SUPPORTING INFORMATION. Lithium Metal Anodes with An Adaptive Solid-Liquid Interfacial Protective Layer

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1 SUPPORTING INFORMATION Lithium Metal Anodes with An Adaptive Solid-Liquid Interfacial Protective Layer Kai Liu 1, Allen Pei 1, Hye Ryoung Lee 2, Biao Kong 1, Nian Liu 1, Dingchang Lin 1, Yayuan Liu 1 Chong Liu 1, Po-chun Hsu 1, Zhenan Bao 3, Yi Cui 1, 4 * These authors contributed equally to this work. 1 Department of Materials Science and Engineering, Stanford, California , USA 2 Department of Electrical Engineering, Stanford, California , USA 3. Department of Chemical Engineering, Stanford University, California 94305, USA, 4 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA. * yicui@stanford.edu EXPERIMENTAL SECTION Rheological measurements. A parallel-plate rheometer (ARES-G2, TA Instrument) was used to measure the rheology. The diameter of the parallel plate is 8 mm and the thickness is set as 0.6 mm. Frequency sweep was performed at 0.05% strain at a fixed temperature (25 C). Fabrication of electrodes. Silly Putty Registered trademark of Binney and Smith, Inc, and is manufactured by Dow Corning Corporation under the name Dow Corning 3179 Dilatant Compound. For the fabrication of SP coated electrode, a Cu foil disk (10/16 inch) was heated to 130 C on a hot plate. Silly Putty was then melted at 130 C and coated onto the Cu disk using a sharp blade. The thickness of Silly Putty was measured to be ~1 µm. The statically crosslinked PDMS was prepared by mixing an elastomer base and curing agent (Dow Corning) in a 70:1 weight ratio. The mixture was degassed in vacuum at room temperature for 20 minutes and coated onto a Cu disk using a sharp blade. Uncrosslinked PDMS-coated electrodes were prepared similarly, without addition of curing agent. Commercial battery separators (Celgard 2325) were placed onto the cooled polymer modified electrodes, and the electrodes were pressed flat between two glass slides. The electrodes were degassed again in vacuum, then transferred into an Ar glovebox to be used in battery assembly. PDMS was S1

2 cured at room temperature for 5 days. Pristine Cu disks were used as the bare Cu control electrodes. All performance tests and cycling experiments used Cu-foil type electrodes. Only the cross-sectional SEM image used the Cu-coated glass electrode, due to the ease of breaking the electrode to see the interior. Electrochemical Testing. Galvanostatic cycling was performed using a 96-channel battery tester (Arbin Instruments). The as-fabricated working electrodes were assembled in 2032-coin cells (MTI Corporation) with Li metal foil (Alfa Aesar) serving as both the counter-electrode and reference electrode. The electrolyte used was 1 M lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 volume ratio) with 1% wt. lithium nitrate (LiNO 3 ) as an additive. 60 µl of electrolyte was used in each coin cell to standardize the testing. The batteries were first cycled once between 0 V and 2 V to pretreat the electrodes (no Li deposition). Cycling was performed by deposition of 1 mah of Li onto the Cu working electrode, followed by Li stripping up to 2 V. Additional cycling data and statistics for SP-coated and bare Cu electrodes are available in Figure S12. Cross Sectional SEM Electrodes. 10 nm of chromium (Cr) followed by 100 nm of copper (Cu) was evaporated onto clean glass coverslips. Glass was used as the electrode substrate due to the easy breaking of the electrode. Tearing or cutting Cu foil electrodes damages the morphology of the Li deposits and polymer coating. Silly putty was coated onto the Cu as previously discussed. A pouch cell was fabricated with the Cu-coated glass as the working electrode and Li metal as the counter-electrode and reference electrode. S2

3 Figure S1. Coulombic efficiencies of high capacity and high current Li metal deposition/stripping on control bare Cu electrodes (black square) and SP-coated electrodes (red circle). a, Cycling was performed with current density 1 ma cm -2 and areal capacity 2.5 mah cm -2. b, Cycling was performed with current density 3 ma cm -2 and areal capacity 1 mah cm -2. Note S1 (Calculation of diffusion coefficient of Li + in the SP coating) We measured the conductivity of the SP coating using a two blocking electrode setup, where a thin layer of SP wetted by electrolyte was sandwiched between two stainless steel spacers. The ionic conductivity of the swollen coating is calculated to be 0.1 ms/cm. The diffusion constant can be estimated using σ = µne and the Einstein relation. The mobility is thus 1.03x10-6 cm 2 /V s and the diffusion coefficient of Li + is calculated to be 2.66x10-8 cm 2 /s. Note S2 (Calculation of strain rate) Figure S2. Schematic showing the growing granular Li pillar with a thin layer of SP coated on it. During charging, Li metal starts to nucleate on the Cu substrate underneath the SP coating layer. As Li continues to deposit, granular Li begins to grow, as can be seen in Figure 4e in the main text. The stain rate that the growing granular Li exerts on the SP coating layer can be calculated as : S3

4 Ɣ where Ɣ is the stain rate, 1 2 is the rate at which the two sides of SP move away from each other. is the thickness of the SP layer (~1 µm). 1 2 During normal charging, the maximum stain rate should come from the sidewall of the granular Li pillar because is largest there. That is why the lithium filaments always extrude from the gaps between the granular Li pillars, as indicated by Figures 4b and c in the main text. In our experiment, the vertical growth rate (v 1 ) of the bulk granular Li is ~5 µm h -1. Thus, the stain rate (v 2 =0) during normal Li deposition (without dendrites) can be roughly calculated as 10-3 s -1. Note: Based on Monroe and Newman s model 1, the tip growth velocity of a Li metal dendrite is where J is the local current density, V Li is the molar volume of Li, and F is Faraday s constant. For a current density of 1 ma cm -2, the tip growth rate is 4.84 µm h -1. Enhanced electric fields and Li ion flux near the sharp Li protrusion can enhance the tip current density and increase the tip growth rate. Thus, the maximum strain rate resulting from abnormal filamentary or dendritic Li growth shooting from cracks in the electrode will increase the strain rate locally, causing the SP to stiffen even more at the growth location, further suppression Li growth. Figure S3. The magnified SEM image showing the SP uniformly cover the deep ridges and fissures on the Li metal anode. S4

5 Figure S4. The flowability of SP on the Li electrode. a, SEM image of the SP coating on Cu foil after stripping Li. Note the good coverage and conformity of the SP to the bumpy surface. The roughness likely results from SEI formation. b, Optical microscope image of SP coating on Li metal foil with a large crack cut by razor blade. The SP-coated Li was submerged into the battery electrolyte after imaging. c, The same location after 1 hour in electrolyte, showing partial sealing of the gap. SP is anticipated to be able to heal more completely during cycling because the stripping process may bring the fractured surfaces into closer contact. Figure S5. FTIR spectra of SP before and after coated onto Li metal. The spectra was not changed, indicating that the chemical property of the SP protective layer was not affected by the reductive environment of Li metal anode. S5

6 Figure S6. Low magnification SEM image of Li deposited on SP-coated Cu electrode after 75 cycles of Li deposition and stripping and removal of SP coating. Cycling was performed with current density 1 ma cm -2 and areal capacity 1 mah cm -2. Figure S7. The characterizations of the SEI layer. a and b, SEM images of remaining SEI after stripping away Li from the electrodes that were cycled 75 times. a, SEI from Li grown on control Cu electrode. The SEI is porous and rough, reflecting damage and cracking resulting from Li deposition/stripping cycles. b, SEI from Li grown on SP coated Cu electrode. The SEI is more compact and smooth compared to the control SEI. Cycling was performed with current density 1 S6

7 ma cm -2 and areal capacity 1 mah cm -2. c and d, Impedance spectroscopy after different cycles and fully stripping Li for (c) the bare Cu electrode and (d) the SP coated electrode. The SP coated electrodes exhibit a decreasing resistance with cycling without change of the shape of the semi-circle, indicating the relatively stable interface between the electrode and electrolytes (Fig. S6d). However, the bare Cu electrode develops a second semicircle in the Nyquist plot after 70 cycles, representative of the formation of a porous interphase 2 (Fig. S7c). Note also that the control electrode has already begun showing severe capacity decay after 70 cycles, offering some insight into the role of SEI buildup in cell failure. Figure S8. Homemade optical microscope Li deposition cell. Optical cells used to test Li deposition on (a) bare Cu electrodes and (b) SP-coated Cu electrodes. 10 nm Cr followed by 200 nm Cu was evaporated onto glass slides (dimension 3 inch x 1 inch) using a mask to obtain the Cu electrode pattern as shown. SP was coated onto the Cu as described previously. LFP-coated Al foil was assembled onto the glass slide, and a thin glass coverslip was placed over the electrodes and sealed with epoxy and an ionomer resin. Electrolyte was infiltrated into the space between the glass slides and sealed fully. The black residue seen in (b) is from carbon tape used to mount the samples to the microscope. Cu tape and silver paste was used to improve electrical contact between the battery tester and the cell. A fixed current of 4 ua was applied to deposit Li. S7

8 Figure S9. Top-view SEM images of Li metal deposited on a static crosslinked PDMS-coated Cu electrode after 75 cycles of Li deposition and stripping. Arrows highlight some Li filaments that are present over the entire electrode. Cycling was performed with current density 1 ma cm -2 and areal capacity 1 mah cm -2. Note S3 (Simulation Details) The physics model used was Electrodeposition, Tertiary Nernst-Planck. The majority of the simulation cell consists of the electrolyte-impregnated separator with thickness 50 µm. The diffusion coefficient of Li + in the electrolyte was set to 1x10-5 cm 2 s -1. The electrodeposition module in COMSOL uses the Einstein relation (D = µk b T/q) to calculate the ionic mobility, µ, and the diffusion coefficient chosen closely matches that calculated from experimental electrolyte conductivities. From literature [3], the ionic conductivity of 1M LiTFSI in 1:1 (v/v) DOL:DME is 11 ms/cm, giving sing a Li + ion mobility of 1.14x10-4 cm 2 /V s (σ = µne). Then, applying the Einstein relation, an experimental diffusion coefficient of 2.93x10-6 cm 2 /s is obtained. This is quite close to the value of 1x10-5 cm 2 /s used in the simulation. A polymer coating of 1 µm thickness, modeled as an electrolyte layer, was generated on the anode surface. The cathode is set as the upper boundary of the 50 µm electrolyte layer. The diffusion coefficient of Li + in the bulk polymer coatings was set to 1x10-9 cm 2 /s. To generate the non-uniformity in the polymer coatings with no S8

9 pinholes, the diffusion coefficient of Li + in the polymer coatings was spatially modified following a Gaussian function with full-width half-maximum 0.5 µm and maximum amplitude 1x10-7- cm 2 /s at the center of the coating (Equation 1,2). 1 (1). (2) This represents an intermediate conductivity resulting from the flowable polymer coating filling in the pinhole. To give the polymer the dynamic solid-liquid behavior, a feedback mechanism was added to the spatially-varying Gaussian conductivity profile. The Gaussian was decreased by a factor of, where V is the volume change of each mesh element in the polymer coating as the polymer coating deforms while Li metal is deposited, and A is a scaling factor. This scaling was chosen to correspond with Arrhenius behavior. The overpotential of deposition was set to -500 mv vs Li/Li + at the working electrode. S9

10 Figure S10. Simulation on the effect of dynamic stiffening of the SP on the formation and growth of Li electrode. a and b, The simulation cell geometry in COMSOL for (a) the control electrode with a rigid, non-flowable and non-dynamic polymer coating with a 0.5 µm pinhole and for (b) both the electrode with a flowable and non-dynamic polymer coating and the electrode with the flowable and dynamic polymer (SP). c and d, Simulation visualization Mesh element volume change. Simulation snapshots of Li deposition on (c) flowable, non-dynamic and (d) flowable, dynamic polymer coated electrodes. These are the same snapshots as Figure 5c and Figure 5d, but with the color map representing the relative volume change of each mesh element compared to its initial volume. Note the high amount of compression in (c) near the tip of the Li filament. This compression leads to the suppression feedback in conditions for (d), resulting in a much more uniform Li deposition. e, f and g, Growth rate comparison of central Li growth vs. bulk Li growth for various polymers for the simulation. The growth rates of Li deposited on S10

11 electrodes with (e) rigid, non-dynamic, (f) flowable, non-dynamic, and (g) flowable and dynamic polymers. The orange line represents the height of the Li pillar at the origin over time. The blue line shows the height of the Li bulk 4.5 microns away from the central Li pillar over time. The grey line shows the difference between the central pillar height and bulk Li height, the slope of which represents the relative growth rate difference between the non-uniformity and the bulk. It is clear that only for SP-coated electrodes does the growth rate plateau upon suppression by the polymer coating. The other electrodes exhibit consistent faster growth of the Li pillar in the center compared to the growth rate of the bulk Li away from the center. Figure S11. Full-cell discharge capacity of SP-coated Li metal anode paired with a traditional LiFePO 4 cathode. The Coulombic efficiency of the cathode is plotted on the secondary y-axis. The current density is 0.25 ma/cm 2 for the first cycle, and 1 ma/cm 2 for later cycles. We did a full cell test of SP coated Li metal anode with a typical cathode, LiFePO 4 (LFP). We predeposited 5 mah/cm 2 of Li onto SP-coated Cu foil to serve as the anode, and paired it with LFP electrodes with active material loading of ~7 mg/cm 2 (~1.28 mah/cm 2 ). The cell maintained a high average CE of 99.5% and a stable average capacity of mah/g for over 50 cycles (Figure S11), indicating the strategy of SP coating is promising for practical battery system applications. S11

12 Figure S12. Extra coulombic efficiency plots for Li metal deposition/stripping on SP-coated electrodes and bare Cu electrodes. These data represent the best 4 cells of each type. Cycling was performed with current density 1 ma cm -2 and areal capacity 1 mah cm -2. The average number of cycles to reach 90% CE for SP-coated electrodes was 86 cycles with standard error 7.2 (n=16), while the average number of cycles to reach 90% CE for bare Cu electrodes was 49 cycles with standard error 3.0 (n=9). Video S1: Optical Microscope videos of optical cell of the bare Cu and SP-coated anode. Video S2: Compiled Simulation results on the Li metal deposition with rigid & non-dynamic, flowable & non-dynamic, and flowable & dynamic stiffening polymer coatings. References (1) Monroe, C.; Newman, J. J. Electrochem. Soc. 2003, 150, A1377-A1384. (2) Lu, D. P.; Shao, Y. Y.; Lozano, T.; Bennett, W. D.; Graff, G. L.; Polzin, B.; Zhang, J. G.; Engelhard, M. H.; Saenz, N. T.; Henderson, W. A.; Bhattacharya, P.; Liu, J.; Xiao, J. Adv. Ener. Mater. 2015, 5, (3) Kim, H.-S.; Jeong, C.-S. B. Kor. Chem. Soc. 2011, 32, S12