Li-Ion-Permeable and Electronically Conductive Membrane

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Li-Ion-Permeable and Electronically Conductive Membrane Comprising Garnet-type Li6La3Ta1.5Y0.5O12 and Graphene Towards Ultra-stable and High-Rate Lithium Sulfur Batteries Patrick Joo Hyun Kim, Sumaletha Narayanan, Jinze Xue, Venkataraman Thangadurai, and Vilas G. Pol ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00519 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Energy Materials

Li-Ion-Permeable

and

Electronically

Conductive

Membrane Comprising Garnet-type Li6La3Ta1.5Y0.5O12 and Graphene Towards Ultra-stable and High-Rate Lithium Sulfur Batteries Patrick J. Kim,a Sumaletha Narayanan,b Jinze Xue,a Venkataraman Thangaduraib and Vilas G. Pola* a

Davidson School of Chemical Engineering, Purdue University, West lafayette, 47907, USA

b

Department of Chemsitry, University of Calgary, Calgary, Alberta, T2N 1N4, Canada

*Corresponding authors Prof. Vilas G. Pol (V. G. Pol) E-mail: [email protected], Tel: +1 765-494-0044

Keywords: Graphene; Garnet Solid-state electrolyte; High ionic conductivity; Lithium sulfur batteries; Li6La3Ta1.5Y0.5O12

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ABSTRACT State-of-the-art lithium sulfur (Li-S) batteries suffer from serious systemic issues which are mainly derived from polysulfide shuttling effect, poor sulfur utilization and low Coulombic efficiency. These fundamental challenges impede the practical use of sulfur cathode in commercial battery, albeit its higher theoretical storage capacity compared to intercalation electrodes based Li ion batteries, including Graphite-LiCoO2; Graphite-LiFePO4 and Graphite -Li(Ni,Mn,Co)O2 cells. In this article, we designed a multifunctional membrane, comprising of graphene nanosheet and Li-stuffed garnet solid-state electrolyte (SSE) composite, in order to synergistically enhance both cycle stability and rate capability of general sulfur cathode in a facile and effective way. With the synergistic contribution of graphene nanosheet and SSE, the sulfur cathode exhibited a superior capacity of 1165 mAh g1

at 0.5 C and retained an excellent discharge capacity of 947.03 mAh g-1 (81 % of initial

capacity) over 200 cycles when a Gr/SSE-separator was employed. In addition, the sulfur cathode with a Gr/SSE-separator delivered a remarkable discharge capacity of 643 mAh g-1 even at 4 C. These results are attributed to three main benefits of Gr/SSE layer: (a) synergistically enhanced electrical and Li-ion conductivity of interlayer, (b) improved electrolyte wettability and (c) well-entangled architecture of graphene nanosheet and SSE powder.


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1. Introduction Demand for electrochemical energy storage devices with high power density and energy density have been rapidly grown over the past few years to keep pace with the development of state-of-art electronic applications (e.g. electric vehicle, portable devices, etc.).1-5 Among several candidates, especially, lithium sulfur (Li-S) batteries have received tremendous attention from academic and industrial fields, due to the high theoretical capacity (1672 mAh g-1) with average working potential at 2.1 V, excellent cost:benefit ratio, environmental benignity, etc.1-4 However, the inherent systemic weaknesses, mainly deriving from polysulfide shuttling effect, poor sulfur utilization and low Coulombic efficiency of sulfur cathode impede the practical use in commercial battery market. In order to address the abovementioned challenges, many effective strategies have been approached to optimize the electrode: a) impregnation of sulfur into micro-/mesoporous framework,1,

6-11

b)

encapsulation of sulfur reservoir with an additional protective layer,12-14 c) alteration of interfacial and crystalline characteristics of sulfur containers.15-19 All these approaches have shown significant improvements in preserving sulfur species locally within cathodic side and improving the electrochemical kinetics and reactions. Considering the procedure step and cost of following these strategies, eventually these are not ideal approaches to advance the development of practical Li-S cell; because each preparation step is too complicated and requires sophisticated techniques. In a different approach, Manthiram’s group first suggested a new type of cell configuration with a functional interlayer and a modified separator.20-23 The introduction of a conductive layer, such as CNT, graphene, and carbon black, in-between a sulfur cathode electrode and a general separator not only physically blocks the migration of polar polysulfide species, but also provides an additional conductive pathway to the sulfur cathode 3 ACS Paragon Plus Environment


electrode.24-30 In an analogous approach, the modification of a polypropylene (PP) separator with functional materials was also intensively explored.31-34 Both approaches have shown significant potentials to overcome the fundamental issues of Li-S system in a facile way. However, many of the candidate materials utilized as interlayer or deposition layer (on a separator) were mainly limited to the carbonaceous nanomaterials and the functional materials, which have chemical affinity or repulsive interaction with polar polysulfide species in order to curb the polysulfide shuttling and thus improve the cycle stability.7, 13, 25, 28-29, 35-36 Together with long-term cycle stability, high rate capability is also crucial for the development of Li-S cells. Relatively, the strategy to improve the rate capability of Li-S cell by an introduction of interlayer has hardly investigated. In this study, we designed a multifunctional membrane, comprising of graphene nanosheet and Li-stuffed garnet solid-state electrolyte (SSE), in order to synergistically improve both cycle stability and rate capability of general sulfur cathode in a facile and effective way. Graphene has many materialistic benefits in terms of electrical conductivity, surface area, and mechanical strength. These characteristics facilitate the improved electrochemical kinetics and the effective confinement of out-diffused polysulfide species within the cathodic side.36-38 Garnet-type solid-state-electrolyte has received intense attention in Li-based batteries due to its high ionic conductivity at room temperature and excellent chemical stability against Li metal.39-41 In order to exploit all these properties, garnet-type Li6La3Ta1.5Y0.5O12 was used, for the first time, as Li-ion diffusion catalyst and homogenize it with conductive graphene nanosheet via ball milling in order to achieve both high electrical conductivity and ionic conductivity.39, 41 Then, homogenized graphene/solid-state electrolyte (Gr/SSE) composite was coated onto a PP separator to use it as a multifunctional membrane. With the aids of synergistic contribution of graphene nanosheet and SSE, specific capacity and cycle retention of the sulfur cathode with a Gr/SSE-separator were dramatically 4 ACS Paragon Plus Environment

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improved over 200 cycles. In addition, it also delivered an excellent discharge capacity of 643 mAh g-1 even at a high current rate of 4 C.

2. Results and discussion Figure 1 illustrates the role of Gr/SSE layer as a hybrid interlayer in Li-S batteries. During the discharge, Li-ion migrates from Li metal electrode to cathode and reduces sulfur species in sulfur electrode. High-order polysulfide species, which were diffused out of sulfur electrode migrate towards Li metal anode owing to concentration gradient. At this stage, Gr/SSE layer first filters most of reduced polysulfide species by the dense structure of graphene nanosheet and provides an additional conductive path for sulfur cathode electrode. Not only it facilitates the effective suppression of polysulfide shuttling effect, but also it improves the electrical conductivity of a total cell configuration. In addition, solid-stateelectrolyte (SSE), Li6La3Ta1.5Y0.5O12, which was impregnated in the Gr/SSE layer efficiently promotes the diffusion of Li-ion back and forth throughout the Gr/SSE-separator and thus leads to the rapid electrochemical reaction of sulfur cathode, even at high currents.



Figure 1. Schematic illustrates the role of a GR/SSE-separator. The impregnated SSE powder in Gr/SSE promotes the rapid and efficient electrochemical reaction during discharge.

Figure 2a shows photos of a punched separator with a diameter of 15 mm and a bent separator (inset), which indicates strong mechanical properties (e.g. adhesion and robustness) between Gr/SSE layer and polypropylene (PP) separator. Figure 2b presents the scanning electron microscopy (SEM) image of a pristine PP separator with many pores and gaps, consistent with previous reports.33 As shown in Figure 2c, the morphology of SSE powder showed uneven and rugged shapes with 1-2 µm diameters. Figure 2d displays twodimensional graphene nanosheets with sizes of 1-5 µm. After ball-milling the graphene nanosheets and SSE powders altogether, GR/SSE composites were coated onto PP separator via tape casting. The top-view SEM image of a Gr/SSE-separator shows no pores and cracks throughout the whole area (Figure 2e). The thickness of a Gr/SSE separator was ~11 µm (Figure 2f). To characterize the crystalline phase of each graphene nanosheet, SSE, and 6 ACS Paragon Plus Environment

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Gr/SSE composite, X-ray diffraction (XRD) analysis was carried out and presented in Figure 2g. The crystalline phase of SSE powder is well matched to the pattern of garnet-type Li6La3Ta1.5Y0.5O12.39 Even after a ball-milling process, Gr/SSE powder displayed the analogous peaks as SSE powder but slightly increased intensity around 23o due to the presence of graphene nanosheet, indicating that SSE powders were well impregnated into graphene nanosheets and the crystalline phase of SSE was not damaged even after a physical homogenizing process. To ascertain the change of crystalline structure and surface characteristics after ball-milling process, Raman spectroscopy measurement were further performed. As shown in Figure 2h, graphene nanosheet showed two significant peak positions at 1334 cm-1 (D) and 1581 cm-1 (G) and SSE powder exhibited several peaks, associated with the crystalline structure and chemical bonds of Li6La3Ta1.5Y0.5O12, in the frequency range from 1800 cm-1 to 60 cm-1.27 In accordance with XRD result, Raman spectrum of Gr/SSE powder also exhibited the overlapped peak positions of graphene nanosheet and SSE without changes. It implies that Gr/SSE composite has a good homogeneity and ball-milling process does not influence on the crystallinities of each graphene and SSE powder. In order to investigate the specific surface area of each graphene, SSE, and Gr/SSE composite, Brunauer Emmett Teller (BET) analyses were carried out (Figure 2i). The specific surface areas of each graphene nanosheet, SSE, and Gr/SSE composite correspond to 56.1, 2.08 and 24.2 m2 g-1, respectively. The Gr/SSE showed an intermediate value of specific surface area among these three samples, which indirectly supports that graphene nanosheets and SSE powders were homogenized well.



Figure 2. Characterization. (a) Photograph of a Gr/SSE-separator; Top view SEM images of (b) a pristine separator, (c) SSE powder, (d) Gr powder and (e) a Gr/SSE-separator; (f) cross-sectional SEM view of a Gr/SSE-separator; (g) XRD analysis; (h) Raman spectra; (i) BET measurements.

Figure 3a presents the initial voltage profiles of each electrode at a current density of 0.5 C. The sulfur cathode (with sulfur areal loading mass of 1.8 mg cm-1) delivered a discharge capacity of 953 mAh g-1, presenting clear discharge and charge plateaus. When a SSEseparator was placed into the sulfur electrode, the initial discharge capacity decreased to 778 mAh g-1 and the potential gap difference between discharge and charge became wider due to 8 ACS Paragon Plus Environment

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the poor electrical conductivity of inorganic-based solid electrolyte. In contrast, when a conductive Gr-separator and a Gr/SSE-separator were employed as membranes in Li-S cell, the discharge capacity increased to 1118 mAh g-1 and 1165 mAh g-1, respectively, due to the improved kinetics of total battery configuration by an additional conductive layer. Figure 3b shows the cycle performances of each electrode at a current density of 0.5 C over 200 cycles. The sulfur cathode retained its discharge capacity of 214 mAh g-1 (22.4 % of initial capacity) over 200 cycles, being attributed by the massive loss of active sulfur material during repetitive cycles. In case of the sulfur cathode with a SSE-separator, it exhibited a relatively high cycle retention of 47.5 % over 200 cycles, which is ascribed to the physical blocking of polysulfide species by a SSE layer. When Gr-separator was introduced in Li-S cell, the cycle retention of sulfur cathode was dramatically improved to 69 % of initial capacity over 200 cycles due to the compact and conductive characteristics of graphene nanosheet.36 Especially for the sulfur cathode with a Gr/SSE-separator, it delivered a superior cycle retention (81 % of initial capacity, over 200 cycles) to the other electrodes. As shown in Figure S1, the cycle voltammetry (CV) curve of the sulfur cathode with a Gr/SSE-separator was repetitively overlapped during 10 cycles, which further supports the electrochemical stability. To substantiate the evidence that SSE powder impregnated in Gr/SSE film enables the rapid Liion diffusion (or transport) throughout the Gr/SSE film and, thus, leads to efficient and fast electrochemical reaction, sulfur cathodes with different separators were evaluated at current densities from 0.2 C to 4 C (Figure 3c). The specific capacity of sulfur cathode shows nearly zero at a high current density of 2 C due to the poor electrochemical kinetics, whereas that of the sulfur cathode with a SSE-separator showed a relatively high capacity around 366 mAh g1

. This result directly supports that SSE can assist the electrochemical reaction by providing

Li-ion-conducting channels. The sulfur cathode with a Gr-separator showed improved rate capabilities, as comparison with the sulfur cathode, due to the enhanced electrical 9 ACS Paragon Plus Environment


conductivity and the effective suppression of polysulfide shuttling. With the synergistic contribution of graphene nanosheet and SSE powder, the sulfur cathode with a Gr/SSEseparator exhibited an excellent discharge capacity of 643 mAh g-1 even at a high current density of 4 C. It is attributed to three main characteristics of Gr/SSE layer: a) enhanced electrical and ionic conductivity of interlayer and b) improved electrolyte wettability by inorganic-based solid electrolyte, and c) well-entangled architecture of graphene nanosheet and SSE powder. To clearly see the rate capability of each electrode, rate retention curve is provided in Figure S2. To characterize the interfacial behaviors and Li-ion diffusivity throughout each separator, electrochemical impedance spectroscopy (EIS) tests were performed after one cycle (discharge/charge) and presented in Figure 3d. The Ohmic resistances of the sulfur cathodes with a pristine separator, a SSE-separator, a Gr-separator and a Gr/SSE-separator exhibited analogous values of 5.0, 7.0, 5.1, and 5.0 Ohm, respectively. Interestingly, the charge transfer resistance of each electrode exhibited definitely different values. All electrodes delivered slightly decreased charge transfer resistance when an additional layer was introduced in Li-S cell, as comparison with the sulfur cathode electrode. These are attributed to the suppression of polysulfide shuttling effect and the reactivation of out-diffused polysulfides by an additional protective layer. Especially for the sulfur cathode with Gr/SSE-separator, it showed the lowest charge transfer resistance of 25.8 Ohm due to the dramatic improvements of interfacial kinetics between Gr/SSE layer and sulfur cathode and the enhanced electrolyte wettability by inorganic compound-based SSE.42 In addition, as for the sulfur cathode with a Gr/SSE-separator, the straight line of Nyquist curve at low frequency, which is associated with the diffusion rate of Li-ion throughout bulk layer (semi-infinite Warburg diffusion), showed the steepest slope among electrodes, implying that the co-existence of SSE and liquid electrolyte synergistically improves the Li-


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ion diffusivity.43-45 After 200 cycles, overall impedances of each cell were increased due to the side reaction and polysulfide shuttling effect (Figure S3).

(b)

Pristine separator SSE-separator Gr-separator Gr/SSE-separator


1500

Specific capacity (mAh g-1)

Voltage (V vs. Li+/Li)

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(c)1500

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1C

-Z'' (ohm)

Specific capacity (mAh g-1)

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1C 2C 4C

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Figure 3. Comparison of electrochemical performances of Li-S cells with different separators. (a) Initial voltage profiles; (b) cycle performances; (c) rate performances; (d) Nyquist plots.

In order to corroborate the concrete evidence that SSE impregnated within Gr/SSE layer can promote the efficient and rapid electrochemical reaction, comparison study was further 11 ACS Paragon Plus Environment


carried out by substituting SSE powder with similar-sized ZnO powder (Figure 4). To verify whether each ZnO powder and SSE powder has chemical affinity with polysulfide species, polysulfide absorption tests were carried out (Figure S4). The polysulfide solution (Li2S8) was prepared by following the previous reports.33 Each ZnO powder and SSE powder was put into Li2S8 solution followed by thoroughly being mixed for 10 min. After one day, no color change of Li2S8 solution was observed for both samples, implying that each ZnO and SSE has no chemical interaction with polysulfide species. Figure 4a-b illustrates the role of each interlayer system during discharge. During discharge, SSE powders which are impregnated within Gr/SSE layer assist the Li-ion diffusion throughout a dense layer and, thus, lead to the rapid and effective electrochemical reaction even at high current levels (Figure 4b). By contrast, there is no significant improvement on the Li-ion diffusivity by ZnO powder, which hardly improves the electrochemical kinetics and reactions of sulfur cathode at high current levels (Figure 4a). Figure 4c presents the rate performances of Li-S cells with a Gr/SSE-separator and a Gr/ZnOseparator. At a current density of 0.2 C, the discharge capacity value and the trend of capacity fading showed analogous behavior during the first few cycles. However, the rate retention of the sulfur cathode with a Gr/SSE-separator and a Gr/ZnO-separator displayed remarkable distinction as current density increases from 0.5 C to 4 C. At a current density of 4 C, the sulfur cathode with a Gr/SSE-separator delivered a discharge capacity as high as 643 mAh g-1 (rate retention of 56 %), whereas the sulfur cathode with a Gr/ZnO-separator exhibited a discharge capacity as low as 198 mAh g-1 (rate retention of 17.8 %) (Figure S5). These can be evidenced by the Nyquist plots of these two cells. As shown in Figure 4d, the sulfur cathode with a Gr/SSE-separator has a much smaller charge transfer resistance of 25.9 Ohm than the sulfur cathode with a Gr/ZnO-separator (121.5 Ohm). In addition, the sulfur cathode with a Gr/SSE-separator exhibited a much steeper slope of linear region than the sulfur cathode with 12 ACS Paragon Plus Environment

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a Gr/ZnO-separator. These results imply that Gr/SSE layer has a higher ionic conductivity and a better Li diffusion ability than Gr/ZnO layer.46-48 In addition, the co-existence of Listuffed SSE and liquid electrolyte synergistically improve the Li-ion diffusivity during electrochemical reaction.

Figure 4. Electrochemical behaviors of Li-S cells with a Gr/ZnO-separator and a Gr/SSE-separator. Schematic illustration of the sulfur cathode with a) Gr/ZnO-separator and b) Gr/SSE-separator during discharge at high current levels; c) rate performances; d) Nyquist plots.



To practically demonstrate the potential of our-designed membrane for Li-S cell, the sulfur cathodes with different ratio of sulfur were tested at 0.5 C over 400 cycles (Figure 5). Each of sulfur electrode was precisely matched to the sulfur loading mass of 2.5 mg cm-1 to directly ascertain the electrochemical behaviors as a function of ratio of sulfur to carbon. Figure 5a-c displays the voltage profiles of each sulfur cathode at 1st and 400th cycle. The polarization gap of electrode becomes wider and the discharge capacity decreases as the ratio of sulfur to carbon increases from 60 to 70 %, due to the increased resistance of overall electrodes. Figure 5d shows the cycle performances of each sulfur cathode with a Gr/SSE-separator. The sulfur cathode (60 % sulfur) with a Gr/SSE-separator delivered an initial discharge capacity of 933 mAh g-1 and retained its capacity of 677.4 mAh g-1 (72.6 % of initial capacity) over 400 cycles. As for the sulfur cathode (70 % sulfur) with a GR/SSE-separator, it retained a less capacity of 450.8 mAh g-1 (57.7 % of initial capacity) than the other two electrodes, since there is more chance to lose the active sulfur mass during repetitive cycles. Extrapolating from this result, it is confirmed that the ratio of sulfur to carbon is paramount to both cycle retention and discharge capacity value, and it could be easily optimized by controlling the areal sulfur mass and the ratio of sulfur to carbon in the sulfur cathode.


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Figure 5. Long-term cycle tests of Li-S cells with a Gr/SSE-separator. The voltage profiles of each (a) sulfur cathode (60 % S), (b) sulfur cathode (65 % S), (c) sulfur cathode (70 % S) at 1st and 400th cycle; (d) cycle performances at 0.5 C over 400 cycles.

In order to further ascertain the substantial effect of a Gr/SSE-separator on the protection of Li metal from polysulfide shuttling effect, Li-S cells were disassembled after a few cycles, and the surface of Li metals and separators were observed by SEM measurements (Figure 6). In the case of the sulfur cathode with a pristine separator, Li metal showed many black spots on the surface and the SEM image of black spots shows porous and interconnected particles with diameter of 2-10 µm over the surface of Li metal (Figure 6a). The XPS data of these black spots corresponds to low order polysulfide species (Figure S6), which is well consistent with previous result.33 By contrast, Li metal, which was tested in the sulfur cathode with a Gr/SSE-separator, displayed no noticeable black spots over the surface of Li metal. In 15 ACS Paragon Plus Environment


addition, Li metal showed clean and flat surface without large agglomerates as shown in the SEM image (Figure 6b). The cycled pristine separator shows the analogous morphology as a fresh pristine separator, implying that pristine separator has no ability to physically capture high order polysulfide species during cycles (Figure 6c). On the other hand, Gr/SSE-separator was covered by large amount of polysulfide species after a few cycles (Figure 6d-e). These results directly support the effects of Gr/SSE-separator on confining the polysulfide within the cathodic side.

Figure 6. Post-diagnosis of Li-S cells after a few cycles. The photographs and SEM images of cycled Li metal tested with (a) a pristine separator and (b) a Gr/SSE-separator; the topview SEM images of (c) a cycled pristine separator; (d) a cycled Gr/SSE-separator (low magnification); e) a cycled Gr/SSE-separator (high magnification). 16 ACS Paragon Plus Environment

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3. Conclusions In this study, we demonstrated a multifunctional membrane, comprising of graphene nanosheet and Li-stuffed garnet solid-state electrolyte (SSE), in order to synergistically enhance the cycle stability and rate capability of general sulfur cathode in a facile and effective approach. When a Gr/SSE-separator was employed in Li-S cell, the sulfur cathode exhibited a superior capacity of 1165 mAh g-1 (at 0.5 C) to the other electrodes and retained an excellent discharge capacity of 947.03 mAh g-1 (81 % of initial capacity) over 200 cycles. In addition, the sulfur cathode with a Gr/SSE-separator delivered a remarkable discharge capacity of 643 mAh g-1 even at 4 C. These results are attributed to three main benefits of Gr/SSE layer: a) synergistically enhanced electrical and ionic conductivity of interlayer, b) improved electrolyte wettability and c) well-entangled architecture of graphene nanosheet and SSE powder. Unlike the previous reports, which mainly dealt with the combination of functional materials and conductive nanomaterials, the strategy of incorporating Li-stuffed SSE into the well-stacked carbon platform to improve both cycle stability and rate capability has not been approached yet. Moreover, our designed membrane showed promising results in achieving long term cycle stability over 400 cycles, even though it was coupled with general sulfur cathode electrode with high areal sulfur loading; this strategy will open a new avenue to advance the development of excellent membranes for high performance Li-S batteries and can be further extended to other excellent ion-conductive materials.

4. Experimental Section

4.1. Fabrication of general sulfur cathodes 17 ACS Paragon Plus Environment


First, the slurry for general sulfur cathode was prepared by putting 30 mg Super P, 60 mg sulfur and 10 mg Polyvinylidene fluoride (PVdF) binder into N-Methyl-2-pyrrolidone (nMP) solvent and followed by homogenizing it with a planetary mixer (Thinky). As-prepared sulfur cathode slurry was coated onto Al-foil via tape-casting and dried at 50 oC in convection oven for one day to fully get rid of remaining nMP solvent. The areal sulfur loading of sulfur electrode was modulated by changing the blade thickness and was precisely controlled to get uniform sulfur cathode (1.8 mg cm-2). For the preparation of the sulfur electrode with high sulfur loading mass (2.5 mg cm-2), we used the doctor blade with high thickness. The preparation of sulfur electrode with different ratio of sulfur was also prepared by following the above-described procedure, except for the composition ratio.

4.2. Preparation of Gr/SSE-separator and other membranes The synthesis of SSE powder, Li6La3Ta1.5Y0.5O12, was followed by previous work.39 Before fabricating a Gr/SSE-separator, 350 mg graphene nanosheet and 150 mg SSE powder were ball-milled for two hours in order to get homogenized Gr/SSE composite. Then, 40 mg Gr/SSE powder and 10 mg PVdF binder were dispersed into N-Methyl-2-pyrrolidone (nMP) solvent by planetary mixer (2000 rpm, 20 min) to prepare the slurry. As-prepared slurry was directly laminated onto a PP separator (Celgard 2500), followed by drying at 50 oC in convection oven for one day to fully dry nMP solvent. As fabricated Gr/SSE- separator was cut into circular shape with a diameter of 15 mm and utilized as membrane for Li-S batteries. The preparation of a SSE-separator and a Gr-separator followed the same procedure as Gr/SSE-separator. The average areal mass of each material (Gr, Gr/SSE, and SSE) on separators is ~ 0.8 mg cm-2. As for the preparation of Gr/ZnO-separator, it also followed the same procedure and the ratio of graphene to ZnO is seven to three. The SEM image of Gr/ZnO-separator was provided in Figure S7. 18 ACS Paragon Plus Environment

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4.3. Characterizations The morphologies of PP separator, SSE-separator, Gr-separator and Gr/SSE-separator were observed by a FE-SEM (FEI NOVA NanoSEM 450). The galvanostatic discharge/charge test, cycle and rate performance of each sulfur electrode were evaluated by MTI battery tester. The EIS curves of each Li-S cell were analyzed by an potentiostat/galvanostat (Gamry Instruments Reference 600 Electrochemical Workstation) in the frequency range from 100 kHz to 0.1 Hz at 5 mV. The electrolyte for Li-S cell was 1.0 M lithium bis(trifluoromethanesulfonyl)imide

(LiTFSI)

in

1,3-dioxolane

(DIOX)

and

1,2-

dimethoxyethane (DME) (v:v=1:1) (Sigma Aldrich) dissolved with 1 wt% of LiNO3. The Raman spectra of graphene nanosheet, SSE powder, and Gr/SSE composite were analyzed by Thermo Scientific DXR Raman Microscope (532-nm laser). X-ray diffraction (XRD) studies were performed by X-ray diffractometer (Rigaku SmartLab) to ascertain the crystalline phase of graphene nanosheet, SSE powder, and Gr/SSE composite. The specific surface area of graphene nanosheet, SSE powder, and Gr/SSE composite were measured by Brunauer, Emmett, and Teller (Micromeritics Tristar 3000). In order to avoid the charging effect of SEM images for cycled separators, Pt layer was deposited onto the separator via Pt coater. Additional SEM images of sulfur cathode electrode before and after cycling were provided in Figure S8.

SUPPORTING INFORMATION

Cycle voltammetry curves of each electrode (Figure S1); Rate retention of each electrode (Figure S2); EIS curves of each electrode after 200 cycles (Figure S3); Polysulfide absorption test (Figure S4); Comparison of rate retention of sulfur cathodes with a Gr/ZnO-separator and a Gr/SSE-separator (Figure S5); XPS spectrum of black spots over Li metal (Figure S6); 19 ACS Paragon Plus Environment


SEM images and EDS mapping images of Gr/ZnO-separator (Figure S7); SEM images of sulfur cathodes before and after cycles ((Figure S8).

Notes The authors declare no competing financial interest.

Corresponding author *E-mail: [email protected]

Acknowledgement The authors would like to acknowledge Purdue University, and the Davidson School of Chemical Engineering for their financial support. The authors wish to thank the Office of Naval Research for supporting Li and Na metal battery safety project under Naval Enterprise Partnership Teaming with Universities for National Excellence (NEPTUNE Phase I and II) at Purdue Center for Power and Energy Research provided under grant number N00014-15-12833. The authors also want to thank the Office of Naval Research for supporting the project under grant number N00014-18-1-2397. The Li-S part of the included research was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under contract no. DE-EE0006832 and under the Advanced Battery Materials Research (BMR) Program.

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Table of contents (TOC)


Li-Ion-Permeable and Electronically Conductive Membrane

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