Supporting Information

Inhibiting Polysulfide Shuttle in Lithium–Sulfur Batteries throughLow-Ion-Pairing Salts and a Triflamide SolventAbhinandan Shyamsunder, Witali Beichel, Petra Klose, Quan Pang, Harald Scherer,Anke Hoffmann, Graham K. Murphy, Ingo Krossing,* and Linda F. Nazar*

anie_201701026_sm_miscellaneous_information.pdf

Author Contributions

A.S. Writing—original draft: Lead; Writing—review & editing: Equal.

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EXPERIMENTAL PROCEDURES

Cathode Fabrication. Electrodes were constructed from a 50 wt% Ketjenblack/sulphur composite (KB/S) prepared by melt diffusion. Positive electrodes were cast from a slurry containing 80 wt% KB/S composite, 10 wt% Super P carbon and 10 wt% poly(vinylidene difluoride) binder in N,N-dimethylformamide onto carbon paper (AvCarb P50), with a sulphur loading of 1 mg/cm2. The P50 paper was used as received and the cast electrodes were dried at 60 °C overnight before use.

Electrochemistry. Coin cells(2325, NRC) were fabricated in an argon-filled glovebox using a KB/S electrode, two sheets of CelgardTM separators, a Li foil negative electrode and 25 μL of the electrolyte. For galvanostatic cycling at room temperature, an Arbin BT-2000 instrument was used. For the high-temperature study, a VMP-3 instrument (Biologic) was employed with a Thermoscientific oven. The same instrument was used to perform the cyclic voltammogram and galvanostatic intermittent titrations (GITT). The GITT experiments were conducted by alternating C/30 pulses (20 mins) with OCV periods (120 mins). Cyclic voltammogram studies were performed in coin cells with a lithium foil counter electrode and a stainless-steel working electrode at a scan rate of 0.1 mV/s. Electrochemical Impedance Spectroscopy (EIS) was performed on a VMP-3 instrument (Bio-logic) between 200 kHz and 20 mHz before and after 1 galvanostatic cycle.

Preparation of the electrolyte. The triflamide solvents were synthesised according to a previously reported method [1].(See below) Li[Al(Ohfip)4] was prepared according to a previously reported procedure (SI).[2] LiTFSI salt was provided by the BASF network and was dried in a vacuum oven at 100 ° C for 2 days prior to use. Trifluoromethane sulfonic anhydride, dipropyl amine, and triethyl amine were purchased from Sigma-Aldrich and were used as received. Anhydrous dimethyl amine was purchased from the Linde gas group. Electrolytes were prepared with a concentration of 0.2 M irrespective of the salt used. The water content of electrolytes was measured using a Karl Fischer instrument and water contents of both triflamides was less than 1 ppm. The water content for Li[Al(Ohfip)4] could not be determined via Karl-Fischer since this compound is incompatible with the typical Hydranal Coulomat AG titer reagent.

Ionic conductivity and viscosity. The viscosities at room temperature were measured using a µVISC viscometer (Rheosense) and the ionic conductivities with a 3-star conductimer (Orion) equipped with a 2-electrode epoxy conductivity probe (cell constant of 1.0 cm-1). Temperature dependent viscosities were measured with a programmable Brookfield rotation viscometer (RVDV–III UCP) in an atmosphere of dry air in a specifically home–built glove box (relative humidity in air below 0.1%). The samples were tempered with an external cryostat, which was connected to the viscometer from outside the glove box (accuracy about ±0.1°C). T-dependent electrical conductivities were performed outside the glovebox with the Mettler Toledo SevenMulti conductometer and a 4-pin platinum electrode (Mettler Toledo InLab@710), in an inert home-built cell (Fig.S4). The temperature was regulated by a cryostat connected to the cell.

XPS. XPS analysis was performed on a Thermo ESCALAB 250 instrument, using a monochromatic Al-Kα source. The samples were transported under an argon atmosphere into the chamber. All spectra were fitted with Gaussian-Lorentzian functions and a Shirley-type background. The binding energy values were all calibrated using the C 1s peak at 284.8 eV.

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Operando X-ray diffraction (XRD). Operando X-ray diffraction (XRD) was performed using a PANalytical Empyrean diffractometer outfitted with a PIXcel detector. The diffraction patterns were collected using Cu-Kα radiation. Operando XRD (X-ray diffraction) was performed using a home-made Swagelok-type cell with a glassy-carbon window. The diffraction patterns were collected in Bragg–Brentano geometry. The cell was cycled at C/30 rate with a pattern collection time of 15 mins (20-55°). Patterns were group-merged by three for peak area quantification. The Li2S, α-S8 and β-S8 are quantified by integrating the peak area of their corresponding major peaks, (022), (222) and (200), respectively. Ketjen black with 70 wt% of hand-ground sulfur was utilized as the cathode for optimal signal acquisition and electrochemistry.

Synthesis of DMT and DPT: The sulphonamides were synthesised according to the following procedure. Amine (2.56 mmol) and triethyl amine (2.73 mmol) were dissolved in anhydrous dichloromethane at -78 ˚C. Trifluoromethanesulfonic anhydride was dissolved in dichloromethane, added dropwise over a period of 15 mins and stirred at -78 ˚C for 2 hours. Water was then added and the reaction mixture was allowed to reach room temperature. The aqueous phase was twice extracted with dichloromethane and the combined organic phases were washed with brine and dried over anhydrous magnesium sulphate, filtered, and evaporated to dryness. The resulting residue was distilled under vacuum to afford the corresponding sulfonamides.

N,N-dimethyl triflamide (DMT): Yield 85%. Analytical data are in agreement with those previously reported.[1] N,N-dipropyl triflamide (DPT) : Yield 88%. Analytical data are in agreement with those previously reported.[1] Synthesis of Li[Al(Ohfip)4] : [2] Due to air- and moisture sensitivity, all manipulations were undertaken using standard vacuum (better than 5x10-2 mbar in the vacuum line) and Schlenk techniques as well as in a glove box with an argon or nitrogen atmosphere (H2O and O2 < 1 ppm). All solvents were dried by conventional drying agents[3] and distilled afterwards. Their residual moisture content was found by Karl-Fischer titration to be about 5 ppm. Hexafluoroisopropanol was stirred over P4O10 at 60 °C and distilled afterwards (H2O content < 5 ppm). Purification of LiAlH4 Commercially available LiAlH4 was purified by extraction with Et2O in an extraction frit, which yields a white powder.[3] All traces of Et2O are removed by weighing the sample constant in vacuo (approx. 10-3 mbar) with heating to 80 °C (Caution: Do not heat higher! Use a water bath, since LiAlH4 explodes at about 120 °C). Usually the LiAlH4 cake has to be ground very finely inside a glove box and is then again exposed to vacuum prior to reaching a constant weight. Depending on the quality of the LiAlH4, up to 30% of the starting material are impurities (Al, Al oxides and Li3AlH6), which remain insoluble on the frit plate. Synthesis of Li[Al(OCH(CF3)2)4] In the glove box, 1 equivalent of purified, very finely ground LiAlH4 was weighed into a two-necked Schlenk vessel. After the addition of hexane (approx. 50 mL hexane per 1g LiAlH4), a gas cooler with

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the bubbler as well as a dropping funnel were connected to the Schlenk vessel. Due to the low boiling point of the alcohol, a reflux condenser connected to a cryostat and set to a temperature of -15 °C has to be used to avoid evaporation. 4.1 equivalents of hexafluoroisopropanol were then added dropwise to the LiAlH4 / hexane suspension. After all the alcohol was added, the reaction mixture was heated under reflux until the gas evolution ceased. To test if all LiAlH4 was consumed, a few drops of the suspension were dropped into a beaker containing water. The reaction is complete when no reaction occurs. The Schlenk vessel was then cooled down to -25 °C to allow complete precipitation of the product. The supernatant solvent was decanted off and the remaining solvent was then removed in vacuo (10-3 mbar) until constant weight of the sample was obtained (isolated yield: approx. 95 to 99%).

Pulsed-Gradient Stimulated Echo (PGSTE)-NMR: Spectra were recorded on a BRUKER AVANCEIII HD 300 spectrometer using a BBFO probehead with standard z-gradient. δ values of 1.1 up to 1.5 ms were used, while Δ ranged from 200 up to 1000 ms. Samples were measured under an argon atmosphere in a sealed NMR tube. During an experiment, the gradient was varied linearly from 2% to 100% of the preset gradient strength. Prior to a diffusion measurement, 90° pulses were determined. Measurements were recorded at 298 ± 1 K and 323 ± 1 K using a stimulated echo sequence.

Self-diffusion constants were extracted from 7Li (D+) and 19F (D-) spectra by using the integrals of the NMR signals and their fitting with the Stejskal–Tanner equation[4]

3exp

)0,,(

),,( 222

Dg

gI

gI

with I = signal intensity, = gradient pulse width, = diffusion time, g = gradient magnitude, D = self

diffusion constant and g: gyromagnetic ratio.

The limiting ideal conductivity ( NMR) of an electrolyte was calculated from the corresponding

conductivities of its constituents (for a 1:1 salt AnionCationNMR ). The latter were calculated from

the Nernst-Einstein equation:

Tk

cDq iiii

2

with qi = elementary charge, Di = self-diffusion constant, ci = concentration, k = the Boltzmann constant and T = temperature.

Finally, the ionicity (I) is calculated as the ratio between the experimentally determined conductivity

( exp) and NMR

.

NMR

I

exp

For I = 0 the electrolyte completely consists of ion pairs, for I = 1 the electrolyte is fully dissociated. Values in between gives a measure of the amount of ion pairs vs. dissociated species. E.g. for I = 0.50 50% of the electrolyte consists of ion pairs while the remaining species are dissociated ions. See Refs.[5] for a detailed account on the concept of ionicity and its application to ionic liquids.

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List of Supporting Figures and Tables :

Figure S1. Electrochemical data recorded at a scan rate of 0.1 mV/s on a stainless-steel electrode at 25 °C for the triflamide electrolytes. a) CV of DMT- LiAl(Ohfip)4; b) CV of DPT- LiAl(Ohfip)4; c) anodic voltage sweep for both DMT- LiAl(Ohfip)4 (red curve) and DPT-LiAl(Ohfip)4 (black curve) showing comparison of their oxidation onset potential.

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Figure S2. Discharge capacity (left axis) and coulombic efficiency (right axis) as a function of cycle number for Li-S cells with 0.2 M LiAl(Ohfip)4 salt in DMT (red markers) and DPT (black markers).

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a

Figure S3. Nyquist plots obtained for DOL/DME-1M LiTFSI (blue markers), DMT-0.2M LiAl(Ohfip)4 (red markers) and DPT- 0.2 M LiAl(Ohfip)4 ( black markers), a) before cycling and b) after cycling. The impedance at the electrode-electrolyte interface (approximated by the intercept of the semicircle at low frequency with the x-axis) is low for the DMT electrolyte before cycling, and is even decreased on cycling, whereas the DPT system exhibits the opposite behavior. This demonstrates the superior performance of the DMT over the DPT electrolyte (and DOL/DME), and confirms the DPT cell suffers from various factors, including poor wetting of the electrode by the electrolyte, and lower ionic conductivity.

b

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Figure S4. Comparison of the galvanostatic profiles for Li-S cells run at C/20 (blue curve), C/5 (red curve) at 50 °C with 0.2 M LiAl(Ohfip)4 in DMT.

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Figure S5. Photographs of separators soaked in DOL, obtained from cells that underwent 30 cycles at 50 °C at a current rate of C/20 (left) and C/5 (right) using the 0.2 M LiAl(Ohfip)4 electrolyte

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Figure S6. Galvanostatic intermittent titration technique (GITT) curves of a sulfur electrode discharging and charging in 0.2 M LiAl(Ohfip)4-DMT at a) 25 °C and b) 50 °C, obtained by alternating C/30 pulses (20 mins) with OCV periods (120 mins).

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Figure S7. Discharge capacity as a function of cycle number for Li-S cells with 0.2 M LiAl(Ohfip)4 salt in DMT at 25 °C (blue markers), 50 °C (black markers), 0.2 M LiTFSI salt in DMT at 25 °C (maroon markers) and 50 °C (red markers).

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Figure S8: a) Double walled cell for temperature dependent measurements equipped with an Inlab@710 conductivity electrode; Variation of ionic conductivities(b) and viscosities (c) of DMT with 0.2 M LiAl(Ohfip)4 salt at temperatures from 25 to 70 °C.

270 280 290 300 310 320 330 340 3500

1

2

3

4 Li[Al(Ohfip)

4] 0.2 M in DMT

Li[TFSI] 0.2 M in DMT

T [K]

[

mS

/cm

]

270 280 290 300 310 320 330 340 350

1

2

3

4

5

6 Li[Al(Ohfip)

4] 0.2 M in DMT

Li[TFSI] 0.2 M in DMT

T [K]

[

cP

]

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a

b

Figure S9. a) Rate capability data for the 0.2 M LiAl(Ohfip)4 system at 50 °C; b) discharge capacity (left axis) and coulombic efficiency (right axis) as a function of cycle number for Li-S cells tested at 50 °C at a rate of 2C.

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Li 1s

S 2p

F 2p

Al 2p

Figure S10. XPS analysis of a lithium electrode subjected to 5 cycles,stopped at charge and subsequent washing with THF and drying overnight. BG = background.

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Table S1: Physical properties of triflamide solvents

Solvent Boiling

point

( °C)

Density

(g/cm3)

Water

content

(ppm)

Ionic

conductivity

(0.2 M

LiAl(Ohfip)4 )

Viscosity

(0.2 M

LiAl(Ohfip)4 )

DMT 175 1.234 <1 1.25 mS/cm 5.98 cP

DPT 188 1.249 <1 0.24 mS/cm 9.65 cP

Table S1: Self diffusion coefficients (D+/-S, S = DMT) of the 0.2 M electrolyte formulations at different temperatures (T), along with the theoretical ideal conductivities (σNMR), experimental conductivities (σexp) and ionicities (I).

T [K]

D+∙10-9 [m2∙s-1]

D- ∙10-9 [m2∙s-1]

DDMT∙10-9 [m2∙s-1]

σNMR [mS∙cm-1]

σexp [mS∙cm-1]

I

LiTFSI 293 0.26 0.26 0.56 3.91 0.43 0.11

323 0.44 0.44 0.91 6.10 0.635 0.10

LiAl(Ohfip)4] 293 0.26 0.28 0.54 4.06 1.96 0.48

323 0.42 0.45 0.88 6.03 2.87 0.48

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References

[1] S.-T. Fu, S.-L. Liao, J. Nie and Z.-B. Zhou, J. Fluorine Chem. 2013, 147, 56-64. [2] I. Krossing and A. Reisinger, Coord. Chem. Rev 2006, 250, 2721-2744. [3] W. L. Armarego and C. L. L. Chai, Purification of laboratory chemicals, Butterworth-Heinemann, 2013. [4] E. O. Stejskal and J. E. Tanner, J. Chem. Phys. 1965, 42, 288-292. [5] a) K. Ueno, H. Tokuda and M. Watanabe, Phys. Chem. Chem. Phys. 2010, 12, 1649-1658; b) A. Rupp, N. Roznyatovskaya, H. Scherer, W. Beichel, P. Klose, C. Sturm, A. Hoffmann, J. Tübke, T. Koslowski and I. Krossing, Chem. - Eur. J. 2014, 20, 9794-9804.

Author Contributions

A.S carried out the synthesis of the solvents, the electrochemical experiments and conceived the study along with W.B, G.K.M, I.K and L.F.N. PK carried out the synthesis of the salt, measured the temperature-dependent viscosities and ionic conductivities under the supervision of WB. QP carried out the operando XRD studies and helped with the XPS measurements. HS and AH carried out the diffusion measurements and the non-standard processing of the data. AS, WB, GKM, IK and LFN wrote the paper.