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were acquired by selecting solely the zero‐loss peak (zero‐ loss filtering) with a 15 eV energy slit. Samples were prepared for TEM by sonicating the pristine carbon material in ethanol for 3 min, placing a drop of the suspension on a holey carbon‐ coated copper grid, and subsequently activating the material at 200 °C under vacuum (10‐4 mbar). Elemental analysis was performed by the Molecular and Biomolecular Analysis Ser‐ vice (MoBiAS) at ETH Zürich. The elemental composition was determined by a combination of combustion analysis in O2 us‐ ing a CHNS instrument (TruSpec Micro, LECO Corp., with IR and thermal conductivity detectors) and a chromatograph equipped with a thermal conductivity detector to determine the sulfur content (HEKAtech, Eurovector SRL). Fluorine con‐ tent was determined by digestion of the sample as per the Schöniger method and quantification by ion chromatography. NMR Spectroscopy. 19F and 13C solid‐state nuclear mag‐ netic resonance (NMR) spectroscopy was performed using a Bruker 9.4 T spectrometer equipped with an Avance III con‐ sole and a double resonance 2.5 mm solid‐state probe head (with which the proton channel could be tuned to 19F frequen‐ cies). Samples were prepared for NMR as described in the Supporting Information, and filled into a 2.5 mm zirconia ro‐ tor in an argon glovebox. All experiments were performed at room temperature, either in static mode or while spinning the sample at 10 kHz magic angle spinning (MAS) frequency. The 19F chemical shifts were referenced to CFCl3 and the 13C chem‐ ical shifts were referenced to Si(CH3)4. The number of transi‐ ents acquired was 1024 for 19F NMR experiments and 2048 for 13C NMR experiments. All spectra were acquired without decoupling using one‐pulse excitation sequences with pulse lengths of 5.75 μs for 19F (corresponding to a 90° pulse) and 1.1 μs for 13C (corresponding to a 30° pulse). The recycle delay was set to 5 s. 19F solution‐state NMR spectroscopy was performed using a Bruker 11.7 T spectrometer equipped with a PABBO probe head and an Avance III console. Samples were filled into con‐ ventional 5 mm glass NMR tubes. Sealed glass capillaries con‐ taining C6D6 were added to the sample for locking to the deu‐ terium signal of the deuterated benzene. The number of tran‐ sients acquired was 256. Inverse gated H‐1 pulse sequences were used with pulse lengths of 15.0 μs for 19F. During the ac‐ quisition of 19F NMR spectra, a WALTZ16 decoupling se‐ quence was performed using 80 μs proton decoupling pulses. Electrochemical Cell Materials. The following materials were used in the preparation of electrochemical cells: potas‐ sium bis(fluorosulfonyl)imide (KFSI, >99.9%, Suzhou Fluolyte Co.), ethylene carbonate (EC, battery grade, BASF), dimethyl carbonate (DMC, battery grade, BASF), potassium (99.5%, cubes in oil, Sigma‐Aldrich), and glass microfiber discs (0.67 mm × 257 mm, GF/D grade, catalogue number 1823‐257, Whatman). Electrolyte Preparation. The electrolyte was prepared by slowly mixing KFSI powder and EC/DMC solvent (in a 1:1 ra‐ tio, by weight) under inert Ar atmosphere (<0.1 ppm H2O/O2) in the concentration specified (typically 4.8 M). A highly exo‐ thermic reaction takes place upon mixing, resulting in the eventual formation of a viscous, transparent liquid. Current Collector Coating. To improve cycling stability un‐ der high‐voltage conditions, the stainless steel coin‐type cell caps (316L, Hohsen Corp.) were coated with TiN by pulsed DC magnetron sputtering using a titanium target under a flowing ACS Applied Materials & Interfaces Page 14 of 17 Ar/N2 atmosphere (held at a molar ratio of 3.6:1 at a total flow rate of 105.5 sccm) at a pressure of 0.5 Pa, similar to a previ‐ ously described method.73, 74 The substrate and target were both pre‐sputtered for 5 min in pure Ar before deposition. The target was then intentionally poisoned under a flowing Ar/N2 atmosphere (held at a molar ratio of 2.75:1 at a total flow rate of 112.5 sccm) for 5 min. During deposition, the target power and temperature were set to 0.58 W cm‐2 and 200 °C, respec‐ tively. The sides of the current collectors, parallel to the sput‐ tering beam and thus less covered with TiN were further pro‐ tected with a thin layer of Araldite Rapid two‐component glue. Electrochemical Cell Preparation. Stainless steel coin‐ type cells (316L, Hohsen Corp.) were assembled in a glovebox under inert Ar atmosphere (<0.1 ppm H2O/O2). A thin potas‐ sium film pressed onto stainless steel was used as both the reference and counter electrodes. The working electrode was prepared without the use of any binder, conductive additive, or solvent, and the electrolyte was used as prepared above. To assemble the cell, the cathode material (in dry powder form) was homogeneously dispersed onto the TiN‐coated stainless‐ steel cell cap. A single glass microfiber disc was then placed on top as the separator, and saturated with ~300 μL electro‐ lyte, added by wetting the exposed separator at the edge of the cell. Lastly, a disc of stainless‐steel containing a thin layer of potassium was then placed on top and the cell was shut, pressing all the components into contact. Each cell contained loadings of 0.6‐4.1 mg of active cathode material. Electrochemical Measurements. The prepared cells were electrochemically cycled after a waiting time of 2 h (under open circuit conditions to allow complete wetting of the car‐ bon cathode), typically between 2.65‐4.7 V vs. K/K+, on a multi‐channel workstation (CT2001A, 0.005‐1 mA, Lanhe Corp.). Cyclic voltammetry was performed using a separate multi‐channel workstation (MPG‐2, Bio‐Logic SAS). To determine the gravimetric quantities of specific energy and power of the cathode alone, the measured mass of the dry carbon material (prior to first charge) was used: typically 0.6‐ 4.1 mg. To determine the corresponding volumetric quanti‐ ties, the bulk density of each cathode material was measured in the dried state: e.g., 0.2 g mL‐1 for ZTC. Conversions of cath‐ ode‐specific to full‐cell quantities were then made by several methods to account for the additional mass and volume of the electrolyte and other components (see the Supporting Infor‐ mation for details regarding all calculations). ASSOCIATED CONTENT Supporting Information. Synthesis details, SEM and TEM im‐ aging, N2 adsorption analysis, cyclic voltammetry, additional galvanostatic cycling, 19F static NMR and 13C MAS NMR spectra, elemental analysis, and full‐cell energy/power density calcula‐ tions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *nstadie@montana.edu, mvkovalenko@ethz.ch Author Contributions The manuscript was written through contributions of all au‐ thors. 9 ACS Paragon Plus EnvironmentPDF Image | Zeolite-Templated Carbon as the Cathode
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