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Synergistic Ultra-thin Functional Polymer Coated Carbon Nanotube Interlayer for High performance Lithium Sulfur Batteries Joo Hyun Kim, Jihoon Seo, Junghyun Choi, Donghyeok Shin, Marcus Carter, Yeryung Jeon, Chengwei Wang, Liangbing Hu, and Ungyu Paik ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06190 • Publication Date (Web): 20 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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ACS Applied Materials & Interfaces

Synergistic Ultra-thin Functional Polymer Coated Carbon Nanotube Interlayer for High performance Lithium Sulfur Batteries Joo Hyun Kima, Jihoon Seoa, Junghyun Choia, Donghyeok Shina, Marcus Carterb, Yeryung Jeona, Chengwei Wangb, Liangbing Hub*, and Ungyu Paika*

a

WCD Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea

b

Department of Materials Science and Engineering, University of Maryland, College Park,

MD 20742-2115, USA

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Abstract Lithium-sulfur (Li-S) batteries have been intensively investigated as a next-generation rechargeable battery due to its high energy density of 2600 W∙h kg-1 and low cost. However, the systemic issues of Li-S batteries, such as the polysulfide shuttling effect and low Coulombic efficiency, hinder the practical use in commercial rechargeable batteries. The introduction of a conductive interlayer between the sulfur cathode and separator is a promising approach that has shown the dramatic improvements in Li-S batteries. The previous interlayer work mainly focused on the physical confinement of polysulfides within the cathode part, without considering the further entrapment of the dissolved polysulfides. Here, we designed an ultrathin poly(acrylic acid) coated single-walled carbon nanotube (PAA-SWNT) film as a synergic functional interlayer to address the issues mentioned above. The designed interlayer not only lowers the charge transfer resistance by the support of the upper current collector but also localizes the dissolved polysulfides within the cathode part by the aid of a physical blocking and chemical bonding. With the synergic combination of PAA and SWNT, the sulfur cathode with a PAA-SWNT interlayer maintained higher capacity retention over 200 cycles and achieved better rate retention than the sulfur cathode with a SWNT interlayer. The proposed approach of combining a functional polymer and conductive support material can provide an optimistic strategy to overcome the fundamental challenges underlying in Li-S batteries.

Keywords: Ultra-thin coating; Carbon nanotube interlayer; Lithium-Sulfur battery; Hydrogen bonding; Poly(acrylic acid) 2 ACS Paragon Plus Environment

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Introduction As the depletion of fossil fuels and natural resources are becoming critical socio-economic issue, the demand for secondary rechargeable battery with low cost, high energy density has dramatically increased.1-4 Currently, conventional Li-ion batteries based on the lithium insertion-compound anodes and cathodes have shown stable operation and high energy density, when compared to other rechargeable batteries. However, the inherent limitations of materials (e.g. LiCoO2), such as low energy density, hinder the practical applications aimed towards the large-scale smart grid and electric vehicle (EV). In this regard, lithium-sulfur (LiS) battery with high energy density (2600 W∙h kg-1) at a safe operating voltage (around 2.1 V vs. Li/Li+) is considered as a promising candidate for the next-generation of rechargeable batteries.5-7 Moreover, the sulfur has a great cost-benefit due to its natural abundance.8 Even though the Li-S battery can address many issues arising from the previous Li-ion battery, several critical hurdles still must be overcome for the practical utilization of the Li-S battery.3 These challenges are mainly associated with the conversion mechanism to sulfur during the reaction process. Lithiated sulfur species (Li2Sn2- , 4≤n≤8) can easily dissolve into the electrolyte, thus resulting in the polysulfides shuttling effect which is related to the loss of active material, lithium metal corrosion, and overcharging phenomenon.3 To address the challenges mentioned above, the localization of the polysulfide species within the cathode part is required. Many approaches have been explored in order to confine the active materials inside the sulfur cathode electrode by (a) designing the carbonsulfur nanocomposites,9-13 (b) encapsulating the sulfur cathodes with functional polymer such as PEDOT:PSS, and polyaniline, etc.,14-16 and (c) engraving the hydrophilic functional groups on the surface of carbon nanomaterial.17-18

These attempts have shown the dramatic

enhancement in electrochemical performances of Li-S batteries. However, each preparation 3 ACS Paragon Plus Environment


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step of the sulfur composites requires expensive and elaborate techniques, which hinders realistic applications in the commercial battery market.17, 19-20 In a different approach, instead of modifying the sulfur composites itself, a new Li-S battery configuration with a conductive interlayer was first suggested by Manthiram’s group.17,

19-20

The introduction of carbon

interlayer, such as mesoporous carbon,21 MWNT,22 and graphene,23 in-between separator and sulfur electrode can suppress the migration of polysulfides to Li-metal and enhance the electrical conductivity of sulfur cathodes. These approaches have the potential to overcome the weaknesses of Li-S batteries with a simple approach. However, the previous interlayer work mainly focused on the physical confinement of mobile polysulfides within the cathode, without considering further entrapment of the dissolved polysulfides which diffuse across the interlayer. A few approaches, such as elements doping (nitrogen, sulfur, etc.) or metal oxide decoration (RuO2, MnO2, etc.), have investigated chemical bonding within the interlayer to reutilize the polysulfides for further improvement of cycle performance in Li-S batteries.24-35 Herein, we first design a ultrathin poly(acylic acid)-coated single walled carbon nanotube (PAA-SWNT) film as a synergic functional interlayer for efficiently suppressing the diffusion of dissolved polysulfides by a hydrogen-bonding interaction and a physical blocking. The freestanding SWNT film has been known to be an ideal carbon scaffold for its high conductivity and flexibility, which allows it to work as a sulfur reservoir and dual current collector when inserted in-between a sulfur cathode and separator.22 PAA possesses carboxyl groups throughout the chain, which was shown to be effective in suppressing the polysulfide shuttling effect by forming hydrogen bonds.36 Comparisons with the sulfur cathode with a pristine SWNT interlayer, the sulfur cathode with a PAA-SWNT interlayer showed a higher specific capacity and a better cycle retention, even at a high current density.


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Experimental Section Preparation of freestanding SWNT film First we put 0.5 g of sodium dodecyl sulphate (SDS) into 49.5 g of DI water to prepare the surfactant solution. Then 3 mg of SWNTs were added into the surfactant solution, followed by tip sonication (200 W, 40 min) to prepare a homogeneous solution. The as-prepared SWNT solution was filtered through a mixed cellulose acetate (MCE) membrance via vacuum filtration system, followed by washing with enough DI water to eliminate the remaining SDS. Then, the resulting product was dried in dry room for 1 day to obtain a SWNT film from the membrane filter. To completely remove the remaining SDS, SWNT film was dipped into nitric acid for 30 min.

Synthesis of PAA-SWNT film For the modification of the SWNT film with PAA, a freestanding SWNT film was treated with ultraviolet ozone (UVO) for 20 min, which can increase the wettability of carbon substrate with inherent hydrophobic properties.37-39 The resulting SWNT film was dipped into the PAA solution (10 %) and washed with DI water. The PAA-SWNT film was dried in a vacuum oven at 50 oC for overnight. The thickness of both SWNT interlayer and PAASWNT interlayer corresponded to ~7 µm. The areal loading weights of a SWNT and a PAASWNT were 0.75 mg cm-2 and 0.82 mg cm-2, respectively.

Fabrication of sulfur cathode The sulfur cathode was prepared by mixing sublimed sulfur (65 %), super P (25 %), and PVDF binder (10 %) to make slurry. To satisfy the recent requirements for the high sulfurloaded electrode, we casted the slurry onto the Al foil with a large thickness using a doctor blade. The loading areal density of sulfur was matched to 2.7 mg cm-2 and the thickness of 5 ACS Paragon Plus Environment


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sulfur cathode is around 91 µm. The 1 M lithium bis(trifluoromethanesulphonyl)imide (LiTFSI)-1,3-dioxolane (DIOX):1,2-dimethoxyethane (DME) (v:v=1:1) with lithium nitrate (LiNO3) (1 wt%) was used as an electrolyte. Same amount (~80 µl) of electrolyte was applied in each cell. Each cell was evaluated within the potential window of 1.5-3.0 V. During the fabrication process, the introduced interlayer should cover the whole area of sulfur cathode electrode in order to prevent the polysulfides leak.

Characterization The morphologies of SWNT and PAA-SWNT were analyzed by a field emission scanning electron microscope (JSM 4700F) and by a transmission electron microscopy (TitanTM 80300). FTIR spectra (FT-IR-4200) were recorded in the range 2500-1000 cm−1 with a FT-IR Spectrometer by the KBr pellet method. The percentage of PAA loading weight in PAASWNT film was measured by TGA (TA Instruments) in the temperature range of 80 to 800 oC at a ramping rate of 10 oC min-1 in air. The EIS of sulfur cathode, sulfur cathode / SWNT interlayer, and sulfur cathode / PAA-SWNT interlayer were evaluated by potentiostat (Autolab PGSTAT 302N) in the frequency range of 250 kHz to 100 mHz with an excitation amplitude of 10 mV. The Raman spectrum (NRS-3100) was measured in the range of 2000100 cm-1.

Results and Discussion Figure 1 illustrates the role of PAA-SWNT interface layer as an effective blocking agent, preventing dissolved polysulfides from traveling to the lithium metal during the discharge. During the discharge process, reduced polysulfides diffused out of sulfur cathode and started to migrate toward lithium metal due to the concentration gradient of polysulfies.40 The main advantages of utilizing a PAA-SWNT interlayer in Li-S batteries are two-fold. The 6 ACS Paragon Plus Environment

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PAA-SWNT interlayer can substantially inhibit the migration of dissolved polysulfides to the lithium metal by acting as a physical barrier. The interlayer chemically anchors the nonfiltered polysulfides onto the PAA graft on the SWNT by forming hydrogen bonds. These two contributions of PAA as a functional polymer and SWNT as a blocking layer effectively mitigated the diffusion of dissolved polysulfides species toward the lithium anode with a physical and chemical approach, which contributed to the improved electrochemical performances of Li-S batteries.

Sulfur Cathode

PAA-SWNT Interlayer

Separator

Lithium Anode

Lithium ion Polysulfide Chemically captured Polysulfide

Figure 1 Schematic to illustrate the role of PAA-SWNT interface layer in effectively blocking the dissolution of polysulfide to lithium metal during the discharge. The cell configuration is composed of sulfur cathode, PAA-SWNT interlayer, separator, and lithium metal.



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The scanning electron microscopy (SEM) images and schematics of the SWNT interlayer and the PAA-SWNT interlayer are displayed in Figure 2a and 2b, respectively. We fabricated a freestanding SWNT film by the vacuum filtration method, which is a facile and rapid method to prepare the uniform film. The morphology of SWNT interlayer showed the one-dimensional well-entangled structure of SWNT film. After grafting the PAA polymer on the SWNT, no obvious changes in the morphology was observed, implying that PAA is slightly coated onto the surface of SWNT scaffold with no agglomerate. To characterize the structure of PAA-SWNT, transmission electron microscopy (TEM) was examined. As shown in the TEM images (Figure 2c and 2d), ultrathin PAA layer was formed through the highly crystallized SWNT backbone, which can be clearly distinguish by the differences in morphologies. To manifestly ascertain the formation of PAA on the SWNT film, we carried out the Fourier-transform infrared spectra (FT-IR) analysis (Figure 2e). The FT-IR spectrum of PAA-SWNT exhibited the strong peak in the range of 1700-1720 cm-1, compared to pristine SWNT, which is ascribed to the carboxyl groups in the PAA chain. The loading mass of PAA in the PAA-SWNT film was revealed by thermogravimetric analysis (TGA) (Figure 2f). The weight percentage of PAA in the PAA-SWNT was around 10 %.41


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Figure 2 Characterization of interlayers. SEM images and photograph images of a) SWNT and b) PAA-SWNT, TEM images of c) SWNT and d) PAA-SWNT, e) FTIR spectrum, and f) TGA analysis for each of SWNT and PAA-SWNT interlayer.

The electrochemical performances of Li-S battery with our designed carbon interlayer were evaluated by coin-type half cell (2032R type) and presented in Figure 3. Figure 3a shows the initial discharge-charge curves of the pure sulfur cathode, the sulfur 9 ACS Paragon Plus Environment


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cathode with a SWNT interlayer, and the sulfur cathode with a PAA-SWNT interlayer. Due to the inherently low electrical conductivity of pure sublimed sulfur, the capacity of the pure sulfur cathode at 1 C (1 C = 1672 mAh g-1) was as low as 379 mAh g-1. In addition, the pure sulfur cathode only showed a lower plateau unlike the other two electrodes. This could be due to the dissolution of sulfur during the discharge.2, 42 When the conductive carbon interlayer of the SWNT or the PAA-SWNT was inserted, the specific capacities of the sulfur cathode with a SWNT and the sulfur cathode with a PAA-SWNT increased to 767 mAh g-1 and 770 mAh g-1, respectively. The improved specific capacity was attributed to the increased utilization of sulfur and the improved redox reaction kinetics with the help of the conductive interlayer. A comparison of the peak potential differences between the lower plateau of the discharge and charge curves, the sulfur cathode with a PAA-SWNT interlayer exhibited smaller gap between the potential plateaus than the others, which indicates that PAA-SWNT interlayer can effectively facilitate reversible electrochemical reactions.14 The cycle performance and the Coulombic efficiency for each electrode are evaluated at a constant current density of 1 C and plotted in Figure 3b. The pure sulfur cathode showed fast capacity decay during the repetitive cycling due to the loss of active material and the poor Coulombic efficiency. After 200 cycles, the discharge capacity of the pure sulfur cathode decreased to 136 mAh g-1 (36.0 % of the initial capacity). With the aid of the SWNT interlayer as a physical barrier and a conductive support, the sulfur cathode with a SWNT interlayer showed an improved specific capacity of 437 mAh g-1 (57.0 % retention of the initial capacity) after 200 cycles and a high Coulombic efficiency of around 97 %. However, the SWNT interlayer still has limitations to efficiently curb the migration of polysulfides species due to the porous architecture of the film. In comparison to the above cases, the sulfur cathode with a PAA-SWNT interlayer delivered a much improved discharge


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capacity of 573 mAh g-1 even after 200 cycles (74.4 % retention of the initial capacity) and maintained the highest Coulombic efficiency of 99 % with no decline. These results clearly demonstrate that the PAA-SWNT film is an effective interlayer to hold the dissolved polysulfides on the cathode side and reactivate those captured polysulfide species. Figure 3c provides the rate capability of each electrode at different current densities from 0.1 C to 2 C. In the first few cycles, the sulfur cathode with a PAA-SWNT interlayer showed a stable cycling performance at the current density of 0.1 C, compared to the other electrodes. The sulfur cathode with a PAA-SWNT interlayer exhibited a high specific capacity of 592 mAh g-1 with rate retention of 53 % even at the high current density of 2 C, whereas the pure sulfur cathode and the sulfur cathode with a SWNT interlayer delivered low capacities of 227 mAh g-1 and 501 mAh g-1, respectively. The superior rate capability and stable operation at high current density are mainly attributed to (1) the improved conductivity and (2) the efficient capture of polysulfides species within the cathode part by the synergistic contributions of PAA polymer and SWNT scaffold. To further analyze the surface resistance on the interface between sulfur cathode and interlayer, we carried out an electrochemical impedance spectroscopy (EIS) experiment in a frequency range of 250 kHz to 100 mHz with an amplitude of 10 mV. Figure 3d provides the Nyquist plots of each electrode. The intersection point of the semi-circle and real axis implies the Ohmic resistance between electrolyte and electrode.43 The Ohmic resistances of the pure sulfur cathode, the sulfur cathode with a SWNT interlayer, and the sulfur cathode with a PAA-SWNT interlayer exhibited similar values of 3.5 Ω, 3.0 Ω, and 3.3 Ω, respectively. On the other hand, the diameters of semi-circle corresponding to charge transfer resistance present distinctly different values. The pure sulfur cathode showed a high polarization resistance of 21 Ω, whereas the sulfur cathode with a SWNT interlayer 11 ACS Paragon Plus Environment


and the sulfur cathode with a PAA-SWNT interlayer exhibit low polarization resistance of 13.9 Ω and 13.3 Ω, respectively. The improved reaction kinetics of the sulfur cathode with a conductive interlayer was ascribed to the enhanced electrical conductivity with SWNT as an excellent current collector.10 Moreover, it was reported that PAA also had the effect of lowering the reaction bulk resistance when it was slightly formed on the sulfur cathode.36 The synergic effect of combining PAA polymer and SWNT contributed to lowering the reaction resistance, thereby efficiently improving the electrochemical performances of Li-S batteries.

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Figure 3 Electrochemical performances of the pure sulfur cathode, the sulfur cathode/SWNT interlayer, and the sulfur cathode/PAA-SWNT interlayer. a) Initial discharge-charge curve, b) Cycle performance and Coulombic efficiency, c) Rate performance, and d) Nyquist plot. 12 ACS Paragon Plus Environment

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In order to shed light on the chemical binding between Li2S8 and PAA, we sealed the Li2S8 and PAA powder in a vial, and then mixed the compounds for 1 day (Figure 4a). We selected Li2S8 as diffusion carrier due to its high solubility in the electrolyte, which was prepared by following the previous report from Cui group.44 The dark yellow color Li2S8 solution became light yellow, indicating strong adsorption ability of PAA. The results clearly demonstrated the intrinsic ability of PAA to adsorb the polysulfide species by a chemical interaction. To analyze the chemical binding interaction between PAA and Li2S8, Raman spectra of precipitated PAA-Li2S8 powder and PAA powder were analyzed immediately after washing by centrifugation, and drying. Figure 4b exhibits the Raman spectra in the range of 1500-1900 cm-1. The peaks of PAA centered at 1689 cm-1 are assigned to the stretching of C=O band from carboxyl groups. After reacting PAA powder with the Li2S8, the main bands assigned to C=O groups are positively shifted to the peak around 1715 cm-1 due to the stretching of C=O by hydrogen bonding interactions.45 To verify the substantial effect of PAA-SWNT film on blocking the diffusion of polysulfides through the membrane, we prepared the polysulfides diffusion test bottle as shown in Figure 4c. Three diffusion test bottles were prepared to see the diffusion rate of polysulfides throughout the membrane. For the diffusion test bottle with a separator, the polysulfides started to diffuse through the membrane due to the difference in concentration, when the diffusion test bottle was tipped inside fresh electrolyte. In contrast to the test bottle with a separator, the polysulfides more slowly diffused out of the membrane when the interlayer introduced. After one day elapsed, the three diffusion test bottles appeared visually different in color. The bottle with a separator changed to the brown color, which suggests fast diffusion of polysulfides through the membrane. The bottle inserted with a SWNT film, the color of solution that filter through the SWNT film was lighter than that of solution leaking 13 ACS Paragon Plus Environment


out through a separator, indicating that SWNT film can suppress the migration of polysulfides by acting as a physical barrier. Compared with the bottles mentioned above, the bottle inserted with a PAA-SWNT film showed the lightest color, indicating the most efficient suppression of polysulfides migration through the PAA-SWNT film by both physical and chemical blocking.

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Figure 4 Verification of chemical binding between PAA and Li2S8, and diffusion test. Adsorption test with the sealed vial containing a) the Li2S8 solution and the Li2S8 solution with PAA powder. b) Raman spectra of PAA powder and PAA-Li2S8 powder. c) Configuration of diffusion test bottle; it is composed of a separator, an interlayer, and Li2S8


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solution. The diffusion test bottle with a separator contains no interlayer. The polysulfides diffuse out of the membrane, when the diffusion test bottle is dipped in the fresh electrolyte.

We carried out the SEM study to investigate the morphological changes of each interlayer after discharge (Figure 5). The cell was discharged to 1.5 V at 0.05 C for 1 cycle. The side of the SWNT film which faces the sulfur cathode, showed the mechanically filtered polysulfide species on the surface of the interconnected structure of SWNT, which can be clearly distinguished by comparing with the morphology of pristine SWNT (Figure 5c). Conversely, the images of the back side, which faces the lithium metal, display no visible captured sulfur species on the surface of SWNT since pristine carbon has no chemical interaction with polysulfides and the smaller polysulfides which passed through the porous interlayer cannot visualized on the surface by SEM (Figure 5e).46 Unlike the SWNT interlayer, the SEM images of the PAA-SWNT interlayer morphology show that polysulfide species were attached on both sides of interlayer, which can be attributed to the hydrogen bond between PAA and the smaller polysulfides which passed through the porous interlayer (Figure 5(d-f)). The SEM study directly provides the concrete evidences for the effect of PAA-SWNT film as a synergic conductive interlayer on capturing the dissolved polysulfides within the cell.



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Figure 5 Morphology analysis of interlayers after discharge. a-b) The illustrations showing both sides of each interlayer during the discharge. c) Front side and e) back side of SWNT interlayer, d) front side and f) back side of PAA-SWNT interlayer. The front side faces the sulfur cathode and the back side faces the lithium metal. The inset shows the enlarged SEM images of each carbon interlayer. The arrow indicates the sulfur species on the surface of interlayer. 16 ACS Paragon Plus Environment

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Conclusions In this article, we first designed a freestanding PAA-SWNT film as a synergistic functional interlayer to diminish the polysulfides shuttling effect and improve the electrical conductivity of sulfur cathode. Compared with the sulfur cathode with a SWNT interlayer, the sulfur cathode with a PAA-SWNT interlayer electrode delivered a high initial discharge capacity of 770 mAh g-1 at 1 C and maintained its specific capacity of 573 mAh g-1 at the 200th cycle with higher capacity retention and better Coulombic efficiency. Moreover, the rate capability of the sulfur cathode with a PAA-SWNT interlayer exhibited a higher rate retention of 53 % even at a high current density of 2 C than that of the pure sulfur cathode and the sulfur cathode with a SWNT interlayer. The excellent electrochemical performance of the sulfur cathode with a PAA-SWNT interlayer arose from three main advantages: 1) efficient physical blocking of the dissolved polysulfide by the porous scaffold, 2) chemical interactions of PAA with the dissolved polysulfides via hydrogen bonding, and 3) low charge transfer resistance. Unlike the previous approach to block the polysulfides with physical barriers, the concept of combining conductive interlayer and functional polymer can provide an optimized strategy to address the fundamental challenges of Li-S batteries. The proposed concept can be further extended to other types of functional polymers as well as other conductive support materials, which will open new avenues to improve the electrochemical performances of Li-S batteries.

Coresponding Author *E-mail: [email protected] Tel: +82-2-2220-0502 *E-mail: [email protected]

Tel: +1-301-405-9303

Acknowledgements 17 ACS Paragon Plus Environment


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This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) which granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20142020104190).

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