Electrochemical Characterization

The characterization of the CDC nanospheres (see section 5.1.2) and the CDC mesofoams (see section 5.2.1) was performed by Dipl. Chem. Katja Pinkert (Institute for Complex Materials, Leibniz Institute for Solid State and Materials Research Dresden). The carbon materials were suspended in acetone and mixed with a polyvinylidene-difluoride (PVDF)-acetone solution, resulting in a carbon:PVDF mixture (95:5 by weight) without conductive agent. The slurry was uniformly dropped on a platinum coin current collector with a diameter of 12 mm and dried at 80°C for 12 h. Each electrode comprised about 5 mg of active material. Two electrodes were assembled in a symmetrical electrode configuration, separated by a Whatman GF/D glass microfiber filter (GE Healthcare Life Sciences, USA), and soaked with the aqueous electrolyte (1 M H2SO4).

The sandwich was placed in a Swagelok-type test cell. Electrochemical measurements were carried out at 25°C using a multichannel VMP3 potentiostat–galvanostat (Bio- Logic, France). The capacitance determination of the symmetrical two-electrode cells was accomplished by cyclic voltammetry (CV) experiments at different scan rates. Five cycles were measured at each potential scan rate and the capacitance was calculated from the 5th _{cycle. The differential specific capacitance for CV plots at different scan}

rates is calculated according to Equation 18, where Cspec is the differential specific

capacitance in F/g (based on the mass of electroactive material in a single electrode), Ispec is the specific response current density at the applied potential step in A/g (based

on the mass of electroactive material in a single electrode), and ν is the potential scan rate in mV/s.

𝐶_{𝑠𝑝𝑒𝑐} =𝐼𝑠𝑝𝑒𝑐

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The integral specific capacitance is calculated according to Equation 19, where Ispec is the

specific response current density in A/g (based on the mass of electroactive material in a single electrode) integrated over the applied potential window V2-V1.

𝐶_{𝑠𝑝𝑒𝑐} =_{ν (𝑉}1

2−𝑉1) ∫ 𝐼𝑠𝑝𝑒𝑐𝑑𝑉

𝑉2

𝑉1 (19)

Galvanostatic charge-discharge (C-D) measurements up to 0.9 V were performed at current densities from 1-20 A/g (based on the mass of electroactive material in a single electrode). The specific capacitance was calculated according to Equation 20, where dV/dt is the slope of the discharge curve in V/s.

𝐶_{𝑠𝑝𝑒𝑐}= 2𝐼𝑠𝑝𝑒𝑐

(𝑑𝑉_𝑑𝑡) (20)

Potentiostatic impedance spectroscopy was carried out in the frequency range from 1 mHz-100 kHz with a 10 mV alternating current (AC) amplitude. The specific capacitance was calculated according to Equation 21 where f is the operating frequency in Hz, Im(Z) is the imaginary part of the total device resistance in Ω, and m is the mass of electroactive material in a single electrode.

𝐶_{𝑠𝑝𝑒𝑐} = (_{2 𝜋 𝑓 𝐼𝑚(𝑍) 𝑚}2 ) (21)

The characterization of the CDC aerogels (see section 5.3.2) and the Kroll-Carbons based on silica or alumina templates (see section 5.6.2) was performed at GeorgiaTech. The materials were ground into powders in a mortar and were suspended in ethanol under mild sonication. A suspension of polytetrafluoroethylene binder (PTFE, 60 wt% in water, Sigma Aldrich) was added and the resulting slurry of 5 wt% PTFE and 95 wt% of carbon was concentrated by slow evaporation of ethanol at 80°C under constant stirring. The highly viscous mixture was then dried on a glass plate and mixed with razor blades. When the mass became dry with a rubberlike (clay) consistency, it was rolled to a thickness of ~150 μm between aluminum foil sheets using a roll mill. The resulting composites were dried over night at 80°C under vacuum.

For the measurements in the aqueous electrolytes, electrodes of ~1 cm2 _{(2-3 mg active}

material) were cut out and the device assembly took place under air atmosphere. A high- purity gold foil (Sigma Aldrich, USA) was used as the current collector and a commercially available Dreamweaver Silver separator (Dreamweaver International,

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USA) was placed between the electrodes. The sandwich was assembled in a beaker-type cell configuration and held together using Teflon slabs and screws. Sufficient wetting of the porous carbon electrodes with sulfuric acid was ensured by adding an excess of electrolyte solution to the beaker followed by a treatment under vacuum at RT for 1 h. For the measurements in the ionic liquid electrolyte EMIBF4, (> 98%, IoLiTec Ionic

Liquids Technologies GmbH, Germany) and the organic electrolyte 1 M TEABF4 in AN,

the devices were assembled in a stainless steel coin cell configuration in an argon filled glovebox. Carbon coated aluminum foil was used as the current collector and the above mentioned Dreamweaver product as the separator. In case of the organic electrolyte, a GORE membrane (W.L. Gore and Associates, USA) of 25 μm in thickness and with 60% porosity was used (4-5 droplets of the electrolyte were used for the wetting of the electrodes and the separator and the excess amount was removed during compression of the coin cell).

For electrochemical measurements, aluminum contacts were fixed to the coin cells. CV measurements were performed on a Solartron 1480A (AMETEK Advanced Measurement Technology, USA) from -0.6-0.6 V (aqueous electrolyte), 0-2.0 V (organic electrolyte), or -2.0-2.0 V (ionic liquid electrolyte) at scan rates of 1-1000 mV/s. The gravimetric capacitance of each electrode at different scan rates was calculated from the CV data according to Equation 22 where dU/dt is the scan rate, m is the mass of active material in a single electrode, and I(U) is the total current.

𝐶_{𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒}= 2𝐶_{𝑐𝑒𝑙𝑙}= (_{(𝑑𝑈/𝑑𝑡)𝑚}2 ) {∫_−0.6𝑉0.6𝑉 𝐼(𝑈)𝑑𝑈− ∫_−0.6𝑉0.6𝑉 𝐼(𝑈)𝑑𝑈}1_21.2𝑉1 (22)

C-D experiments at charge/discharge current densities of 0.1-20 A/g (based on the mass of a single electrode) were carried out with an Arbin SCTS supercapacitor testing system (Arbin Instruments, USA). The specific capacitance was calculated according to Equation 23 where I is the total current, dU/dt is the slope of the discharge curve, and m is the mass of active material in a single electrode.

𝐶_{𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒} = 2𝐶_{𝑐𝑒𝑙𝑙} = (_{(𝑑𝑈/𝑑𝑡)𝑚}2𝐼 ) (23)

EIS measurements were performed on a Gamry Potentiostat (Gamry Instruments, USA) from 100 kHz-1 mHz with a 10 mV alternating current (AC) amplitude.

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Electrode Materials in Li-S Batteries

Electrochemical characterization of the CDC mesofoams (see section 5.2.1) and the KCs from titania templates (see section 5.6.1) were performed by M. Sc. Sören Thieme (Fraunhofer Institute for Material and Beam Technology (IWS) Dresden). The carbon/S nanocomposites were prepared by combining pristine sulfur (Sigma Aldrich, ≥ 99.5%) with finely ground carbon material in a defined C:S weight ratio. After homogenization in a mortar, the mixture was transferred into a ceramic crucible and heated to 155°C for 12 h under air to perform the melt infiltration of sulfur. The cathodes were prepared by homogeneously mixing multiwalled carbon nanotubes (MWCNT, NanocylNC 7000 series) as the conducting agent and poly(tetrafluorethylene) (PTFE, ABCR) binder with the carbon/S nanocomposites in a defined weight ratio followed by intensive grinding at elevated temperature. The as-prepared self-supporting cathode foil was laminated onto a 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.

For electrochemical characterization, the carbon/S composite cathode (working electrode), one layer of Celgard 2500 separator (Celgard, USA), and a lithium metal chip (Pi-Kem, 99.0%, diameter 15.6 mm, thickness 250 μm) were stacked and subsequently sealed airtight in 2016 coin cells. Prior to stacking, the cathode was thoroughly wetted with 8 μl liquid electrolyte per mg of sulfur 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 mixture (1:1 by

volume) of 1,2-dimethoxyethane (DME, Aldrich, 99.5%, anhydrous) and 1,3-dioxolane (DOL, Aldrich, 99.8%, anhydrous). The whole cell assembly took place in an argon-filled glove box. The long-term stability of the carbon/S composite cathode was investigated by galvanostatic cycling at room temperature at different current rates with a Cell Test System (BASYTEC, Germany) in a voltage range of 1.8-2.6 V vs. Li/Li+_.

The electrochemical testing of the PMMA-CDCs as Li-S cathode components was performed at GeorgiaTech. The S/PMMA-CDC composites and polyacrylic acid (PAA, Polysciences) as a binder were mixed in water:ethanol (1:3 by weight) to prepare a slurry for casting an electrode. The ratio of S/PMMA-CDC to PAA binder was 85:15 by weight. No conductive additives were used. The slurry was stirred at room temperature for 1 h and cast on an aluminum foil. After drying overnight at RT under vacuum, coin

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cells were assembled with 1 M, 3 M, and 5 M LiTFSI in dimethoxyethane (DME):1,3- dioxolane (DIOX) (1:1 by volume) as electrolyte, a celgard2400 (Celgard) separator and a pure Li foil (Alfa Aesar, 99.9%) as anode. 0.2 M LiNO3 (Alfa Aesar, 99.99%) was added

to the electrolyte as an additive. The cells were equilibrated for 24 h before operation. The average sulfur surface loading was ~0.5 mg/cm2_{. The coin-cells were assembled}

inside an argon-filled glovebox and cycled with different C-rates in the range 3.0-1.2 V vs. Li/Li+ _{in galvanostatic mode using an Arbin battery test system (Arbin Instruments).}

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