a new route for the preparation of mesoporous carbon ... · a new route for the preparation of...
TRANSCRIPT
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
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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.
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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
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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.
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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
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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.
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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).
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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.
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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
.
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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
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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
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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
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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
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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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