Effects of Polymer Coatings on Electrodeposited Lithium Metal

Transcription

1 Effects of Polymer Coatings on Electrodeposited Lithium Metal Jeffrey Lopez 1#, Allen Pei 2#, Jin Young Oh 1, Ging-Ji Nathan Wang 3, Yi Cui 2,4*, and Zhenan Bao 1* 1 Department of Chemical Engineering, 2 Department of Materials Science and Engineering, and 3 Department of Chemistry, Stanford University, Stanford, California 94305, USA; 4 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA. * Corresponding author: yicui@stanford.edu, zbao@stanford.edu S1

2 Materials and Methods Materials and Polymer Characterization. The self-healing polymer (SHP) coating was synthesized and processed according to previously reported methods. 24 To make the self-healing elastomer (SHE) a version of the amine oligomer used to make the SHP was synthesized with increased branching and chemically crosslinked according to previously reported methods. 34 Poly (ethylene oxide) (M v 600,000, Aldrich), poly (vinylidene fluoride) (M w 534,000, Aldrich), poly (vinylidene fluoride-co-hexafluoropropylene) (M w 455,000, Aldrich) polyurethane (Tecoflex SG-80A, M w 57,773, Lubrizol), and poly (dimethylsiloxane) (Silsoft SE30, M w 455,609, Momentive) were used as received. Differential scanning calorimetry (DSC) experiments were carried out on a DSC-Q2000 (TA Instruments) with a temperature range of -70 to 150 C or 200 C and a heating and cooling ramp rate of 10 C/min. Tensile testing was carried out on an Instron 5565 load frame with a 100 N load cell. A strain rate of 100 %/min was used for all samples. Dynamic mechanical analysis (DMA) was carried out on a DMA-Q800 (TA Instruments) at room temperature using a strain of 0.1%. Samples were measured in tension. Rheometry was carried out on an Ares-G2 (TA Instruments) at 25 C using a strain of 1%. An advanced peltier system (APS) was used as the bottom geometry and a 25 mm parallel plate was used as the top geometry. Contact angle measurements were carried out using a goniometer and software from First Ten Angstroms with both water and diiodomethane as the liquids. Surface energy was calculated using the Owens-Wendt geometric mean method. Dielectric constant was calculated from capacitance measured at 100 Hz using an Agilent E4980A LCR meter. SEC analysis was performed on the PDMS samples using a Tosoh high-temperature EcoSEC equipped with a single TSKgel GPC column (GMHHR-H; 300 mm 7.8 mm) calibrated with monodisperse polystyrene standards. The mobile phase was 1,2,4-trichlorobenzene with a flow rate of 1 ml/min under 180 C. SEC analysis was performed on the PU samples using an Dionex Ultimate 3000 instrument outfitted with an ERC Refractomax 520 refractometer. The columns were Jordi Resolve DVB 1000 Å, 5m, 30 cm x 7.8 mm and a Mixed Bed Low, 5m, 30 cm x 7.8 mm, with a Jordi Resolve DVB Guard Column, 1000 Å, 5m, 30 cm x 7.8 mm, 5 cm x 7.8 mm. DMF with 10 mm LiBr was used as eluent at 1 ml min 1 at room temperature. Monodispers poly (ethylene glycol) standards were used to calibrate the GPC system. Analyte samples at 2 mg ml 1 were filtered through a nylon membrane with 0.2 mm pore size before injection (20 µl). S2

3 Battery Fabrication and Electrochemical Testing. Working electrodes consisted of either a 2 cm 2 bare copper current collector or a Cu foil disc coated with the designed polymer. To coat the copper current collector, the polymers were dissolved and coated according to the conditions in Table S1. Solutions were passed through a 0.45 µm PTFE filter to remove any particulates before spin coating. After spin coating the electrodes were baked overnight at 80 C under vacuum to remove any residual solvent. Coin cells were assembled using 2032 cell casings. A 1 cm 2 Li metal foil was paired with the bare or polymer-coated current collector. The electrolyte was 1 M lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) in 1,3-dioxolane and 1,2- dimethoxyethane (volume ratio 1:1) with 1 wt % lithium nitrate (LiNO 3 ). A commercially available polymer separator (Celgard 2325) was used between the two electrodes. The same amount of electrolyte was added to each cell to standardize the tests (75 µl). Cells were rested for 2 hours for SEM and 10 hours for cycling. Deposition tests were carried out by first precycling the cell 1 time between 0 V and 1 V vs. Li/Li +. Then, for the deposition step, the halfcell voltage was first discharged to 0 V at 1 ma cm -2, and then 0.1 mah cm -2 Li was deposited at the same current. This is to avoid parasitic capacity draw from capacitance in the double layer and polymer coating. Coulombic efficiency tests were carried out by first precycling by depositing 5 mah cm -2 and then stripping fully to 1 V at current 0.5 ma cm -2. Then, a reservoir of 5 mah cm -2 Li is deposited, followed by 10 cycles of deposition and stripping of 1 mah cm -2 Li from the Li reservoir, all at 0.5 ma cm -2. Finally, the remaining Li is all stripped to 1 V and the CE is calculated from the amount stripped. 42 Li electrodes were rinsed using clean 1,3- dioxolane and transferred to the SEM chamber by a sealed Ar-filled vessel. A FEI Quanta 200 SEM was used. Images were analyzed in ImageJ, and particle statistics sample sizes range from around , taken from different images of different regions of the same electrode. Homebuilt microelectrodes using 25 µm tungsten (W) wire embedded in glass were used. Microelectrodes were polished with 100 nm grit lapping disks and cleaned after each test. Electrochemical cells consisted of a W microelectrode (working electrode) and Li foil (counter & reference electrode) submerged in electrolyte. CV scans were swept at 200 mv/s from +1 V to -240 mv and back to +0.5 V. Exchange current densities were calculated by fitting tafel slopes of CV scans. Raw data was filtered using a Savitsky-Golay filter (EC-Lab software) to remove noise and capacitive current was subtracted. The tafel plot of the log current vs. cell polarization was linearly fit over the voltage range -150 mv to -200 mv to extract the tafel slope. The S3

4 exchange current was then calculated based on the intercept of the tafel slope and the y-axis. 6 or more independent tests using fresh microelectrode surfaces were taken for the data shown. Additional characterization of the polymer coating properties For PEO, PVDF-HFP, and PVDF a clear melting point (T m ) can be identified indicating that these three polymers are semi-crystalline. SHP, and SHE all show clear glass transitions below 0 C without any melting. This indicates that these materials are amorphous at room temperature. SHP, PEO, PVDF, and PVDF-HFP all experience significant plastic deformation under strain (Figure S2), which can be identified by the flat or decreasing stress after the initial linear region. The crystalline PEO, PVDF-HFP, and PVDF were much stiffer than either of the other materials with measured Young s moduli of 82 MPa, 335 MPa, and 171 MPa respectively (Figure S2). PU and SHE both display a stress response that is roughly linear with strain (need to show reversibility), indicating that these two materials are elastomers. The SHE has a measured Young s modulus of MPa and the PU has a modulus of 1.40 MPa (Figure S2). The polyurethane has a significantly higher strain at break (>2000%) (Figure S2). Figure S1: Synthetic scheme of a) the self-healing supramolecular polymer and b) the selfhealing elastomer. S4

5 Figure S2: Stress-strain curves for a) self-healing polymer, b) PEO, c) PVDF, d) self-healing elastomer, e) PU, and f) PVDF-HFP. Figure S3: Frequency sweeps using a rheometer to measure shear storage (G ) and loss modulus (G ) for a) self-healing polymer and b) PDMS, and using a DMA to measure tensile storage (E ) and loss modulus (E ) for c) PVDF, d) self-healing elastomer, e) PEO (top) and PU (bottom), and f) PVDF-HFP. S5

6 Figure S4: Differential Scanning Calorimetry (DSC) second cycle traces for a) self-healing polymer, b) PEO, c) PVDF, d) self-healing elastomer, e) PU, and f) PVDF-HFP. S6

7 Additional electrochemical characterization and device processing details Table S1: Properties of the polymer coatings in the battery environment. Polymer Overpotential a [mv] j 0 b [ma/cm 2 ] Swelling [%] σ c [ms/cm] Size [um] PU PVDF- HFP PEO Soluble PDMS SHP Soluble PVDF a Nucleation overpotential determined from galvanostatic cycling. b Exchange current density. c Ionic conductivity Table S2: Spincoating conditions for films reported in the project Polymer Molecular Weight Concentration Spincoating Film Thickness Solvent (g/mol) (g/ml) RPM (µm) Chloroform:Ethanol SHP N/A (1:1) PEO 600, Acetonitrile N-Methyl-2-pyrrolidone PVDF 536, PVDF- HFP 455, N-Methyl-2-pyrrolidone PU 58, Tetrahydrofuran SHE N/A 0.1 Ethanol PDMS 456, Chloroform S7

8 Figure S5: Histogram of the particle size of 0.1 mah/cm 2 of lithium deposited on a bare copper electrode. Figure S6: Representative plots of the process used to process and fit the microelectrode data as described in the Methods section. S8

9 Figure S7: SEM images of 0.1 mah/cm 2 of lithium deposited under a PDMS coating. Taylor Expansion of Equation 4 Equation 4 from the main text: 2 (4) Using γ Li = 398 mj m -2 [1] and assuming Φ 1: (S1) Taking the Taylor expansion around γ poly = 30 mj m -2 : (S2) (S3) From the factors in front of each term we can see that the linear term is a satisfactory approximation for the interfacial energy in the region that we are interested in. Therefore Γ can be simplified as follows: (S4) (S5) (S6) S9

10 Reactivity The reactivity of different functional groups was estimated from their relative bond strengths. The relevant functional groups are listed below in order of decreasing reactivity (increasing bond strength). amide urea urethane > amine > ether > C CF 3 > C F siloxane Surface Energy Calculation Surface energy was calculated using the Owens-Wendt method. The contact angle of both water and diiodomethane was measured on polymer films spin coated on glass. Equation S7 can be derived from Young s equation (S8) and the geometric mean for the interfacial energy (S9). (S7) (S8) 2 (S9) With known liquids (in this case water and diiodomethane), equation S7 takes the form of a line with a slope (square root of the polar component of the polymer surface energy) and intercept (square root of the dispersive component of the polymer surface energy). All the other parameters are known from the liquid or the contact angle measurement. Using a minimum of two different liquids, the surface energy of the polymer can be calculated. Three contact angle measurements were taken for each liquid on each polymer. S10

11 Supplemental References (1) Taylor, J. W. XCVIII. the Surface Energies of the Alkali Metals. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 1955, 46 (379), S11