Supporting Information

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1 Supporting Information Nucleation and Growth of Lithium Peroxide in the Li O2 Battery Sampson Lau and Lynden A. Archer * * laa25@cornell.edu Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States 1. Cathode preparation. A cathode slurry was prepared by mixing 360 mg of Super P carbon (TIMCAL), 40 mg of polyvinylidene fluoride (PVDF, Sigma-Aldrich), and 1800 mg of n-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) in a ball mill (Fritsch PULVERISETTE 23) at 40 Hz for at least 20 minutes. The slurry was coated on a sheet of carbon paper (Toray TGP-H-030) with a doctor blade set to a height of 80 μm. The coated carbon paper was dried at 100 C under vacuum for 12 hours and transferred into an argon-filled glovebox (O2 < 0.2 ppm, H2O < 1.0 ppm; Innovative Technology) without exposure to air. 5/8-inch (15.88 mm) diameter disks were punched from the carbon paper to form the individual coin cell cathodes. The weight of the active layer (total weight minus carbon paper weight) averaged 1.0±0.1 mg. 2. Electrolyte preparation. Electrolyte components were dried for 24 hours and stored in the argon-filled glovebox prior to use. The salt, trifluoromethanesulfonate (LiCF3SO3; Sigma-Aldrich), was dried at 100 C under vacuum and the solvent, tetraethylene glycol dimethyl ether (TEGDME; Sigma-Aldrich), was

2 dried over 3 Å molecular sieves. The electrolyte, a 1.0 M LiCF3SO3 solution in TEGDME, was prepared by mixing with a Teflon-coated stir bar. 3. Coin cell assembly. A 1/2-inch (12.7 mm) diameter hole was punched in the top (cathode) side of each CR2032 case (Pred Materials). The following components were added in sequence: 1. Stainless steel wire cloth disk, 3/4-inch (19 mm) disk diameter, inch (0.140 mm) wire diameter (McMaster-Carr). 2. The cathode disk, prepared as described in section Whatman GF/D glass fiber separator, 3/4-inch (19 mm) diameter μl of electrolyte, prepared as described in section Lithium metal, 0.75 mm thick (Alfa Aesar), punched to 1/2-inch (12.7 mm) diameter. 6. Stainless steel spacer disk, 15.5 mm diameter, 0.5 mm thick. 7. Stainless steel wave spring (MTI Corporation). The polypropylene gasket and the bottom (anode) side of the CR2032 case were added to complete the assembly, which was crimped to a pressure of 14 MPa using a hydraulic coin cell crimper (BT Innovations). It should be noted that the electrolyte was dropped only at the center of the separator; we found that attempting to spread the electrolyte over the entire separator introduced inconsistencies in wetting. In addition, the electrolyte-facing side of the lithium metal foil was scraped with a spatula prior to use to remove surface oxides and impurities. 4. Testing environment. Testing was performed in a custom-built hermetically sealed chamber, shown in Figure S1. The cells were loaded into the main chamber while in the glovebox. The entire apparatus was purged with oxygen (99.999% purity, Airgas) for 15 minutes, then adjusted such that the main test

3 chamber was regulated at a pressure of 1.3 atm. An external oxygen supply tank, initially at 250 psi (1724 kpa), ensured that the system was kept at positive pressure at all times. The cells were allowed to equilibrate for 3 hours prior to electrochemical testing. No significant drop in pressure was observed throughout the duration of the test. Figure S1: Li O2 cell test chamber. (a) External oxygen tank. (b) Main chamber with coin cells. (c) Electrical connectors to battery test channels. 5. Electrochemical tests. Cells were discharged at constant current using a Neware BTS-5V1mA battery testing unit. Discharge was set to automatically stop once the cell reaches a voltage of 2.0 V. 6. Scanning electron microscopy. Discharged cells were disassembled inside the glovebox, and the cathodes were removed and transported to the scanning electron microscope (Zeiss LEO 1550 Field Emission SEM) within an airtight container. The cathodes were loaded onto the stage in the presence of a nitrogen stream.

4 Due to the tendency of Li2O2 to decompose under the electron beam, a low accelerating voltage of 2.0 kv was used. Images were taken with a single pass after focusing on a nearby region. 7. X-Ray Diffraction Cathodes were mounted on a glass microscope slide inside an argon-filled glovebox and coated with paraffin oil to protect them from air during the x-ray diffraction (XRD) measurements. Measurements were done on a Bruker D8 Discover x-ray diffractometer employing Cu-Kα radiation (λ = Å) and fitted with a 2-dimensional detector. Frames were captured with an exposure time of 10 minutes, after which they were integrated along χ (the polar angle orthogonal to 2θ) to yield an intensity vs 2θ plot.

5 8. Additional figures. Figure S2: SEM image of a cathode partially discharged at 2.5 μa/cm 2.

6 η total (V) η total = kt i ln ( ) + ir eα i i (A m -2 ) Figure S3: Tafel plot of onset potentials and corresponding fit to ir-corrected Tafel equation. (α = 0.656, i0 = A m -2, and R = Ω m -2 ) Figure S4: (a) Simulated (lines) and experimental (symbols) discharge curves for a Li O2 cell with Vulcan XC-72 cathode, as prepared by Nazar et al. 1 (b) Plot to obtain J0 and α.

7 Figure S5: XRD spectra of pristine cathode and a cathode discharged at 2.5 μa/cm 2. All peaks can be indexed to Li2O2 (PDF # ) or the carbon paper. References (1) Adams, B. D.; Radtke, C.; Black, R.; Trudeau, M. L.; Zaghib, K.; Nazar, L. F. Energy Environ. Sci. 2013, 6, 1772, DOI: /c3ee40697k.