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Sulfonic Groups Originated Dual-Functional Interlayer for High Performance Lithium−Sulfur Battery Yang Lu,†,‡ Sui Gu,†,‡ Jing Guo,†,‡ Kun Rui,†,‡ Chunhua Chen,§ Sanpei Zhang,†,‡ Jun Jin,† Jianhua Yang,† and Zhaoyin Wen*,† †

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100039 P. R. China § CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: The lithium−sulfur battery is one of the most prospective chemistries in secondary energy storage field due to its high energy density and high theoretical capacity. However, the dissolution of polysulfides in liquid electrolytes causes the shuttle effect, and the rapid decay of lithium sulfur battery has greatly hindered its practical application. Herein, combination of sulfonated reduced graphene oxide (SRGO) interlayer on the separator is adopted to suppress the shuttle effect. We speculate that this SRGO layer plays two roles: physically blocking the migration of polysulfide as ion selective layer and anchoring lithium polysulfide by the electronegative sulfonic group. Lewis acid−base theory and density functional theory (DFT) calculations indicate that sulfonic groups have a strong tendency to interact with lithium ions in the lithium polysulfide. Hence, the synergic effect involved by the sulfonic group contributes to the enhancement of the battery performance. Furthermore, the uniformly distributed sulfonic groups working as active sites which could induce the uniform distribution of sulfur, alleviating the excessive growth of sulfur and enhancing the utilization of active sulfur. With this interlayer, the prototype battery exhibits a high reversible discharge capacity of more than 1300 mAh g−1 and good capacity retention of 802 mAh g−1 after 250 cycles at 0.5 C rate. After 60 cycles at different rates from 0.2 to 4 C, the cell with this functional separator still recovered a high specific capacity of 1100 mAh g−1 at 0.2 C. The results demonstrate a promising interlayer design toward high performance lithium−sulfur battery with longer cycling life, high specific capacity, and rate capability. KEYWORDS: sulfonated reduced graphene oxide, ion selective interlayer, functional separator, Lewis acid−base principle, lithium−sulfur batteries, DFT calculation

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INTRODUCTION

and eco-friendly, which would be crucial for the future potential applications on a large scale.1 However, several issues still hinder the practical utilization of Li−S battery in spite of its great potential.3 First, the intermediate discharge products of sulfur (polysulfides) are highly soluble in the ether-based electrolyte. The diffusion of

With the rapid development of energy storage technology over the past decades, the lithium-ion batteries have almost approached the limitation of their theoretical energy density.1,2 The rechargeable lithium−sulfur (Li−S) battery, due to its extremely high energy density (2600 Wh kg−1) and theoretical capacity (1675 mAh g−1), is considered as one of the most attractive alternatives for the next-generation energy storage chemistries. Furthermore, sulfur is low cost, abundant in nature, © 2017 American Chemical Society

Received: February 14, 2017 Accepted: April 13, 2017 Published: April 13, 2017 14878

DOI: 10.1021/acsami.7b02142 ACS Appl. Mater. Interfaces 2017, 9, 14878−14888

Research Article

ACS Applied Materials & Interfaces the polysulfides between sulfur cathode and lithium anode driven by the concentration gradient, which is known as the shuttle effect, could lead to low Coulombic efficiency, poor cycle stability, self-discharge, and loss of the active material.4 Moreover, sulfur and Li2S/Li2S2 are electrical insulators, which also deteriorate the rate capability and the cycling life. Thus, great efforts have been made to address these problems, including rational design and modification of cathode host materials,5−11 protection of lithium anode,12−15 functionalization of separator or interlayer,16−19 and optimization of electrolyte additives.20−23 Recently, functional separator or interlayer between the separator and cathode has been considered as a highly effective approach to enhance electrochemical performance of Li−S batteries. First, the functional layer serves as a physical barrier to prevent the dissolved polysulfide from further diffusing to the anode.18,24 Additionally, during the discharge−charge process, the designed layer can trap and reuse the soluble polysulfide, acting as a second current collector, enhancing the utilization ratio of the active materials.18 Up to now, many reports have been focused on the carbon-based materials utilized as the functional interlayers in Li−S batteries.17,18,24−26 However, most of the carbon hosts are nonpolar, delivering no obvious chemical interaction with the polar polysulfide. Thus, it is still challenging for nonpolar carbon hosts to offer effective interaction with the polar Li2SX. To impart the polarity of pure carbon matrices, various strategies have been exploited, including addition of polar additives (e.g., TiO2), heteroatom doping, and chemical modification of polar functional groups (e.g., carboxyl groups and hydroxyl groups).19,27−33 Obvious performance enhancements have been achieved and are found to be mainly attributed to the static interaction formed between polysulfide and the carbon scaffold.34 On the other hand, cation selection membranes also possess the ability to separate the cations and anions in the solution. These routes have been utilized in the lithium sulfur batteries. For instance, Zhang and co-workers reported that GO and Nafion membrane with cation selective groups as blocking layer could significantly alleviate the influence from the shuttle effect.35,36 The ion selective groups, such as carboxyl groups and sulfonic groups, are electron-rich, and these functional groups have a strong tendency to attract polar Li2SX.37 However, it should be pointed out that most of the membrane matrices have poor conductivity, which would result in the increase of inner impedance. Meanwhile, during the cycling, the anchored Li2SX on the membranes cannot be reused. Considering the aforementioned problems, we tried to combine the ion selective blocking and polar interaction together to obtain a high-efficiency interlayer. To achieve this, we choose sulfonic groups to modify the electron conductive graphene sheets. The achieved SRGO was introduced on the glass fiber membranes by vacuum filtration. The sulfonic groups could achieve two targets as shown in Scheme 1. After these modification processes, our cells exhibit a high reversible discharge capacity of more than 1300 mAh g−1 and good capacity retention of 802 mAh g−1 after 250 cycles at 0.5 C rate. After 60 cycles at different rates from 0.2 to 4 C, the cell still recovered to a high specific capacity of 1100 mAh g−1 at 0.2 C. The enhanced performances are mainly attributed to the applied interlayer. To further analyze the interaction between sulfonic groups and the polysulfides, the DFT calculation was applied. It was anticipated that sulfonic groups could efficiently anchor the terminal lithium ion in the Li2SX by forming Li

Scheme 1. Schematic Representation of Indicating Blocking Theory for Lithium−Sulfur Battery

bonds. We also excluded the influence of lithium ion from the lithium salt to make our conclusion precisely.

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EXPERIMENTAL SECTION

Synthesis of SRGO. The SRGO was synthesized through a modified aryl diazonium salt coupling reaction according to previously reported methods as shown in Scheme 2.38,39 In a typical procedure, GO (graphene oxide) was first obtained by the modified Hummer method.40 Then the GO was dispersed in deionized water and prereduced by glucose at 95 °C for 2 h. One gram of sulfanilic acid (Aladdin, 99%) was dissolved in 10 mL of 2 wt % NaOH solution. Then sodium nitrite (0.4 g) was added. The obtained solution was

Scheme 2. Schematic Representation of the Synthesis Process of SRGO

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ACS Applied Materials & Interfaces

Figure 1. (a) FTIR spectra of GO, RGO, and SRGO. (b) XPS spectra of GO, RGO, and SRGO. (c) Corresponding high-resolution S 2p XPS spectrum of SRGO. (d) Digital photo of SRGO solution and RGO solution with concentration of 0.1 mg mL−1 after standing for 1 h. 10−3 Ha Å−1, respectively. The calculation of the interaction was set in a 20 Å × 20 Å × 20 Å box. The K point was set as 8 × 1 × 1. The binding energy for the system was defined as the following formula:

gradually added into 10 mL of ice water below 5 °C. After 30 min vigorous stirring, the solution would turn ivory white. Then, the ivory white solution was added dropwise into a 30 mL RGO dispersion (10 mg mL−1) in an ice bath. This mixture was vigorously stirred in an ice bath for another 5 h. Finally, the black solid SRGO was collected, washed with deionized water for several times, and then freeze-dried for 24 h. Preparation of the SRGO Coating Glass Fiber Separator. The obtained SRGO was homogeneously dispersed in water at the concentration of 0.1 mg mL−1. The glass fiber membranes were then coated by the SRGO through vacuum infiltration of the SRGO solution. After coating, the SRGO layers were pressed to stack tightly. The loading amount of SRGO was about 0.15 mg cm−2. Preparation of the Sulfur Cathode. The mixture of Ketjen Black (KB) and sulfur at the weight ratio of 1:2 was heat treated under vacuum at 155 °C for 12 h to obtain the S/C composite. The cathode slurry was composed of 80 wt % S/C composite, 10 wt % super P conductive carbon, 5 wt % carboxyl methylated cellulose, 5 wt % styrene−butadiene rubber binder, and distilled water. The slurry was casted on the aluminum foil. Then the electrode was dried under vacuum for 12 h and cut into 12 mm disks with the typical sulfur loading of 1.2−1.5 and 5 mg cm−2. The mass ratio between cathode and interlayer is 8.325−10.425:1; therefore, the mass ratio between active sulfur and cathode/interlayer is 50.6%−51.2%. CR2025 coin cells were assembled in the glovebox filled with argon with oxygen and water less than 0.1 ppm. The electrolyte consisted of 1 M LiN(CF3SO2)2 (LiTFSI) in the mixture solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) at volume ratio of 1:1 with 0.1 M LiNO3 as the additive. The computational analysis was applied according to the DFT calculation by the package of Dmol3 in the Materials Studio.41 The calculations were all carried out by utilizing Perdew−Burke−Ernzerhof (PBE) exchange-correlation functions with the general gradient approximation (GGA).42 The double numerical plus (DNP) polarization function was applied for the basis set of all the electron in this calculation system. The energy tolerance accuracy, the displacement, and the maximum force were set as 2 × 10−5 Ha, 5 × 10−3 Å, and 4 ×

E binding = Esystem − (E host + Eguest)

(1)

where Esystem, Ehost, and Eguest stand for the total energy for the interaction system, the isolated host, and the soluble sulfur clusters (Li2SX, X = 4, 6), respectively.

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CHARACTERIZATION Morphology images were obtained by a HITACHI S-3400 scanning electron microscope (SEM, Japan) and a transmission electron microscope (TEM JEOL JEM-2100F). All the cells were tested in the voltage range of 1.8−2.6 V (vs Li+/Li) in galvanostatic mode on a LAND CT2001A battery test system (China). The electrochemical impedance spectroscopy (EIS) measurement was conducted by applying the frequency response analyzer technique on an Autolab electrochemical workstation. The test frequency range was from 10 MHz to 0.1 Hz with perturbation voltage of 10 mV. The cyclic voltammograms were also collected on the same electrochemical workstation at the scan rate of 0.2 mV s−1. The Fourier transform infrared spectroscopy (FTIR) spectra were obtained from a Nicolt IS10 FTIR analyzer (America). X-ray photoelectron spectroscopy (XPS) results were carried out by Thermo Scientific ESCALAB 250 X-ray photoelectron spectroscope. Element mapping images were characterized by an energy dispersive spectrometer (EDS) affiliated with the HITACHI SEM. Some of the lithium sulfur batteries were disassembled to achieve the electrodes and separators for the SEM/EDS/XPS. Before the tests, the electrodes and separators were cut into small pieces and fully washed by DME. All the aforementioned operations were carried out in the glove boxes with argon atmosphere. Then all the samples for XPS and SEM 14880


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Figure 2. SEM images of the coated SRGO glass fiber separator: (a) the cross section and the (b) surface. EDS mapping of (c) carbon, (d) oxygen, and (e) sulfur corresponding to (b).

Figure 3. (a) Cycling and (b) rate performance of the cell with SRGO interlayer, RGO interlayer, and pristine separator at 0.5 C rate with voltage range of 2.6−1.8 V and (c) prolong cycling performance of the cell with SRGO interlayer at the rate of 0.5 C (1 C = 1675 mA g−1).

proving the successful introduction of sulfonic groups on the graphene sheets.43 Specifically, the S 2p XPS spectrum (Figure 1c) displays two main peaks both with a separation of 1 eV ranging from 162 to 172 eV, which could be assigned to S 2p1/2 and S 2p3/2.44 The red S 2p3/2 peak at 168.3 eV indicates the presence of S−O bonds of sulfonic groups, while the green S 2p3/2 at 163.9 eV indicates the presence of C−S bonds of psubstituted phenyl groups.44 From the atomic percentage results from XPS, the content of sulfonic groups was 1.35 mmol g−1. Another evidence of the successful sulfonation is that the as-synthesized SRGO shows better dispersion in water but pure RGO coagulates (Figure 1d).45 Further microstructural characterization was carried out to examine the functional separator after coated with SRGO. A SRGO layer with a thickness of nearly 10 μm on the glass fiber separator can be easily observed from SEM images in Figure 2a. It shows a laminar morphology and attaches on the glass fiber separator tightly. As displayed in Figure 2b, the SRGO layer is

characterization were transferred under an argon atmosphere to the testing apparatuses.

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RESULTS AND DISCUSSION Characterization of the Sulfonation of RGO and SRGO Interlayers. Figure 1 shows the appearance of the RGO and its sulfonated products as well as their compared characterizations. As shown in Figure 1a, the FTIR spectra of GO, RGO, and SRGO demonstrated the successful sulfonation of the RGO. As shown, GO exhibits the presence of adsorption peaks located at 1720 cm−1 (υ CO), 1630 cm−1 (υ CC), 1375 cm−1 (υ C− OH), 1250 cm−1 (υ C−O−C), and 1060 cm−1 (υ C−O). After the prereduction, the absorption peaks at 1700, 1375, 1250, and 1060 cm−1 are weakened and even disappear. This result indicates that most of the hydroxyl groups and ether bonds as well as some CO groups were reduced.43 After sulfonation, peaks at 1186, 1126 cm−1 (υ S−O), 1005 cm−1 (υ S−phenyl), and 840 cm−1(υ p-substituted phenyl group) are observed, 14881


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Figure 4. Electrochemical performance of the cell with different interlayers: (a) Typical cyclic voltammetry (CV) curves of the cell with SRGO interlayer at a scan rate of 0.2 mV s−1 and voltage range from 1.8 to 2.8 V. (b) Impedance spectra of the cell with SRGO and RGO interlayers. (c) The fifth discharge curve of the cell with SRGO RGO interlayer and pristine separator. (d) The first−fifth discharge curves of the cell with SRGO interlayer at 0.5 C rate between 2.6 and 1.8 V vs Li+/Li.

at a high rate (≥2 C). After 250 cycles at 0.5 C, the cell with SRGO interlayer still demonstrates a high capacity of 802 mAh g−1, with an average Coulombic efficiency of more than 99% (Figure 3c). Considering the total mass of cathode and interlayer, the initial specific capacity based on the total mass of cathode and interlayer is 733.7 mAh g−1, and the specific capacity after 250 cycles is also maintained at 402.7 mAh g−1 (Figure S4). In order to get rid of the possibility that the SRGO may make contribution to the lithium storage at the voltage range in this work, the SRGO interlayer were tested in isolation at the voltage range between 3 and 1 V. The consequence proved that SRGO did not show lithium storage at the voltage range for this work (1.8−2.8 V). The details could be found in the Supporting Information (Figure S5). The cathode with higher loading mass of sulfur (5 mg cm−2) is also measured in the batteries. The improved performances are also acquired with SRGO interlayers (Figure S6). Figure 4a shows the typical CV curves of the cell with SRGO interlayer at a scan rate of 0.2 mV s−1. In the first scan, two cathodic peaks located at 2.30 and 1.98 V can be observed, while anodic peaks are located at 2.36 and 2.43 V. After the first scan, the cathodic peaks shift to 2.35 V (I) and 2.01 V (II), where peak I is related to the transformation of original sulfur to the soluble polysulfide (SX2−, 4 < X ≤ 8) and the subsequent reduction to Li2S2 and Li2S is represented by peak II. However, the anodic ones still maintain at 2.33 V (III) and 2.42 V (IV). Peaks III and IV correspond to the continuous oxidation process of Li2S/Li2S2 to Li2S4 and Li2S4 to Li2S8/S8,47 respectively. Furthermore, almost overlapping curves of the subsequent scans can be obviously seen. The slight deviation of

loose and porous which could be favorable for trapping the soluble polysulfide.46 Corresponding EDS mappings of carbon, oxygen, and sulfur element are shown in Figures 2c−e, respectively. Taking carbon and oxygen as references (Figures 2c,d), the sulfonic groups are uniformly distributed on the SRGO sheets (Figure 2e). The TEM images of SRGO and RGO precursor were also tested. After the sulfonation, there are no obvious changes about the morphology of RGO precursor, proving that the sulfonation by aryl diazonium salt will not break the structure of RGO (Figure S1). Lithium−Sulfur Battery Tests. The Li−S cells with SRGO interlayer show very promising electrochemical performance. As shown in Figure 3a, better cycling stability can be achieved compared with those without SRGO interlayer. The cell with SRGO interlayer maintained a higher specific capacity of 930 mAh g−1 and a higher average Coulombic efficiency over 99% after 100 cycles at a rate of 0.5 C. In contrast, the Coulombic efficiency of the cell with RGO interlayer gradually decreased from 96% to 93% with a specific capacity of only 644 mAh g−1 after 100 cycles, indicating less effective suppression of polysulfide diffusion by RGO. The battery without any interlayer shows obvious capacity decay and poor Coulombic efficiency. The rate capability of the cells is shown in Figure 3b. The cell with SRGO interlayer shows the specific capacity of 1240 (third discharge), 1019, 866, 673, 550, and 471 mAh g−1 at rate of 0.2, 0.5, 1, 2, 3, and 4 C, respectively. When it returns to 0.2 C after 60 cycles, the discharge capacity recovers to 1100 mAh g−1. Both cycling stability and rate capability of the cell with SRGO interlayer are obviously better than that of the cells with common RGO interlayer. Furthermore, the cell without any interlayer on the membrane abruptly degraded while cycled 14882


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Figure 5. (a) Charge and discharge testing time−voltage curve of the interruption process (the sixth discharge was suspended at 2.1 V for 72 h) and (b) corresponding performance.

Figure 6. Surface of functional separator: (a) fresh SRGO coating before cycling, (b) the RGO coating before cycling, (c) SRGO coating after 300 cycles, (d) RGO coating after 300 cycles, and element mapping of (e) carbon and (f) sulfur corresponding to (c).

the peaks of the first curve from the subsequent cycles could be ascribed to the redistribution of the active sulfur in the SRGO layers.47 As shown in Figure 4b, the fresh cell with SRGO interlayer possess lower impedance, proving that the SRGO also maintained considerable electric conductive property. In order to verify the presence of interaction between the SRGO interlayer and the polysulfide, we compared and analyzed the specific capacity of two discharge plateaus of the cells with different separators, respectively. Figure 4c displays the fifth discharge curves of cells with SRGO, RGO interlayer, and pristine separator. As shown, the fifth discharge capacity of the cell with SRGO interlayer is 1125 mAh g−1, and the specific capacities of the two discharge plateaus are 350 and 775 mAh g−1, respectively. Both the

specific capacities corresponding to the two discharge plateaus of the cell with SRGO interlayers are higher, indicating that the sulfonic groups on the SRGO contribute to better utilization of soluble state polysulfide and more effective conversion of the solid state lithium sulfide. Combining with the discharge curves in Figure 4c, at the end of discharge, an obvious oblique line on the discharge curve of cell with SRGO is observed, which corresponds to the effective reduction process of solid state Li2S2 to Li2S.48 On the contrary, the RGO could only work as physical barrier and common second collector to the polysulfide.19 We speculate that the uniformly distributed sulfonic groups lead to a uniform distribution of the polysulfide. Thus, in further discharge process, the excessive agglomeration of lithium sulfide could be mitigated to a certain extent, 14883


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Figure 7. SEM image and corresponding EDS mapping of the cross section of Li anode after 50 cycles in (a) cell with pristine separator, and (b) the EDS mapping of sulfur corresponding to (a); (c) cell with RGO interlayer on the separator, and (d) the EDS mapping of sulfur corresponding to (c); (e) cell with SRGO interlayer on the separator, and (f) the EDS mapping of sulfur corresponding to (e).

SRGO is much weaker than that of carbon matrices (KB superP) in the cathode. Therefore, with excessive SRGO, the electron conductivity of the whole cathode will decrease, leading to the poor utilization ratio of active sulfur. This test was carried out with following experiment parameters (rate: 0.5 C; voltage cutoff: 2.6−1.8 V; sulfur mass loading: 1.2−1.5 mg cm−2). Figure 6 presents the morphology changes during cycling for both the SRGO interlayer and the RGO interlayer. Figure 6a displays the porous and loose morphology of SRGO sheets. However, slight agglomeration of RGO sheets could be observed in Figure 6b. After 300 cycles at rate of 0.5 C, a swollen morphology of the SRGO sheets is observed (Figure 6c) while the RGO sheets maintain an unchanged surface morphology (Figure 6d). Therefore, it is speculated that during the continuous cycling, certain electrochemical process of sulfur occurred on the surface of SRGO, leading to the swollen morphology. Meanwhile, the unchanged morphology of the RGO demonstrates that the entrapment of polysulfide on the original RGO is limited.17,47 The element mappings of SRGO in Figure 6c certify that after 300 cycles, taking the signal of element carbon as reference (Figure 6e), sulfur is still distributed uniformly on the SRGO sheets after 300 cycles (Figure 6f). From the perspective of anode, the corrosion of lithium anode could reflect the extent of the shuttle effect.14 Figures 7a−f comparatively demonstrate the cross section and the EDS mapping of lithium anode in cells with pristine separator, RGO coating separator, and SRGO coating separator. The cycled

contributing to the effective conversion of solid state lithium sulfide. The self-discharge performance was tested according to the protocol designed by Hart et al.49 At the sixth discharge, the discharge was suspended at 2.1 V for 72 h as shown in Figure 5a. The concentration of the soluble polysulfide will reach the peak value at 2.1 V, where the insoluble sulfide would be observed if the cell is further discharged.4,50 At the period of interruption, the soluble polysulfide will migrate to the lithium anode causing the loss of active material.51 The capacity loss of self-discharge is marked as Δl in Figure 5b, and the loss rate is calculated by Δl/Qfifth (Qfifth is the specific capacity of the fifth discharge). The Δl values of SRGO, RGO, and pristine separator are 189, 231, and 315 mAh g−1, corresponding to loss rate of 16.5%, 22.6%, and 35.6%, respectively. The cell with SRGO interlayer shows the best result of the suppression of the self-discharge. The optimized proportion of sulfur mass and the amount of SRGO is estimated as follows. Since a light SRGO layer will be in great favor of the energy density of the whole cells, the lightest coating layer, formed by a mass loading of 0.15 mg cm−2, was used in this work. In addition, we have tested the cell with different mass loading of SRGO: 0.25, 0.5, and 0.75 mg cm−2 (Figure S7). It is clearly shown that the mass loading of 0.25 mg cm−2 exhibits similar performance as the loading amount of 0.15 mg cm−2 in Figure 3c. When the mass loading of SRGO is increased, the batteries show worse performance. First, the thickness of SRGO layer may influence the free transfer of lithium ion. Second, the electron conductivity of 14884


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Figure 8. Raw XPS spectra of (a) pristine lithium anode after cycling, (b) fresh SRGO interlayer, (c) lithium anode in batteries with SRGO interlayer after cycling, and (d) SRGO interlayer after cycling. (e) and (f) are the fitted XPS spectra of (c) and (d), respectively.

ensure the adsorption of polysulfides on the SRGO layers. In comparison, it could be obviously found that after cycling the Li2S−Li2S2−S compounds are detected on the surface of SRGO interlayers (binding energy of 164, 162−160 eV).25 Hence, the adsorption of polysulfides is completely confirmed on the surface of SRGO. Mechanism Analysis. Herein, we propose a possible mechanism of the performance improvement involving with the SRGO interlayer (Scheme 3). The most significant point is that the anchored sulfonic groups generate the obvious transformation of the surface properties. First, the sulfonic groups are electronegative ion selective groups.53 This ion selective layer could suppress the migration of the polysulfide. From the EIS results, the battery with SRGO modified separator processes lower integral impedance. The sulfonic groups provide the electronegative environment which promotes the transportation of Li+ ion.54 Thus, the polysulfide could not penetrate this negative layer easily, while lithium ion could transport through it fast. As reported, the detailed mechanism is consistent with the work by Zhang et al. and Manthiram et al.36,54 Compared with reported ion selective membrane such as Nafion or GO, SRGO could maintain higher electronic conduction.35,36,39 Hence, we indicate that this layer may have another function. Many works for N-doped, O-doped, and amino-functionalized graphene have testified that the electronegative doped atom or groups are electron-rich hosts, and they

lithium with SRGO interlayer displays smooth morphology (Figure 7e). Meanwhile, the cycled lithium with RGO interlayer and pristine separator show much rougher morphology (Figures 7a,c and Figure S8). In the corresponding EDS mapping (Figures 7b,d,f), the cycled lithium with SRGO interlayer illustrates the weakest signal of sulfur (Figure 7f), proving the minimum level of shuttle effect.13 The XPS S 2p spectra of Li anode and SRGO interlayer are measured to further understand the suspected mechanism including the blocking and the adsorption of polysulfides. Before the test of XPS, all the samples are fully washed by DME. The comparison between the cycled lithium anode (Figures 8a,c) and comparison of the cycled interlayer and fresh SRGO interlayer (Figures 8b,d) show obvious differences. From the view of lithium anode, the content of Li2S will reveal the level of the shuttle effect, so that it helps to evaluate the result of the blocking to the polysulfide. As it is shown in Figures 8a,c,e, the brown area corresponds to the Li2S; it is easy to observe that the Li2S content on the lithium anode of batteries without SRGO interlayer is much higher than that on the lithium anode with SRGO interlayer (binding energy of 162−160 eV).52 The Li2S mainly arises from the shuttle effect and partly arises from the reduction of lithium salt (e.g., peaks of Li2S*SO3 at 163.2 eV, S−C at 162.5 eV, etc.).52 Hence, it can be concluded that the shuttle effect is suppressed by the SRGO interlayer. The XPS spectra in Figures 8b,d,f will help to 14885


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host for polysulfide. SRGO could maintain considerable electronic conduction, making it a second collector for the cathode to reuse part of the soluble polysulfide and to enhance the utilization ratio of sulfur. Moreover, the uniformly distributed sulfonic groups on the SRGO could be regarded as active sites, contributing to mitigate excessive agglomeration of sulfur and lithium sulfide (Figure 6f). This can also be manifested by higher specific capacity of cell with SRGO interlayer (Figure 4c). Consequently, the SRGO layer on the separator as acts as a dual-functional interlayer: ion selective layer and the second current collector, resulting in higher sulfur utilization ratio, better rate capability, and less self-discharge. Finally, in order to present the improvement of our work compare with the state-of-the-art, we summarized some of the proximate works about functional interlayer and separator58−65 (most of in 2016) and the works with approximate processes43,66 in the following tables including the long cycle performance (Table S1) and rate performance (Table S2). We take loading amount of sulfur, current density for test, and the content of LiNO3 as the details of the experiment. Compared with the works with similar sulfur loading amount, our cells could exhibit higher discharge capacity under the same or higher current density in prolong cycles, and rate performance is also better according to the tables. Generally, the lithium− sulfur battery is the hot spot in the field of secondary energy storage. One of the most significant reasons is the easily available sulfur. Hence, when we design the material for a lithium−sulfur battery, we had better take the cost of experimental ingredient and the processes of experiment into consideration. If we utilize the expensive materials, the advantage of sulfur in cost is submerged. Besides, the simplified processes of experiment are advocated. The simple processes are more beneficial to the physical application. In our work, we use the one-step sulfonation process in mild conditions and avoid using toxic materials, which is consistent with the ecofriendly concept.

Scheme 3. Schematic Representation of the Mechanism of the Ion Selectivity and Anchoring to the Polysulfide on the Surface of SRGO

could interact with terminal Li ion in lithium polysulfide via dipole−dipole interaction,28,30,37,55 which could also be proved by the Lewis acid−base principle.56 Therefore, the formation of the lithium bond (Li+−O−) is theoretically possible. It is speculated that some lithium ions are more inclined to form lithium bonds with the oxygen ion on the sulfonic groups. However, in the electrolyte environment, lithium salts also contain lithium. It is hard to confirm the source of the anchored lithium by experiment. Thus, the computational analysis could solve this problem to conclude whether the sulfonic groups help to anchor the polysulfide by lithium bonds. In order to simplify the calculation and to highlight the function of sulfonic groups, we took the benzenesulfonic lithium as host and the sulfur containing cluster (Li2S4, Li2S6) and LiTFSI as guest. Because the system is lithium-rich, the lithium ion is the unique cation in our calculation. The Li2SX (X = 4, 6) act as the guest to demonstrate our opinion since they are the key soluble intermediate product in the discharge process.57 The atoms for benzene groups were fixed in the calculation as the boundary conditions. The details for the calculation are shown in the Experimental Section. As shown in Scheme 4, the binding energy of sulfonic groups with Li2S4, Li2S6, and LiTFSI are −1.61, −1.78, and 3.264 eV,

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CONCLUSION With the prereduction and sulfonation processes, the sulfonic groups were grafted on the RGO sheets. The as-prepared SRGO was coated on the glass fiber separators as dual functional separators. The cell with SRGO coating separator shows an improved cycling stability, rate capability, Coulombic efficiency, and suppression of self-discharge. The EIS spectra indicated the contribution of SRGO interlayer to the decrease of the integral impedance. Further studies on the mechanism reveal that, first, the sulfonic group is a cation selective group alleviating the migration of electronegative polysulfide and, second, the sulfonic group could anchor polysulfide through the Li bond. This conclusion was ensured by the Lewis acid−base principle and computational analysis. SRGO could reuse part of polysulfide acting as a second collector to raise the utilization ratio of sulfur. With advanced functional interlayer, the performance of simple sulfur cathode could be substantially enhanced. Compared to the complicated sulfur cathode design, our work is concise in experiment and easy to be repeated without using any expensive ingredient, presenting the potential of practical utilization and scale-up production.

Scheme 4. Optimized Geometry and Binding Energy for the Interaction Model of (a) Li2S4−Sulfonic Group, (b) Li2S6− Sulfonic Group, and (c) LiTFSI−Sulfonic Group

respectively. This result demonstrates that the binding between soluble polysulfide and sulfonic groups is more stable than the one between LiTFSI and sulfonic groups. The sulfonic groups are more inclined to anchor the polysulfide through the lithium bond owing to the lower total energy of PhSO3−−lithium polysulfide system than that of PhSO3−LiTFSI. The SRGO is not only ion selective blocking layer but also works as a thin

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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.7b02142. 14886


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TEM images of SRGO, discharge and charge profiles at rate tests, discharge and charge profiles for self-discharge tests, the specific capacity calculated on total mass of cathode and interlayer, lithium storage tests of SRGO, the cycling performance of Li−S batteries with high sulfur loading, the performance of Li−S batteries with different amounts of SRGO, the SEM images of the surface of lithium anode after 50 cycles, cycling stability and the test details of works about interlayer published in 2016 and works with approximate processes, rate performance of some latest work about interlayer (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chunhua Chen: 0000-0001-9589-6329 Zhaoyin Wen: 0000-0003-1698-7420 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely appreciate the funding from National Natural Science Fundation of China (NFSC) project No. 51402330, No. 51472261, and No. 51372262. We thank Dr. Yejing Dai and Dr. Yisong Deng for the beneficial discussions to this work.

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