Bimodal Mesoporous CMK-5 Carbon: Selective Pore Filling with Sulfur

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Bimodal Mesoporous CMK-5 Carbon: Selective Pore Filling with Sulfur and SnO2 for Lithium Battery Electrodes Christian Weinberger, Sai Ren, Marc Hartmann, Thorsten Wagner, Didem Karaman, Jessica M Rosenholm, and Michael Tiemann ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00307 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Applied Nano Materials

Bimodal Mesoporous CMK-5 Carbon: Selective Pore Filling with Sulfur and SnO2 for Lithium Battery Electrodes Christian Weinberger1, Sai Ren1, Marc Hartmann1, Thorsten Wagner1, Didem. Ş. Karaman, Jessica M. Rosenholm, Michael Tiemann* 1

Department of Chemistry, University of Paderborn, Warburger Str. 100, D-33098 Paderborn,

Germany. E-mail: [email protected] 2

Pharmaceutical Sciences Laboratory, Faculty of Science and Engineering, Åbo Akademi

University, Turku, Finland KEYWORDS: ordered mesoporous carbon, nanocasting, selective pore filling, lithium ion battery, powder X-ray diffraction, composite materials, sulfur, SnO2

ABSTRACT

Ordered mesoporous CMK-5 carbon exhibits two distinct pore systems that can be modified individually. This work demonstrates how one of the pore system can be selectively filled with elemental sulfur, while the other pore system remains empty. The resulting sulfur-carbon

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composite material with high residual porosity can be used as the cathode material in lithiumsulfur battery cells. We present a systematic investigation of the loading of CMK-5 carbon with variable relative amounts of sulfur and compare the results to the preparation of SnO2 (as well as TiO2, Mn2O3/Mn3O4, NiO) nanoparticle-loaded CMK-5 carbon.

INTRODUCTION Ordered mesoporous carbon materials have recently been proposed as conductive host matrices for electrochemically active species in lithium battery electrodes.1-3 In particular, mesoporous CMK-3 carbon has been suggested as a host for elemental sulfur in lithium-sulfur cells.4-6 CMK-3 consists of periodically arranged, linear cylindrical rods of amorphous carbon with diameters in the range of a few nanometers; the voids between adjacent rods constitute a continuous system of mesopores of a few nanometers pore width.7 These pores have been filled with sulfur, yielding a carbon-sulfur composite material that serves as the cathode in Li-S cells. The carbon network provides electrical contact with a large interface to sulfur and limits outdiffusion of polysulfide species, thereby reducing capacity fading. Likewise, porous carbon materials are also being discussed as matrices for redox-active metal oxides in Li ion cell anodes. In this case the underlying concept is to constrain the metal oxide nanoparticle size (by the pore size dimensions) and to provide room for the substantial volume expansion during chargedischarge cycles (by leaving parts of the pore volume empty). Mesoporous CMK-3 carbon has been used as a host for SnO2 for this purpose.8 A more sophisticated system for the above-described functions is mesoporous CMK-5 carbon which has a similar nanostructure as CMK-3 carbon (with the same p6mm space group symmetry at the meso scale), but consists of hollow tubes of amorphous carbon instead of solid rods (see Scheme 1).9-11 Hence, two distinct pore systems exist in CMK-5 carbon, i.e. the voids between


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adjacent tubes (inter-tubular pores, as in CMK-3 carbon) and, in addition, the interior of the tubes (intra-tubular pores). The two pore systems can be addressed independently during the synthesis of CMK-5 (by a structure replication process using mesoporous SBA-15 silica12 as a structural mould). It is possible to load only one of the two pore systems with guest species8,13-15 or to modify/functionalize the pore walls16 of only one type of pores. This might turn out as a great utility for the design of Li cell electrode materials, since dual function is possible: one pore system may host a guest species (such as sulfur for cathodes or metal oxide nanoparticles for anodes), while the other pore system may be used for efficient electrolyte penetration. This concept has been demonstrated for SnO2 nanoparticles residing in the intra-8 or inter-tubular13 pores of CMK-5 carbon as well as for Co3O4 nanoparticles14 in the intra-tubular pores. The respective composite materials were successfully tested as anodes in Li cells. Also, Fe2O3 has been created selectively in the intra-tubular pores of CMK-5 carbon for catalytic application.15 Here we present the loading of the intra-tubular pores of CMK-5 carbon with variable amounts of sulfur and compare the results to loading with SnO2. We demonstrate that the loading can be monitored by a combination of N2 physisorption analysis and powder X-ray diffraction. Some preliminary tests show the general applicability of sulfur-loaded CMK-5 carbon as a cathode material in Li-S cells.


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Scheme 1. Schematic diagram of synthesis options for the incorporation of guest species (e.g. sulfur) in mesoporous carbon derived from SBA-15 silica. CMK-3 carbon exhibits only one type of pores that can accommodate the guest species, resulting in a non-porous composite (A). CMK-5 carbon features two distinct pore systems (intra- and inter-tubular pores), both of which can be filled at once to yield a non-porous composite (B); alternatively, the intra-tubular pores can be filled selectively (prior to silica etching) which leads to a composite that is still porous (C). RESULTS AND DISCUSSION The cylindrical pores of mesoporous SBA-15 silica were coated with a layer of carbon by carbonizing furfuryl alcohol and oxalic acid inside the pores as described in the Experimental Section. The resulting carbon-silica composite material, labeled CMK-5@SBA-15,9,10 still contains cylindrical (intra-tubular) mesopores, yet with a smaller diameter than SBA-15 silica alone (see below). In the next step, the intra-tubular pores of the CMK-5@SBA-15 composite


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were filled with either elemental sulfur (S) or tin dioxide (SnO2). Since the silica phase is still present at this time, the inter-tubular pores do not yet exist (they are entirely blocked with silica) and therefore cannot be filled with the guest species. Sulfur was then introduced into the pores as a melt; the relative amount of sulfur was varied. Formation of SnO2 was accomplished by repeated cycles of pore filling with tin(II) chloride (SnCl2) and subsequent conversion to SnO2 by thermal treatment under air. All procedures are described in the Experimental Section. The resulting composite materials contain variable amounts of the respective guest species (S or SnO2) in the pores of CMK-5@SBA-15, as shown in Tables 1 and 2. Please note that all wt% values of loading with guest species refer to the silica-free CMK-5 carbon samples (which will be discussed below); they were determined by EDX analysis (SnO2) or thermogravimetric analysis (sulfur), as described in the supplementary information section (Figure S1). The low-angle powder X-ray diffraction (XRD) diagrams of both series of products are shown in Figure 1. The reflections correspond to the two-dimensional hexagonal p6mm space group symmetry of the mesopore system; they are indexed as 10, 11, and 20, the latter two reflections being very weak. All reflections gradually decrease in intensity upon increasing quantities of the two guest species; the 11 and 20 reflections eventually become indiscernible. The presence of the guest species reduces the low-angle X-ray scattering contrast between carbon/silica and the mesopores. These findings confirm that the guest species are located inside the pores in variable relative amounts. This aspect will be elucidated in more detail below.


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Figure 1. Low-angle X-ray diffraction (XRD) diagrams of mesoporous CMK-5@SBA-15 carbon/silica composite materials containing variable amounts of sulfur (left) and SnO2 (right) in the mesopores.

Figure 2 shows the wide-angle powder XRD diagrams of the same samples. For the sulfurcontaining samples no wide-angle reflections are detected (except for two broad ones that originate from the sample holder), indicating that sulfur is present in an amorphous state, regardless of the degree of sulfur loading in the pores. This finding stands to reason given the fact that the thermodynamically stable crystalline modification of sulfur, orthorhombic α-S8, exhibits relatively large unit cell dimensions (a = 1.0465 nm, b = 1.2866 nm, c = 2.4486 nm17). A sufficiently large, XRD-detectable array of repeat units must therefore not be expected in the confined space of the pores (3.6 nm in diameter) of the CMK-5@SBA-15 host material. The absence of crystalline sulfur is therefore frequently considered as a strong indication for its location inside the pores, as any sulfur deposited outside the pores would likely occur in the


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crystallized state.4 In case of SnO2 the XRD signature of the tetragonal cassiterite α-SnO2 structure (JCPDS card No. 41-1445) is observed. (The cell dimensions of α-SnO2 are a = b = 0.4737 nm and c = 0.3185 nm.18) The line width of the reflections suggests crystal domain sizes below 10 nm (according to the Scherrer method, 110 reflection), as frequently found for SnO2 prepared by this method.19,20

Figure 2. Wide-angle X-ray diffraction (XRD) diagrams of the same samples as in Figure 1.

Figure 3 shows the N2 physisorption isotherms of the materials; the corresponding pore size distribution curves are shown in Figure 4. The CMK-5@SBA-15 host materials (with no guest species in the pores) show two distinct steep inclines in the adsorption isotherm, corresponding to capillary condensation at relative pressures of ca. p/p0 = 0.5 and p/p0 = 0.8. In the desorption isotherm these steps are more pronounced and occur delayed, i.e. shifted towards lower relative pressures of ca. p/p0 = 0.45 and p/p0 = 0.7, respectively. These findings suggest the occurrence of


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two types of pores with average pore diameters of 3.6 nm and 7.4 nm (Figure 4), respectively. The smaller pore mode (3.6 nm) is attributed to the intra-tubular cylindrical pores inside the CMK-5 carbon phase (which forms a coating of the SBA-15 pore walls), whereas the larger mode (7.4 nm) apparently corresponds to SBA-15 silica pores that have not been coated with carbon (Scheme 2a), presumably due to inhomogeneous infiltration of the carbon precursor compounds. (The original SBA-15 silica material exhibits a slightly larger pore diameter of 7.9 nm. This can be explained by the fact that the synthesis of SBA-15 comprised a maximum temperature of 550 °C for its calcination, whereas the carbonization temperature for the CMK-5 inside the silica pores was 850 °C; a slight shrinkage of the silica lattice at this temperature due to densification is plausible.) The specific pore volumes of the two types of pores, determined by integrating over the respective pore size distribution peaks, are 0.176 cm3 g-1 (smaller pores, 3.6 nm) and 0.099 cm3 g-1 (larger pores, 7.4 nm), respectively.

Figure 3. N2 physisorption isotherms of the same samples as in Figure 1. Plots are vertically shifted for clarity.


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Figure 4. BJH pore size distribution plots calculated from the data in Figure 3 by the BJH method. Plots are vertically shifted for clarity.

Incorporation of the guest species (S or SnO2) into the CMK-5@SBA-15 materials leads to a substantial decrease in the specific pore volumes, but not to any significant change in the average pore sizes (see Figures 3 and 4, Tables 1 and 2). However, a clear difference between the two guest species is obvious. In case of sulfur the smaller pores are filled preferentially. The respective pore volume decreases much sooner with increasing amounts of sulfur than the volume of the larger pores. The volume of the smaller pores is reduced by 99 % (to 0.001 cm3 g-1) upon loading with 53 wt-% sulfur, at which point the volume of the larger pores is reduced by only 3 % (to 0.096 cm3 g-1). Hence, it can be concluded that sulfur is deposited preferably in the smaller pores (Scheme 2b). This stands to reason since the walls of the smaller pores consist of (hydrophobic) carbon to which elemental sulfur, with its low polarity, will adsorb more easily than to the walls of the larger pores which consist of (hydrophilic) SiO2. Contrary to that, SnO2 is taken up by both types of pores nearly simultaneously; their respective volumes decrease by 59


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% (from 0.234 to 0.095 cm3 g-1, smaller pores) and by 44 % (from 0.186 to 0.105 cm3 g-1, larger pores) upon loading with 59 wt-% of SnO2. This indicates that SnCl2 equally adsorbs to both the carbon and the silica pore walls (Scheme 2c).

Scheme 2. Schematic representation of a mesopore in SBA-15 silica. The pore wall is incompletely coated with carbon (a), which results in two distinct pore modes: smaller pores (with carbon pore walls) and larger pores (with silica pore walls). Sulfur resides preferentially in the smaller pores (b), whereas SnO2 is equally located in both pore modes (c).

The next synthesis step comprised of the selective removal of silica from the CMK-5@SBA-15 carbon/silica composites containing the guest species (S or SnO2) and from the guest species-free samples. Figure 5 shows the low-angle powder XRD diagrams of the respective CMK-5 materials. (The wide-angle diagrams are shown in Figure S2 in the supplementary information section; they are very similar to the ones before removal of the silica phase, as expected.) The low-angle diffraction patterns exhibit some significant differences from the respective diffraction diagrams before removal of the silica (see Figure 1). For the pure CMK-5 samples (containing no guest species) the 11 reflection is now the most intense one while the 10 reflection is hardly discernible; this is commonly observed for CMK-5 carbon9,10


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and will be discussed below. Loading with increasing amounts of sulfur slowly reduces the 11 peak intensity while at the same time the 10 peak becomes more intense. In contrast, loading with increasing amounts of SnO2 leads to a sooner decrease of the 11 peak intensity while the 10 remains as weak as in the SnO2-free material.

Figure 5. Low-angle X-ray diffraction (XRD) diagrams of mesoporous the CMK-5 carbon materials after removal of silica containing variable amounts of sulfur (left) and SnO2 (right) in the mesopores.

These observations can be rationalized by considering the electron densities of the three species involved, i.e. carbon, sulfur, and SnO2. The Fourier transform of the electron density distribution in the unit cell (of the p6mm mesopore lattice arrangement) determines the structure factor and, thus, the intensity of the X-ray diffraction peaks. In a simplified model we assume that all electrons scatter X-rays to the same extent, i.e. we consider the relevant electron charge density ρe to be proportional to the mass density ρ of the respective material by


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ߩ௘ =

ߩ ⋅ ܰ஺ ⋅ ܼ ⋅ ݁ ‫ܯ‬

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(1)

where Z is the number of electrons per formula unit, e is the electron charge, M is the molecular/atomic mass, and NA is Avogadro's number. Materials with the same mesopore structure and the same/similar electron density in the pore walls then exhibit the same/similar low-angle X-ray diffraction patterns. As described in the Introduction, pure CMK-5 (with no guest species) consists of cylindrical tubes of amorphous carbon; the tubes are hollow (intra-tubular pores) arranged in parallel with voids between adjacent ones (inter-tubular pores). Scheme 3a depicts two such adjacent hollow tubes; the electron density is zero both between the tubes and inside the hollow center of each tube (disregarding the low quantity of carbon that is actually present between the tubes). The 2D hexagonal p6mm symmetry of the tube arrangement leads to the characteristic low-angle X-ray diffraction pattern described above (strong 11 reflection, weak 10 and 20 reflections).21 For CMK-5 carbon loaded with a guest species the situation is different (Scheme 3b). The guest species resides exclusively in the interior of the tubes and not between adjacent tubes. (The voids between the tubes were still occupied with silica when the guest species was introduced.) The electron density distribution is now determined by three phases: carbon, the guest species (inside the carbon tubes), and air (between adjacent tubes). The electron density of sulfur according to equation (1) is ρe = 100 C m-3 (mass density of sulfur:22 ρ = 2.07 cm3 g-1), nearly identical with that of carbon in CMK-5, ρe = 98 C m-3 (mass density of CMK-5:23 ρ = 2.03 cm3 g-1; mass density of non-porous amorphous carbon:22 ρ = 1.8–2.1 cm3 g-1). Hence, the overall electron density distribution of our sulfur-loaded (53 wt-%) CMK-5 carbon material is approximately the same as that of pure CMK-3 carbon.9-11,21 CMK-3 has the same mesostructure and symmetry as CMK-5, except that it consists of solid carbon rods, instead of hollow carbon


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tubes (Scheme 3c). Consequently, the low-angle X-ray diffraction pattern of our fully sulfurloaded CMK-5 carbon sample resembles that of CMK-3,7 i.e. it exhibits a strong 10 and relatively weak 11 and 20 peaks. This corroborates that sulfur is located inside the CMK-5 carbon tubes. SnO2, on the other hand, exhibits a much higher electron density of ρe = 292 C m-3 (mass density of mesoporous SnO2:23 ρ = 6.90 cm3 g-1; mass density of non-porous SnO2 (cassiterite):22

ρ = 7.0 cm3 g-1). This means that SnO2 inside the CMK-5 tubes scatters X-rays much stronger than the carbon phase. A similar case has been reported and discussed recently for Co3O4 particles in a CMK-5 matrix;14 Co3O4 exhibits a comparable electron density of ρe = 257 C m-3 (mass density of non-porous Co3O4:22 ρ = 6.11 cm3 g-1). The extremely low intensity of the lowangle X-ray diffraction peaks in the SnO2-loaded sample (59 wt-%) indicates that SnO2 does not form a continuous phase inside the carbon tubes. Instead, it appears to be present as separate, individual particles distributed randomly over the hollow tubes, leading to low periodicity and, hence, negligible low-angle Bragg diffraction intensity.


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Scheme 3. Schematic representation of (a) two adjacent hollow carbon tubes in CMK-5, (b) adjacent carbon tubes filled with sulfur in CMK-5, and (c) adjacent solid carbon rods in CMK-3. The electron density distribution (depicted along the a axis) in the sulfur-loaded CMK-5 sample is similar as in CMK-3 carbon.

This finding is supported by transmission electron microscopy (TEM); Figure 6 shows an example image of silica-free CMK-5 carbon loaded with 36 wt-% SnO2. (No TEM images of sulfur-loaded samples were recorded due to limitations caused by sulfur sublimation under the high-vacuum conditions in the electron microscope.24) Similar findings are obtained for various other metal oxides (TiO2, Mn2O3/Mn3O4, NiO) as shown in the supplementary information section (Tables S2-S4).


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Figure 6. Transmission electron microscopic (TEM) images of CMK-5 carbon containing individual SnO2 nanoparticles in the intra-tubular pores (46 wt-% SnO2).

The N2 physisorption isotherms and the corresponding pore size distribution curves of the silica-free samples are shown in Figures 7 and 8. All samples exhibit type IV(a) isotherms with H1 hysteresis, as expected for uniform, cylindrical mesopores.25 Similar as in the CMK-5@SBA-15 samples (see Figures 3 and 4), capillary condensation occurs at relative pressures of ca. p/p0 = 0.5 in all adsorption isotherms with hysteresis closure at ca. p/p0 = 0.45 in the desorption branches, corresponding to BJH mesopore diameters of ca. 3.6 nm.


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Figure 7. N2 physisorption isotherms of the same samples as in Figure 5. Plots are vertically shifted for clarity.

Contrary to the CMK-5@SBA-15 samples, however, the second pore type (i.e. the larger pores of ca. 7.4 nm diameter, corresponding to a second capillary condensation step in the isotherms) is now absent, confirming the above-made assumption that they represented unfilled fractions of the SBA-15 silica pores. On the other hand, the silica-free CMK-5 carbon material contains two intrinsic types of pores, as described above; in addition to the intra-tubular pores (more or less filled with the guest species), the inter-tubular pores are now accessible. However, both pore modes have very similar pore sizes which is why they usually cannot be distinguished in the N2 physisorption experiment;11,16 they both contribute to the single capillary condensation step in the isotherms and to the resulting single peak in the pore size distribution curve. The porosity is progressively reduced with increasing amounts of the guest species, consistent with the observations made for the CMK-5@SBA-15 composite materials. The decrease in specific pore


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volume and BET surface area (Tables 1 and 2) occurs quite drastically, partly because they are, by definition, normalized to the sample weight which increases with the gradual filling of the pores with the guest species. Note that a substantial degree of porosity persists even upon loading with high amounts of guest species due to the empty inter-tubular pores.

Figure 8. BJH pore size distribution plots calculated from the data in Figure 7.

It also needs to be stressed that the average pore diameter of the intra-tubular pores does not change upon loading with guest species (this was already apparent from the physisorption data for the materials before the removal if the silica phase) which indicates that each individual segment of the pores is either filled entirely with the guest species or is entirely empty; partial filling by, e.g., coverage of the carbon pore walls with the guest species, can be ruled out as this would result in a decrease of the average pore diameter. This finding supports the above-made assumption that SnO2 is present in the pores as individual nanoparticles rather than forming a continuous phase throughout the network.


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Table 1. Porosity data of sulfur-loaded samples.

SBA-15

CMK-5@ SBA-15

CMK-5f

small mesopores

large mesopores

wt.-% sulfura

DBJHb / nm

Vc / cm3 g-1

DBJHb / nm

-

-

-

7.9

1.291

1.26

451

0

3.6

0.176

7.8

0.099

0.43

388

25

3.6

0.060

7.8

0.104

0.22

132

43

3.6

0.008

7.8

0.096

0.15

71

53

-

0.001

7.8

0.091

0.11

51

0

3.6

1.489

-

-

2.18

1475

25

3.6

1.052

-

-

1.46

883

43

3.6

0.611

-

-

0.87

491

53

3.6

0.376

-

-

0.62

304

Vc / cm3 Vd / cm3 g-1 g-1

aBETe / m2 g-1

a

Data derived from thermogravimetric analysis; loading with respect to silica-free CMK-5. Pore diameter. c Specific pore volume from peak area. d Entire specific pore volume from isotherm (adsorbed volume at p/p0 = 0.95). e Specific surface area. f Silica-free.

b

Table 2. Porosity data of SnO2-loaded samples.


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SBA-15

CMK-5@ SBA-15

CMK-5f

small mesopores

large mesopores

wt.-% SnO2a

DBJHb / nm

Vc / cm3 g-1

DBJHb / nm

-

-

-

7.9

1.327

1.26

519

0

3.6

0.186

7.4

0.234

0.58

432

46

3.6

0.132

7.3

0.151

0.43

323

61

3.6

0.116

7.3

0.119

0.38

294

75

3.6

0.105

7.3

0.095

0.33

257

0

3.6

1.027

-

-

2.13

1642

46

3.6

0.714

-

-

1.48

1152

61

3.6

0.521

-

-

1.04

793

75

3.6

0.509

-

-

0.78

603

Vc / cm3 Vd / cm3 g-1 g-1

aBETe / m2 g-1

a

Data derived from EDX analysis; loading with respect to silica-free CMK-5. b-f See legend to Table 1.

The applicability of SnO2 nanoparticles in the intra-tubular pores of CMK-5 carbon as an anode material in Li ion cells has recently been demonstrated,8 as mentioned in the Introduction. In order to also prove the general concept of using sulfur-loaded CMK-5 carbon as a cathode material in Li-S cells, we have carried out some basic tests, as shown in the supplementary information section. Figure S3 shows the evolution of the specific capacity of a cell assembled from CMK-5 with 53 wt.-% sulfur as the cathode and lithium as the anode material during repeated charge/discharge cycles. The initial charge capacity of 1233 mAh g-1 (1107 mAh g-1 for discharge) decreases by 24 % to 938 mAh g-1 (discharge: by 22 % to 859 mAh g-1) after the first 15 cycles and by a total of 66 % to 419 mAh g-1 (discharge: by 63 % to 406 mAh g-1) after 100 cycles. This strong capacity fading suggests that further improvement in materials synthesis will be necessary in future work; nevertheless, the results provide proof of concept for the suitability


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of CMK-5 as a host for sulfur in Li-S cell cathodes. The charge/discharge profile is shown in Figure S4 in the supplementary information section; it exhibits the typical shape1,2 of step-wise reduction of sulfur in several steps corresponding to distinct lithium (poly)sulphide species. The Coulombic efficiency (quotient of charge and discharge capacity, × 100 %) decreases from initially 90 % to 85 % after 6 cycles which is attributable to non-reversible surface reactions at the electrodes;1 the value then recovers to 97 % after 100 cycles.

EXPERIMENTAL SBA-15 silica was synthesized by a modified literature synthesis.12 16.0 g Pluronic P123 triblock copolymer (Sigma-Aldrich) was dissolved in 480 mL water and 48 mL hydrochloric acid (37 %, Stockmeier). After 24 h 37 mL tetraethyl orthosilicate (TEOS, 99 %; ABCR) was added and the solution was stirred for 24 h at 35 °C and then placed in a glass-lined autoclave at 140 °C for another 24 h. The solid product was collected by filtration, washed with water, and dried at 120 °C overnight. Finally, the as-synthesized product was calcined at 550 °C for 6 h in flowing air (heating ramp 2.5 °C min-1). The CMK-5@SBA-15 carbon/silica composite material was synthesized by a modified literature synthesis.10 SBA-15 silica was grinded in a mortar with a mixture of oxalic acid dihydrate (98 %, ABCR) dissolved in furfuryl alcohol (98 %, ABCR). 9 mg oxalic acid were used for 1 mL furfuryl alcohol. The amount of the precursor solution was calculated by the mass and the total pore volume of SBA-15 silica, multiplied by 1.3 (130 % theoretical loading). The homogeneous mixture was kept at 60 °C for 24 h and then at 90 °C for 48 h. Afterwards, the composite was carbonized under vacuum (150 °C, 2 °C/min; 300 °C, 1 °C/min; 850 °C, 5 °C/min for 4 h).


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Loading with sulfur: A homogeneous mixture of sulfur (99 %, ABCR) and the CMK-5@SBA-15 carbon/silica composite material was prepared by grinding in a mortar. The mixture was heated to 155 °C for one hour to melt the sulfur; by this procedure, the molten sulfur is taken up by the porous material due to capillary force.4 The amount of sulfur per 1 g of the carbon/silica composite was varied from 0.14 g to 0.58 g. Silica was later removed from the sulfur-loaded material by dispersing 1 g of the composite in 100 mL aqueous hydrofluoric acid (12 %, Sigma-Aldrich) and stirring for at least 6 h; this procedure was repeated three times, followed by washing with water and drying overnight at 60 °C. Loading with SnO2: The CMK-5@SBA-15 carbon/silica composite was grinded in a mortar with a saturated solution of SnCl2 (98 %, ABCR) in water and HCl (37%, Stockmeier, 1:1 vol.). The amount of the SnCl2 solution was calculated by the mass and the total pore volume of host composite material, multiplied by 1.3 (loading of 130 %). After drying at 120 °C overnight SnCl2 was converted to SnO2 by heating the material to 500 °C for 4 h (heating rate of 2 °C min-1) under air. The whole procedure was repeated up to 3 times to vary the loading with SnO2. Silica was later removed from the SnO2-loaded material by dispersing 1 g of the composite in 100 mL KOH solution (5 mol L-1, Stockmeier) and stirring for at least 6 h; this procedure was repeated three times, followed by washing with water and drying overnight at 60 °C. Characterization: N2 physisorption analysis was conducted with a Quantachrome Autosorb 6 apparatus. Samples were degassed at room temperature (sulfur-containing samples) or at 120 °C (all other samples), respectively, for 12 h prior to measurement. Pore size distributions were calculated by the BJH model26 from the desorption isotherms. BET surface areas27 were calculated in a pressure range 0.1 ≤ p/p0 ≤ 0.3. Powder X-ray diffraction was carried out on a


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Bruker AXS D8 Advance diffractometer with Cu Kα radiation (40 kV, 40 mA). The step size 2θ = 0.0075° for low-angle (2θ ≤ 5°) and 2θ = 0.02° for wide-angle measurements (20° ≤ 2 θ ≤ 80°) with a counting time of 3 s per step. TEM images were taken with a Jeol JEM-1200EX electron microscope. Thermogravimetric analysis (TGA) was conducted under synthetic air at a heating rate of 5 °C min−1 using a Mettler Toledo TGA/SDTA851e thermobalance. EDX measurements were conducted on a Zeiss NEON 40 scanning electron microscope with a ThermoFisher Scientific UltraDry detector. Battery tests: Li-S test cells were assembled in a gas-tight Teflon housing (10 mm Ø, Swagelok). A disk of the sulfur-loaded CMK-5 carbon material (fabricated by punching out of a doctor-bladed layer of the carbon with polyvinylidene difluoride, MW = 275 000, Sigma-Aldrich, as a binder) on aluminum foil was used as the cathode. Lithium foil served as the anode. Electrodes were separated with a polypropylene foil (Celgard 2500). A solution (1 mol L-1) of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in 1,3-dioxolane was used as the electrolyte. Cell measurements were performed by using a custom-built charging circuit optimized for low capacity Swagelok-type half cells. An adjustable voltage-controlled current source was operated by a microcontroller-based multi-purpose DAQ system, which also records cell voltage and charging current. Data was transferred via a serial connection (USB) to a PC program written in LabVIEW. Based on the recorded current values the cell capacitance was calculated. A standard charging-discharging cycle started with a constant current charging up to 2.5 V cell voltage, followed by a constant voltage operation until the charging current dropped to 20 % of the initial value. The cell was then discharged in constant current mode (100 µA) until a cell voltage of 1.8 V was reached. This cycling protocol corresponds to a C rate of ca. 0.3.


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CONCLUSION In summary, we have demonstrated that elemental sulfur can be selectively filled into the intratubular pores of CMK-5 carbon by melt impregnation. After creating a carbon coating layer at the pore walls of SBA-15 silica (CMK-5@SBA-15), sulfur can be introduced into the pores before the removal of the silica matrix, i.e. before the inter-tubular pores of CMK-5 carbon become accessible. Incomplete coating of the silica pores with scarbon does not compromise the structural quality of the final products, since sulfur is preferentially deposited to carbon pores instead of un-coated silica pores.

ASSOCIATED CONTENT Supporting Information. The following files (PDF) are available free of charge. Explanation of the determination of SnO2 and sulfur content, EDX analysis, thermogravimetric analysis, wide-angle X-ray diffraction measurements, measurement of the coulombic efficiency and specific capacity over 100 cycles of charge and discharge, charge-discharge profiles for selected cycles (1, 10, 25, 100), porosity data of metal oxide (TiO2, Mn2O3/Mn3O4, NiO)-loaded carbon composite materials

AUTHOR INFORMATION Corresponding Author Michael Tiemann, [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources C.W. thanks the German Fonds der Chemischen Industrie (FCI) for a PhD Fellowship (510262).

ACKNOWLEDGMENT C.W. thanks the German Fonds der Chemischen Industrie for a PhD Fellowship. We thank Celgard for the support with the separator material.

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Bimodal Mesoporous CMK-5 Carbon: Selective Pore Filling with Sulfur

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