Supporting Information

Transcription

1 Supporting Information A Fluorinated Alkoxyaluminate Electrolyte for Magnesium-Ion Batteries Jake T. Herb a,b, Carl A. Nist-Lund b, Craig B. Arnold *,b,c a Department of Chemistry, Princeton University, Princeton, NJ 08544, United States b Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ 08544, United States c Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, United States cbarnold@princeton.edu

2 SI-1 Table of Contents Materials, Methods, and Experimental Procedures...SI-2 NMR Spectra...SI-5 Electrochemical Measurements...SI-12 Surface Characterization and Chronoamperometry...SI-17

3 SI-2 Methods Materials. All synthetic procedures were carried out under an atmosphere of argon using standard Schlenk techniques and oven-dried glassware. 1,1,1,3,3,3-Hexafluoro-2-propanol (99%, HFIP), anhydrous 1,2-dimethoxyethane (99.5%, DME), anhydrous inhibitor-free tetrahydrofuran (99.9%, THF), anhydrous diethylene glycol dimethyl ether (99.5%, G2), trimethylaluminum (2 M in hexane), and magnesium methoxide (6-10% in methanol) were used as received (Sigma- Aldrich). Electrochemical and surface measurements. Electrochemical measurements (Solartron 1287) were performed either in an argon-filled glove box (Vacuum Atmospheres) using a standard three electrode setup (Solartron 1287, Gamry Interface 1000E) or in coin cells (Arbin BT2000). Conductivity measurements were made using a conductivity meter paired with an InLab 751 conductivity probe (Mettler Toledo SevenCompact S230). SEM images and EDS spectra were acquired at a 15 kev accelerating voltage (FEI Quanta 200 FEG SEM). XPS spectra were recorded using a ThermoFisher Scientific K-Alpha XPS system. XRD patterns were acquired using a Bruker D8 ADVANCE ECO powder diffractometer. Linear sweep voltammograms for determination of oxidative stability limits were acquired at 5 mv s -1 in a flooded cell using a standard three electrode setup with various metal electrodes of known area that had been polished using 1 µm alumina. The counter and reference electrodes were magnesium foil that were mechanically abraded in the glove box before use (99.95% GalliumSource, LLC). The commercially available disc electrodes used were platinum (2 mm dia., CH Instruments, Inc.), gold (2 mm dia., CH Instruments, Inc.), and glassy carbon (3 mm dia., CH Instruments, Inc.). In house-made 2 mm disc electrodes were made using aluminum 6061, Al (99.999%, Sigma-Aldrich), copper 101, stainless steel 304, and stainless steel 316. Anodic stabilities are reported as the potential at which 0.1 ma cm -2 is reached. Cyclic voltammograms were performed in 2032-type coin cells that were fabricated using a copper cathode, abraded magnesium anode, and Celgard 2400 separator material. A Teflon ring was used to limit the working electrode area in the coin cell. Sweep rates for cyclic voltammograms were typically 10 mv s -1 unless otherwise noted. Limiting cathodic potentials were set either when a defined current value or potential was reached, after which the scan direction was reversed. Galvanostatic plating and stripping experiments were performed in 2032 coin cells with a current density of 0.5 ma cm -2 for 15 minutes per cycle with an oxidative cutoff potential of 1 V vs. Mg/Mg 2+. Coulombic efficiency values were calculated by dividing the charge passed during stripping by the charge passed during plating. Chronoamperometry experiments were performed in coin cells at various potentials for two days using standard Al foil (MTI Corporation), pure Al foil (99.999%, Sigma Aldrich), Cu foil, and

4 SI stainless steel (MTI Corporation). Stirred electrolysis of 2 ml of 0.25 M (3) in DME was performed at 3.25 V vs. Mg/Mg 2+ for 64 hours using a Pt electrode until the current decayed to approximately 0 A (total charge passed during potential hold = 250 mc). The solution was then analyzed with NMR, CV, and LSV. α-mno 2 was used as received (Erachem Comilog). A slurry was made that consisted of 70% active material, 20% Super-P carbon (TIMCAL), and 10% PVDF (Kynar) in 1-methyl-2- pyrrolidinone (99.5%, Sigma-Aldrich), and was mixed using a planetary mixer (DAC 150 FVZ, FlackTek Inc.). The slurry was cast on an aluminum substrate at a thickness of 75 µm and heated in a vacuum oven at 120 C for at least 8 hours. Average active material mass loading was 2 mg cm -2. Full cells used an abraded Mg anode (99.95% GalliumSource, LLC) and Celgard 2400 separator material in a 2032-type coin cell case. A large aluminum foil disc was used to prevent contact between the cathode cap and the rest of the cell. Full cells were cycled at constant current density of 20 µa cm -2, with voltage cutoffs of 0.8 V and 3.5 V. Chevrel phase Mo 6 S 8 was prepared according to a previous report. 1 A slurry of the active material was made that consisted of 85% active material, 7.5% Super-P carbon (TIMCAL), and 7.5% PVDF (Kynar) in 1-methyl-2-pyrrolidinone (99.5%, Sigma-Aldrich), and was mixed using a planetary mixer (DAC 150 FVZ, FlackTek Inc.). The slurry was cast on a copper substrate at a thickness of 40 µm and heated in a vacuum oven at 120 C for at least 8 hours. Average active material mass loading was 1.7 mg cm -2. Full cells used an abraded Mg anode (99.95% GalliumSource, LLC) and Celgard 2400 separator material in a 2032-type coin cell case. The cells were cycled at constant current density of 10 ma g -1, with voltage cutoffs of 0.4 V and 1.8 V. NMR spectroscopic analysis. NMR spectra were acquired with a Bruker Avance III 500 MHz spectrometer, with chemical shifts reported in ppm. Solvents for the NMR experiments were the native electrolyte solvent paired with a lock solvent (CDCl 3, 99.8%, Cambridge Isotope Laboratories) in a 1.7 mm capillary tube (New Era Enterprises, Inc.). For 27 Al NMR spectra, a broad shift centered near 70 ppm resulting from aluminum in the NMR probe was subtracted from experimental spectra (refer to Supplementary Figure 12 for background spectrum). Gaussian curve fits for peak deconvolution were performed using MestReNova 10 software. Synthetic procedures. Aluminum hexafluoroisopropoxide (Al(HFIP) 3, 1), adapted from ref. 22 in main text: To a solution of chilled (0 C) HFIP ( g, 82 mmol, 4.3 eq) in DME was added trimethylaluminum (9.5 ml, 19 mmol, 1 eq). The solvent and any excess HFIP was removed in vacuo, resulting in a clear, colorless, viscous liquid that solidified upon cooling. 27 Al NMR (130 MHz, DME, 25 C): δ 7.34 (ν 1/2 = 718 Hz), (ν 1/2 = 1114 Hz), (ν 1/2 = 98 Hz). 1 H NMR (500 MHz, DME, 25 C): δ 5.24 (s, br, CH), 4.80 (s, br, CH), 4.51 (s, br, CH). 19 F

5 SI-4 NMR (470 MHz, DME, 25 C): δ (d, 3 J HF = 5.9 Hz, CF 3 ), (d, 3 J HF = 6.6 Hz, CF 3 ), (d, 3 J HF = 6.3 Hz, CF 3 ), (d, 3 J HF = 6.3 Hz, CF 3 ) Magnesium hexafluoroisopropoxide (Mg(HFIP) 2, 2): Magnesium methoxide in methanol (30 ml, g, 40 mmol, 1 eq) (Sigma-Aldrich) was added to a Schlenk flask and sparged with argon. The solvent was removed in vacuo. 30 ml of THF was added to the flask, forming an insoluble mixture. HFIP (14.84 g, 88 mmol, 2.2 eq) was added to the magnesium methoxide solution, resulting in a clear and colorless solution. After stirring for 12 hours, the solvent was removed in vacuo, leaving a white powder. 25 Mg NMR (31 MHz, DME, 25 C): δ (ν 1/2 = 435 Hz). 1 H NMR (500 MHz, DME, 25 C): δ 5.21 (m, CH). 19 F NMR (470 MHz, DME, 25 C): δ (s, CF 3 ). 1:2 Mg(HFIP) 2 :Al(HFIP) 3 (3): Electrolyte solutions on the order of 0.25 M by Mg were made by first dissolving two equivalents of aluminum fluoroalkoxide in DME or G2, resulting in a clear solution. The corresponding stoichiometric amount of magnesium fluoroalkoxide was added to this solution, resulting in a slightly cloudy solution. After reacting for a minimum of 96 hours, the solution became predominantly clear with no significant solid precipitates, and was filtered using a 0.45 µm PTFE filter to remove any fine particulate matter. 25 Mg NMR (31 MHz, DME, 25 C): δ (ν 1/2 = 42 Hz). 27 Al NMR (130 MHz, DME, 25 C): (ν 1/2 = 142 Hz). 1 H NMR (500 MHz, DME, 25 C): δ 5.06 (m, CH). 19 F NMR (470 MHz, DME, 25 C): δ (s, br, CF 3 ), (d, 3 J HF = 5.5 Hz, CF 3 ), (d, 3 J HF = 5.7 Hz, CF 3 ), (d, 3 J HF = 6.1, CF 3 ), (d, 3 J HF = 6.0 Hz, CF 3 ) (d, 3 J HF = 6.5 Hz, CF 3 ), (d, 3 J HF = 6.3 Hz, CF 3 ). References: (1) Lancry, E.; Levi, E.; Gofer, Y.; Levi, M. D.; Aurbach, D. The Effect of Milling on the Performance of a Mo 6 S 8 Chevrel Phase as a Cathode Material for Rechargeable Mg Batteries. J. Solid State Electrochem. 2005, 9,

6 SI-5 Figure S1. 19 F NMR spectrum of Al(HFIP) 3 (1) in DME. Figure S2. 27 Al NMR spectrum of Al(HFIP) 3 (1) in DME.

7 SI-6 Figure S3. 1 H NMR spectrum of Al(HFIP) 3 (1) in DME. The minor peaks marked in gray are impurities or peaks related to the solvent. Figure S4. 19 F NMR spectrum of Mg(HFIP) 2 (2) in DME.

8 SI-7 Figure S5. 25 Mg NMR spectrum of Mg(HFIP) 2 (2) in DME. Figure S6. 1 H NMR spectrum of Mg(HFIP) 2 (2) in DME. The minor peaks marked in gray are impurities or peaks related to the solvent.

9 SI-8 Figure S7. 19 F NMR spectrum of 1:2 Mg(HFIP) 2 :Al(HFIP) 3 (3) in DME. Figure S8. 27 Al NMR spectrum of 1:2 Mg(HFIP) 2 :Al(HFIP) 3 (3) in DME.

10 SI-9 Figure S9. 25 Mg NMR spectrum of 1:2 Mg(HFIP) 2 :Al(HFIP) 3 (3) in DME. Figure S10. 1 H NMR spectrum of 1:2 Mg(HFIP) 2 :Al(HFIP) 3 (3) in DME. The minor peaks marked in gray are impurities or peaks related to the solvent.

11 SI-10 Figure S F NMR spectrum of HFIP in CDCl 3. Figure S Al NMR spectrum of probe background.

12 SI-11 Figure S13. NMR spectra of a solution of 3 in DME electrolyzed at 3.25 V vs. Mg/Mg 2+ for 64 hours using a Pt electrode until the current decayed to approximately 0 A (total charge passed during potential hold = 250 mc). a) 27 Al spectrum b) 25 Mg spectrum c) 19 F spectra of pre- and post-electrolyzed solution d) 1 H spectra of pre- and post-electrolyzed solution.

13 SI-12 Figure S14. Cyclic voltammograms of the first three cycles (10 mv s -1 scan rate) of a 0.25 M solution of 3 in DME. Inset shows charge passed during cycle 3.

14 SI-13 Figure S15. Cyclic voltammograms of 0.25 M 1:2 Mg(HFIP) 2 :Al(HFIP) 3 in G2 on a copper substrate (10 mv s -1 ).

15 SI-14 Figure S16. Long term cycling behavior (scan rate 25 mv s -1 ) of 0.25 M 1:2 Mg(HFIP) 2 :Al(HFIP) 3 in DME that had previously undergone a stirred electrolysis at 3.25 V vs. Mg/Mg 2+ for 64 hours using a Pt electrode until the current decayed to approximately 0 A (total charge passed during potential hold = 250 mc).

16 SI-15 Figure S17. Linear sweep voltammograms of 0.25 M Mg[Al(HFIP) 4 ] 2 (5 mv s -1 scan rate) in DME at room temperature on a variety of electrode substrates. The Pt traces indicate linear sweep behavior before and after electrolyzing 2 ml of electrolyte at 3.25 V vs. Mg/Mg 2+ for 64 hours using a Pt electrode until the current decayed to approximately 0 A (total charge passed during potential hold = 250 mc). The aluminum traces show oxidative behavior of aluminum 6061 and % pure aluminum.

17 SI-16 Figure S18. XRD pattern of pristine Mo 6 S 8 (black trace, bottom) and Mo 6 S 8 discharged to 0.4 V (blue trace, top). Note that the XRD pattern for the discharged material Mg 2 Mo 6 S 8 was acquired on a copper current collector, whereas the Mo 6 S 8 pattern represents the powder only. The thin red trace under each pattern recorded in this work represent literature patterns for Mg 2 Mo 6 S 8 (ICSD collection code ) and Mo 6 S 8 (ICSD collection code 86788).

18 SI-17 Figure S19. a) Charge-discharge behavior of potassium-stabilized α-mno 2 cycled at a rate of 20 µa cm -2 using a 0.25 M solution of 3 in DME. b) XPS spectra of the Mn 3s region of the pristine α-mno 2 cathode, after the initial discharge, and after the initial charge.

19 SI-18 Figure S20. Surface characterization of magnesium deposits from 0.25 M 1:2 Mg(HFIP) 2 :Al(HFIP) 3 (3) in DME. a) SEM micrograph of a deposit plated at 3.5 ma cm -2 for one hour with a total passed charge density of 12.6 C cm -2. Inset shows EDX spectrum of deposit. Fitted XPS spectra of Mg deposit (after sputtering ca. 40 nm material): b) Mg 2p, c) Mg 1s, d) O 1s, e) Al 2s, f) Al 2p regions.

20 SI-19 Figure S21. Chronoamperometric characterization of % pure Al foil held at 3 V for two days. Electrolyte was 0.25 M (3) in DME. a) SEM image of oxidized surface at 32000x b) Al, c) C, d) O EDS maps of same region e) 500x view of the surface. f) EDS spectrum of mapped region. g) XPS depth profile of F 1s region. The intensity scale is consistent between each plot. The surface was sputtered with an argon ion beam at 3 kv in 30 second intervals. h) Chronoamperometry data from coin cell.

21 SI-20 Figure S22. Chronoamperometric characterization of % pure Al foil held at 3.5 V for two days. Electrolyte was 0.25 M (3) in DME. a) SEM image of oxidized surface at 500x b) Al, c) C, d) O EDS maps of same region. e) EDS spectrum of mapped region. f) XPS depth profile of F 1s region. The intensity scale is consistent between each plot. The surface was sputtered with an argon ion beam at 3 kv in 30 second intervals. g) Chronoamperometry data from coin cell.

22 SI-21 Figure S23. Surface characterization of % Al standard. a) SEM image of Al surface at 500x. b) EDS spectrum of the point marked spectrum 11.

23 SI-22 Figure S24. Chronoamperometric characterization of standard grade Al foil held at 3 V for two days. Electrolyte was 0.25 M (3) in DME. a) SEM image of oxidized surface at 6000x. b) EDS spectrum of the indicated area. c) Chronoamperometry data from coin cell. d) XPS depth profile of F 1s region. The surface was sputtered with an argon ion beam at 3 kv in 30 second intervals. e) 500x view of the surface.

24 SI-23 Figure S25. Chronoamperometric characterization of standard grade Al foil held at 3.5 V for two days. Electrolyte was 0.25 M (3) in DME. a) SEM image of oxidized surface at 2000x. b) EDS spectrum of the point marked spectrum 4. c) EDS spectrum of the point marked spectrum 5. d) XPS depth profile of F 1s region. The surface was sputtered with an argon ion beam at 3 kv in 30 second intervals. e) Chronoamperometry data from coin cell. f) 500x view of the surface.

25 SI-24 Figure S26. Surface characterization of standard aluminum foil. a) SEM image of standard Al surface at 500x. Iron-rich inclusions appear as small white spots in image. b) EDS spectrum of the point marked spectrum 3. c) EDS spectrum of the point marked spectrum 4.

26 SI-25 Figure S27. Chronoamperometric characterization of SS 304 held at 3 V for two days. Electrolyte was 0.25 M (3) in DME. a) Chronoamperometry data from coin cell. b) SEM image of SS 304 surface at 500x c) EDS spectrum of point marked spectrum 1. d) EDS spectrum of point marked spectrum 2.

27 SI-26 Figure S28. Chronoamperometric characterization of SS 304 held at 4 V for two days. Electrolyte was 0.25 M (3) in DME. a) Chronoamperometry data from coin cell. b) SEM image of SS 304 surface at 500x c) EDS spectrum of point marked spectrum 3. d) EDS spectrum of point marked spectrum 4.

28 SI-27 Figure S29. Surface characterization of SS 304 standard. a) SEM image of SS 304 surface at 500x. b) EDS spectrum of the point marked spectrum 7. c) EDS spectrum of the point marked spectrum 8.

29 SI-28 Figure S30. Chronoamperometric characterization of Cu foil held at 1.9 V for two days. Electrolyte was 0.25 M (3) in DME. a) SEM image of Cu surface at 500x b) SEM image of Cu surface at 6000x c) EDS spectrum of the highlighted area d) Chronoamperometry data from coin cell.

30 SI-29 Figure S31. Surface characterization of Cu standard. a) SEM image of Cu surface at 500x. b) EDS spectrum of the point marked spectrum 4.