A new route for the preparation of mesoporous carbon ... · A new route for the preparation of...

A new route for the preparation of

mesoporous carbon materials with

high performance in lithium-sulphur

battery cathodes

Martin Oschatz,a Sören Thieme,

b Lars Borchardt,

a Martin

R. Lohe,a Tim Biemelt

,a Jan Brückner,

b Holger Althues

b

and Stefan Kaskel*a,b

a Department of Inorganic Chemistry, Dresden University of Technology, Bergstrasse 66, D-

01069 Dresden, Germany b

Fraunhofer Institute for Material and Beam Technology, Winterbergstraße 28, D-01277

Dresden, Germany

Electronic Supplementary Information (ESI†)

Electronic Supplementary Material (ESI) for Chemical CommunicationsThis journal is © The Royal Society of Chemistry 2013

Supplementary Figures

Figure S1. Kroll-Carbon synthesis. Infiltration of TiO2 nanoparticles (1) with sucrose is followed by

polymerization and gives the precursor composite (2). After carbonization, the carbon/TiO2 composite (3) is

subsequently transformed to Kroll-Carbon (4) by carbochlorination.

Figure S2. Kroll-Carbon structure. The thermogravimetric analyses of KC (obtained from P 25 template)

under oxidative conditions (air atmosphere) in a) shows complete combustion due to the absence of oxidic

impurities. XRD patterns of the composite materials in the different synthesis stages and of the KC obtained

from P 25 template in b) proves the absence of both titanium carbide after the carbonization and impurities after

the carbochlorination step due to a complete conversion of the titanium dioxide template to volatile TiCl4 and

CO.


Figure S3. Kroll-Carbon porosity. Semi-logarithmic nitrogen physisorption isotherms (measured at -196°C) of

KC from Degussa P 25 templates. The adsorption in the low pressure regime is associated with the filling of

narrow micropores.

Figure S4. Kroll-Carbon porosity. a) represents the QSDFT pore size distributions of Kroll-Carbons obtained

from different sorts of TiO2 templates. B) shows the QSDFT fit between the used carbon slit/cylindrical


adsorption branch kernel (dotted line) and the experimental data (nitrogen physisorption, measured at -196°C;

empty diamonds) for the KC obtained from the P25 templates. The good fit between the calculated and the

experimental data indicates that this kernel calculates a reliable PSD.

Figure S5. Kroll-Carbon porosity. TEM image and schematic illustration of the hierarchical pore structure

present in the Kroll-carbons from Degussa P 25 templates.

Figure S6. Kroll-Carbon structure. SEM image of KC prepared from Degussa P 25 template particles.


Figure S7. Kroll-Carbon structure. SEM images of the Degussa P 25 template (left) and Kroll-carbon (right)

at equal magnifications.

Figure S8. Conformability of the carbochlorination reaction. Photographs of the Degussa P 25/sucrose

composite (left) and the KC material (right).

Figure S9. Structural analysis of KC/S cathodes and -composites. The representative SEM micrographs of

the highly loaded KC/S_64 cathode in a) and b) show the cathode surface with inter-particular porosity and


uniformly distributed multi-walled carbon nanotubes throughout the whole cathode providing long-lasting

electrical contact of the KC/S particles. The cathode surface is smooth but reveals a wide-open and highly

accessible inter-particular porosity throughout the active material layer. The large-sized, irregularly-shaped KC/S

composite particles (up to 20 µm) reduce the cathode compressibility (density) and lead to a loosely packed

active layer. (c) are thermal analyses of KC/S composites (air atmosphere) after melt infiltration showing good

accordance between the experimental and theoretical values of 33:67 (1:2 composite), 25:75 (1:3 composite),

20:80 (1:4 composite).

Figure S10. Structural analysis of KC/S cathodes. XRD patterns of pristine sulphur, the PTFE binder, and the

KC/S composites of different compositions. The intense PTFE peak can be observed for all cathodes. In contrast,

the reflections corresponding to bulk α-sulphur completely disappear even at high loadings of 72 wt% because

sulphur is completely melt-infiltrated and well dispersed inside the KC framework.


Figure S11. Electrochemical characterization of KC/S cathodes. Cycling stabilities and coulombic

efficiencies of the KC/S_60 and KC/S_53 cathodes.

Figure S12. Electrochemical characterization of KC/S cathodes. Discharge/charge voltage profiles of the

10th cycles for KC/S cathodes measured at a constant current of 167 mA g-1

(C/10).


Figure S13. Characterization of the CMK-3 reference material. Nitrogen physisorption isotherms (measured

at -196°C) and corresponding QSDFT pore size distribution (inset). The isotherm is typical for ordered

mesoporous carbon materials with narrow pore size distribution. The BET surface area is 1396 m2 g

-1 and the

pore volume was determined to 1.42 cm3 g

-1.

Figure S14. Electrochemical characterization of CMK-3/S reference cathodes. Cycling stabilities and

coulombic efficiencies of the CMK-3/S_60 and CMK-3/S_53 cathodes. The curve of the KC/S_72 cathode is

shown for comparison.


Figure S15. Electrochemical characterization of the KC/S_72 cathode. Measured at variable C-rates. Filled

and empty symbols represent the capacities per mass of sulphur and per mass of the cathode, respectively. The

sulphur surface loading of the cathode sample is 4.92 mg(S) cm-2

. Thus, the highest rate of 1.672 A g-1

(1C)

corresponds to an extremely high current density of 8.23 mA cm-2

.


Supplementary Tables

Table S1. Nitrogen physisorption (measured at -196°C) data summary of mesoporous Kroll-Carbons prepared

from TiO2 template particles of different size.

Template

Template specific

surface area

(m2 g

-1)

KC specific

surface area

(m2 g

-1)

KC total

pore volume

(cm3 g

-1)

KC mesopore

diameter

(nm)

KC micropore

volume

(cm3 g

-1)

Degussa P 25 50 1989 3.12 18 0.30

Degussa P 90 100 1979 2.89 11 0.28

„in-house

TiO2-NP“ 152 1872 2.28 8.5 0.32

Table S2. Characteristics of the prepared KC/S composite cathodes.

Cathode

foil

KC:S composition

(weight ratio)

KC/S:MWCNT:PTFE

composition

(weight ratio)

Active layer

thickness

(µm)

Sulphur surface

loading

(mg cm-2

)

KC/S_53 1:2 8:1:1 95 2.83

KC/S_60 1:3 8:1:1 85 3.32

KC/S_64 1:4 8:1:1 89 4.55

KC/S_72 1:4 18:1:1 83 4.92


Experimental Section

Kroll-Carbon synthesis. For the synthesis of Kroll-Carbons, 2 g titanium dioxide

nanoparticles (P 25 and P 90 particles were purchased from Evonik, Germany; LW-S particles

were purchased from Sachtleben, Germany) were carefully mixed in a glass Petri dish with a

10 ml aqueous solution of 2.5 g sucrose (Roth Chemicals; 99%) to which was added 30 mg of

sulphuric acid (Sigma Aldrich; 96% in water). The resulting white dispersion was then left at

100°C for 3 h and at 160°C for another 3 h to achieve complete polymerization of the

disaccharide. About 2 g of the black hydrocarbon/titanium dioxide composite was then placed

in a quartz boat inside a quartz tube (inner tube diameter 30 mm) in a horizontal tubular

furnace and flushed with 150 ml min-1

argon for 1 h. The furnace was heated up to 900°C

with a heating rate of 5 K min-1

and annealed for 1 h under constant argon flow. Subsequently,

the gas flow was changed to a mixture of 80 ml min-1

chlorine and 70 ml min-1

argon for 2 h

while keeping the temperature at the same level. The furnace was cooled down to room

temperature under an argon flow of 30 ml min-1

. A post reductive treatment was performed in

the same tube with 80 ml min-1

hydrogen at 600°C for 2 h.

TiO2 nanoparticle synthesis. In house made TiO2 nanoparticles were produced by a flame

spray pyrolysis process as introduced by Madler et al.46

. A 0.58 M solution of titanium

tetraisopropoxide dissolved in a 11:5 (v:v) mixture of toluene and acetonitrile was fed with

5 ml/min through a commercial nozzle (NPS10, Tethis) and ignited by a surrounding

supporting-flame to prevent the main flame from self-extinguishing. The supporting-flame

gas flow consisted of 1.5 slm CH4 and 3.0 slm O2 (slm denotes a flow of gas in liters per

minute at standard conditions). The liquid feed was delivered by a micro annular gear pump

(mzr-2905, HNP Mikrosysteme GmbH) and dispersed by 7 slm O2 with a dispersion gas

pressure drop of 2.6 bar, which was adjusted by the annular gap around the liquid feed

capillary. Additionally a sheath gas flow of 5 slm O2 was supplied through a porous metal


ring to ensure full combustion of the precursor solution. All gas flows were controlled by

thermal mass flow controllers (Bronkhorst). The synthesized particles were collected on a

binder free glass fiber filter (GF/A Whatman) with the help of a rotary vane pump

(Vacuubrand RZ 9).

Composite preparation. Kroll-Carbon/sulphur (KC/S) nanocomposites with different

composition were prepared by combining pristine sulphur (Sigma Aldrich, 99.5%) with

finely ground Kroll-Carbon in different KC:sulphur weight ratios of 1:2, 1:3 and 1:4. After

thorough homogenization the mixture was transferred into a ceramic crucible and heated to

155°C for 12 h under air to perform the melt infiltration of sulphur. This strategy was used to

adjust the degree of carbon host pore-filling in the resulting KC/S nanocomposite, which was

31 (1:2), 47 (1:3), and 62 vol% (1:4 composite) depending on the KC-to-sulphur ratio in the

mixtures and the void total pore volume of KC (3.1 cm3 g

-1). Assuming that all sulphur

encapsulated in the Kroll-Carbon matrix is completely converted into lithium sulfide under a

volume expansion of about 79%, the theoretical maximal carbon pore-filling was calculated to

be 56 vol.%, thus being somewhat lower than the value adjusted for the 1:4 composite. All

values are calculated based on the density of solid α-sulphur (2.07 g cm-3

) and solid lithium

sulfide (1.66 g cm-3

).

Electrode preparation. The as-prepared KC/S nanocomposites were ground together with

multi-walled carbon nanotubes (MWCNT, Nanocyl NC 7000 series, 90 %) conducting agent

in a mortar. After the addition of poly(tetrafluorethylene) (PTFE, ABCR) binder the mixtures,

which exhibit a composition of 8:1:1 (KC/S composite:MWCNT:PTFE) by weight

corresponding to 53 (1:2), 60 (1:3) and 64 wt% (1:4 composite) sulphur in the complete

cathode, were thoroughly homogenized. Moreover, one cathode sample with very high

sulphur content of 72 wt% was prepared by increasing the KC/S composite (1:4) content

under reduction of both conducting agent and binder content, corresponding to a weight ratio

of 18:1:1 (KC/S composite:MWCNT:PTFE). Intensive grinding at elevated temperature


caused the agglomeration of the raw material particles and thus formation of tiny sheets,

which were subsequently rolled out to a thin cathode film. The composite cathode foils are

denoted as KC/S_53, KC/S_60, KC/S_64 and KC/S_72 according to their sulphur content in

weight percent. The self-supporting cathode foils were laminated onto carbon coated,

expanded aluminum current collector (Benmetal, 99.5 % with 20 % Electrodag EB-012).

Circular electrode discs (diameter 12 mm, area 1.131 cm2) were punched out for

electrochemical characterization.

Characterization. Nitrogen physisorption (-196°C) measurements were performed on a

Quantachrome Autosorb 1C apparatus. Specific surface areas were calculated using the BET

equation in the P/P0 range from 0.05 to 0.20. Total pore volumes were determined from the

amount adsorbed at 0.99 P/P0. Pore size distributions were calculated using the Quenched

Solid Density Functional Theory (QSDFT) method for carbon (slit/cylindrical pores,

adsorption branch kernel). Micropore volumes were calculated from the cumulative pore

volumes at a pore diameter of 2 nm. FESEM (Field Emission Scanning Electron Microscopy)

investigations were carried out with a Stereoscan 260 SEM with EDX analysis system using

SE (Secondary Electrons) and BSE (Backscattered Electrons) detectors, respectively.

Elemental analyses using EDX were obtained as a mean value of three measurements in a

magnification of 3000. The TEM investigations were carried out on a Cs-corrected JEOL

JEM-2010F microscope (JEOL, Japan). X-ray diffraction patterns were collected on a Stoe

Stadi-P or on a PANanalytical X´Pert Pro with CuKα1 radiation ( = 0.15405 nm). The

thermal analyses were performed using a Netzsch STA 409 PC LUXX with a heating rate of 5

or 10 K min-1

under oxidizing conditions (synthetic air).

Electrochemistry. For the electrochemical characterization the KC/S composite cathode

samples (working electrode), a high-porosity polypropylene separator (Celgard 2500) and a

lithium metal chip (Pi-Kem, 99.0 %, diameter 15.6 mm, thickness 250 µm) were stacked,

thoroughly wetted with 8 µl liquid electrolyte per mg of sulphur consisting of 1 M lithium


bis(trifluoromethylsulfonyl)imide (LiTFSI, Sigma Aldrich, 99.95%) and 0.25 M lithium

nitrate additive (LiNO3, Alfa Aesar, 99.98%, anhydrous) dissolved in a 1:1 mixture (v:v) of

1,2-dimethoxy ethane (DME, Aldrich, 99.5%, anhydrous) and 1,3-dioxolane (DOL, Aldrich,

99.8%, anhydrous) and subsequently airtight sealed in 2016 coin cells (MTI Corp.). The cell

assembly took place under inert atmosphere in an Argon-filled glove box. All chemicals

except for LiTFSI, DME and DOL were used as received. To remove residual water LiTFSI

was dried at 120°C under vacuum for 88 h prior to use, whereas DME and DOL were dried

and stored over 3 Å molecular sieve. The long-term stability of the KC/S composite cathodes

was investigated by galvanostatic cycling at a constant rate of 167 mA g-1

(C/10) at room

temperature with a BASYTEC Cell Test System (CTS). To prevent an irreversible

decomposition of lithium nitrate additive the discharge cutoff voltage was adjusted to 1.8 V

vs. Li/Li+. The charge process was terminated at 2.6 V vs. Li/Li

+ to inhibit the irreversible

conversion of electrolyte components.


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