Porous Carbon Paper as Interlayer to Stabilize the Lithium Anode for

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Porous Carbon Paper as Interlayer to Stabilize the Lithium Anode for Lithium−Sulfur Battery Ling-Long Kong,† Ze Zhang,† Ye-Zheng Zhang,† Sheng Liu,*,† Guo-Ran Li,† and Xue-Ping Gao*,†,‡ †

Institute of New Energy Material Chemistry, School of Materials Science and Engineering, National Institute for Advanced Materials and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Nankai University, Tianjin 300350, China S Supporting Information *

ABSTRACT: The lithium−sulfur (Li−S) battery is expected to be the high-energy battery system for the next generation. Nevertheless, the degradation of lithium anode in Li−S battery is the crucial obstacle for practical application. In this work, a porous carbon paper obtained from corn stalks via simple treating procedures is used as interlayer to stabilize the surface morphology of Li anode in the environment of Li−S battery. A smooth surface morphology of Li is obtained during cycling by introducing the porous carbon paper into Li−S battery. Meanwhile, the electrochemical performance of sulfur cathode is partially enhanced by alleviating the loss of soluble intermediates (polysulfides) into the electrolyte, as well as the side reaction of polysulfides with metallic lithium. The Li−S battery assembled with the interlayer exhibits a large capacity and excellent capacity retention. Therefore, the porous carbon paper as interlayer plays a bifunctional role in stabilizing the Li anode and enhancing the electrochemical performance of the sulfur cathode for constructing a stable Li−S battery. KEYWORDS: lithium−sulfur battery, lithium anode, interlayer, porous carbon paper, surface morphology

1. INTRODUCTION Along with the extraordinary development of current society, it is critical to acquire high energy density storage and conversion systems, especially in the field of portable electronic devices, transportation vehicles, and large-scale energy storage devices.1−4 In particular, a lithium−sulfur (Li−S) battery with a high theoretical energy density of 2600 W h kg−1 is considered one of the most promising candidates for high-energy batteries. Sulfur possesses a high theoretical capacity of 1675 mA h g−1 when reacting with lithium by transferring two electrons. Besides, elemental sulfur is sufficient, economical, and nontoxic, giving extensive application prospects to the Li−S battery system. However, the practical application of Li−S battery is largely plagued by several problems, such as the intrinsic insulation of sulfur and lithium sulfides, large volume change (∼80%), and “shuttle effects” resulting from the soluble lithium polysulfides in the cathode and anode.5−9 In particular, the degradation of lithium anode in the battery is severe and still difficult to be solved. Considerable efforts have been focused on improving the electrochemical stability of the sulfur cathode and lithium anode. On the cathode side, sulfur/carbon composites,10−17 sulfur/oxides composites,18−20 and conductive polymer coating21,22 have proven effective to limit the dissolution of polysulfides. Otherwise, modifying the separator via surface functionalization23−25 can also improve the cycle performance © XXXX American Chemical Society

of the cathode. On the anode side, the surface modification,26−30 electrolyte optimization,31−37 and using novel negative electrodes38−40 can stabilize or substitute for the lithium anode. However, most of these methods are designed for either sulfur cathode or lithium anode alone. Effective ways to protect cathode and anode simultaneously are desired for stable Li−S battery. Furthermore, the recent progress on the cell construction by inserting a carbon interlayer between the sulfur cathode and separator reveals a unique merit of the functional inserted component.41−49 The interlayer can be fabricated by coating metal oxides, such as V2O5,42 Al2O3, and TiO2,43 on the commercial separators. In addition, some carbon materials such as graphene,45,46 conductive carbon black, and multiwalled carbon nanotubes47,48 have been employed to prepare carbon papers as interlayer for Li−S battery. Obviously, the interlayer can be generally believed to block the diffusion of lithium polysulfides, facilitate the utilization of sulfur species, and achieve a high performance of the sulfur cathode in terms of high discharge capacity and long-term stability. Moreover, the inserted carbon interlayer can diminish the concentration of polysulfides near the metallic lithium, which may have a great Received: September 4, 2016 Accepted: November 2, 2016 Published: November 2, 2016 A

DOI: 10.1021/acsami.6b11188 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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diffraction (XRD, Rigaku mini FlexII) and X-ray fluorescence (XRF, Rigaku Super mini). The sulfur content in the S/C composite was confirmed by a thermogravimetric analyzer (TGA, METTLER TOLEDO), as reported previously.17 The morphology and microstructure of the carbon powders and interlayer were identified by scanning electron microscopy (SEM, Supra 55VP, Zeiss) and transmission electron microscopy (TEM, JEM-2100, JEOL). To characterize the electrodes’ properties, coin cells with and without interlayer were disassembled in the charged states. The cycled cathode (including interlayer), anode, and separator were thoroughly rinsed by DME in the glovebox. The rinsing solutions were tested by UV−visible absorption spectrophotometry (UV−vis, Varian Cary 100 Conc). After complete evaporation of the solvent, the cycled interlayer and Li anode were measured by SEM (Supra 55VP, Zeiss), combined with EDAX detector. The surface chemistry of the lithium metals after cycle was detected by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe) with Mg Kα radiation of 1253.6 eV.

impact on the deposition process and surface morphology of lithium anode. However, exploring the effect of the interlayer on the deposition process and surface morphology of the lithium anode is insufficient. In this work, we focus mainly on the effect of porous carbon interlayer on stabilizing the lithium anode morphology. Porous carbon paper derived from corn stalks is fabricated as interlayer for Li−S battery. The bifunctional role toward the protection of both anode and cathode is explored. By introducing the bifunctional interlayer into the battery, metallic Li anode with the smooth surface morphology and Li−S battery with the improved cycle stability are expected.

2. EXPERIMENTAL SECTION 2.1. Preparations of Porous Carbon and Interlayer. For preparing the porous carbon, corn stalks were first peeled off, dehydrated for 12 h at 100 °C, and carbonized at 400 °C for 1 h under argon atmosphere. The pretreated sample with gray color was finely ground and marked as GC. The GC was activated with KOH at 850 °C in Ar, resulting in black carbon powders. The black carbon powders were subsequently washed using diluted HCl and deionized H2O to neutral, followed by drying for 24 h at 50 °C to get activated porous carbon (APC). To prepare the interlayer, the mixture of APC, conductive carbon black (Super P), and binder (polytetrafluoroethylene, PTFE) (75:10:15, mass ratio) was dispersed in alcohol and roll-pressed, followed by punching into disks with a diameter of 14 mm. 2.2. Preparations of the Sulfur/Carbon Composite and Sulfur Cathode. Bamboo carbon was obtained by carbonizing the bamboo50 and activated by Ni powders51 at 800 °C for 4 h. Bamboo carbon was used to prepare S/C composite for the cathode. The S/C composite was obtained by a common melt−diffusion method by thoroughly grinding the bamboo carbon powders with sublimed sulfur powders (Alfa Aesar, 99.5%) in a mass ratio of 30:70, as reported previously.17 Then, the mixture was transferred into a sealed PTFE vessel and heated for 12 h at 155 °C to get S/C composite with 65.54 wt % S as a typical crystalline sulfur (presented in Figure S1). To get the sulfur cathode, the slurry was fabricated by adding S/C composite, conductive carbon black (Super P), and binder (PVdF) at a mass ratio of 70:20:10 into N-methyl-2-pyrrolidone (NMP) and mixing for 4 h. The cathode was prepared by coating the slurry on Al foil, evaporating solvent and punching into disks with a radius of 6 mm with a sulfur loading of 1.0−1.5 mg cm−2, as reported previously.17 2.3. Cell Assembly and Electrochemical Measurement. Before assembling, sulfur electrode and interlayer were dried in an oven for 12 h. Testing cells (2032-type) were assembled with metallic lithium foil and separator (Celgard 2400) in the hydrophobic and anaerobic glovebox, as reported previously.17 Bis(trifluoromethanesulfonly)imide (LiTFSI, 1.0 M) with LiNO3 (0.2 M) dissolved in 1,3-dioxolane (DOL, J&K) and 1,2-dimethoxyethane (DME, J&K) (v/v, 1:1) was used as electrolyte. The volume of the electrolyte used in each cell was 90 μL. Galvanostatic tests of the cells were performed between 1.8 and 2.6 V (vs Li/Li+) using LAND instruments (CT2001A, Wuhan Jinnuo, China). The mass of sulfur in the cathode was used to calculate the capacity. Cyclic voltammetry (CV) measurements were conducted using electrochemical workstation (LK2010, Tianjin Lanlike, China) at a scanning rate of 0.1 mV s−1 as reported previously.17 Electrochemical impedance spectra (EIS) were measured by electrochemical interface (Solartron, 1287A/ 1255B) with a disturbance amplitude of 5 mV in the frequency range of 10 mHz to 100 kHz. 2.4. Materials and Electrode Structure Characterization. Brunauer−Emmett−Teller (BET) surface area and pore structure measurements were measured by a volumetric sorption analyzer (BK122W, Beijing JWGB Sci & Tech Co., Ltd.) based on the nitrogen adsorption/desorption characterization at the liquid nitrogen temperature. The phases and compositions of the GC powders, APC powders, APC paper, and S/C composite were characterized by X-ray

3. RESULTS AND DISCUSSION The textural properties of APC powders and as-prepared carbon paper with a thickness of about 50 μm (Figure S2) are characterized by N2 physical adsorption/desorption experiments (Figure 1). Obviously, APC powders possess abundant

Figure 1. (a) Nitrogen adsorption and desorption isotherms of APC powders and APC paper with relevant (b) mesopore distribution and (c) micropore distribution.

mesopores and micropores and present type IV isotherm combining with distinct type I broader knee. As calculated, the surface area of APC powders is 2543.9 m2 g−1 along with the pore volume of 1.6 cm3 g−1 and average pore size of 3.7 nm (Table S1). The micropore volume of APC powders is 1.3 cm3 g−1 with a pore width of 0.6 nm. Similarly, the APC paper as interlayer still has a large surface area of 1263.9 m2 g−1, B


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Figure 2. (a) XRD patterns of GC, APC powders, and APC paper. SEM images of (b) GC and (c) APC powders; (d) TEM image of APC powders.

combined with a pore volume of 1.4 cm3 g−1 and an pore width of about 3.9 nm. The micropore volume of the APC paper is 0.4 cm3 g−1 with a narrow micropore size distribution near at about 1 nm. These results prove that the APC powders and paper possess large specific surface area and pore volumes along with abundant mesopore and micropore structures. In particular, the abundant micropores in the APC paper with fairly small pore size may have some benefits on confining soluble polysulfides. It is revealed from XRD patterns (Figure 2a) that broad peaks at 24 and 43° can be observed, indicating graphite-like amorphous structure in the APC powders. The impurities such as KCl in GC powders are removed by the washing process as demonstrated by XRF in Figure S3. Similarly, the APC paper has amorphous structure, except for the instinct diffraction peak of PTFE binder at about 18°. The morphology of the GC and APC powders is shown in Figure 2b−c. The GC after precarbonization at 400 °C exhibits some basic skeleton of pristine corn stalk tissues with some macropores. The APC shows relatively smooth surface in the shape of a plate or sheet, and its cross section (red rectangle in Figure 2c) appears loose and porous as shown in the inset graph. To further identify the pores in the APC, TEM is carried out (Figure 2d). Obviously, abundant nanosized pores are shown in the APC substrates, coincident with the result from the above BET test. To explore the role of interlayer on protecting of metallic lithium anode in Li−S cells, the surface morphology of the cycled Li anodes with and without interlayer are characterized by SEM images (Figure 3). Usually, the pristine lithium metal possesses a relatively flat and smooth surface (Figure S4). After 200 cycles in the Li−S cell without interlayer, the surface of Li anode undergoes tremendous structure changes, accompanying pulverization, cracks, and dendrites (Figure 3a). The surface destruction is mainly attributed to the nonuniform electrochemical dissolution/deposition processes of Li anode, consuming the electrolytes, polysulfides, and active lithium, and forming unstable SEI films. In comparison, the Li anode in the Li−S cell with interlayer exhibits a relatively smooth and intact surface as shown in Figure 3c, demonstrating that the interlayer plays a positive role in maintaining the surface integrity of Li anode and alleviating the repeated destruction/ formation of the SEI films. Otherwise, the SEI film on Li anode can be observed on the cross-sectional images (Figure 3b and 3d). The thickness of the SEI film on Li anode in the Li−S cell without interlayer is about 18 μm, and the uneven surface of the SEI film consists of cracks and dendrites (red rectangle in Figure 3b). The broken SEI film here may induce the more severe losses of electrolyte and polysulfides, along with the continual corrosion of the metallic Li surface. By comparison, the SEI film on Li anode from the Li−S cell with interlayer is flat and dense with a thickness of nearly 3 μm, far thinner than that on Li anode in the Li−S cell without interlayer. Hence, the

Figure 3. SEM images of top surface and cross section of Li anodes after 200 cycles in the cells (a, b) without interlayer and (c, d) with interlayer, respectively. Element analysis of Li anode surface (e, f) without and (g, h) with interlayer after 200 cycles.

dense and robust SEI film is formed on Li anode by introducing the interlayer into the cell, which can effectively maintain the surface structural stability of Li anode and enhance the electrochemical behavior of the Li−S cell. Element distribution on the cycled Li anodes is analyzed by EDS. The positive effect of the interlayer is illustrated by the reduction of element S on the lithium anode surface from 16.03 to 12.56 wt % (Figure 3e−h). Furthermore, the amount of element F on the top surface of Li anode from the Li−S cell with interlayer is greater than that of the Li anode from the cell without interlayer, indicating the formation of a LiF-rich layer on the Li anode surface. Subsequently, the surface distribution of elemental S and F is detected as shown in Figure 4. It can be seen that elemental S and F are primarily distributed in the C


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Figure 4. SEM images and elemental mappings of S and F of Li anodes in Li−S cells (a−c) without interlayer and (d−f) with interlayer after 200 cycles.

crack area of the uneven Li surface with some aggregations in Figure 4b,c, originating from the inclusion of the lithium salt in the surface crack region and the imperfect SEI film. By contrast, the dissembled Li anode from the cycled Li−S cell with interlayer shows a relatively homogeneous distribution of elemental S and F (Figure 4e,f) on the unbroken Li anode surface. Moreover, the amount of elemental S corresponding to Figure 4e is evidently lower than that in Figure 4b, illustrating less consumption of polysulfides and electrolyte in the cell with interlayer. Undoubtedly, introducing the interlayer into the Li− S cell can stabilize the Li anode with a flat surface structure and reduce elemental S accumulation on the metallic Li surface. Meanwhile, the thin and robust SEI film without noticeable cracks and dendrites is formed, indicating the decreased sidereaction between soluble polysulfides and Li anode. It is proved here that the soluble intermediates are effectively confined on the cathode chamber in the repeated discharge/charge processes by introducing the interlayer into the Li−S cell. The surface chemistry of the Li anode in Li−S cells after 200 cycles is characterized by XPS. As indicated in Figure 5, the chemical composition is different for the cycled Li anodes from Li−S cells with and without interlayer. For the S 2p and F 1s spectra of Li anode in the traditional Li−S cell, core levels are attributed to the decomposed products of Li salts, such as S2− S*2O6 (164.0 eV), Li2SO3 (167.0 eV), −NSO2CF3 (169.0 eV), −NSO2CF3 (687.8 eV), and LiF (684.8 eV).33,34 In contrast, peaks in S 2p spectra of the cycled Li from the Li−S cell with interlayer are assigned to Li2SO4 (168.5 eV), Li2SO3 (167.0 eV), S2−S*2O6 (164.0 eV), S−S*O3 (161.7 eV), and Li2S2/Li2S (∼160.0 eV).33,34 S2−S*2O6 and S−S*O3 in Figure 5 represent the bridging sulfur on the Li anode surface.33 More insoluble LixSOy species are generated on the Li anode surface, which can effectively passivate and protect Li anode.33 Meanwhile, the increasing intensity of LiF with the decreasing intensity of −NSO2CF3 gives a powerful proof of a LiF-rich SEI layer formed on the cycled Li surface by introducing the interlayer into the cell. LiF and LixSOy can construct a stable and robust SEI layer on the Li anode surface. The further corrosion of Li can be reduced by blocking the soluble polysulfides in the electrolyte. The surface integrity of Li anode can be kept in the long cycle under the complicated chemical environment, which will benefit to stabilize the electrochemical performance of the Li−S cell.

Figure 5. S 2p and F 1s core levels of XPS spectra for the Li anodes in testing cells (a, c) without and (b, d) with interlayer after 200 cycles.

It is widely accepted that LiNO3 as additive in the electrolyte plays an important role in maintaining the stability of Li anode, especially in forming the SEI film on the Li anode surface. Hence, the effect of the interlayer on Li anode in the controlled electrolyte (without LiNO3) is also verified. In the controlled electrolyte, the surface pulverization, cracks, and even dendrites are ubiquitous on the cycled Li surface in the cell without interlayer (Figure 6a). In comparison, metallic Li anode from the cell with the interlayer still possesses the smooth and dense surface morphology after 100 cycles as shown in Figure 6b. Obviously, the interlayer in the system is indeed beneficial to D


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Figure 6. Surface morphology of the Li anodes taken from the cells (a) without interlayer and (b) with interlayer after 100 cycles.

following cycles. While the interlayer is used here, polysulfides can be effectively confined on the cathode chamber, and the irreversible deposition of sulfur species is weak. Moreover, the utilization of sulfur is more sufficient and potential polarization is weak with scanning, leading the increase of current density at ∼2.3 V as shown in Figure 7b. In addition, the peak currents, peak potentials, and the integrated areas of cathodic/anodic processes of the Li−S cell with interlayer are well-maintained after successive cycles, demonstrating excellent reversibility and stability. The interlayer in the Li−S cell has no adverse effect on the cathode electrochemical reaction but enhances the electrochemical stability. The initial five discharge and charge potential profiles of the testing cells are presented in Figure 8. Two potential plateaus at

stabilize the Li anode morphology after long cycling in the electrolyte without LiNO3. Here, the porous interlayer is also expected to improve the electrochemical performance of the sulfur cathode. Subsequently, a series of measurements is performed to verify the positive effect of the interlayer in the Li−S cell system. The cyclic voltammograms (CVs) of the cells with and without interlayer are shown in Figure 7. All CVs display two cathodic

Figure 7. Cyclic voltammograms (CVs) of Li−S cells (a) without and (b) with interlayer at the potential region of 1.8−2.6 V with a scan rate of 0.1 mV s−1.

peaks, ascribed to the conversion of sulfur to high-order polysulfides and the subsequent reduction of soluble polysulfides to Li2S2/Li2S, respectively. As for the Li−S cell without interlayer, the second cathodic peak appears to an evident shift to a lower potential during the first cathodic process, which may be caused by autologous large impedance and polarization. However, the second reduction peaks of the Li−S cell with interlayer hardly shift to low potential. The anodic process occurs at the potential range of 2.3−2.4 V, which reveals the reversible conversion reaction of insoluble Li2S2/Li2S to lithium intermediates and further to S8/Li2S8. Nevertheless, the anodic potentials of the Li−S cell without interlayer slightly shift to high potential as the cycle proceeds, manifesting a relatively slow kinetics process and consequently inferior reversibility. In the anodic process, the peak currents at ∼2.3 V are relatively weak in Figure 7a, while the peak currents are strong in Figure 7b. In the Li−S cell without interlayer, insoluble Li2S2/Li2S are easily deposited on Li anode surface and separator, leading to a large potential polarization in the

Figure 8. Initial 5 discharge−charge potential profiles of Li−S cells at 100 mA g−1 (a) without interlayer and (b) with interlayer.

about 2.3 and 2.1 V (vs Li/Li+) are observed during the discharge process, indicating the typical two-step reactions of sulfur to soluble intermediates Li2Sx (4 ≤ x ≤ 8) and further to insoluble Li2S2/Li2S, which are basically in agreement with the cathodic characteristics in CVs. The charge plateau is located at the range of 2.3−2.4 V (vs Li/Li+), representing the conversion of Li2S2/Li2S to long-chain polysulfides and even S8/Li2S8. In addition, the capacity stays steady after two cycles in the Li−S cell with interlayer. After 5 cycles, the Li−S cell with interlayer shows a discharge capacity of 1267.3 mA h g−1, much larger than 859.2 mA h g−1 of the Li−S cell without interlayer. Comparing CVs and discharge/charge curves of the two cells, it E


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Figure 9. (a) Cycle performance of Li−S cells at 100 mA g−1; (b) rate performance of the Li−S cells at 100, 200, 500, 1000, and 2000 mA g−1. (c) Long cycle performance of the Li−S cell with interlayer at 1000 mA g−1.

can be observed that the introduction of interlayer does not alter the electrochemical redox process of the Li−S cell but enhances the electrochemical performance by confining the dissolution of polysulfides and stabilizing the surface morphology of Li anode. The cycle performance of testing cells with and without interlayer is presented in Figure 9. Meanwhile, the preprepared interlayer separately as cathode is also examined with only a few capacities (Figure 9a). The initial discharge capacity of the Li− S cell without interlayer is 1010.2 mA h g−1, which is far smaller than that of the cell with interlayer. It is noted that the initial discharge capacity of the cell with interlayer exceeds the theoretical capacity of sulfur slightly, while the charge capacity is much smaller than the discharge capacity. The irreversible capacity is attributed to the capacitive energy storage of the porous carbon in the interlayer, which is usually observed in carbon materials for Li batteries.46 The capacity of the Li−S cell with interlayer remains at 1133.5 mA h g−1 after 100 cycles, much higher than that of the Li−S cell without interlayer. Notably, the capacity fading of the Li−S cell with interlayer is only 0.27% per cycle, while the capacity decaying of the Li−S cell without interlayer is 0.58% per cycle. Furthermore, the Coulombic efficiency (CE) is evidently improved for the Li−S cell with introducing the interlayer, which is nearly 100% even after 100 cycles, while it is about 95% for the Li−S cell without interlayer (Figure S5a), indicating the effectively suppression of shuttle effects with the assistance of the porous interlayer. Moreover, the rate performance of testing cells at different current densities is also tested (Figure 9b). Obviously, the Li−S

cell with interlayer shows excellent rate capability in contrast with the cell without interlayer at the gradually changed current densities. The cycle performance of the Li−S cell with interlayer is given in Figure S5b. The Li−S cell delivers large discharge capacities of 1018.6, 1073.2, and 867.7 mA h g−1 at 500, 1000, and 2000 mA g−1, respectively, indicating that the cell still remains good utilization of sulfur and excellent cycle stability at different current densities after introducing the interlayer into the cell. The long cycle performance of the Li−S cell at 1000 mA g−1 is presented in Figure 9c. After the initial three cycles of activation at 100 mA g−1, the Li−S cell with interlayer shows a large discharge capacity of 1195.45 mA h g−1 with a Coulombic efficiency of 99.9% at 1000 mA g−1. After 300 cycles, the cell still retains the discharge capacity of 982.61 mAh g−1 with a capacity loss of ∼0.06% per cycle (after third cycle). The average Coulombic efficiency of the cell is above 99%. The improvement on the cycle performance of the Li−S cell with interlayer is ascribed to the positive effect of porous carbon substrate in the paper. To further verify the effect, the Li−S cell in the electrolyte without LiNO3 is tested (Figure S5c). The cycle testing shows that the interlayer is still helpful to improve the cycle stability of the cell even without adding LiNO3 into the electrolyte. Obviously, the Li−S cell with interlayer delivers a similar discharge capacity in comparison with that of the cell with interlayer and LiNO3 (Figure 9a). The Coulombic efficiency can be stabilized at about 95%, showing that the interlayer can effectively confine the polysulfides in the absence of LiNO3 and enhance the utilization of the sulfur active materials. F


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251.4 Ω with a Warburg diffusion impedance (Zw) of 58.2 Ω before cycling, while the cell with interlayer possesses a smaller Rct of 44.2 Ω and a relatively larger Zw of 218.3 Ω. The decrease in the charge-transfer resistance is caused by the applied interlayer acting as another superior current collector. After cycling, the charge-transfer resistance is gradually increased from 4.3 to 5.1 Ω, indicating more insulated and inactive substances generating on the surface of cathode and separator in the cell without interlayer. In comparison, the cell with interlayer displays evidently diminishing Rct from 76.6 to 8.0 Ω and Zw from 178.4 to 14.96 Ω after 100 cycles. It means that the interlayer takes positive effect on decreasing the formation of adverse species on the surface of separator and cathode,43 enhancing the utilization of sulfur active materials and improving the cycle stability of the Li−S cell. To further explore the origin of the positive function by introducing the interlayer, the cells with and without interlayer are disassembled and examined after cycling in the charged state. Different soaking solutions of cathode (including interlayer) and Li anode washed by DME display varied colors (Figure 11a). The obvious yellow solution of the cell without

Electrochemical impedance spectra (EIS) of Li−S cells with and without interlayer are measured after different cycles (Figure 10). The initial impedance plots for the cell without

Figure 10. EIS spectra of the Li−S cell (a) without interlayer and (b) with interlayer after charging to 2.6 V at 100 mA g−1. (c) Equivalent circuit for simulating the experimental data (the dotted portion only suits the Li−S cell without interlayer). Rs represents the solution resistance, R1 signifies the resistance from the insoluble Li2S/Li2S2 film, CPE1and CPE denote the constant phase element, Rct stands for the charge-transfer resistance, and Zw indicates the semi-infinite Warburg diffusion impedance.

Figure 11. Digital photos of (a) soaking solutions of cathode (including carbon interlayer) combined with lithium anode in DME and (b) separators used in the Li−S cells after cycle. From left to right: Li−S cell without interlayer and with interlayer, respectively. (c) Sealed vials of lithium polysulfides solution (0.01 M Li2S6 dissolved in DOL/DME solvents, diluted to 0.002 M by DME before use). APC powders (10 mg) were added into a glass vial, and then Li2S6 solution (2 mL) was poured into glass vial. A blank glass vial was also filled with the same Li2S6 solution for reference. (d) UV/vis absorption spectra of standard Li2S6 solution and soaking solution corresponding to the cells with and without interlayer.

interlayer are consisted of a semicircle (high- to mediumfrequency region) and a sloped line (low-frequency region). After one cycle, another semicircle appears in the high- to medium-frequency region. The first semicircle is ascribed to the formation process of the insoluble Li2S/Li2S2 film on the cathode surface, and the second semicircle belongs to the charge-transfer process. However, the impedance plots of the cell with interlayer only consist of a semicircle in the high- to medium-frequency region and a sloped line in the low frequency region before and after cycling. Here, the semicircle from the insoluble Li2S/Li2S2 film disappears, illustrating a positive role of the interlayer on preventing the formation of passivation film on the cathode surface. R1 and Rct are denoted as the resistance from the passivation film and the chargetransfer resistance, respectively. The equivalent circuit shown in Figure 10c is used to simulate the spectra with data listed in Table S2. The cell without interlayer presents a primary Rct of

interlayer indicates that free soluble intermediates exist in the bulk electrolyte without effective restrictions, reflecting incomplete transformation of polysulfides to S8/Li2S8. However, the solution in the cell with interlayer is almost colorless, suggesting that the most of polysulfides can be trapped between the interlayer and cathode and even oxidized to S8/Li2S8. Meanwhile, the separators (the side close to the cathode) of the G


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Figure 12. SEM images of (a) the pristine interlayer and (b) the cycled interlayer. EDS mappings of (c) elemental C and (d) elemental S of the cycled interlayer corresponding to the yellow rectangle in b.

cells with and without interlayer show different phenomena (Figure 11b). Massive yellow and brown substances can be observed on the separator surface in the cell without interlayer. Nevertheless, a few colored species can be discovered on the separator surface of the cell with interlayer. It means few substances are deposited on the separator during the electrochemical reaction and that polysulfide intermediates can be effectively confined by the interlayer.43 To further prove this conclusion, APC powders (10 mg) are immersed into the Li2S6 solution to test the interaction with polysulfides (Figure 11c). After 0.5 h, the solution varies clearly from fresh yellowgreen to transparent, demonstrating the strong adsorption of APC powders to polysulfides. Predictably, the interlayer in the Li−S cell is beneficial to confine polysulfides near the cathode, acting as porous microreactors that allow the reaction of Li+ and polysulfide ions. UV−visible absorption spectroscopy is also used to support this viewpoint (Figure 11d). A certain concentration of Li2S6 solutions is tested as reference. The characteristic peaks at approximately 260, 300, and 350 nm can be assigned to S62− species.52 The UV/vis spectrum of the soaking solution in the cycled cell without interlayer exhibits a curve similar to that of the reference solution, indicating an approximate concentration of the free polysulfides in the Li−S cell. However, such characteristic peaks of polysulfides almost disappear in the UV/vis spectrum of the solution in the cell with interlayer, implying the strong effect of the interlayer on soluble polysulfides. SEM images of the interlayer before and after cycling are presented in Figure 12. Some large pores can be seen on the interlayer surface with a relatively loose morphology before and after cycling, beneficial to the infiltration of the electrolyte and the soluble polysulfides through the interlayer. Meanwhile, the corresponding EDS mappings of the local region of the cycled interlayer (Figure 12c,d) exhibit that elemental C and S are uniformly distributed without agglomeration. Evidently, the

soluble polysulfide intermediates can be confined and anchored in the pores of the interlayer, decreasing the surface area and pore volume of the cycled interlayer (Figure S8). In particular, the drop of microporous volume (0.2 cm3 g−1) is significant for the cycled interlayer. It is demonstrated that the interlayer can prevent the migration of the soluble polysulfide intermediates, similar to the roles of the mesopores and micropores in the carbon matrix for S/C composites. Obviously, the excessive migration of the soluble intermediates to the Li anode can be inhibited by the porous interlayer, resulting in the less deposition of Li2S2/Li2S on the Li anode surface. During the charging process, the surface layer of the Li anode maintains stable without remarkable structural changes caused by the transformation of the deposited Li2S2/ Li2S and further reduces the unnecessary consumption of polysulfides, electrolyte, and Li metal. The diffusion model as in eq 1 is proposed to interpret the phenomenon of nonuniform metal electrodeposition.53,54 τ=

πDn2F 2C02 4(Jt a )2

(1)

where τ is the “Sand’s time” representing the time when Li dendrites start to grow, D is the diffusivity, F is Faraday’s constant, n is the valence of the Li salt, C0 is the original concentration of the Li salt, J is the practical current density of the electrode, and ta is the transference number of anions, defined as (μa/(μa + μc)) where μa and μc stand for the mobility of anions and cations, respectively. The increase of the Sand’s time means the growth delay of Li dendrites. Therefore, lower current density, higher Li salt concentration, and smaller anion transference number are the key approaches to retard the dendrites on the Li surface and form a uniform morphology. According to the absorbance intensity at λ = 410 nm in Figure 11d, the concentration contrast of the polysulfides in the cells is H


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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Li−S Battery Assembly on Porous Carbon Paper

obvious. The mobility of anions (μa) is decreased for minor concentration of the polysulfides in the electrolyte, leading to the reduction of ta (ta = μa/(μa + μc)). As a result, τ in eq 1 is increased by decreasing the concentration of the polysulfides. With the introduction of the porous interlayer, the concentration of free polysulfides in the electrolyte on the anode side is dramatically decreased, along with the decrease of ta.37 Therefore, Sand’s time can be prolonged to bring about the smooth surface on the anode side. The carbon interlayer adopted in the Li−S battery is proved to stabilize metallic Li anode and enhance the utilization of cathode active materials by restricting/trapping soluble polysulfide intermediates in abundant micropores. The working mechanism of the porous interlayer in the Li−S cell can be explained in Scheme 1. In the discharge process, the sulfur would be gradually reduced to soluble polysulfides. In the Li−S cell without interlayer, polysulfides are readily dissolved in the electrolyte and diffused to the Li anode, leading to a serious corrosion of Li, noticeable overcharge phenomena, and relatively inferior availability of cathode materials. The carbon interlayer with high surface area and abundant micropores can reduce the migration of soluble polysulfide intermediates to separator and Li surface, avoiding the serious corrosion on Li anode caused by polysulfides and stabilizing the surface morphology of Li anode. Meanwhile, the electrochemistry reaction of sulfur is restricted to a great extent on the cathode side by trapping soluble polysulfides into the porous carbon interlayer.

In particular, the working mechanism of stabilizing Li anode by introducing the carbon interlayer may provide a new insight on battery fabrication for developing a stable Li−S battery.

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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.6b11188. TGA curves, X-ray diffraction patterns, SEM images, Xray fluorescence patterns, electrochemical curves, N2 sorption isotherms, and electrochemical parameters from EIS spectra are included (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86-22-23500876. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the New Energy Project for Electric Vehicles in National Key Research and Development Program (2016YFB0100200), NFSC (21573114, 51502145, and 21421001), and MOE Innovation Team (IRT13022) of China are acknowledged.

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4. CONCLUSIONS The carbon interlayer is prepared from corn stalks carbon. The abundant micropores in the carbon interlayer provide sufficient space for anchoring the soluble polysulfides during the electrochemical reactions in Li−S cells. The effective confinement of soluble polysulfides by the carbon interlayer can decrease the side reaction on Li anode side, resulting in the uniformly electrochemical dissolution/deposition processes of Li anode with a relatively smooth surface morphology. As result, after introducing the interlayer, the high utilization of the sulfur active material, low capacity fading (∼0.06% per cycle), and high Coulombic efficiencies (∼100%) are obviously obtained. Therefore, the carbon interlayer plays a bifunctional role in stabilizing the surface morphology of Li anode and enhancing the electrochemical performance of sulfur cathode.

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Porous Carbon Paper as Interlayer to Stabilize the Lithium Anode for

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