Classification: PHYSICAL SCIENCES (Engineering) Title: Nonflammable Perfluoropolyether-based Electrolytes for Lithium Batteries

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1 Classification: PHYSICAL SCIENCES (Engineering) Title: Nonflammable Perfluoropolyether-based Electrolytes for Lithium Batteries Authors: Dominica H. C. Wong 1, Jacob Thelen 2, Yanbao Fu 3, Didier Devaux 3, Ashish A. Pandya 1, Vince Battaglia 3, Nitash P. Balsara 2,3,4*, Joseph M. DeSimone 1,5* Affiliations: 1 Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, 27599, USA. 2 Department of Chemical Engineering, University of California, Berkeley, California, 9472, USA. 3 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California, 9472, USA. 4 Materials Science Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California, 9472, USA. 5 Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, 27695, USA. Corresponding author: Joseph M. DeSimone Department of Chemistry The University of North Carolina at Chapel Hill CB# 329, 257 Caudill Chapel Hill, NC Tel: (919) desimone@unc.edu Keywords: fluorinated polymers lithium-ion batteries nonflammable electrolytes 1

2 SI Appendix: Supporting Text Materials: Fluorolink D1 (kinematic viscosity 7 cst), Fluorolink D1/H, Fomblin ZDOL, and Fomblin ZDOL 4 were obtained from Solvay Solexis. Lithium bistrifluoromethylsulfonyl imide (LiTFSI) was obtained from Acros Organics. LiNi 1/3 Co 1/3 Mn 1/3 O 2 (LiNMC) powder was obtained from Umicore (Cellcore MX-6). Battery-grade acetylene black (AB) with an average particle size of 4 nm and a material density of 1.95 g/cm 3 was acquired from Denka Singapore Private Limited. The binder polyvinylidene fluoride (PVDF 11) was supplied by Kureha, Japan. Anhydrous N-methylpyrrolidone (NMP) was purchased from Aldrich Chemical Company. Characterization of PFPE-diols: The ratio of m and n monomers in PFPEs was determined by 19 F NMR(1). Fig. S9 and Table S2 describes PFPE 1 -diol. The ratio m:n is 7:3 and the total number of repeating units, m+n, is 1. This analysis was repeated for all different molecular weights of PFPE-diols studied (Table S3). Because the synthesis of PFPE-DMC only involved end group modification without changes to the oligomer backbone, the number of repeating units is assumed to be the same as its PFPE-diol reagent. Synthesis of PFPE-DMC: PFPE 1 -DMC, PFPE 14 -DMC, PFPE 2 -DMC, and PFPE 4 -DMC was obtained from PFPE 1 -diol, PFPE 14 -diol, PFPE 2 -diol, and PFPE 4 -diol at 85%, 82%, 85%, and 88% yields respectively. 1 H NMR analysis of PFPE 1 -DMC is shown in Fig. S1 and Table S4. Thermogravimetry analysis of PFPE electrolytes: T d (5%) of all other electrolytes were measured and tabulated in Table S5. All T d s are found to be above 2 C. Effect of salt solvation on glass transition temperatures of PFPE electrolytes: Below saturation, addition of LiTFSI generally resulted in slight T g elevations (Table S6). This is not surprising, as similar results were reported with PEO (2). Electrochemical performance of PFPE-based electrolytes: Fig. S11 illustrates the temperaturedependent conductivity performances of the 8 different PFPE-based electrolytes studied at their maximal LiTFSI loading. Due to the larger salt solvation capacities of PFPE-DMC electrolytes, these materials can achieve higher conductivity than their hydroxy analogs. Determination of t + using AC impedance spectroscopy: The lithium-ion transference number, t +, was measured using Li/electrolyte/Li symmetrical cells assembled in 2325 coin cell hardware. Alternating current impedance spectroscopy measurements were made between 38.7 C and 85.5 C in the frequency range of 1 MHz to 1 mhz, with a 2 mv excitation signal. t + was evaluated with the procedure described by Bouchet et al.(3), where the impedance spectrum (Fig. S3) are modeled by an electrical equivalent circuit (Fig. S4) that permits to extract the electrolyte and the diffusion resistance, R e and R d, respectively. The Lithium transference number is calculated by 2

3 t + = R e + R d This technique has also been compared with the steady state technique which combines a dc polarization with ac measurements (4). This method consists of an initial ac impedance measurement to determine the lithium interfacial resistance, R int followed by a chronoamperometry using a dc voltage, ΔV, of 2 mv to monitor the current evolution over time from its initial I! value. When a steady state current, I!, is obtained the interfacial resistance, R int, is measured. t + is determined by: t + = I (ΔV - I R int I (ΔV - I R int ) For accuracy, the interfacial resistance of the symmetrical cells was measured every hour during the chronoamperometry period by applying an AC signal to the DC one. These intermittent measurements enables one to follow the t + evolution over time. The electrical model takes into account the apparatus resistance (R c ) and inductance (L c ). The electrolyte resistance (R e ) is in parallel with the electrolyte pseudocapacitance (CPE e, Constant Phase Element). The Lithium/electrolyte interface phenomena can be decomposed in two contributions, one due to the passive layer modeled by R int1 in parallel with CPE int1 and the other one, R int2 in parallel with CPE int2, corresponding to the electronic transfer. W d is a short Warburg element related to the diffusion resistance (R d ). Determination of t + using the Bruce and Vincent method: In addition to AC impedance spectroscopy, the transference number of PFPE 1 -DMC was also determined using a potentiostatic polarization technique(4) (Fig. S5). Similar high transference values for the lithium cation were observed. Both approaches were also used to determine t + of SEO electrolytes. Supporting Figures and Tables R e ) Figure S1. Photograph of PFPE/LiTFSI electrolyte (A) below and (B) above the salt saturation limit. 3

4 PFPE-diol LiTFSI r=.5 Transmittance (%) r=.4 r=.2 r=.1 PFPE-diol Wavenumber (cm-1) Figure S2. Infrared spectra of PFPE 1 -diol/litfsi blends compared to pure PFPE 1 -diol and LiTFSI Im(Z) / Ohm Re(Z) / Ohm Figure S3. Nyquist plot obtained from AC Impedance for PFPE 1 -DMC electrolyte. Open symbol is the experimental spectra while the line is the modeling using electrical equivalent cicuit. Figure S4. Electrical model used to fit the Nyquist plot. 4

5 Current / ma I vs. t - Cell 1 I vs. t - Cell 2 t+ Cell 1 t+ Cell Transference Number Time / h Figure S5. t+ measurements of cells containing 5 µm PFPE1-DMC electrolyte using the Bruce and Vincent method using a 2 mv polarization at 38.8 C. Figure S6. Nyquist plot obtained from AC Impedance for Polysytrene-PEO co-block polymer electrolyte. t+ was calculated to be.12. 5

6 Figure S7. Cyclic voltammograms of PFPE 1 -DMC at 1 mv.s -1 from a Li/Electrolyte/Stainless steel cell obtained at every fifth cycle at 4 C (blue), 6 C (green), 8 C (orange), and 1 C (red). Figure S8. Cycle performance of LiPF 6 -ethylene carbonate/diethyl carbonate(1m, 1:2 vol, Daikin Co.) batteries. 6

7 Figure S9. 19 F NMR of PFPE 1 -diol in CDCl 3. Figure S1. 1 H NMR of PFPE 1 -DMC in CDCl 3. 7

8 σ (S cm -1 ) K diol 1.4K diol 2K diol 4K diol 1K DMC 1.4K DMC 2K DMC 4K DMC Temperature (1/K) Figure S11. Temperature-dependent conductivity of PFPE-based electrolytes at maximal LiTFSI loading. Table S1. VFT parameters calculated for PFPE 1 -DMC electrolytes Value StDev Units A 5.E-4 6.7E-5 (S/cm)*K^(1/2) B kj/mol T K Table S2. Chemical Shift Analysis of 19 F NMR of PFPE 1 -diol Signal Chemical Shift Group Monomer Ratio a OCF 2 CF 2 OCF 2 OCF 2 CF 2 O- 3 b OCF 2 CF 2 OCF 2 OCF 2 OCF 2 - c CF 2 OCF 2 OCF 2 OCF 2 OCF 2 - d OCF 2 CF 2 OCF 2 CH 2 OH Terminal Group 2 e OCF 2 OCF 2 CH 2 OH f OCF 2 CF2OCF 2 CF 2 OCF2 CF 2 O- 7 g OCF 2 CF 2 O CF 2 CF 2 OCF 2 OCF 2 - i CF 2 OCF 2 OCF 2 CF 2 OCF 2 OCF 2 - CF 2 O Table S3. Determination of m:n and number of repeating units for PFPEs PFPE m:n No. repeating units PFPE 14 -diol 1:3 13 PFPE 2 -diol 5:7 24 PFPE 4 -diol 1:1 48 Table S4. Chemical Shift Analysis of 1 H NMR of PFPE 1 -DMC Signal Chemical Shift Group a 4.5 -CF 2 CH 2 OCOOCH 3 n CF 2 CF 2 O m 8

9 b 3.9 -CF 2 CH 2 OCOOCH 3 Table S5. T d of PFPE-diols and PFPE-DMCs. PFPE PFPE 14 -diol PFPE 2 -diol PFPE 4 -diol PFPE 14 - DMC PFPE 2 - DMC T d (5%) 22 C 28 C 327 C 221 C 23 C 337 C PFPE 4 - DMC Table S6. T g of PFPE electrolytes with and without LiTFSI Electrolyte r T g (no LiTFSI) ( C) T g (with LiTFSI) ( C) PFPE 1 -diol PFPE 14 -diol PFPE 2 -diol PFPE 4 -diol PFPE 1 -DMC PFPE 14 -DMC PFPE 2 -DMC PFPE 4 -DMC References 1. Tchistiokov A, Fontana S, & Tonelli C (26) Spearation of bifunctional perfluoropolyethers (PFPEs) having -CH2OH termination from their mixtures with - CH2OH monofunctional PFPEs. US 26/966 A1 2. Tominaga Y, Takizawa N, & Ohno H (2) Effect of added salt species on the ionic conductivity of PEO/sulfonamide salt hybrids. Electrochimica Acta 45(8 9): Bouchet R, Lascaud S, & Rosso M (23) An EIS Study of the Anode Li/PEO-LiTFSI of a Li Polymer Battery. Journal of The Electrochemical Society 15(1):A Bruce P & Vincent CA (1987) Steady State Current Flow in Solid Binary Electrolyte Cells J. Electroanal. Chem. 225: