Supporting Information

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1 Supporting Information Conditioning-Free Electrolytes for Magnesium Batteries Using Sulfone-Ether Mixtures with Increased Thermal Stability Laura C. Merrill and Jennifer L. Schaefer*, University of Notre Dame, Notre Dame, IN, USA 1

2 Experimental Methods Electrolyte Preparation All chemicals used are available commercially. Anhydrous tetrahydrofuran (THF, Sigma-Aldrich), was dried over 3 Å molecular sieves for at least 48 hours prior to use. Butyl sulfone (99%, Sigma Aldrich; BS) and sulfolane (99%, Sigma Aldrich; SL), were distilled under high vacuum, to remove trace impurities. All solvent mixtures were dried over sieves for at least 48 hours prior to use. The THF was about 7 ppm moisture after drying on sieves and the butyl sulfone mixtures were about 15 ppm moisture, as determined by KF titration. The authors note that it is not advised to regularly use sulfones in KF titration as the sulfone group alters the stoichiometry of the KF reaction and can cause irreproducible readings over time. Magnesium bis(hexamethyldisilazide) (97%, Sigma Aldrich) and magnesium chloride (99.9 %, anhydrous, Sigma Aldrich) were used without further purification. The electrolyte was prepared by first dissolving the necessary amount of Mg(HMDS)2 into the desired solvent, then MgCl2 was added to the solution which was then allowed to stir until transparent. The effects of purity of the Mg(HMDS)2 were tested. Mg(HMDS)2 was recrystallized from anhydrous heptane in a glove box. The performance of the recrystallized salt was compared to that of the salt as-purchased. Given the similar electrochemical performance, the salt was used as-purchased unless otherwise stated. Figure S1: Cyclic voltammograms of Mg(HMDS) 2 4 MgCl 2 in 50 THF/50 BS solvent mixtures where the Mg(HMDS) 2 was aspurchased (dashed) or recrystallized (solid). Mg foil was used as the reference and counter electrodes and Pt wire was used as the working electrode. A scan rate of 5 mv/s was used. 2

3 Figure S2: SEM and EDS results of Mg deposition from Mg(HMDS) 2 4 MgCl 2 in 50 THF/50 BS solvent mixtures where the Mg(HMDS) 2 was as-purchased (a) or recrystallized (b). A constant potential was applied to induce magnesium deposition. Figure S3: Galvanostatic cycling of Mg-Cu coin cells containing Mg(HMDS) 2 4 MgCl 2 in 50 THF/50 BS electrolyte where the Mg(HMDS) 2 was as-purchased (outlined) or recrystallized (filled). A -.25 ma current was applied to deposit magnesium on the copper for 1000 seconds then stripped using the reverse current until the cell reached 1.5 V. 3

4 Electrochemical Characterization Cyclic voltammetry was done using a 3 electrode glass jar cell (closed and sealed with parafilm) inside an argon filled glovebox (oxygen and moisture < 2ppm). About 1 ml of electrolyte was used for each test. No visible solvent evaporation was observed throughout cycling. Magnesium ribbons (GalliumSource, LLC) were used as the reference and counter electrodes after the oxide layer was mechanically removed. Platinum wire (0.25 mm diameter, 99.9 %, Alfa Aesar) was used as the working electrode. A PARSTAT MC1000 (Princeton Applied Research) potentiostat was used to take the measurements. A scan rate of 5 mv/s was used for all voltammograms. Increased temperature tests were conducted using an oil bath in an argon filled glovebox; the three electrode cell was held at open circuit for 1 hour at increased temperature prior to characterization. Cyclic voltammograms ranged between -0.5 and 2V for THF and 50 THF/50 BS based electrolytes, and -0.8 and 2 V for 30 THF/70 BS and 50 THF/50 SL based electrolytes. Oxidative stabilities were measured by scanning until significant currents were observed. Electrochemical deposition was completed using a similar set up, by applying a suitable reduction potential for 5 hours. Galvanostatic cycling was completed on coin cells using a PARSTAT MC1000 potentiostat. The coin cells were either magnesium copper or symmetric magnesium, size 2032, with a glass fiber separator. A current of ma was applied for 1000 s, to induce magnesium deposition onto the copper electrode, then a current of 0.25 ma was applied for 1000 s or until the system reached 1.5 V. Conductivity measurements were acquired using a Novocontrol Broadband Dielectric/Impedance Spectrometer with a brass-brass symmetric cell from 20 C and 60 C, in 10 C increments. An AC voltage of 100 mv was applied and the frequency ranged between 3 x 10 6 and 0.1 Hz. Scanning Electron Microscopy (SEM) A FEI Magellan 400 was used to image magnesium deposition using a voltage of 10 kv and current of 13 pa. A Bruker energy dispersive x-ray spectrometer (EDS) was used for elemental analysis of the deposits. The energy resolution of the EDS is 123 ev and the SEM has a resolution of 0.7 nm. Electrodes were washed with THF then left under vacuum for at least 1 hr prior to characterization. Samples were transferred to the instrument using a Pelco SEM pin stub vacuum desiccator to minimize air exposure upon transfer. Mass Spectrometry Mass spectrometry measurements were taken using a Bruker microtof-q II mass spectrometer using a direct infusion method via airtight Hamilton syringes to minimize exposure to ambient atmosphere. The nebulizer gas was at 4 and the pressure was at 0.4 bar. A flow rate of 20 µl/min and the electrolyte was diluted with THF to prevent complications within the machine. The spectrometer measured a range of m/z vales from 150 to Nuclear Magnetic Resonance Spectroscopy NMR measurements were completed using a Bruker AVANCE III HD 400 Nanobay Spectrometer. Deuterated benzene (99.5 % D, Cambridge Isotopes) was used as the NMR solvent and 100 µl of sample was used for each measurement. 4

5 Thermal gravimetric analysis (TGA) A Mettler Toledo TGA was used for thermal measurements of the electrolytes. Samples were analyzed under a nitrogen atmosphere from 30 C to 350 C with a scan rate of 10 C/min. X-Ray Photoelectron Spectroscopy (XPS) A PHI VersaProbe II was used for XPS measurements of electrodes after cycling. A 100 W x-ray beam was focused to a 100 µm diameter. The full XP spectra were with a ev pass energy over 7 scans. Individual element spectra were taken with a 23.5 ev pass energy over 20 scans to achieve higher resolution. Samples were washed with THF then dried under vacuum prior to measurement. When specified, samples were sputtered with a 2 kv Ar + ion gun over a 2x2 mm area for one minute. Spectra was shifted to a ev carbon peak. The samples were transferred to the equipment in a vacuum desiccator to minimize exposure to air, however exposure was unable to be completely avoided. The samples were loaded onto a platen and into the spectrometer under ambient atmospheric conditions. Figure S4: TGA of Mg(HMDS)2 4 MgCl2 in 50 THF/50 SL 5

6 Figure S5: Cyclic voltammograms from Figure 2 with a narrower current scale. The left side shows the full graph and the right side shows the corresponding voltammogram zoomed in along the x and y axis to see the baseline. 6

7 Figure S6: Cyclic voltammograms (a) and conductivity measurements (b) of Mg(HMDS)2-4 MgCl2 in THF (black), 50 THF/50 BS (blue), and 30 THF/70 BS (green). For the voltammetry, the working electrode was platinum wire and the reference and counter electrodes were magnesium ribbon. A scan rate of 5 mv/s was used. Figure S7: Cyclic voltammograms beyond the 1st cycle for Mg(HMDS) 2-4 MgCl 2 in THF and 50 THF/50 BS. 7

8 Figure S8: Stability limit comparison of recrystallized Mg(HMDS) 2-4 MgCl 2 in 50 THF/50 BS (solid line) and THF (dashed line) against copper, stainless steel (SS), aluminum, and platinum 8

9 Figure S9: Mass spectra of Mg(HMDS)2-4 MgCl2 in (a) 50 THF/50 BS, (b) 50 THF/50 SL, and (c) THF. Peaks marked with an x represent MgCl + ions and peaks marked with an * represent Mg2Cl3 + ions. 9

10 Table S1: List of identified cation peaks from mass spectra in Figure S3. m/z i % Species THF Electrolyte MgCl(THF) MgCl(THF) H2O MgCl(THF)2 H2O MgCl(THF) MgCl(THF)2 H2O Mg2Cl3(THF)2 H2O Mg2Cl3(THF) Mg2Cl3(THF)2 H2O Mg2Cl3(THF)3 Butyl Sulfone THF Electrolyte MgCl(Butyl Sulfone) Mg2Cl3(Butyl Sulfone) H2O Mg2Cl3(Butyl Sulfone)2 H2O MgCl(Butyl Sulfone) MgCl(Butyl Sulfone)2 H2O Mg2Cl3(Butyl Sulfone) Mg2Cl3(Butyl Sulfone)2 H2O Mg2Cl3(Butyl Sulfone)2 2H2O MgCl (Butylsulfone) Mg2Cl3(Butyl Sulfone)3 Sulfolane THF Electrolyte MgCl(Sulfolane) MgCl(Sulfolane) H2O MgCl(Sulfolane)2 H2O MgCl(Sulfolane) MgCl(Sulfolane)2 H2O MgCl(Sulfolane)2 5H2O Mg2Cl3(Sulfolane) Mg2Cl3(Sulfolane)2 H2O MgCl(Sulfolane) Mg2Cl3(Sulfolane)3 10

11 Figure S10: 1 H NMR spectra of (a) Mg(HMDS)2-4 MgCl2 in 50 THF/50 BS and (b) 50 THF/50 BS. The peak marked with a + is representative of TMS, and x of the HMDS anion. The NMR spectra was calibrated to the benzene-d6 solvent peak at 7.16 ppm. Figure S11: Galvanostatic cycling data of Mg - Mg coin cells with the Mg(HMDS)2-4 MgCl2 electrolyte in 50 THF/50 BS at 20 and 50 C and corresponding zooms from to seconds. A current of 0.25 ma was applied for 1000 s then reversed. 11

12 Figure S12: SEM of copper electrode and EDS results after stripping following 50 deposition/dissolution cycles in a Mg-Cu cell with Mg(HMDS)2 4 MgCl2 in 50 THF/50 BS electrolyte. The material seen was remaining following stripping to 1.5 V. The silicon content was left out of the EDS analysis due to the presence of silicon fibers on the electrode from the separator. Figure S13: XPS of magnesium electrode following 50 deposition/dissolution cycles in a Mg-Cu cell with recrystallized Mg(HMDS)2 4 MgCl2 in 50 THF/50 BS electrolyte. Full spectra is pre-sputtering and shows assignments. 12

13 C 1s (at %) Mg 2p (at %) Si 2p (at %) S 2p (at %) Cl 2p (at %) O 1s (at %) 24% 27% 4% 1 % 4% 41% Figure S14: XPS of individual elements before sputtering on same magnesium electrode following 50 deposition/dissolution cycles in a Mg-Cu cell with recrystallized Mg(HMDS)2 4 MgCl2 in 50 THF/50 BS electrolyte. Mg 0 (49 ev), MgO (50 ev), and MgCl x (52 ev) are present in the Mg 2p region, and SO 2 (168 ev) and MgS x (162 ev) are present in the S 2p region. C 1s (at %) Mg 2p (at %) Si 2p (at %) S 2p (at %) Cl 2p (at %) O 1s (at %) 10% 40% 3% 1 % 2% 44% Figure S15: Individual elemental regions and tabulated elemental percentages are shown after sputtering. Notably Mg 0 (49 ev), MgO (50 ev), and MgCl x (52 ev) are present in the Mg 2p region, SO 2 (168 ev) and MgS x (162 ev) are present in the sulfur spectra. The ratio of SO 2 (168 ev) to MgS x (162 ev) intensities decreased after sputtering. 13