Li 2 OHCl Crystalline Electrolyte for Stable Metallic Lithium Anodes

Transcription

1 Supporting Information Li 2 OHCl Crystalline Electrolyte for Stable Metallic Lithium Anodes Zachary D. Hood, 1,2, Hui Wang, 1, Amaresh Samuthira Pandian, 1 Jong Kahk Keum 1,3 and Chengdu Liang 1,* 1 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831, USA 2 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, 30332, USA 3 Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831, USA These authors contributed equally to this publication Experimental Methods: Synthesis of the LiOH-LiCl electrolytes: Fast-cooled LiOH-LiCl crystalline electrolytes were prepared in a nickel crucible by mixing appropriate molar ratios of LiOH (Sigma Aldrich, 98%) and LiCl (Sigma Aldrich, 99%) and heating to 350 C for 30 minutes to achieve a homogeneous melt, and then cooled quickly to room temperature (this process took about 20 minutes). All reagents were used without further purification. The precursors were dried on a Schlenk line for 4 hours prior to moving the powders to the Argon-filled glove box. Slow-cooled anti-perovskite LiOH-LiCl crystalline electrolytes were prepared in a nickel crucible using the same precursors; the nickel crucible was then sealed with a copper gasket in a bomb reactor and heated to 350 C for 24 hours and cooled slowly at 8 C/ hour to 250 C, held at this temperature for 24 additional hours, and cooled to room temperature at 25 C/ hour. The samples were then ground to a fine powder with a mortar and pestle and ball milled (8000M Spex Mixer Mill) using a mixture of 3 mm and 5 mm Y-ZrO 2 ball milling media in a 1:25 (solid electrolyte: media) mass ratio in a HDPE vial. All processes were carried out under Argon, as the LiOH-LiCl electrolytes and precursors are sensitive to moisture and air. Structural and electrolyte characterization: Crystallographic phase characterization was conducted with a PANalytical X Pert Pro Powder Diffractometer with Cu-K α radiation. All samples were prepared in an Argon-filled glove box and sealed with Kapton films S1

2 on quartz slides. Rietveld refinement was completed using HighScore Plus, a software package developed by PANalytical. To investigate the phase transitions, LiOH-LiCl crystalline electrolytes were sealed with Kapton films and silver paste on quartz slides. The quartz slides were places in an Ashton Paar Align.Stage Hot Stage, which was heated between 30 C-200 C in 10 C increments; the temperature was maintained for 30 minutes prior to collecting crystallographic data. Phase transition reversibility was also confirmed by ramping the hot stage from 200 C to 30 C in 10 C increments. A Zeiss Merlin Scanning Electronic Microscope (SEM) was used to collect images of the pellet surface at 5.0kV. Energy-dispersive X-ray spectroscopy (EDX) was completed with a gun acceleration of 10.0kV to observe surface and cross-sectional elemental distribution in the molten lithium exposed Li 2 OHCl crystalline electrolyte. Samples were placed on carbon conductive tape and sealed under Argon prior to collecting SEM images. After ball milling the LiOH-LiCl samples, about 140 mg of each sample were coldpressed at 300 MPa into a pellet with a diameter of 1/2" and sealed in a pressurized cell developed by our group. For electrochemical impedance spectroscopy (EIS), pellets were pressed with Al/C blocking electrodes and a Solartron 1260 coupled with a Maccor environmental chamber was used to determine Arrhenius activation energy measurements from C. All EIS measurements were completed from 1 MHz 1Hz with amplitude 100 mv. Phase reversibility was confirmed through EIS by ramping cells from C. A Maccor multifunction Model 4200 battery cell cycler collected all cell cycling data. Specialized pressurized cells developed by our group were used to assemble Li/Li 2 OHCl/ Li symmetric cells. First, ball-milled Li 2 OHCl was cold-pressed at 300 MPa to form a continuous membrane across the pressurized cell s dye. Next, lithium foil ( 15 mg) was placed on each side of the solid electrolyte membrane. Carbon mesh was placed on each side of the lithium to prevent molten lithium leakage into the cell. After sealing the cell, the symmetric cell was moved to a Fischer Scientific TM Isotemp TM forced air oven at 195 C for 2 hours prior to collecting data. All processes for cell fabrication (for cell cycling and EIS measurements) were completed under inert atmosphere as LiOH-LiCl electrolytes are sensitive to moisture and air. S2

3 Figure S1. XRD patterns at room temperature for as-synthesized LiOH-LiCl crystalline electrolytes from a) uncontrolled fast cooling from 350 C and b) slow cooling at 8 C/ hour from 350 C to 250 C and holding at 250 C for 24 hours. Figure S2. Impedance spectra of fast-cooled Li 2 OHCl measured at a) 25 to 80 C and b) 100 to 200 C. All measurements were completed from 1 MHz 1Hz with amplitude 100 mv. The total ionic conductivity is determined by using the intercept between the semi-circle or semi-arc and straight line as total resistance. S3

4 Figure S3. Teflon cast and plunger used to prepare LiOH-LiCl membranes. Figure S4. SEM images of Li 2 OHCl after the molten salt was poured into Teflon casts, showing (a) the surface of Li 2 OHCl when no pressure was applied to the surface of the pellet, (b) a closeup of Figure S3a. Applying pressure to the surface of the melt allows for a dense Li 2 OHCl membrane to be fabricated (Figure 3). S4

5 Figure S5. Molten lithium cyclability in a Li/Li 2 OHCl/Li symmetric cell with a current density of 1.0 ma cm -2 at 195 C, demonstrating stability between the molten lithium anode and the crystalline electrolyte for 14,000 minutes. Figure S6. Molten lithium cyclability in a Li/Li 2 OHCl/Li symmetric cell at different current densities (0.1, 0.5 and 1.0 ma cm -2 ) at 195 C. S5

6 Figure S7. SEM images of Li/Li 2 OHCl/Li symmetric cell surface layers with EDX mapping of chlorine in green and oxygen in red. The SEI is mainly composed of Li 2 O. Figure S8. SEM image of Li/Li 2 OHCl/Li symmetric cell showing a cross section of the SEI (a) after 40 and (b) after 160 charge/ discharge cycles. The SEI was uniform across electrolyte and measures 50 µm for both cells, demonstrating that the SEI layer stabilizes the molten lithium anode with Li 2 OHCl. S6