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Classification: PHYSICAL SCIENCES (Engineering)

Title: Nonflammable Perfluoropolyether-based Electrolytes for Lithium Batteries

Authors: Dominica H. C. Wong1, Jacob Thelen2, Yanbao Fu3, Didier Devaux3, Ashish A. Pandya1, Vince Battaglia3, Nitash P. Balsara2,3,4*, Joseph M. DeSimone1,5*

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, 94720, USA. 3 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California, 94720, USA. 4 Materials Science Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California, 94720, 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# 3290, 257 Caudill Chapel Hill, NC 27599-3290 Tel: (919) 962-2166 desimone@unc.edu Keywords: fluorinated polymers

lithium-ion batteries nonflammable electrolytes

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SI Appendix: Supporting Text Materials: Fluorolink D10 (kinematic viscosity 70 cSt), Fluorolink D10/H, Fomblin ZDOL, and Fomblin ZDOL 4000 were obtained from Solvay Solexis. Lithium bistrifluoromethylsulfonyl imide (LiTFSI) was obtained from Acros Organics. LiNi1/3Co1/3Mn1/3O2 (LiNMC) powder was obtained from Umicore (Cellcore® MX-6). Battery-grade acetylene black (AB) with an average particle size of 40 nm and a material density of 1.95 g/cm3 was acquired from Denka Singapore Private Limited. The binder polyvinylidene fluoride (PVDF 1100) 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 19F NMR(1). Fig. S9 and Table S2 describes PFPE1000-diol. The ratio m:n is 7:3 and the total number of repeating units, m+n, is 10. 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: PFPE1000-DMC, PFPE1400-DMC, PFPE2000-DMC, and PFPE4000-DMC was obtained from PFPE1000-diol, PFPE1400-diol, PFPE2000-diol, and PFPE4000-diol at 85%, 82%, 85%, and 88% yields respectively. 1H NMR analysis of PFPE1000-DMC is shown in Fig. S10 and Table S4. Thermogravimetry analysis of PFPE electrolytes: Td (5%) of all other electrolytes were measured and tabulated in Table S5. All Tds are found to be above 200°C. Effect of salt solvation on glass transition temperatures of PFPE electrolytes: Below saturation, addition of LiTFSI generally resulted in slight Tg 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 temperature-dependent 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 20 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, Re and Rd, respectively. The Lithium transference number is calculated by

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t+ = ReRe + Rd

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, Rint

0 followed by a chronoamperometry using a dc voltage, ΔV, of 20 mV to monitor the current evolution over time from its initial 𝐼! value. When a steady state current, 𝐼!, is obtained the interfacial resistance, Rint∞0 , is measured. t+ is determined by:

t+ = I∞(ΔV - I 0Rint0 )

I 0 (ΔV - I∞Rint∞ )

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 (Rc) and inductance (Lc). The electrolyte resistance (Re) is in parallel with the electrolyte pseudocapacitance (CPEe, Constant Phase Element). The Lithium/electrolyte interface phenomena can be decomposed in two contributions, one due to the passive layer modeled by Rint1 in parallel with CPEint1 and the other one, Rint2 in parallel with CPEint2, corresponding to the electronic transfer. Wd is a short Warburg element related to the diffusion resistance (Rd). Determination of t+ using the Bruce and Vincent method: In addition to AC impedance spectroscopy, the transference number of PFPE1000-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

Figure S1. Photograph of PFPE/LiTFSI electrolyte (A) below and (B) above the salt saturation limit.

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Figure S2. Infrared spectra of PFPE1000-diol/LiTFSI blends compared to pure PFPE1000-diol and

LiTFSI.

Figure S3. Nyquist plot obtained from AC Impedance for PFPE1000-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.

5001000150020002500300035004000

Wavenumber (cm-1)

Tra

nsm

ittan

ce (%

)

LiTFSI

r=0.05

r=0.04

r=0.02

r=0.01

PFPE-diol

PFPE-diol

650 700 750 800 850 900 9500

50

100

150

200

250

300

350

Re(Z) / Ohm

-Im(Z

) / O

hm

880 890 900 910 920 9300

10

20

30

40

50

 

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Figure S5. t+ measurements of cells containing 50 µm PFPE1000-DMC electrolyte using the Bruce and Vincent method using a 20 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 0.12.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0 5 10 15 20 25 30 35 40 45 50

Transference Num

ber C

urre

nt /

mA

Time / h

I vs. t - Cell 1 I vs. t - Cell 2 t+ Cell 1 t+ Cell 2

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Figure S7. Cyclic voltammograms of PFPE1000-DMC at 1 mV.s-1 from a Li/Electrolyte/Stainless

steel cell obtained at every fifth cycle at 40°C (blue), 60°C (green), 80°C (orange), and 100°C (red).

Figure S8. Cycle performance of LiPF6-ethylene carbonate/diethyl carbonate(1M, 1:2 vol, Daikin Co.) batteries.

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Figure S9. 19F NMR of PFPE1000-diol in CDCl3.

Figure S10. 1H NMR of PFPE1000-DMC in CDCl3.

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Figure S11. Temperature-dependent conductivity of PFPE-based electrolytes at maximal

LiTFSI loading. Table S1. VFT parameters calculated for PFPE1000-DMC electrolytes Value StDev Units A 5.0E-04 6.7E-05 (S/cm)*K^(1/2) B 0.47 0.13 kJ/mol T0 271.3 6.9 K Table S2. Chemical Shift Analysis of 19F NMR of PFPE1000-diol Signal Chemical Shift Group Monomer Ratio

a -51.7 -OCF2 CF2OCF2OCF2 CF2O-

3 b -53.4 -OCF2 CF2OCF2OCF2 OCF2-

c -55.1 -CF2 OCF2OCF2OCF2 OCF2- d -80.6 -OCF2 CF2OCF2CH2OH Terminal Group 2 e -82.8 -OCF2OCF2CH2OH f -88.9 -OCF2 CF2OCF2CF2OCF2 CF2O-

7 g -90.6 -OCF2 CF2O CF2CF2OCF2OCF2- i -92.0 -CF2 OCF2OCF2CF2OCF2 OCF2-

Table S3. Determination of m:n and number of repeating units for PFPEs

PFPE m:n No. repeating units PFPE1400-diol 10:3 13 PFPE2000-diol 5:7 24 PFPE4000-diol 1:1 48

Table S4. Chemical Shift Analysis of 1H NMR of PFPE1000-DMC

Signal Chemical Shift Group a 4.5 -CF2CH2OCOOCH3

2.4 2.6 2.8 3.0 3.2 3.410-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

Temperature (1000/K)

σ (S

cm

-1)

1K diol

2K diol4K diol1K DMC

1.4K diol

1.4K DMC2K DMC4K DMC

CF2On

CF2CF2Om

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b 3.9 -CF2CH2OCOOCH3 Table S5. Td of PFPE-diols and PFPE-DMCs. PFPE PFPE1400-diol PFPE2000-diol PFPE4000-diol PFPE1400-

DMC PFPE2000-DMC

PFPE4000-DMC

Td (5%) 220°C 208°C 327°C 221°C 230°C 337°C Table S6. Tg of PFPE electrolytes with and without LiTFSI

Electrolyte r Tg (no LiTFSI) (°C) Tg (with LiTFSI) (°C) PFPE1000-diol 0.04 -89 -90 PFPE1400-diol 0.02 -98 -96 PFPE2000-diol 0.01 -112 -112 PFPE4000-diol 0.01 -117 -117

PFPE1000-DMC 0.04 -95 -91 PFPE1400-DMC 0.01 -101 -101 PFPE2000-DMC 0.03 -113 -112 PFPE4000-DMC 0.01 -118 -117

References 1. Tchistiokov A, Fontana S, & Tonelli C (2006) Spearation of bifunctional

perfluoropolyethers (PFPEs) having -CH2OH termination from their mixtures with -CH2OH monofunctional PFPEs. US 2006/0009660 A1

2. Tominaga Y, Takizawa N, & Ohno H (2000) Effect of added salt species on the ionic conductivity of PEO/sulfonamide salt hybrids. Electrochimica Acta 45(8–9):1285-1289.

3. Bouchet R, Lascaud S, & Rosso M (2003) An EIS Study of the Anode Li/PEO-LiTFSI of a Li Polymer Battery. Journal of The Electrochemical Society 150(10):A1385.

4. Bruce P & Vincent CA (1987) Steady State Current Flow in Solid Binary Electrolyte Cells J. Electroanal. Chem. 225:1-17.