Mesoporous Carbon Interlayers with Tailored Pore Volume as

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Mesoporous Carbon Interlayers with Tailored Pore Volume as Polysulfide Reservoir for High-Energy Lithium–Sulfur Batteries Juan Balach, Tony Jaumann, Markus Klose, Steffen Oswald, Jürgen Eckert, and Lars Giebeler J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512062t • Publication Date (Web): 05 Feb 2015 Downloaded from http://pubs.acs.org on February 10, 2015

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The Journal of Physical Chemistry

Mesoporous Carbon Interlayers with Tailored Pore Volume as Polysulfide Reservoir for High-Energy Lithium–Sulfur Batteries Juan Balach,*a Tony Jaumann,a Markus Klose,a Steffen Oswald,a Jürgen Eckert,ab and Lars Giebelerab a

Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Institute for Complex

Materials, Helmholtzstraße 20, D-01069 Dresden, Germany. b

Technische Universität Dresden, Institut für Werkstoffwissenschaft, Helmholtzstraße 7, D-

01069 Dresden, Germany.

ABSTRACT: The lithium–sulfur (Li–S) battery is one of the most promising candidates for the next generation of rechargeable batteries owing to its high theoretical energy density which is four- to five-fold greater than those of state-of-the-art Li–ion batteries. However, its commercial applications have been hampered by the insulating nature of sulfur and by the poor cycling stability caused for the polysulfide shuttle phenomenon. In this work, we show that Li–S batteries with a mesoporous carbon interlayer placed between the separator and the sulfur cathode not only reduces the internal resistance of the cells but also that its intrinsic mesoporosity provides a physical place for trapping soluble polysulfides as well as to alleviate the negative impact of the large volume change of sulfur. This improvement of the active

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material reutilization allows to obtain a stable capacity of 1015 mAh g–1 at 0.2 C after 200 cycles despite the use of a conventional sulfur-carbon black mixture as cathode. Furthermore, we observe an excellent capacity retention (~0.1% loss per cycle, after the second cycle), thus making one step closer towards feasible Li–S battery technology for applications in electric vehicles and grid-scale stationary energy storage systems.

KEYWORDS: mesoporous carbon, tailored pore volume, carbon interlayer, high-performance lithium–sulfur battery.

1. INTRODUCTION The inevitable fossil fuel depletion and the increasing global energy consumption have become more and more crucial in the last decades by rising living standards of a growing world population. The utilization of alternative renewable energy sources (e.g. solar and wind energies) and the implementation of all-electric vehicles (EVs) would be the most suitable, ecologicallyfriendly and cost-effective pathway to meet future energy demand growth and the replacement of traditional energy sources. However, the development of a cost-efficient electrical energy storage system able to ensure high energy density and long cycle life for plug-in electric vehicles (PEVs) or grid-scale stationary storage still remains a challenge. Among the known rechargeable battery technologies, lithium–ion (Li–ion) batteries offer the highest energy density and output voltage.12

Nonetheless, the use of conventional intercalation oxide cathodes (e.g., LiMn2O4, LiCoO2 and

LiFePO4) limit the capacities of Li–ion batteries to less than 300 mAh g−1, which is insufficient to address the actual energy demand.3 Alternatively, conversion reaction battery systems that can support multiple electrons per molecule, offer both large gravimetric capacity and high energy


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density. Among the various types of conversion reaction batteries, the lithium–sulfur (Li–S) battery has gained tremendously in importance in the past years owing to its high theoretical specific capacity (1672 mAh g−1) and energy density (2600 Wh kg−1), which are several times larger than those of the traditional Li–ion batteries.4-5 Furthermore, the natural abundance, the relative low cost and the environmentally benign nature of sulfur are prompting the Li–S technology to be the leading candidate for next-generation rechargeable batteries.5-6 Despite the outstanding features of sulfur as cathode material, there are several issues related to the low active material utilization and poor cycle stability of Li–S batteries that have been hindered commercial application of the Li–S technology. The low utilization of active material is generally caused by the insulating nature of sulfur (σ = 5 10−30 S m−1) and its reduced discharge product Li2S which exhibits almost twice the volume of sulfur and thus appear even more insulating. The rapid capacity fading over cycling is attributed to the high solubility of the lithium polysulfide (LiPS) intermediates (Li2Sn, 4 < n ≤ 8) in the electrolyte. These polysulfides migrate to the metallic lithium anode where they are discharged to highly insulating Li2S and other side products thus contaminating the anode or leads to an almost inert passivation layer. Furthermore, solid Li2S can be reoxidized during charging and can migrate back to the sulfur cathode. This so-called shuttle effect significantly lowers the efficiency of the discharge–charge process. To overcome these issues and enhance the overall electrochemical performance of Li–S batteries, various approaches have been proposed, including the development of new electrolytes,7-9 the application of alternative binders,10-11 the design of novel sulfur-polymer composites12-14 and sulfur-carbon composites,15-17 the coating protection of the lithium anode and the sulfur cathode,18-19 the modification of the separator20-22 and alternative anode materials.23 Considering the structure of the sulfur cathode, the most promising strategy is to confine sulfur


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into the cavity of hollow/porous carbon structures, which enhances the charge transport properties as one aspect, but also can buffer the volume change as well as encapsulate/trap the soluble LiPS intermediates to suppress the polysulfide shuttling.24-30 However, most sulfurcarbon composites still exhibit undesirable capacity fading after several cycles due to the inexorable escape of the active material from the structured carbon host. In an effort to tackle the polysulfide leak towards the lithium anode, the modification of the cell configuration by insertion of a free-standing, bifunctional carbon interlayer between the separator and the sulfur cathode has been proposed recently.31-32 Compared with the traditional Li–S cell configuration, these "advanced" Li–S batteries6 with carbon nanotubes-, microporous carbon-, or hierarchical carbon-based interlayers commonly used as polysulfide barrier have significantly improved the sulfur utilization and the cycling stability with capacities above 1000 mAh g−1 and 850 mAh g−1 after 100 and 200 cycles, respectively.31-33 In addition, this novel cell configuration allows the use of a sulfur-carbon black mixture as cathode with high sulfur loading. These cathode electrodes can be prepared very straightforward and thus allow high energy densities. However, the physical features of the mentioned carbon substrate for the interlayers are somewhat unsuitable to efficiently retain LiPS intermediates due to their relative low pore volume and the lack of a proper confinement space to accommodate the large volume expansion during the sulfur/Li2S conversion reaction. In this contribution, we investigate the feasibility of a mesoporous carbon interlayer as both second current collector and effective LiPS reservoir for improving the cyclic performance of Li–S batteries. It will be shown that the metallic lithium anode can successfully be protected from the shuttle effect and degradation phenomena resulting in a prolonged cycle life. Pristine mesoporous carbons with similar mesopore size (~12 nm) and tunable pore volume were


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prepared by polymerization of resorcinol with formaldehyde in the presence of colloidal aqueous silica solution and their subsequent carbonization and silica template removal. We find that Li–S batteries using a mesoporous carbon interlayer with large pore volume outperforms conventional Li–S cell design. Our setup shows remarkable high discharge capacity (1015 mAh g–1 after 200 cycles at 0.2 C) and excellent capacity retention (~0.1% loss per cycle, after second cycle) in comparison with up-to-date Li–S cells.

2. EXPERIMENTAL SECTION 2.1 Synthesis of mesoporous carbon For the preparation of mesoporous carbons (MPCs) with different pore volume but similar mesopore size, 1 g of resorcinol (Sigma-Aldrich, ≥ 99 wt%) was dissolved in 1.7 ml of formaldehyde solution (Fluka, 36.5–38 wt%, contains ~10% (w/w) methanol as stabilizer) and mixed with a desired amount of commercial colloidal silica solution (Sigma-Aldrich, LUDOX HS-40, 40 wt% of 12 nm-in-diameter silica). After 10 min. under magnetic stirring at room temperature, the well-dispersed mixture was transferred into an oven at 90 ºC. It turned pale white and solidified within 20 min. due to the strong alkaline medium of the ammonium hydroxide-stabilizer silica (pH = 9.2–9.9) which acts as catalyst for the resorcinol-formaldehyde polycondensation.34-35 The gel was cured and dried for additional 20 h. The obtained brown silica-containing resorcinol-formaldehyde polymer was further carbonized in an argon atmosphere at 900 ºC and maintained at that temperature for 2 h. Afterwards, the solid silicacarbon product was first finely ground and then chemically etched using 20 wt% hydrofluoric acid (Merck 40 wt%) solution to remove the silica templates. Finally, the carbon powder was repeatedly washed with deionized water and dried at 100 °C overnight before use.


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The obtained mesoporous carbon powders were named as MPCX, where X corresponds to the used initial silica-to-resorcinol mass ratio. For instance, MPC3 was obtained from the initial mixture of 3 g of silica (5.77 ml of LUDOX HS-40) and 1 g of resorcinol.

2.2 Mesoporous carbon interlayer preparation Mesoporous carbon interlayers (MPCIs) were prepared by mixing finely grounded MPC powders with Super P Li Carbon (BASF) and polytetrafluoroethylene binder (PTFE, SigmaAldrich) at an 85:5:10 weight percent ratio, respectively. Then, the mesoporous carbon blends were roll-pressed and dried in a vacuum oven at 80 °C for 20 h. Finally the obtained MPCIs were punched into circular disks (diameter: 16 mm; thickness: 30–35 µm; weight: ~0.9 mg cm–2) for assembling cells.

2.3 Sulfur cathode preparation The sulfur cathode was prepared by a simple ball-mill mixing of 70 wt% commercial elemental sulfur (Sigma-Aldrich, 99.98 wt%), 20 wt% SuperP Li carbon and 10 wt% polyvinylidene fluoride (PVDF, Solvay Solef® 21216) in an N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich, 99 wt%) solution to form a homogeneous viscous paste. Then, the slurry was coated onto an aluminum foil current collector via doctor blade technique and dried at 50 °C for 20 h in air. Finally, the sulfur cathode was cut into circular disks of 12 mm. The sulfur loading of the pure sulfur cathode was ~1.7 mgS cm–2. For a fair comparison, conventional Li–S cells were tested using a cathode with 40 wt% of sulfur, which corresponds to the sulfur ratio in the cathode when the mass of the carbon interlayer is accounted into the 70 wt% sulfur cathode.


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2.4 Characterization Scanning electron microscopy (SEM) was carried out with a Gemini 1530 from LEO at 15 kV acceleration voltage. Energy-dispersive X-ray (EDXS) measurements were taken on the SEM with a Bruker EDXS spectrometer. For transmission electron microscopy (TEM), a Philips CM20 equipped with a field emission gun (FEG) working at 200 kV acceleration voltage was used. For sample preparation, one drop of the sample dispersion was placed on a 200 mesh carbon coated copper grid and dried in air. Nitrogen sorption experiments were carried out using a Quantachrome Quadrasorb SI instrument and data analysis was performed by the Quantachrome Quadrawin 4.0 software. Prior to the measurement, the samples were degassed under dynamic vacuum at 150 ºC for 24 h. Sulfur-containing samples were not heated during vacuum exposure. Specific surface areas were calculated at a relative pressure p.p0–1 = 0.05–0.2 using the multi-point Brunauer–Emmett–Teller (BET) method. The total pore volume was determined at p.p0–1 = 0.97. The pore size distributions (PSDs) were obtained using the Quenched Solid Density Functional Theory (QSDFT). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI 5600 spectrometer (Physical Electronics) equipped with a hemispherical analyzer operated at a pass energy of 29 eV. Monochromated Al Kα radiation (350 W) was used applying partly low energy electrons for charge compensation. To avoid any contact of the samples with air, a transfer chamber (Physical Electronics) was used for the sample transport from the argon-filled glove box to the XPS spectrometer. In order to examine the inside part of the carbon interlayer, a sputtering procedure was performed using Ar+ ions at 3.5 keV for 10 min (sputter rate: 3.5 nm/min at SiO2). A similar procedure was performed for the fresh sulfur cathode used as reference. All spectra were calibrated using the C 1s level (284.8 eV) as a reference. The mesoporous carbon interlayers after discharge–charge cycles were


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washed with DOL inside the glove box prior to SEM, nitrogen physisorption and XPS investigation.

2.5 Cell Assembly and electrochemical measurements CR2025-type coin cells were assembled in a dry argon-filled (H2O < 1 ppm, O2 < 0.1 ppm) glove box. The electrolyte contained 1 M lithium bis(trifluoromethylsulfonyl) imide salt (LiTFSI, BASF, dried at 100°C in vacuum overnight) in a mixed solvent of 1,3-dioxolane (DOL, Sigma-Aldrich, 99.8 wt%, anhydrous) and 1,2-dimethoxyethane (DME, Sigma-Aldrich, 99.5 wt%, anhydrous) (1:1, vol%), with 0.25 M lithium nitrate (LiNO3, Merck, > 99.995 wt%, anhydrous, dried at 100 °C in vacuum overnight) as additive. A Li metal foil (Chempur, diameter 13 mm, thickness 250 µm) was used as anode material and reference electrode. A microporous polypropylene membrane (Celgard® 2500, diameter 16 mm, thickness 25 µm) was used as separator. The mesoporous carbon interlayer was placed between the sulfur cathode and the separator. The cells were cycled at room temperature with a BaSyTec Cell Test System (CTS) in a voltage window of 1.8–2.6 V at various cycling rates from 0.2 C to 4 C, based on the mass and theoretical capacity of sulfur (1672 mAh g–1). The cut-off potential of 1.8 V helps to avoid an irreversible reduction at ~1.6 V which results from the LiNO3 co-salt.36 Cyclic voltammetry (CV) tests and electrochemical impedance spectroscopy (EIS) data were obtained with a VMP3 potentiostat (Bio-logic). The cyclic voltammograms were recorded in a voltage window of 1.8– 2.8 V at a scan rate of 0.1 mV s−1 while the EIS measurements were recorded from 200 KHz to 100 mHz with an AC voltage amplitude of 5 mV at the open-circuit voltage of the cells. Prior to


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carrying out the long-term cycling tests at 0.5 C and 1 C rates, the cells were cycled at 0.2 C for the first two cycles as conditioning step.

3. RESULTS AND DISCUSSION To study the impact of the pore volume of the MPCIs on the ability to trap LiPS intermediates and to evaluate the effect of such phenomenon on the electrochemical performance of Li–S batteries, MPCs with mesopore sizes of ~12 nm and different pore volume were prepared using resorcinol-formaldehyde resin and commercial colloidal silica particles as carbon precursor and hard template, respectively. We choose silica nanoparticles as the mesostructure template since they offer a high control over the pore diameter of the carbon material while the pore volume in the carbonaceous sample can be easily adjusted by varying the silica/carbon precursor ratio during the initial stage. The textural properties of the MPC powders were characterized by nitrogen physisorption experiments and the corresponding results are summarized in Table S1. Figure 1 shows the respective isotherms obtained at -196 °C and the corresponding PSDs. In all three cases distinct type IV isotherms with sharp H1-type hysteresis loops are observed, indicating the presence of a pronounced mesoporosity in the carbon materials (Figure 1a). The type-H1 shape is often associated with the existence of agglomerates or compacts of roughly uniform spherical particles with high degree of pore size uniformity.37 The PSDs derived from a QSDFT model exhibited narrow size distributions centered at ~12 nm in all three MPCs (Figure 1b). This value is highly congruent with the sizes of single silica particles used as hard templates. Certainly, there is a very clear trend that, with increasing mass ratio of silica to resorcinol (e.g. from 1 to 3), the pore volume increases considerably (from 1.088 cm3 g–1 to 3.231 cm3 g–1) as well as the specific surface area (from 630 m2 g–1 to 900 m2 g–1). These results clearly


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demonstrate that the physical properties of the carbon materials can be easily tailored by adjusting the amount of silica template in the initial polymerization stage.

Figure 1. (a) Nitrogen physisorption isotherms of pristine MPC powders and (b) the corresponding QSDFT pore size distributions (PSDs) derived from the respective adsorption branches employing a kernel for spherical- and slit-shaped pores.

The morphology of the three MPCs was examined by SEM and TEM, as shown in Figure 2. The SEM images (Figures 2a–c) reveal that all samples exhibit disordered mesoporous structures with uniformly distributed pores. However, the sample prepared with the lowest amount of silica template (MPC1, Figure 2a) presents a relatively high amount of non-porous carbon (Figure 2a, white circles), which results in a carbon network with lower pore density. This observation explains the low specific surface area of the MPC1 when it is compared with the samples MPC2 and MPC3. TEM investigation shows the typical amorphous carbon structure for the MPCs (Figures 2d–f). Although the MPC1 sample displays a dense close packing structure (Figure 2d), the increases of the silica content improves the pore distribution throughout the carbon matrices (Figures 2e and 2f). This observation demonstrates the good structural replication of the


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corresponding silica template after carbonization of the silica/resorcinol-formaldehyde material and consequent silica etching.

Figure 2. SEM (a–c) and TEM (d–f) images of pristine MPCs obtained with different amount of silica template. (a and d) MPC1, (b and e) MPC2 and (c and f) MPC3.

We applied these mesoporous carbons in Li–S cells by placing a MPCI between the separator and the sulfur cathode not only to improve the electrical conductivity but also to take advantage of the mesoporous framework of MPCI as effective container for both trapping soluble LiPS intermediates to suppress the sulfur shuttle mechanism and buffering large volume expansions involved in the conversion reaction between sulfur and Li2S. The concepts of the conventional Li–S cell and the Li–S cell with a MPCI are shown in Scheme 1. During charging in conventional Li–S batteries soluble LiPS species are formed, which leak into the electrolyte and migrate throughout the porous polypropylene separator (Celgard® separator) to the anode side, causing contamination of the lithium foil and self-discharge of the cell by the shuttle effect


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(Scheme 1a).38 Our modified Li–S cell configuration is expected to boost faster internal electron transport as well as retaining the specific capacity of the cell by preventing large accumulation of the active material onto the separator through the early trapping of soluble LiPSs into the mesoporous network of the carbon interlayer and the further reutilization thereof (Scheme 1b). To corroborate this premise, a series of electrochemical measurements were performed. The cathode resistance before cycling was evaluated by EIS measurements of Li–S cells with and without MPCI, as shown in Figures 3a and 3b. The Nyquist plots for these cathodes consist of a single semicircle in the high-to-medium frequency region which is ascribed to the charge transfer resistance (RCT), while the leaning line at low frequency is related with mass transfer processes.39 Compared with the Li–S cell without carbon interlayer, the Nyquist plots in the middlefrequency region extracted from EIS data revealed a significant decrease of the RCT by over 79% after insertion of, e.g., MPC3I in the Li–S cell (Figure 3a). The significant reduction of the cathode resistance is caused by the use of the high electrically conductive carbon interlayer framework as a second current collector.31 After ten discharge–charge cycles the EIS spectra for the conventional Li–S cell showed a high increase of the internal resistance from 148 Ω to 386 Ω (Figures 3a and 3b). This behavior describes the formation of a passivation film by the deposition of reduced/oxidized LiPSs onto both the separator/cathode electrode interface and the separator/metallic lithium interface.40 On the other hand, the EIS spectra recorded after ten cycles for the Li–S battery with MPC3I exhibited a decrease of the RCT values from 31 Ω to 17 Ω (Figure 3a and inset of Figure 3b, respectively). The reduction of the RCT may be attributed to the redeployment of the physically stable active material occupying a more electrochemically favorable site through enlarging the contact area between the active material and the carbon


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interlayer after the initial cycles.41 In addition, an eased contamination of the Li anode by insoluble LiPSs could also be responsible for a lowered RCT. Figure 3c shows the discharge–charge voltage profiles obtained during the initial ten cycles at a C rate of 0.2 for the Li–S battery with MPC3I. Two voltage plateaus are observed after the first discharge process, which are related with the typical two-step sulfur reduction reactions. The upper discharge plateau at ∼2.30 V and the lower discharge widespread plateau at ∼2.1 V are attributed to the conversion of elemental sulfur (S8) to long-chain polysulfides (Li2Sn, n = 4–8) and

their

subsequent

reduction/precipitation

to

short-chain

polysulfides

Li2S2/Li2S,

respectively.42 On the other hand, the charge curve presents two continuous plateaus at ∼2.28 V and ∼2.37 V which correspond to reversible oxidation reactions of Li2S2/Li2S to S8/Li2S8.43 In the subsequent cycles, the discharge and charge capacity were kept practically constant, suggesting high capacity retention and excellent cycle stability. The 1st, 3rd, 6th and 10th of the cycle voltammograms of the cell with MPC3I cycled between 1.8 and 2.8 V at 0.1 mV s–1 are presented in Figure 3d. The CV curves display cathodic and anodic peaks at potentials consistent with those obtained from the discharge–charge voltage profiles analyzed above (Figure 3c). A low overpotential is observed during the initial cathodic/anodic scan which is attributed to the conversion and reorganization process of the sulfur-related species.41 However, after successive voltammetry cycles, the peak current and the integrated area of the cathodic as well as anodic peaks are well retained, indicating good redox reaction reversibility and, hence, superior cell stability. The absence of further polarization is explained by the good contact between the conductive carbon interlayer network and the active materials which provide faster electronic kinetics. In contrast, the CV curves of the conventional Li–S cell show spread peaks as well as a shift of both the reduction peaks to lower potentials and the oxidation peaks to higher potentials,


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indicating slow kinetic processes and thus inferior reversibility (Figure S1). Note that the actual carbon content in the entire Li–S cell set-up is equal for both types. The enhanced electrochemical stability of the reconfigured Li–S cell can be ascribed not merely to the improved electrical conductivity, but also to the trapping of soluble LiPSs into the mesoporous matrix of the carbon interlayer and their availability for further reutilization.

Scheme 1. Schematic configuration of the conventional Li–S cell (a) and the Li–S cell with a mesoporous carbon interlayer placed between the separator and the sulfur cathode (b).

Figure 3. EIS spectra of the Li–S cells with and without MPC3I before cycling (a) and after ten cycles (b). (c) Discharge–charge voltage profile at 0.2 C and (d) cyclic voltammograms at a scan rate of 0.1 mV s−1 of the Li–S cells with MPC3I.


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In order to investigate the effect of the pore volume of the MPCIs on the cycle performance of the Li–S batteries, galvanostatic cycling experiments were carried out. Figure 4a shows the cycling performance of Li–S cells with and without carbon interlayers based on the total amount of sulfur in the cathode, under a constant rate of 0.2 C. While the conventional Li–S cell delivered an initial discharge capacity of 1037 mAh g–1, the cells with MPC1I, MPC2I and MPC3I present higher initial discharge capacities of 1297, 1388 and 1364 mAh g–1, respectively. Furthermore, the cell without carbon interlayer displays poor cycling stability with a low capacity of 460 mAh g–1 after 100 cycles. However, the insertion of MPC1I, MPC2I and MPC3I into the conventional cells improves extraordinarily the cycling performance of the batteries with outstanding capacities of, respectively, 886, 1013 and 1104 mAh g–1 after 100 cycles, corresponding to a more than two-fold increase compared to our conventional Li–S cell. Moreover, an improvement of the Coulombic efficiency is observed with increasing pore volume of the mesoporous carbons that comprise the carbon interlayers. While the Li–S cell without carbon interlayer presents a Coulombic efficiency of 97.8% after 100 cycles, the Li–S batteries with incorporated MPC1I, MPC2I and MPC3I exhibit notable Coulombic efficiencies of, respectively, 98.7, 99.2 and 99.6%, after 100 cycles. The improvement of the cycling performance is ascribed to the sulfur-related species physically retained in the hollow interior and/or adsorbed onto the surface of the electrical conductive mesoporous carbon interlayer which allow the reutilization of the active material and hence higher capacities are achieved.31 Recently, Park et al. demonstrated that the pore volume rather than the pore size of a disordered mesoporous carbon fundamentally affects the cyclic performance of Li–S batteries,44 which supports our results. The polysulfide-trapping phenomenon is explained considering the carbon interlayer as a three-dimensional disordered mesoporous carbon framework, in which the high


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disordered porosity generates a strong tortuosity that suppresses the diffusion of large polysulfide species to the anode side. It is worth to mention that any possible surface area-dependence of the MPCIs regarding to the cycle performance of Li–S cells was discarded by additional electrochemical performance tests carried out with poly(vinylidene difluoride)-derived mesoporous carbon interlayers which have similar surface area but different pore volume compared to the MPC3I studied here (Figure S4). On the whole, these results highlight the crucial importance of large pore volume instead of high surface area in the carbon interlayer to trap the LiPS intermediates produced during the discharge–charge process and avoid their migration to the anode and further lithium/separator contamination. As the MPC3I presents the most promising results to prevent LiPSs migration due to its exceptional physical properties (e.g. high specific surface area and large pore volume), we focused on this carbon interlayer in the following studies. The rate capability of the Li–S batteries with and without MPC3I was investigated. Figure 4b shows the capacity of the Li–S cell containing MPC3I cycled at a current rate from 0.2 C to 4 C (1 C = 1672 mAh). The first specific discharge capacity at a low current rate of 0.2 C is 1357 mAh g–1, which is 81% of the theoretical capacity of sulfur and, thus, a notable value compared to literature despite the use of a sulfur-carbon black mixture with high sulfur loading as cathode.45 When the rate increases from 0.2 C to 0.5 C and 1 C, the discharge capacities decreased from 1190 mAh g–1 to 1080 and 940 mAh g–1, respectively. Even at higher rates of 2 C and 4 C, the battery delivers high capacities of 730 and 350 mAh g–1, respectively, exhibiting a favorable high rate performance. Furthermore, an excellent reversible capacity of 1170 mAh g–1 is recovered when the current density is reduced back to 0.2 C. Furthermore, the subsequent 40 cycles (from cycle 60th to 100th) now cycled at 0.5 C exhibit minor capacity loss. After this exhaustive cyclic treatment, our advanced


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Li–S cell configuration still shows an excellent electrochemical performance and promising long-term cycling stability. As we expected, the rate performance of the conventional Li–S battery is much lower compared to the Li–S cell with MPC3I although the actual carbon content is equal. At a low current rate of 0.2 C, the conventional Li–S cell has an initial capacity of 895 mAh g–1, roughly half of the theoretical capacity of sulfur. Moreover, at high current rates (1–4 C) the battery just provides negligible capacities. These differences in capacity lie in the insulating nature of the active material which is unable to transport electrons in an efficient way and the cathode-generated LiPS intermediates tend to agglomerate during the repetitive cycling process, decreasing the availability of the active material to react with Li+ and thereby lower values in capacity are obtained. These results prove that Li–S cells with a mesoporous carbon interlayer exhibit better capacity retention upon cycling compared to the conventional cell configuration. The long-term cycling performance of the Li–S cell with MPC3I measured at Crates of 0.2, 0.5 and 1 was investigated as shown in Figure 4c. The cells show initial discharge capacities of 1364, 1060, and 966 mAh g–1 at 0.2, 0.5, and 1 C, respectively. After 200 cycles, the cells at 0.2, 0.5, and 1 C rates retain capacities of 1015, 746, and 650 mAh g–1, respectively. Moreover, the cells exhibit remarkable capacity decays of only ~0.1% per cycle (after second cycle) and high average Coulombic efficiencies of ~98%, indicating a significantly lowered migration of LiPSs to the anode.


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Figure 4. (a) Cycling performance of the Li–S cells with MPC1I, MPC2I, MPC3I and without carbon interlayer at 0.2 C. (b) Rate performance of the Li–S cells with and without MPC3I. (c) Long-term cycling performance of the Li–S cell with MPC3I at 0.2, 0.5 and 1 C.

Further evidence for suppression of the LiPS shuttle phenomenon is found by a modification of the electrolyte. In this work we used LiNO3 as additive in the electrolyte due to the capability to form a protective NOx-film on the metallic lithium anode. This interface ensures a minimum of LiPSs reduction/deposition onto the Li metal anode and prevents further contamination upon cycling and, hence, maximizes the Coulombic efficiency.36 Though, a major drawback of LiNO3 is its consumption during cycling causing a continuous decrease of the Coulombic efficiency and its promoting ability of undesired dendrite formation. Considering these facts, additional electrochemical tests were performed using the electrolyte without LiNO3 in order to highlight the shuttle effect suppression when Li–S cells with MPCI barrier are used on the cathode side


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(Figure S5). As shown in Figure S5a, the second discharge–charge voltage profiles of the conventional Li–S cell using the electrolyte without LiNO3 exhibit a low Coulombic efficiency of 67%, which is attributed to a strong migration of LiPSs to the anode side. In contrast, the Li–S cell with MPC3I measured in analogous conditions delivers similar charge and discharge capacities with a high Coulombic efficiency of 96%, indicating improvement against the polysulfide shuttle (Figure S5b). Although the Li–S cell with MPC3I shows a Coulombic efficiency of ~99% when the LiNO3 additive is used (Figure S5c), we want to point out that the cycling performance of Li–S cells with M PC3I deliver similar capacities upon cycling, independently if they are measured with or without LiNO3 (Figure S5d). Overall, the high performance delivered by the cells with MPC3I is ascribed to the conductive carbon interlayer barrier, which effectively reduces the internal resistance of the pure sulfur cathode and assists in a fast transport of electrons and lithium ions during the redox reactions of sulfur. Moreover, the soluble LiPS intermediates generated in the cathode during cycling, are trapped and confined by the carbon interlayer framework, allowing reutilization of the active material. Additionally, the volume expansion of sulfur is accommodated within the available void space of the MPCI, avoiding possible rupture of the interlayer. In order to prove the above assumptions, the Li–S cell with MPC3I was disassembled after 200 cycles in the charged state and both the morphology and the physical properties of the cycled carbon interlayer were characterized. As shown in Figure 5a, the cross sectional SEM of the MPC3I reveals a stable structure with no evident damage, while the corresponding EDXS mapping shows carbon and sulfur signals uniformly distributed with no evidences of agglomerated active material, suggesting that the soluble LiPSs are captured and retained into the carbon interlayer matrix. In addition, SEM images of the MPC3I surface towards the cathode


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further confirm the absence of any large insulating S/Li2S-inactive layer formation on the surface of the interlayer (Figure S6). To confirm the presence of elemental sulfur into the cycled MPC3I, XPS measurements were performed after erosion of surface contamination by ion sputtering (Figure S7). The high-resolution spectrum of the S 2p binding energy region of the fresh sulfur cathode exhibits an S 2p doublet centered at 164.0 eV with intensity ration of 2:1 and separation of 1.2 eV, which is comprised of S 2p3/2 and S 2p1/2 components due to spin-orbit splitting (Figure S7a). This S 2p doublet is characteristic of elemental sulfur commonly observed in sulfur-impregnated carbon materials.46 In the case of the cycled MPC3I (Figure S7b), the S 2p spectrum shows the S 2p doublet positioned at 164.1 eV corresponding to elemental sulfur, which is direct evidence for the formation of S8 in the carbon interlayer. Besides, two additional S 2p doublets around 168.4 eV and 162.4 eV are observed, which are assigned to sulfate and terminal sulfur species, respectively.47-48 Furthermore, the S-to-C ratios of the fresh sulfur cathode and cycled MPC3I were estimated to be 0.38 and 0.16, demonstrating the presence of elemental sulfur inside the MPC3I pore system. These results validated our hypothesis that the carbon interlayer acts as a reservoir for the confinement of sulfur-related species. Further evidence for this was found by the decreased surface area and pore volume of the cycled MPC3I, as displayed in Figures 5b and 5c. Compared with the MPC3 powder, the fresh carbon interlayer MPC3I offers a slightly decreased but still high surface area and pore volume of 773 m2 g–1 and 2.698 cm3 g–1, respectively. However, the MPC3I after 50 cycles shows a significant decrease of both surface area and pore volume with values of 198 m2 g–1 and 0.821 cm3 g–1, respectively. Interestingly, the MPC3I before and after cycling maintains a mesoporous size distribution centered around 12 nm which is consistent with the average diameter of the mesopores of the corresponding pristine MPC3 powder. The almost completely filled pores suggest that the


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soluble LiPSs are retained into the porous carbon network. In addition, the free pore volume remained in the carbon interlayer facilitates good buffering of the volume expansion of sulfur during reduction and thereby a preservation of the mesoporous carbon framework. These studies demonstrate that the carbon interlayer can limit polysulfide migration by its interception inside the mesoporous carbon, keeping the material active for further reutilization.

Figure 5. (a) Cross sectional SEM image and the corresponding EDXS elemental mapping of the cycled MPC3I. Scale bar: 20 µm. (b) Nitrogen physisorption isotherms and (c) the corresponding QSDFT pore size distributions of the pristine MPC3 as well as the fresh and cycled MPC3I.

4. CONCLUSIONS We have demonstrated that Li–S batteries with large pore volume, mesoporous carbon-based interlayers placed between the separator and sulfur cathode exhibit a reduced internal resistance


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of the cells. Moreover, the intrinsic mesoporosity of the interlayers provides a physical place for trapping soluble polysulfides as well as to alleviate the negative impact of the large volume change of sulfur. These characteristics of the mesoporous carbon interlayer improve the reutilization of active material and thus prolong the cycle life of Li–S batteries. We highlight the use of colloidal silica template in the reaction media during the polymerization of resorcinol and formaldehyde and its subsequent carbonization and template etching produces mesoporous carbon powders with high surface area and large pore volume (900 m2 g–1 and 3.231 cm3 g–1, respectively). The physical features and more particularly the pore volume of the mesoporous carbon framework show a high impact on the final properties of the carbon interlayer as soluble polysulfide container. Overall, the rate performance and long cycling life of our reconfigured Li– S cells well exceeds that of conventional Li–S cells despite the use of high amounts of sulfur in the sulfur-carbon black cathode material. This study provides a new perspective for the fabrication of porous carbon interlayer and may have a major impact on the development of advanced Li–S batteries.

ASSOCIATED CONTENT Supporting Information Available: Complete author list for ref 30; Detailed experimental synthesis, TEM inspection and BET analyses of poly(vinylidene fluoride)-derived MPCs; Cycling performance of Li–S cells with hFMPC1 and coFMPC2 as carbon interlayers; CV of the conventional Li–S cell; Voltage profiles of Li–S cells with and without LiNO3; SEM images and EDXS analysis of the MPC3I before and after cycling; High-resolution S 2p XP spectra of the fresh sulfur cathode and the cycled MPC3I after 200 cycles; Data table with the physical


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properties of the mesoporous carbon powders. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Phone: +49 351 4659 693, fax: +49 351 4659 452, e-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank Andrea Voß, Anne Voidel and Ronny Buckan for their valuable technical support. We gratefully acknowledge financial support from the Federal Ministry of Education and Research (BMBF) of Germany through the Excellent Battery – WING center “Batteries Mobility in Saxony” (Grant nos. 03X4637B and 03X4637C).

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Table of Contents Graphic


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Mesoporous Carbon Interlayers with Tailored Pore Volume as

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