Metal–Organic-Framework-Based Gel Polymer Electrolyte with

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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Metal−Organic-Framework-Based Gel Polymer Electrolyte with Immobilized Anions To Stabilize a Lithium Anode for a Quasi-SolidState Lithium−Sulfur Battery Dian-Dian Han,† Zhen-Yu Wang,† Gui-Ling Pan,*,‡ and Xue-Ping Gao*,† †

Institute of New Energy Material Chemistry, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China ‡ Key Laboratory of Functional Polymer Materials of the Ministry of Education, College of Chemistry, Nankai University, Tianjin 300071, China ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by BOSTON COLG on 05/07/19. For personal use only.

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ABSTRACT: A lithium−sulfur (Li−S) battery is widely regarded as one of the most promising technologies for energy storage because of its high theoretical energy density and cost advantage. However, the shuttling of soluble polysulfides between the cathode and the anode and the consequent lithium anode degradation strongly limit the safety and electrochemical performance in the Li−S battery. Herein, a metal−organic-framework (MOF)-modified gel polymer electrolyte (GPE) is employed in a Li−S battery in order to stablize the lithium anode. In view of the abundant pores in the MOF skeleton, the as-prepared GPE not only immobilizes the large-size polysulfide anions but also cages electrolyte anions into the pores, thus facilitating a uniform flux of Li ions and homogeneous Li deposition. Cooperated with a sulfur−carbon composite cathode, the lithium with MOF-modified GPE exhibits a uniform surface morphology and dense solid electrolyte interphase (SEI) film, thus delivering good cycle stability and high-rate capability. KEYWORDS: lithium−sulfur battery, lithium anode, gel polymer electrolyte, quasi-solid-state, metal−organic-framework repeated plating/stripping process will not only cause “dead Li” coupled with the capacity loss but also make the solid electrolyte interphase (SEI) unstable.27,28 Furthermore, the Li anode faces a much more complex environment in a typical Li−S battery, where the insoluble Li2S/Li2S2 gathers on the lithium anode as a result of the shuttling of soluble polysulfides. In response, it is significant to stabilize the Li anode for the application of highenergy Li−S batteries. Aiming to address the above issues, numerous strategies are developed, including liquid electrolyte optimization,29−31 artificial SEI film formation,32−34 and application of a novel anode.35−37 Although such methods are effective to stabilize the lithium anode to a certain extent, most of which are dedicated to regulate the ion distribution during repeated cycling. To achieve the purpose of high-safety, a quasi-solid-state Li−S battery has attracted great attention, in which electrolytes are usually gel polymer electrolyte (GPE) or an interphase between the liquid electrolyte and the solid-state electrolyte.38,39 In view of the effective encapsulation of the liquid electrolyte, GPE not only possesses relatively higher ionic conductivity but also inhibits the growth of lithium dendrites to some extent and improve the battery safety.40 Furthermore, the content of the

1. INTRODUCTION To date, higher energy density batteries have attracted much attention for meeting the ever-growing energy demand.1−3 On account of a higher theoretical capacity (1675 mA h g−1) and abundant reserves of sulfur, lithium−sulfur (Li−S) batteries have been considered as the most promising candidates for newgeneration battery systems.4 However, there are many issues limiting the development of a Li−S battery, such as the intrinsic insulation of sulfur, the unfavorable “shuttle effect” resulting from soluble polysulfides, and the severe lithium anode degradation.5−11 Nowadays, much endeavor has been devoted to address these problems by encapsulating sulfur into various carriers, such as carbon materials,12−16 metal compounds,17−20 and metal−organic framework (MOF)-based materials.21−24 Such methods could suppress the migration of soluble polysulfides between the anode and the cathode to some extent based on the consideration of enhancing the cycle stability of the cathode. However, protection of the lithium anode should be also considered for practical application of Li−S full cells. Because of the ultrahigh theoretical capacity of 3860 mA h g−1 and the extremely low electrochemical potential of −3.04 V, the Li anode is recognized as the “Holy Grail” for secondary batteries, typically using sulfur as the cathode in Li−S batteries.25,26 Nevertheless, the severe degradation of the Li anode grievously hazards the safety and cyclability of the Li−S battery. The inhomogeneous distribution of Li during the © XXXX American Chemical Society

Received: February 27, 2019 Accepted: April 26, 2019

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DOI: 10.1021/acsami.9b03682 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of MOF−PVDF GPE with anions immobilized for the lithium−sulfur battery. solution on the as-prepared PVDF film. Afterwards, the obtained membrane was immersed in the organic electrolyte for 12 h to receive the specified GPE, which was abbreviated as MOF−PVDF. For reference, the bare PVDF membrane was dipped in the same organic electrolyte, which consisted of dissolving 1 M bis-(trifluoromethane)sulfonimide lithium and 0.2 M LiNO3 in 1:1 (in volume) 1,3dioxacyclopentane (DOL) and 1,2-dimethoxyethane (DME). 2.2. Preparation of the Sulfur Cathode. The cathode material was obtained via the melt-diffusion method. Sulfur and graphene were fully milled at a mass ratio of 7:3 and then heated at a temperature of 155 °C for 12 h. Then, the above sample was mixed with Super P (as conductive agent) and PVDF (as binder) in the organic reagent of Nmethyl-2-pyrrolidone (7:2:1, by weight ratio). After the above mixture was placed on the collector uniformly and layed under the circumstances of 60 °C overnight, the cathode was acquired through punching the above film to disks at a diameter of 10 mm. The areal sulfur content in the as-prepared cathode was regulated at 0.8−1.2 mg cm−2. 2.3. Material Characterization. The obtained samples were characterized via X-ray diffraction (Rigaku Mini FlexII). The morphology of the as-prepared samples and the relevant sulfur content in the sulfur/graphene composite were determined on a scanning electron microscope (SEM) (Supra 55VP) and a thermogravimetric (TG) analyzer (METTLER TOLEDO, TGA/DSC1), respectively. The composition analysis of the obtained samples was characterized by an X-ray photoelectron spectroscopy (XPS) instrument with an instrument model of Thermo Scientific ESCALAB 250Xi. The electrolyte uptake of all membranes was calculated according to the equation ΔM = (Ms − M0)/M0 × 100%; herein, Ms and M0 represent the weight of soaked and dried films, respectively. The amount of organic electrolyte was computed by the formula ΔV = (Ms − M0)/(ρ × Msulfur). Here, ρ and Msulfur represent the density of organic electrolyte and the weight of sulfur in the S/C composite cathode, respectively. Fetch a certain volume of liquid electrolyte to measure the weight and then calculate the ρ value. 2.4. Electrochemical Measurement. Li−S cells with 2032-type were assembled in the glovebox, and the lithium metal was served as the counter and reference electrode. Under the same conditions, the cells using a Celgard 2325 separator were assembled with an electrolyte-tosulfur (E/S) ratio of ∼40 μL mg−1. The charge/discharge measurements were performed on LAND CT2001A systems (Wuhan Jinnuo, China) between 1.7 and 2.8 V (vs Li/Li+). Cyclic voltammetry (CV) tests were conducted on a CHI 600A workstation at a scan rate of 0.1 mV s−1. To measure the ionic conductivity, the obtained GPE was assembled with two stainless steel sheets using a Zahner IM6ex instrument in the frequency range of 100 kHz to 100 mHz, while the amplitude was controlled at 5 mV. The ionic conductivity was calculated via the following formula σ = d/(RA), where, R represents the resistance of the bulk electrolytes, d represents the thickness of all membranes, and A represents the effective area of GPE. The transference number (t+) of Li ions was tested using Li/GPE/Li symmetrical cells through the potentiostatic polarization technique. The applied equation was demonstrated as below t+ = Iss(ΔV − I0R0)/

liquid electrolyte in the Li−S battery with GPE is less than that in the conventional batteries with liquid electrolyte (normally >20 μL/mg(sulfur)), which is beneficial to increase the energy density of the battery.41 Because of the large dielectric constant (ε = 8.4) and strong C−F bonds, poly(vinylidene fluoride) (PVDF) electrolyte is a good candidate for building the quasisolid-state Li−S battery.42 However, the PVDF electrolyte exhibits poor interfacial stability with the Li anode, thus limiting the application in the Li−S battery. According to space-charge theory, a space-charge region caused by ion transport could lead to the growth of Li dendrites, thus making it rewarding to modify the electrolyte with ion selectivity to regulate the diffusion of Li ions and the corresponding anions.43 As well known, MOFs, constituted of transition metal cations and organic ligands, are a kind of porous material.44,45 Based on the unique structure by the open metal sites and organic functional groups, MOF-based materials are all equipped with high porosity and ionic selectivity, which are widely employed in the fields of gas separation,46−49 chemical catalysis,50−53 and drug delivery.54,55 Nowadays, many efforts are devoted to applying MOF-based materials to the separators in Li−S batteries on account of their excellent ionic selectivity.56−59 Herein, a novel GPE with immobilized anions is employed in the lithium−sulfur battery in order to stabilize the lithium anode. The Mg(II)-based MOF material (Mg-MOF-74) is constituted of metal species (Mg2O2(CO2)2) and 2,5-dioxido1,4-benzenedicarboxylate (DOBDC), which has an appropriate pore size of 10.2 Å and abundant Lewis acidic sites.60−62 Hence, Mg-MOF-74 material is chosen to modify the PVDF-based GPE to obtain a MOF-modified GPE. As illustrated in Figure 1, the obtained GPE could not only immobilize the large-size polysulfide anions but also encage TFSI− anions in the pores, thus facilitating a uniform flux of Li ions and homogeneous lithium deposition. As a result, the lithium anode in the Li−S battery using MOF−PVDF GPE exhibits a uniform morphology and stable SEI films. Meanwhile, the rate capability and cycle life of the quasi-solid-state lithium−sulfur battery are highly enhanced by the bi-functional GPE with a modification of the MOF.

2. EXPERIMENTAL SECTION 2.1. Synthesis of MOF-Modified PVDF-Based GPE. The preparation method of the PVDF membrane was referred to our previous work.5 The MOF-modified PVDF GPE was obtained by the vacuum-filtration method. First, certain amounts of Mg-MOF-74 (15 mg, Nanjing JC Nano Tech Co., Ltd.) and polyvinylpyrrolidone (30 mg) were dispersed in ethanol (5 mL) and sonicated for 1 h. Then, the MOF-modified polymer film was obtained by filtering the above B

DOI: 10.1021/acsami.9b03682 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. SEM images of the prepared Mg-MOF-74 (a) PVDF film (b) and MOF−PVDF film (c). Insets in (b,c) display the corresponding cross section of the bare PVDF and MOF−PVDF films, respectively. The impedance spectra of the cells with various electrolytes and the relevant ionic conductivity at room temperature (d). The uptake capacity of all membranes (e) and the photographs of the as-prepared films after heating at 150 °C for 30 min (f).

Figure 3. Li plating/stripping behavior of Li/Li symmetrical cells using various electrolytes at a low current density of 0.5 mA cm−2 (a) and the relevant results of the voltage difference (b). SEM pictures of Li in the Li/Li symmetrical cells using a commercial separator (c), PVDF GPE, (d) and MOF− PVDF GPE (e) after plating/stripping for 20 cycles at a current density of 0.5 mA cm−2. Current−time curves of the Li/Li symmetric cell with MOF− PVDF electrolyte (f) and the inset indicates the electrochemical impedance spectra of the corresponding cell. (g) t+ values of Celgard, PVDF, and MOF−PVDF GPE. I0(ΔV − IssRss), where, I0 and R0, respectively, represent the initial current and resistance, Iss and Rss represent the ones at the final stable state, and ΔV set as 10 mV.

sional hexagonal channels with a pore size of 10.2 Å.60,61 As shown in Figure 2a, Mg-MOF-74 possesses a rod-like morphology with various lengths. After the phase inversion and the following drying process, a bare PVDF film with a thickness of 25 μm shows an abundant pore structure caused by the appropriate exchange between dimethylformamide solution and water (Figure 2b). After filtrating using the obtained PVDF

3. RESULTS AND DISCUSSION Normally, the Mg-MOF-74 skeleton, constituted of metal species (Mg2O2(CO2)2) and DOBDC, possesses one-dimenC

DOI: 10.1021/acsami.9b03682 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces film, Mg-MOF-74 is distributed uniformly to construct a dense MOF-modified PVDF film with a total thickness of ∼50 μm (Figure 2c). In order to inspect the ionic conductivity, all electrolytes are placed between two stainless steel sheets for measurement, and then the results are shown in Figure 2d. Clearly, the ionic conductivity of the Celgard separator maintains a minimum value of 3.43 × 10−4 S cm−1, while PVDF electrolyte and MOF−PVDF electrolyte show relatively large ionic conductivities of 4.26 × 10−4 and 6.72 × 10−4 S cm−1, respectively. Although the resistance value of the MOF-modified PVDF electrolyte becomes slightly larger than that of the bare PVDF electrolyte, the corresponding ionic conductivity is obviously improved owing to the increased thickness after MOF modification. The polymer membranes and Celgard 2325 are dipped into the organic electrolyte to detect the corresponding uptake capacity. As illustrated in Figure 2e, the bare PVDF film has a higher electrolyte uptake (249.3%) than the commercial separator (149.8%) because of the excellent affinity of PVDF to liquid electrolyte. Considering the porous feature for MOF material, the electrolyte uptake of the polymer film modified by the MOF is increased further up to 256.1%, superior than the unmodified one. In consequence, the electrolyte usages (E/S) of the cells with MOF−PVDF GPE and unmodified-PVDF electrolyte are ∼16 and 9 μL mg(sulfur)−1, respectively. Furthermore, the usage of liquid electrolyte in the Li−S battery is obviously large (∼40 μL/mg(sulfur), Table S1). To probe the electrochemical stability, an asymmetric coin cell of Li/MOF− PVDF/stainless steel is conducted for the CV test at 1 mV s−1 (Figure S1). Obviously, there is a couple of redox peaks between −0.5 and +0.5 V, corresponding to the lithium plating/stripping process. However, no significant redox peaks can be observed in the voltage range of 1.7−2.8 V, demonstrating the good stability of the obtained MOF−PVDF electrolyte at the voltage range used. To inspect the thermal stability of the obtained films, all the samples are subjected to the environment with heating at 150 °C for 0.5 h. As shown in Figure 2f, both the PVDF and MOF−PVDF films demonstrate excellent thermal stability with moderate thermal shrinkage, while there is a severe contraction in the Celgard separator after heating at 150 °C. In consequence, PVDF-based GPE is beneficial for constructing the quasi-solidstate Li−S battery with high-safety. To further investigate the interaction of the MOF material during the lithium plating/stripping process, Li/GPE/Li symmetrical cells are operated at a current density of 0.5 mA cm−2 with an areal capacity of 1.5 mA h cm−2. As presented in Figure 3a,b, the cell with the Celgard separator undergoes large voltage variation throughout the cycles, followed by increasing the voltage gap from 0.0833 to 0.1847 V. Meanwhile, the polarization voltage of the Li/Li symmetrical cell using PVDF GPE is augmented to 0.125 V after 90 cycles. All the phenomena can be ascribed to the inferior stability with lithium for Celgard and PVDF electrolyte. Nevertheless, the voltage difference of the Li/Li symmetrical cell using MOF−PVDF electrolyte is merely 31 mV during 120 cycles without the presence of short-circuit. In addition, the cell with the MOF-modified electrolyte also maintains stability when the current density is enhanced to 2 mA cm−2, fully demonstrating a positive influence on Li electrodeposition of the MOF material (Figure S2). Aiming to directly inspect the surface morphology during the Li plating/stripping process, the SEM technology is used to characterize the lithium surface of the symmetric cells using various electrolytes after 20 cycles. In Figure 3c−e, the lithium surface of the cell with MOF−PVDF GPE is relatively smooth without any obvious

bumps, while some rod-like lithium appears on the lithium of Li/ Li symmetrical cells with Celgard and PVDF GPE, indicating that Mg-MOF-74 is in favor of the homogeneous lithium plating/stripping process. Moreover, a similar change tendency in the Li morphology can be also observed in Figure S3, where the lithium electrode in symmetric cells with different electrolytes undergoes plating/stripping for 20 cycles at a larger current density of 2 mA cm−2, further demonstrating the preferable regulation of lithium plating/stripping by Mg-MOF74 material. The relevant ionic diffusivity in the obtained electrolyte is expressed in terms of transference number (t+) of Li ions, which could be calculated by the experiment and a specific formula. The conventional liquid electrolytes exhibit lower t+ values of 0.2−0.4, in good agreement with our experimental value of 0.37 (Figures S4a and 3g). The t+ value of the PVDF electrolyte approaches 0.50, which is mainly attributed to the high dielectric constant (ε = 8.4) of PVDF.63,64 In contrast, the MOF−PVDF electrolyte possesses a much higher t+ value of 0.66, confirming that the MOF material could fix the counter anions effectively. Considering that the pore size (10.2 Å) of the Mg-MOF-74 sample is larger than the length of TFSI− anions (7.9 Å), the MOF material could encage large-sized anions, thereby limiting the migration of TFSI−.65 In addition, with a large quantity of Mg2+ ions, the Mg-MOF-74 material provides abundant unsaturated coordination sites for Lewis bases, thus inducing the formation of more Lewis acidic sites. Based on the Lewis acid−base effect, the Mg-MOF-74 material can strongly interact with TFSI− anions as Lewis bases, further confining the diffusion of TFSI− anions in the channel.61,62 Considering that FSI− anions possess the same charge number and a similar structure to TFSI− anions,29 the immobilization of FSI− anions in MgMOF-74 material is also based on the steric hindrance and the Lewis acid−base effect. For MOF-modified electrolytes, the restriction of TFSI− anions in Mg-MOF-74 material is relatively stronger as compared with FSI− anions according to the steric hindrance. On the other hand, the interaction of small FSI− anions with Mg-MOF-74 material becomes stronger than that of large TFSI− anions due to the Lewis acid−base effect. The multiple factors may result in competitive interaction with different anions in Mg-MOF-74. In comparison with small FSI− anions, large TFSI− anions may have a positive impact on t+ of the electrolyte, in which the effect of the steric hindrance is dominant.65 Moreover, Figure 4 shows a schematic illustration

Figure 4. Schematic illustration of the transport of Li ions in the skeleton of the Mg-MOF-74 material.

of the transport of Li ions in the skeleton of Mg-MOF-74 material. As reported previously,43 the mobile anions from the electrolyte with smaller t+ easily move toward the direction contrary to the counter cations, thus impeding the cation transportation. Owing to the steric hindrance and Lewis acid− base effect, the Mg-MOF-74 material could limit TFSI− anions inside the well-ordered pores effectively. When large TFSI− D

DOI: 10.1021/acsami.9b03682 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. SEM images (a−c) of the surface and the relevant cross section of lithium ions in the cells using Celgard 2325 (a,j), PVDF GPE (b,k), and MOF−PVDF GPE (c,l) after 100 cycles, correspondingly. The relevant elemental mapping (d−i) images on the top of lithium ions from the cells using Celgard 2325 (d,g), PVDF GPE (e,h), and MOF−PVDF GPE (f,i).

Figure 6. F 1s and S 2p characteristic peaks of XPS spectra of the Li anodes of the cells using Celgard 2325 (a,d), bare PVDF GPE (b,e), and MOFmodified GPE (c,f) after cycling for 100 times.

the vessel with a Celgard separator after placing for 20 h, resulting in the slight yellow appearance of the solution on both sides. In comparison, both vessels with the MOF−PVDF film experience the ignored color variation during resting for 20 h, implying that the MOF material is beneficial for limiting the diffusion of polysulfides. Given that the diameter of polysulfides is significantly larger than the pore size of Mg-MOF-74 (10.2 Å), the MOF-modified electrolyte can well impede the shuttling of polysulfides.57,66 In view of the investigation above, all GPEs are used in Li−S batteries with a sulfur content of 71.17% (Figure S6). After 100

anions are immobilized, it is more convenient for Li-ions to move from the electrolyte to the lithium anode with 1D channels based on space-charge theory. Therefore, the MOF-modified GPE is more stable toward the lithium anode due to the effective fixing of anions. In order to explore the interaction of MOF−PVDF GPE with soluble polysulfides, the diffusion experiments of polysulfides are carried out in H-type glass containers, which are assembled with various films between 20 mM Li2S6 in DOL/DME on the left and blank DOL/DME on the right. As shown in Figure S5, polysulfides can be diffused quickly from the left to the right in E

DOI: 10.1021/acsami.9b03682 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. CVs (a) of the cell using MOF−PVDF GPE. The discharge and charge curves in the first three weeks (b) of the cell using MOF−PVDF GPE at 0.1 C. The cycling ability at a rate of 0.1 C (c) and 1 C (d) (1 C = 1675 mA g−1) of the cells with various electrolytes. Rate performance (e) at various rates of the cells with different electrolytes.

To deeply understand the chemical composition on the Li anode surface, XPS is performed on the lithium in the cells using various electrolytes after 100 cycles. As shown in Figure 6, F and S are core constituents on the lithium surface. In F 1s spectra, the organic −NSO2CF3 (688.3 eV) and inorganic LiF (684.7 eV) are dominant on the lithium surface in cells with different electrolytes. For S 2p spectra, six characteristic peaks can be observed at binding energies of 160.3, 161.6, 163.1, 167.2, 168.6, and 170.3 eV, where the peaks at 160.3, 161.6, 167.2, and 168.6 eV are attributed to Li2S, Li2S2, Li2SO3, and Li2SO4, respectively. Besides, a larger intensity of Li2SO3 and Li2SO4 could also be observed on the Li anode in the cell using MOF−PVDF electrolyte, accompanied with the low intensity of Li2S/Li2S2. This means that the formation of the SEI film with inorganic components is effective to suppress the deposition of Li2S/Li2S2 on the Li anode. Overall, employing Mg-MOF-74 into PVDFbased electrolyte is beneficial for optimizing the composition of SEI films on the Li anode surface due to the effective immobilization of anions. To get an insight into the redox behavior during the electrochemical process, CV tests are conducted on the Li−S cells with a sulfur loading of 0.8−1.2 mg cm−2. As illustrated in Figures 7a and S7, two cathodic peaks and an equal number of anodic peaks can be seen for cells with various electrolytes, indicating the typical two-step redox of the as-prepared S/C composite. More importantly, sharper redox peaks are shown for the cell with MOF−PVDF GPE, suggesting lower polarization and excellent reversibility. The initial discharge/charge curves of the cells with obtained electrolytes can further confirm the above results as presented in Figures 7b and S8. For the long-term

cycles, the cells are disassembled to probe the surface morphology of the lithium anode. As presented in Figure 5a, the lithium anode of the cell with Celgard experiences serious surface changes along with pulverization and cracks, which are mainly attributed to the inhomogeneous Li electrodeposition and the grievous corrosion by soluble polysulfides. In contrast, relatively minor changes are observed on the surface of the lithium anode for the cell using PVDF electrolyte; however, obvious cracks are still shown. Differently, the Li anode of the cell using the MOF-modified electrolyte displays a fairly smooth morphology after 100 cycles with relatively lower S element content (3.50 wt %) and higher F element content (12.45 wt %) (Table S2). In addition, the distributions of S and F elements on the Li anode in the cell with MOF−PVDF electrolyte are more homogeneous than those with Celgard and PVDF electrolyte (Figure 5d−i). Meanwhile, SEI information can be compared from the cross section of Li anodes (Figure 5j−l). Obviously, the SEI film of the cell using Celgard 2325 emerges a heterogeneous morphology with a thickness from 4 to 9 μm, caused by the excessive consumption of soluble polysulfides toward the Li anode. Furthermore, the cell with blank PVDF GPE exhibits a patchy SEI film, signifying that the surface of the Li anode could also be corroded by soluble polysulfides. In comparison, a dense and smooth SEI film at a thickness of 5 μm is observed on the Li anode of the cell using MOF-modified GPE. Considering that the Mg-MOF-74 material could immobilize the anions (TFSI−) and improve the distribution of Li ions, the MOF-based electrolyte is in favor of stabilizing the morphology and the SEI film on the lithium anode surface, corresponding to the experimental results above. F

DOI: 10.1021/acsami.9b03682 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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cycles at 0.1 C rate, the cell with MOF−PVDF GPE delivers relatively low capacity decay from the initial 1383.1−981.1 mA h g−1 after 200 cycles with a capacity fading of 0.14% per cycle (Figure 7c). In comparison, the cells with the Celgard separator and PVDF electrolyte possess the comparable initial discharge capacities (1379.8 and 1376.6 mA h g−1, respectively), while the reversible capacities are quickly decreased to 633.1 and 837.8 mA h g−1 over 200 cycles, correspondingly. After gradual activation at low rates (0.1/0.2/0.5 C), the cell with MOF− PVDF GPE can be charged/discharged repeatedly at 1.0 C rate. After 250 cycles, the cell manifests a relatively higher capacity of 778.4 mA h g−1 with a low capacity fading of 0.09% per cycle (Figure 7d). Compared with the electrochemical ability of other Li−S batteries using different electrolytes, the cell with MOF− PVDF GPE exhibits relatively good cycle performance (Table S3). Moreover, the cell with MOF−PVDF GPE also presents superior rate performance as verified in Figures 7e and S9. Cycling at 0.2/0.5/1.0/2.0 C rates, the corresponding discharge capacities are 1195.3, 1083.9, 996.7, and 860.5 mA h g−1. However, the cells with Celgard and PVDF GPE exhibit lower rate capacities at different rates, especially at 2.0 C rate with reversible capacities of only 736.0 and 797.2 mA h g−1, respectively. Overall, introducing the MOF material into the GPE is beneficial for stabilizing the Li anode and limiting the diffusion of polysulfides, thus resulting in a relatively superior electrochemical performance. Because of the unique pore structure with a pore size of 10.2 Å, the Mg-MOF-74 material can not only block the soluble polysulfides from diffusing to the Li anode but also cage TFSI− anions, thus resulting in a uniform flux of Li ions and a stable Li anode. Therefore, the Mg-MOF-74 material with the anions immobilized has the potential application for GPE of the lithium−sulfur battery.

Xue-Ping Gao: 0000-0001-7305-7567 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the 973 Program (2015CB251100), National Natural Science Foundation (21573114), and Fundamental Research Funds for the Central Universities of China.

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4. CONCLUSIONS In summary, a MOF-modified GPE with anions immobilized is applied in a Li−S battery for acquiring a stable Li anode. Because of the natural pore structure with a pore size of 10.2 Å, the MgMOF-74 skeleton shows bifunctional characteristics, not only blocking soluble polysulfides from shuttling to the Li anode but also caging TFSI− anions based on the steric hindrance and the Lewis acid−base effect, thus facilitating a homogeneous flux of Li ions. As a result, the cell with MOF-modified GPE exhibits an excellent cycling life and enhanced rate performance; meanwhile, the lithium anode manifests a uniform morphology and a dense SEI film even after long-term cycles. This novel strategy of an anion-immobilized GPE to stabilize the Li anode is also feasible for other lithium metal batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03682.

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REFERENCES

Voltage−time curves, polarization curves, diffusion experiments of soluble polysulfides, CV curves, and rate capability curves of all the samples (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.-L.P.). *E-mail: [email protected] (X.-P.G.). G

DOI: 10.1021/acsami.9b03682 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.9b03682 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX