Effective Suppression of the Polysulfide Shuttle Effect in Lithium-Sulfur Batteries by Implementing rGO-PEDOT:PSS-Coated Separators via Air-Controlled Electrospray.

Lithium-sulfur (Li-S) batteries have been earning significant attention because of their high energy density and cost efficiency. Albeit these outstanding qualities, the polysulfide shuttling effect and low electrical conductivity of the sulfur active material in this battery chemistry results in poor cycling performance. In an attempt to overcome these problems, a hybrid structure of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) and reduced graphene oxide was developed and coated on the surface of a conventional separator using air-controlled electrospray. Implementing these coated separators in Li-S batteries led to lower polarization and stymied the polysulfide shuttling effect through the combining effects of electrostatic, physical, and chemical interactions. Our results reveal that the capacity and rate capacity are drastically improved via coating the separator, leading to more than twice the capacity of over 800 mA h g-1 after 100 cycles at 0.5 C rate, when it is compared to those with the pristine separator. Overall, this hybrid coating material shows great promise in enhancing the current Li-S battery technology.

Introduction

In recent years, development of electric vehicles and smart grids has been on the rise. To accommodate such high-power requiring inventions, energy-storage devices with high energy densities are utmost necessary.[1,2] Conventional lithium-ion batteries have been able to accomplish great success in the energy-storage sector, but they are still not sufficient to meet the power demands of electric vehicles and grid-scale applications.[1,3,4] Hence, many groups have attempted to study different battery chemistries, and lithium–sulfur batteries (Li–S) are rising as one of the next-generation energy-storage devices.

Lithium–sulfur (Li–S) batteries have been attracting intense attention because of its high theoretical capacity (1675 mA h g–1) and energy density (2600 W h kg–1). In addition, this battery system utilizes sulfur as the active material, which is nontoxic and abundant.[5−8] Despite these striking advantages, there are some limitations to this technology that remain unsolved. First, sulfur is electrically insulating, which causes high cell polarization and under-utilization of active materials.[7,8] Second, the polysulfide shuttle effect, which is caused by the dissolution of intermediate lithium polysulfide species in the electrolyte leads to irreversible loss of sulfur, resulting in rapid capacity fading and poor Coulombic efficiency of the cells.[5,7,9] Third, the huge volume expansion of sulfur during the repeated charging/discharging processes disrupts the structural integrity of the electrodes, which leads to poor electrical contact with the conductive additives and current collectors.[10]

Many efforts have been devoted to addressing these challenging issues in Li–S batteries by developing and applying multiple types of porous nanostructured sulfur electrodes.[8,10] Multidimensional carbon/sulfur composites based on carbon nanofiber web structures,[11−13] carbon nanotubes,[14−17] and reduced graphene oxide (rGO) sheets[18−20] have been created as a mainstream approach because of their high specific surface area and electrical conductivity. These innovative strategies involving composite materials not only improve the reaction kinetics but also alleviate the degree of polysulfide migration by localizing the sulfur particles in the cathode compartment of the cell. However, adding such an inactive carbon material leads to an overall lower energy density of the cell. In addition, fabricating and processing nanostructured electrode materials can be time-consuming and difficult as it most often requires multiple steps.[4,9,21]

Utilizing coated separators in Li–S batteries has been sought out as a potential solution.[6,9,22−27] The Manthiram group has demonstrated that the highly conductive carbon layers on the separator suppress the polysulfide shuttle effect through physical trapping and provide enhanced conductivity by acting as an additional current collector, which improves the electrochemical performance.[22] However, the weak adsorption between carbon and polysulfides induces problems with long-term capturing abilities, leading to a limited cycling stability.[24,28] As a result, to reinforce the chemical interaction with polysulfides, polar materials such as graphene oxide,[28,29] inorganic oxide material,[30,31] metal–organic framework,[32] conducting polymer,[33] and boron-nitride nanosheets[34] have been investigated as coating materials. Albeit such progress in polysulfide chemisorption on polar surfaces, further investigations on coated separators are still needed because the functional groups reduce the electrical conductivity. Therefore, it is crucial to design an appropriate structure and material for coated separators that are highly conductive in an effective manner for suppressing the polysulfide shuttle effect. Recently, Cui et al. have conducted studies on applying conducting polymers to Li–S cathodes. The polar heteroatoms in these polymer shells had strong interactions with polysulfides to form chemical bonds, preventing polysulfide dissolution.[35,36] The resulting Li–S battery performances indicated that the mesoporous carbon/sulfur composite coating layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) turned out to be superior over other conducting polymers. Despite this fact, an additional augmentation to increase the conductivity of PEDOT:PSS along with the physical support could undoubtedly improve the performance of Li–S batteries even further.

Herein, we propose a facile strategy to uniformly coat the surface of the separator with hybrid rGO–PEDOT:PSS structures. Through the means of air-controlled electrospraying, rGO–PEDOT:PSS can be coated not only uniformly but also swiftly on a designated substrate. By applying rGO–PEDOT:PSS-coated separators to Li–S batteries, prominent benefits can be achieved. First, the conductive coating layer promotes electron transfer, which leads to low polarization and fast redox reaction kinetics. Second, the polar nature of PEDOT:PSS induces chemical interactions with polysulfides, and the chemical adsorption of polysulfide species onto PEDOT:PSS hampers the redox shuttle effect. Third, the well-developed layer structure of rGO–PEDOT:PSS provides physical trapping sites for polysulfide, which in turn mitigates the polysulfide shuttling effect. By utilizing this method, we were able to achieve improved cycling performances for Li–S batteries through the synergistic effects of rGO and PEDOT:PSS.

Results and Discussion

The rGO–PEDOT:PSS-coated separator was prepared via a facile air-controlled electrospray method, which deposits the material of interest on a designated substrate through a high-speed air stream to overcome many shortcomings that conventional electrospray setups have. The air-controlled electrospray process is similar to the conventional electrospray process, but additional high speed air flow is incorporated. To carry out air-controlled electrospraying, coaxial nozzle configuration is utilized. The inner needle conveys the electrically charged solution, whereas the outer shell supplies the high-speed air flow. The effective combination of two driving forces, high electric field and high-speed air flow, can provide enhanced breakup and deformation of the drops, and thus offer (i) a much higher production rate, (ii) a better control of dispersion of nanoinclusions in the drop, and (iii) smaller drop size and better control of directing drops toward the collector with more uniform and thin coatings.[11,37]

To prepare the solution for electrospraying, an aqueous solution containing PEDOT:PSS was added into the rGO solution under gentle stirring and mild sonication to promote the dispersion of rGO in the solvent. As clearly shown in Figure S2, the dispersion of rGO in the solvent was significantly improved after incorporating PEDOT:PSS, possibly due to noncovalent stabilization.[38,39] To elaborate, the hydrophobic component of the polymer chains tends to wrap around the surface of rGO via π–π interactions, whereas the hydrophilic PSS dissociates rGO from the solvent molecules. Such an interaction could lead to a more uniform distribution of the rGO–PEDOT:PSS hybrid, which would promote the structural integrity and electron transfer.

Figure a presents the schematic cell configuration of the Li–S batteries with the rGO–PEDOT:PSS-coated separator. The coated separator was placed between the cathode and the anode to suppress the polysulfide shuttle effect during electrochemical reactions. The air-controlled electrospraying deposition process resulted in an excellent adhesion of rGO–PEDOT:PSS onto the commercial Celgard separator (Figure b,c), as demonstrated by the folding test presented in Figure d. Scanning electron microscopy (SEM) was employed to characterize the surface morphology of the rGO–PEDOT:PSS hybrid coating on the separator. As shown in Figure e–g, a dense layer of rGO–PEDOT:PSS covering the nanopores on the surface of the separator was observed. This conductive layer could physically prevent the lithium polysulfide ions from migrating to the anode compartment and improve the transport of electrons and lithium ions, providing low polarization and fast redox reaction kinetics.

Figure 1

(a) Schematic cell configuration of the Li–S batteries with the rGO–PEDOT:PSS-coated separator. Digital photographs of the as-prepared rGO–PEDOT:PSS-coated separator: (b) front side, (c) back side, and (d) folded. SEM image of (e) the Celgard separator and (f) top surface and (g) cross-section of rGO–PEDOT:PSS-coated separator.

The hybrid structure of PEDOT:PSS and rGO was characterized using the Fourier transform infrared (FTIR) analysis, with the spectra of rGO, PEDOT:PSS, and rGO–PEDOT:PSS presented in Figure S3. rGO exhibited very small absorption bands widely ranging from 700 to 1200 cm–1, owing to potential carbon–carbon bonds and oxygen-containing bonds.[40] Meanwhile, the pristine PEDOT:PSS exhibited two notable absorption bands at 1325 and 1511 cm–1 corresponding to C–C or C=C stretching from the quinoid conformation structure and C–C or C=C stretching from the thiophene ring, respectively. The absorption band at 1182 cm–1 is assigned to the symmetric vibration from the −SO3 group in the PSS chain. The vibration at 1147 cm–1 corresponds to C–O–C stretching from the ethylenedioxy group. In addition, C–S vibrations from the thiophene ring were also observed, which is verified by the absorption bands occurring at 975 and 876 cm–1.[41,42] Although the rGO–PEDOT:PSS spectra were shifted slightly, all of the absorption bands are from either pure rGO or pristine PEDOT:PSS, confirming the hybrid structure from the interactions.

The redox behavior of the sulfur cathodes with the rGO–PEDOT:PSS-coated separator was initially evaluated by cyclic voltammetry (CV) measurements at a scan rate of 0.1 mV s–1. The CV data for the neat Celgard polypropylene (PP) separator is also given for comparison. As shown in Figure a, the cell with the neat separator exhibited two broad cathodic peaks and an anodic peak because of the sluggish kinetic process.[31] In comparison, the incorporation of rGO–PEDOT:PSS coating resulted in well-defined redox peaks with an increase in the current density, suggesting the improved redox reaction kinetics and utilization of the active materials in the cells.[31,43] Furthermore, the redox peaks of the cathode with rGO–PEDOT:PSS coating of each cycle highly overlapped (Figure S4), indicating lower cell polarization and excellent electrochemical reversibility.

Figure 2

(a) CV curves at a scan rate of 0.1 mV s–1 in the potential window of 1.6 to 2.8 V. (b) Galvanostatic charge/discharge profiles at a current density of 0.1 C.

Figure b shows the galvanostatic charge/discharge profiles of cells after an initial activation process. As presented, both cells exhibited two typical discharge plateaus, which is in agreement with the CV curves. The upper discharge plateau corresponds to cyclic elemental sulfur reduction to long-chain lithium polysulfide, and the lower discharge plateau pertains to the formation of short-chain insoluble polysulfide (Li2S2–Li2S) products.[7] The discharge plateaus for the cell with the rGO–PEDOT:PSS-coated separator were found to be longer and flatter and have lower polarization than the cell with only the neat separator. Obviously, the cell with the coated separator delivered a larger discharge capacity value of 1249.4 mA h g–1, whereas the cell with the neat separator exhibited a limited discharge capacity of 834.3 mA h g–1, confirming high sulfur utilization enabled by the coated separator.

Subsequently, the cycling performance of the cells was investigated further over 100 cycles at a current density of 0.5 C. As displayed in Figure a, the cells assembled with coated separators were able to deliver larger discharge capacities and maintain a relatively stable cycling performance with a higher Coulombic efficiency over their counterparts. After repeated cycling, the retained discharge capacity value was as high as 812.8 mA h g–1. In contrast, the cell with the neat separator showed a lower discharge capacity of 296.8 mA h g–1, which probably resulted from the limited sulfur utilization and continuous dissolution of lithium polysulfide into the electrolyte. The rate capability of the cells was also measured at various current densities and is presented in Figure b. As expected, the cell assembled with the neat separator suffered from poor cycling behavior. Even under a mild condition of 0.1 C, the cell continued to maintain lower discharge capacity values. Moreover, the difference in discharge capacities between the cells was observed to be conspicuous at high current densities in which the effect of redox reaction kinetics was more significant. When the current density increased to 2 C, the cell with the coated separator still achieved 38.9% of its initial capacity, whereas the discharge capacity retention of the cell with the neat separator was around 4.21%. After returning to the initial current density of 0.1 C, a reasonable discharge capacity of 1069.7 mA h g–1 was able to recover, indicating an improved reversibility and stability.

Figure 3

(a) Cycling performance and Coulombic efficiency at 0.5 C for 100 cycles for Li-S cells with pristine and coated separators. (b) Rate capability at different current densities.

The overall enhanced electrochemical performance of the cell with the coated separator can be attributed to several factors. First, the increase in amount of conductive pathways that the rGO–PEDOT:PSS hybrid structure provides facilitated redox electron transfer and reduced interfacial resistance, allowing for a high conversion rate of lithium polysulfide into favorable chemical compounds for charge storage. Second, a well-developed coating layer not only serves as a physical barrier but also gives rise to a polar interaction for lithium polysulfides to suppress the loss of the active material, which in turn enhances reutilization/reactivation of the trapped active material and thus contribute to the higher capacitive behavior.

As such, a visual depiction of lithium polysulfide species and its trapping in rGO–PEDOT:PSS is presented in the inset of Figure a. Note that Li2S6, which is soluble in the electrolyte, was selected as the representative polysulfide species. Li2S6 solution was prepared to visibly clarify whether rGO–PEDOT:PSS can physically and chemically trap a well-known hydrophobic polysulfide species. A color change was expected of the Li2S6 solution once the rGO–PEDOT:PSS was added, and this would strongly imply that the active material would be localized in the cathode compartment of the cell that has a rGO–PEDOT:PSS-coated separator. As depicted in the Figure a inset, Li2S6 in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) solvent was a red-brown colored solution. Remarkably, adding rGO–PEDOT:PSS powders into the polysulfide solution resulted in color fading. Ultraviolet–visible (UV–vis) absorption characterization was also conducted to confirm the concentration changes of Li2S6 solutions. It has been previously reported that the noticeable absorption band in the 400–500 nm region is related to Li2S6.[43,44] As shown in Figure a, we observed a sharp decrease in the absorption band, suggesting that the rGO–PEDOT:PSS exhibited strong interactions with Li2S6 molecules. According to other literature, polar atoms in PEDOT offer chemical binding with lithium polysulfides to form a chelated coordination structure.[35] In this study, the as-prepared sample is composed of PEDOT:PSS and rGO sheets, which could provide a synergetic effect for adsorption of lithium polysulfide. These observed results are consistent with the behavior of the cycled separators. As depicted in Figure S5, the pristine PP separator for the reference cell after cycling was vividly yellow because of the inevitable contamination caused by the dissolved lithium polysulfides in the electrolytes.[45] On the other hand, the rGO–PEDOT:PSS-coated separator with regions where the coating was peeled off appeared more white, indicating that the migration of soluble polysulfide was relatively confined on the cathode side rather than diffusing to the anode side.

Figure 4

(a) UV–vis absorption spectra of Li2S6 solution before and after the addition of rGO–PEDOT:PSS. Inset image shows visualized adsorption of polysulfide after the addition of rGO–PEDOT:PSS. (b) S2p XPS spectra of the rGO–PEDOT:PSS-coated separator before and after cycling. (c–e) SEM micrographs of the morphology of the lithium metal anode surface before cycling, after cycling with the pristine PP separator, and cycling with rGO–PEDOT:PSS-coated separator, respectively.

To better understand the adsorption of polysulfide, X-ray photoelectron spectroscopy (XPS) was employed. As shown in Figure b, bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and polysulfide peaks were observed after cycling in the S2p spectra. The peak of LiTFSI arising from the electrolyte salt was detected at around 170 eV of binding energy, whereas the lower energy peak in the range between 162 and 165 eV was attributed to the insoluble Li2S–Li2S2–S compound.[46,47] These results suggest that improved electrochemical performance could be correlated with the adsorption of polysulfide species within the hybrid coating, which could help suppress the shuttling effect, leading to high reutilization/reactivation of the entrapped active materials in the cathode.

A further manifestation of the alleviated polysulfide shuttling effect was confirmed by analyzing the cycled lithium metal anodes. The SEM micrographs of the morphology of the lithium metal anode surface after cycling with the pristine PP separator and cycling with the rGO–PEDOT:PSS-coated separator were compared (Figure c–e). As shown in Figure d, a rough lithium metal surface was observed along with irregular aggregates in the cell with the neat separator, which is associated with the formation of the passivation layer (Li2S2–Li2S) derived from a reaction between dissolved lithium polysulfides and the lithium metal anode during cycling.[29] This reaction is a major cause of active material loss and inner resistance build up, resulting in the fast degradation of cycling stability. In contrast, in the cell with the coated separator, the surface of lithium metal was found to be smoother (Figure e), which is a clear indication of the protected lithium metal surface against corrosion.[29,44] This result also demonstrates that the lithium polysulfide shuttle effect and parasitic reactions are highly restricted by the coated separator.

Finally, we proceeded to evaluate the electrochemical impedance responses of cells before and after cycling. The relevant equivalent circuit models for analyzing the impedance spectra are presented in Figure S6. As shown in Figure a, each Nyquist plot before cycling is composed of a depressed semicircle in the medium- to high-frequency region and an oblique line in the low-frequency region. The former is coupled with the bulk resistance (R0) from the electrolyte and the charge-transfer resistance (Rct) at the interface between the electrode and the electrolyte, whereas the latter corresponds to Warburg diffusion impedance (Ws) of lithium ions in the electrode.[44] It was observed that the cell with the coated separator exhibited lower charge-transfer resistance, which could be ascribed to the improved electrical conductivity of the cell with the coated separator, as the conductive layer works as a second current collector in the cell.[22,31] After cycling (Figure b), two depressed semicircles and an oblique line for Warburg impedance were observed. The first depressed semicircle is related to resistance from the formation of the insulating surface layer (Rsurf), and the region for the second semicircle corresponds to the charge-transfer resistance.[44]R0 increased for both cells, and the difference before and after cycling was larger in the cell with the neat separator, resulting from the increase in the viscosity of the electrolyte, which is caused by excessive polysulfide dissolution.[48] Such observation strongly hints that polysulfide shuttling was stymied to a stronger degree in the cell with the coated separator and caused more effective lithium-ion diffusion. Additionally, smaller Rsurf and Rct values for the cell with the coated separator were detected, indicating that the polysulfide shuttling behavior and deposition of insoluble Li2S2–Li2S layer on the electrode were suppressed during cycling. We speculate that the reduced Rsurf and Rct in the cell with the coated separator could be attributed to more efficient chemical activation and reutilization of the sulfur active material and improved transport of lithium ions and electrons.

Figure 5

Electrochemical impedance spectra (a) before cycling (b) after cycling.

Conclusions

In conclusion, Li–S batteries with rGO–PEDOT:PSS-coated separators were successfully fabricated by using the method of air-controlled electrospray process. The electrically insulating nature of sulfur was accommodated by adding the conductive rGO–PEDOT:PSS, and the polysulfide shuttling effect was hampered by chemical and physical interactions with rGO–PEDOT:PSS. The improved electrical conductivity and suppressed polysulfide shuttling behavior allowed for enhanced reaction kinetics and reutilization/reactivation of the active material. Electrochemical evaluations revealed that the rGO–PEDOT:PSS-coated separator could deliver a high specific capacity and enhanced cycling performance compared to Li–S batteries without the rGO–PEDOT:PSS coating. Therefore, this study suggests high prospects of this coating material being applied in Li–S batteries and advancing the current energy-storage technology.

Experimental Section

General Procedure for Air-Controlled Electrospray of rGO–PEDOT:PSS

rGO–PEDOT:PSS-coated separators were fabricated by utilizing the air-controlled electrospray process. Typically, to prepare rGO–PEDOT:PSS hybrid structures, pristine rGO is dispersed in deionized water through mild sonication. For this solution, a PEDOT:PSS solution (Sigma-Aldrich, 1.3 wt % dispersion in water) was added at a rGO–PEDOT:PSS weight ratio of 1:1 and stirred vigorously to make a homogeneous solution before spraying. The air-controlled electrospray was carried out onto a 25 μm-thick Celgard PE separator using a Harvard Apparatus PHD 2000 Infusion syringe pump with a coaxial needle set under room temperature. The solution was supplied to the inner 12-gauge needle, and 20 psi air flow rate was applied thorough the 17-gauge outer shell. The working voltage was set at 15 kV, with 15 cm in distance from the needle tip to the collector and 0.08 mL min–1 of solution feeding rate. The mass loading of the hybrid coating was around 0.6 mg cm–2.

Preparation of Li2S6 Solution

Sulfur (Spectrum Chemical) and Li2S (Sigma-Aldrich) were added to DME and DOL (1:1 in volume) to form a molar ratio of 5:1. Afterward, the solution was heated to 90 °C and vigorously stirred for 12 h under an argon atmosphere to yield the Li2S6 Solution.

Characterization Methods

FTIR spectra were recorded with a Nicolet Magna-IR 560 spectrometer using an average of 180 scans with a resolution of 4 cm–1. Thermogravimetric analysis was carried out with a TGA 500 analyzer (TA Instruments) at a heating rate of 10 °C min–1 under a N2 atmosphere. SEM images were taken using a Tescan Mira3 field emission scanning electron microscope. UV–vis absorption spectra were obtained by using a spectrophotometer (SpectraMax 384), and XPS measurements were conducted with a surface science instrument equipped with a monochromatic Al Kα X-ray source (1468.6 eV). Electrochemical characterizations of the coated separator were performed using 2032-type coin cells consisting of the Li metal anode (MTI Corporation) and a slurry coated sulfur cathode. The cathode material contained 70 wt % sulfur/carbon composite material, 15 wt % poly(vinylidene difluoride) (Sigma-Aldrich, Mw = 534 000), and 15 wt % Super P (TIMCAL). The default composite material consisted of 80 wt % sublimated sulfur and 20 wt % Super P (Figure S1), and sulfur was infiltrated into the Super P at 155 °C for 12 h. The electrodes were dried at 60 °C under vacuum for 12 h and then punched into circular disc pieces with a diameter of 15 mm. The mass loading of the sulfur active material on the cathode was around 1.0 mg cm–2. The electrolyte was 1 M LiTFSI and 0.1 M lithium nitrate (LiNO3) in a 1:1 volume ratio of DME and DOL, all purchased from Sigma-Aldrich. All cells were assembled in an argon-filled glovebox. CV and electrochemical impedance spectroscopy measurements were performed using a potentiostat/galvanostat (Princeton PARSAT 4000). Galvanostatic charge/discharge measurements were carried out in the voltage range of 1.8–2.8 V using a battery cycler (MTI) at room temperature. All of the current densities and specific capacities calculated in this study were based on sulfur mass.