. (e) Upper plateau discharge capacities of batteries with different separators at various cycles.

The cell with IM modified separator also has superior rate performance. As shown in Figure 3, the rate performances were also evaluated at various current densities from 0.05 C to 1C rate, for 5 cycles at each C rate. Compared to the pristine cathode, the discharge capacities of the cell with IM modified separator at each current density from 0.05C to 1C is superior to the cell with pristine separator. Figure 3a and b show the discharge/charge profiles of the cell with pristine separator and IM modified separator at the rate of 0.05C, 0.2C, 0.5C, 1C. The decreasing reversible capacity and discharge/charge plateau with higher rate can be explained by the fact that at high rate, the ionic motion within an electrode and across an electrode/electrolyte


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interface is not fast enough for charge distribution to reach equilibrium.29 The cell with pristine separator shows more severely capacity fading and polarization than the cell with IM modified separator does. Especially at the high rate of 1C, the lower discharge plateau of the cell with pristine separator almost disappeared and the cell delivered extremely low capacity close to 150 mAh g-1, which may be attributed to sluggish ionic diffusion kinetics of the cell with pristine separator.29 Through coating the inactive cathode material onto the separator, the cell with IM modified separator offers a large reaction interface between active particles inside and the electrolyte. It can be seen that the cell with IM modified separator delivered stabilized reversible capacity at various rates is 1026 mAh g-1 at 0.05C, 762 mAh g-1 at 0.2C, 702 at 0.5C, and 625 mAh g-1 at 1C (Figure 3c). Nonetheless, the capacity is recovered after reduced the rate of charge/discharge. Even after cycled at a high rate of 1C, the discharge capacity could recover to its original capacity (760 mAh g-1) when the current density was returned back to 0.2C. The result revealed that the structure of IM modified separator is stable even when the cathode was cycled at a high current density. Additionally, the cell also showed excellent cycling performance at the rate of 0.5C. The batteries assembled with IM modified separator shows improved capacities and coulombic efficiency at 0.5C. After 150 cycles, the batteries with pristine separator maintained the reversible capacity of 321 mAh g-1. The battery with IM modified separator maintained highest reversible capacity of 644 mAh g-1, the capacity retention is about 84% and capacity decay of 0.11% per-cycle. Besides, the coulombic efficiency was stable and above 95% after 150 cycles. Further upper plateau discharge capacity also has been studied. As shown in Figure 3e, batteries with IM modified separator improved the upper plateau discharge capacity. The comparison of upper plateau discharge capacities has also shown that the IM modified separator exhibit the stable and highest capacity of 246 mAh g-1 after 150 cycles. In



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order to investigate the role of gelatin in the IM modified separator, we have changed gelatin into the same amount of PVDF to prepare the modified separator as the control sample (Figure s1). Lower reversible capacity of 504 mAh g-1 and upper plateau discharge capacities after 150 cycles could be observed from the batteries with modified separator using PVDF binder. This comparison indicated that gelatin in the IM modified separator may have better capability to improve the re-utilization of sulfur species dissolved in the electrolyte.30 A gradual increase of the discharge capacity for the first 20 cycles can also be observed, indicating an electrochemical activation step in the cells, which is attributed to the electrolyte diffusion through the thin carbon layer, pores and space between the carbon particles and the distribution of the active material during the charge/discharge process.31


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Figure 4.(a) The surface SEM observation and elemental mapping of the IM modified separator after 50 cycles. (b) The surface SEM observation of the modified separator with PVDF as binder after 50 cycles. (c) FTIR of gelatin and the mixture of gelatin and Li2S8. (d) Nyquist plots, the equivalent circuit and table of fitted values (insert) for Li-S cells with pristine separator and IM modified separator before cycling.

To understand the improved electrochemical performance of cells with IM modified separator, SEM images of the IM modified separator after 50 cycles have been carried out. As shown in Figure 4a, sulfur are uniformly coated on carbon layer surface to form a thick layer without any visible aggregation of sulfur species, implying the thin cathode inactive material layer absorbed and captured migrating polysulfide. The elemental mapping further revealed the homogeneous distribution of sulfur, which confirmed the thin carbon layer could block the polysulfides. The surface SEM image of the modified separator using PVDF binder after 50 cycles showed non-uniform distribution of sulfur species. The result revealed that gelatin can promote uniform distribution of the sulfur species. In view of the reported polymer additives or coats, with functional groups including −NHx and −C=O functional groups all own strong lone-pair electrons, has intensive interaction with Li2S and long chain polysulfides.24 The previous research indicated that gelatin could provide additional adsorption ability to polysulfides.32 The FTIR spectroscope (Figure 4c) of gelatin and gelatin mixed with polysulfides (Li2S8) has been analyses. All spectra show major amide bands (A, B, I, II, and III) and variations in wavenumber.33 The free N-H stretching band (amide A) frequency of gelatin at 3422 cm-1 shift to a lower frequency of 3395 cm-1 after mixed with Li2S8, the N-H bend stretch coupled C-N (amide III) shift from 1237 to 1202 cm-1, and the C-O stretching band located at 1081 cm-1 shifted to 1059 cm-1. All these variations in wavenumber suggested that



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the gelatin could afford extra affinitive interactions with polysulfides.34,35 The S-S bending variation of the mixture (gelatin and Li2S8) appears at ~500 cm-1 and the absorption peak at ~600 cm-1 attributed to the C-S stretching, these results indicated that chemical bonds could be formed between gelatin and polysulfides.36 Therefore, it is reasonable to speculate that the presence of gelatin helps to chemically anchor the sulfur species, and it also explained the improved reversible capacities of batteries with IM (AB/gelatin) modified separators especially the enhanced upper plateau discharge capacities. Electrochemical impedance spectroscopy (EIS) was performed to understand the superior electrochemical performance of cells with IM modified separator. The Nyquist plots for Li-S cells with pristine separator and IM modified separator before discharge/charge were presented in Figure 4d. It can be seen that all cells show typical semicircles at medium frequencies and short inclined line in the low frequency region. The semicircle in the medium frequency region responds to the charge transfer resistance (Rct) related to the kinetic resistance of the electrochemical reaction at the electrode−electrolyte interface. In addition, an inclined line at low frequency reflects the Li ion diffusion into the active mass.37, 38 To understand the results better, the relevant equivalent circuit models were given. In this circuit, Re is the electrolyte resistance including electrode contact resistance. Rct is the resistance of the charge transfer resistance. CPE (constant phase element) denotes the capacitance of each component to reflect the depressed semicircular shape. It can be seen from the table 1, the cell with IM modified separator showed a much lower Rct of 218.1Ω than the cell with pristine separator of 314.1Ω, indicates that the existence of the thin cathode inactive material interlayer on the surface of the separator further decreases the charge-transfer resistance, resulting a high reversible capacity, which is consistent with the rate performance test. Because the thin carbon layer is beneficial in the infiltration of electrolyte, the thin carbon


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layer will act as barrier and more reactive surface will be offered for the reduction/oxidation of sulfur species, thus makes sure the fast the ionic motion within an electrode and across an electrode/electrolyte interface.39 Even with a higher sulfur loading of 5 mg cm-2, the battery with IM modified separator still exhibited a stable cyclability to 20 cycles and the columbic efficiency is above 97.8% (in Figure s2). After cycled at 0.05C for 20 cycles, the reversible capacity of 690 mAh g-1 was obtained. In this work, we explored feasible and practical of isolating part of the inactive ingredient from the cathode and using them as the carbon layer modified the separator to improve the electrochemical performance of Li-S batteries. But much more work are still needed to seek optimal cathode inactive material content for IM modified separator, especially when the cathode with high sulfur loading and content to solve the “two low,”40 low sulfur content and loading in the cathode, which were not suitable for a practical application of Li-S batteries.

Conclusions

In summary, we confirmed that it is feasible and practical for improving the electrochemical performance of Li-S batteries by IM modified separator coating part of the cathode inactive ingredient onto the separator. Regardless of additional material involved into Li-S batteries, the IM modified separator combined the chemical and physical effect to inhibit polysulfides, and the excellent rate capability and stable cycling performance have been obtained. During the discharge/charge process, the IM modified separator combine the chemical interaction of gelatin and physical barrier of AB blocks the transportation of the polysulfides, decreases the shuttle effect of polysulfides and the loss of active material. Moreover, the IM modified separator will not involve any addition material, which would decrease the energy density of Li-S batteries.



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The cells with IM modified separator delivered a reversible capacity of 660 mAh g-1 after 100 cycles, which is higher than that (460 mAh g-1) of the conventional Li-S batteries. AUTHOR INFORMATION Corresponding Author *Corresponding author: E-mail: [email protected] (Yaqin Huang). Supporting Information. Electrochemical performance of the Li-S cells with IM modified separator using PVDF binder has been studied, and the cycling stability of the Li-S cells with IM modified separator at high sulfur loadings has been investigated. ACKNOWLEDGMENT Financial support from the National Science Foundation of China (no. 51272017 and 51432003) is gratefully appreciated.

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Poursamar,

S.

A.;

Lehner,

A.

N.;

Azami,

M.;

Ebrahimi-Barough,

S.;

Samadikuchaksaraei, A.; Antunes, A. P. M. The Effects of Crosslinkers on Physical, Mechanical, and Cytotoxic Properties of Gelatin Sponge Prepared via In-situ Gas Foaming Method as a Tissue Engineering Scaffold. Mat. Sci. Eng., C 2016, 63, 1-9.



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A

Graphene-Pure-Sulfur

Sandwich

Structure

for

Ultrafast,

Long‐Life

Lithium-Sulfur Batteries. Adv. Mater. 2014, 26, 625-631. 40. Miao, L. X.; Wang, W. K.; Wang, A. B.; Yuan, K. G.; Yang, Y. S. A High Sulfur Content Composite with Core-Shell Structure as Cathode Material for Li-S Batteries. J. Mater. Chem. A 2013, 1, 11659-11664.


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TOC graphic


Modified Separator Using Thin Carbon Layer Obtained from Its

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