A Rotating-Ring Disk Electrode Study
Supporting Information
Probing the Lithium-Sulfur Redox Reactions: A Rotating-Ring Disk Electrode Study Yi-Chun Lu †,‡,* Qi He † and Hubert A. Gasteiger† †
Institute of Technical Electrochemistry,
Technische Universität München, Lichtenbergstr. 4, 22 D-85748, Garching, Germany. ‡
Department of Mechanical and Automation Engineering, Shun Hing Institute of Advanced
Engineering, The Chinese University of Hong Kong, Shatin. N.T., Hong Kong SAR, China *E-mail: [email protected]
1
Supporting Experimental Details. Rotating-Ring Disk Electrode Measurement. The nonaqueous rotating-ring disk electrode configuration used in this study was adopted from that reported by Herranz et al.1 The working electrode consisted of a PTFE embedded glassy carbon disc of 5.0 mm in diameter surrounded by a gold ring with an internal diameter of 6.5 mm and an external diameter of 7.5 mm (Pine Research Instrumentation, Durham, NC). Prior to its use, the ring disc electrode was polished with a 0.05 μm alumina suspension (Buehler, Düsseldorf, Germany), cleaned by sonication in ultrapure water, screwed onto a PEEK shaft that was fed through a stopper equipped with a ceramic ball-bearing seal (Pine Research Instrumentation), and then dried for 1 h in an oven at 70 °C. The counter electrode was a platinum wire (99.99+%, Advent, Oxford, England) sealed with a glass fitting and immersed in a glass tube partially filled with electrolyte and terminated by a medium-porosity frit that prevented the diffusion of the species evolved at the Pt counter electrode into the main electrolyte compartment. The reference electrode consisted of a glass tube filled with 0.1 M AgNO3 (99.9999% metals basis, Sigma Aldrich) in acetonitrile (99.8% anhydrous, Sigma Aldrich; also predried over zeolites and containing ≤4 ppm H2O) and sealed with a Vycor 7930 frit (Advanced Glass & Ceramics, Holden, MA) at its tip; the silver wire was embedded in a plastic cap, which sealed the reference compartment against the ambient. The reference electrode was assembled in the glovebox at least 30 min before putting together the rest of the electrochemical cell; once ready, it was partially immersed into a beaker containing the electrolyte of interest, along with a piece of Li foil (99.9%, Chemetall, Frankfurt, Germany) connected to a Ni wire (99.98%, Advent). The potential difference between both electrodes was subsequently measured for a minimum of 60 min, and all potentials in this work are corrected using this potential difference (3.75 and 3.24 V in DMSO and DOL:DME, respectively) and are referred to as “volts vs Li/Li+” or VLi. 4 mM of S8 was dissolved in pure DMSO and DOL:DME and stirred for overnight. The RRDE working electrode, the Pt wire counter electrode, the reference electrode, and a glass bubbler allowing for direct flow of gas into the solution or blanketing atop the electrolyte were all assembled inside the glovebox. Once assembled, the electrochemical cell was taken outside of the glovebox, the working electrode rod was mounted onto the rotator, and the bubbler was connected to a gas line constantly fed with argon (6.0 quality, Westfalen-AG, Münster, Germany). This allowed for a constant overpressure inside the cell and prevented contamination from the atmosphere. All subsequent electrochemical measurements were performed using an AFCBP1 bipotentiostat (Pine Research Instrumentation) controlled with Aftermath software. Prior to 2
the RRDE measurements, ac impedance measurements to determine the Ohmic drop between working and reference electrodes were recorded with a VMP3 multichannel potentiostat (BioLogic, Grenoble, France), applying a 10 mV voltage perturbation (1 MHz to 100 mHz) at open circuit. In the resulting Nyquist plots, the high-frequency intersection with the real axis was taken as the uncompensated solution resistance; the values obtained for 0.2 M LiClO4 in DMSO and 1.0 M LiTFSI in DOL:DME were 185 and 67 Ω, respectively. Measurements of Diffusion Coefficient of Polysulfide. Diffusion coefficient of S8, S82-, and “S84-“ in DMSO and DOL:DME are determined using a method established previously for oxygen and oxygen radicals.1 Briefly, the diffusion coefficient of S8 and polysulfides are determined by fitting a relation between the inverse of the rotation speed and the transient time in potential-stepping experiments described below. Note that in all potential-stepping experiments, the disk potential was stepped from a defined potential that is close to OCV (no reaction happening) but not too far away from where it is stepping to (i.e., -0.8 VAg for DMSO and 0 VAg for DOL:DME). In DMSO: 1. The diffusion coefficient of S8 is measured with a constant ring voltage that consumes S8 on the ring (i.e., -1.5 VAg) and a stepping disk voltage that consumes S8 on the disk (i.e., 1.5 VAg). As soon as the steeping disk voltage is applied, the ring will sense a sudden drop of S8 concentration due to S-reduction at the disk. 2. The diffusion coefficient of S82- is measured with a stepping disk voltage that generating S82- (i.e., -1.5 VAg) and a constant ring voltage that oxidizes S82- generated by the disk (i.e., 0.5 VAg). As soon as the steeping disk voltage is applied, the ring will oxidize the S82- sent from the disk. 3. The diffusion coefficient of “S84-“ is measured with a stepping disk voltage that generating “S84-“ (i.e., -2.25 VAg) and a constant ring voltage that oxidizes “S84-“ generated by the disk (i.e., 0.5 VAg). As soon as the steeping disk voltage is applied, the ring will oxidize the “S84“sent from the disk. In DOL:DME: 1. The diffusion coefficient of S8 is measured with a constant ring voltage that consumes S8 on the ring (i.e., -1.3 VAg) and a stepping disk voltage that consumes S8 on the disk (i.e., 1.3 VAg). As soon as the steeping disk voltage is applied, the ring will sense a sudden drop of S8 concentration due to S-reduction at the disk. 2. The diffusion coefficient of polysulfide is measured with a stepping disk voltage that generating “S84-“ (i.e., -1.8 VAg) and a constant ring voltage that oxidizes “S84-“ generated by 3
the disk (i.e., 0.6 VAg). As soon as the steeping disk voltage is applied, the ring will oxidize the “S84-“sent from the disk. The relationship between the rotation speed and the transient time can be described as Ts = K(/D)1/3-1 where K is a proportionality constant depending on the RRDE’s geometry: K = 43.1[log(r2/r1)]2/3 (for Ts reported in seconds and in rpm).2 For the RRDE used here, r1 = 2.5 mm, r2 = 3.25 mm, the theoretical K is 10.1 rpm-s. We used optical microscopy to measure the actual dimensions and K is determined to be 10.34 (r1 = 2.485 mm, r2 = 3.257 mm). is viscosity and is rotation speed. The transient times measured and the diffusion coefficient calculated from the fitted slope (i.e., (K(/D)1/3, the fitted lines were forced to pass through the origin of the axes (0,0)) are listed in Table S2. The relation between the inverse of the rotation speed and the transient time (Ts) in potential-stepping experiments obtained for S8 and polysulfides performed in DMSO (circle) or DOL:DME (square) is shown in Figure S5. Preparation of Dissolved Polysulfide. A polysulfide sample with nominal composition “Li2S8” is prepared following the method developed by Rauh et al.3 Briefly, lithium sulphide (Sigma Aldrich, 99.98 %, stored in an Ar-glovebox upon received.) and sulfur were mixed with magnetic stirring in various solvents described in Fig. S6 following: Li2S + (n-1)/8S8 Li2S8 where n = 8 in an Ar-filled glovebox to yield Li2S8 concentration between 0.05 mM to 0.5 mM.
4
Supporting Figures.
Figure S1. CVs of GC electrode in (a) 4 mM S8-0.2 M LiClO4 DMSO and (b) 4 mM S8-1.0 M LiTFSI DOL:DME(1:1) at 50 mV/s.
2
Current Density (mA/cm )
1.0
DOL:DME
0.5
0.0
DMSO
DMSO DOL:DME
-0.5
Ec,p -0.0360 -0.0058 Ea,p 0.0489 0.0925 1/2(Ec+Ea) 0.0065 0.0434 difference 0.037
-1.0 -0.4
-0.2
0.0
0.2
0.4
0.6
+
Potential / V vs. Ag/Ag
Figure S2. CVs of GC electrode in 5 mM ferrocene, 4 mM S8-0.2 M LiClO4 in DMSO (blue) and 5 mM ferrocene, 4 mM S8-1.0 M LiTFSI in DOL:DME(1:1) at 50 mV/s. The redox potential (i.e., 1/2*(Ec+Ea)) is 6mV for DMSO and 43mV in DOL:DME.
5
300
(a)
300
Ering = 0.6 VAg
250 Ring current (A)
Ring current (A)
250
Ering = 0.5 VAg
200 150 100
(b)
Ering = 3.84 VLi Ering = 4.25 VLi
200 150 100 50
50 0 0
DOL:DME
DOL:DME
100 rpm
DMSO
-1
2
Disk current (mA/cmdisk)
-1
0
DMSO
2
Disk current (mA/cmdisk)
0
-2
400 rpm
-3 900 rpm
-4 -5
1600 rpm
50 mV/s
-6 -2.5
-2.0
-1.5
-1.0
-0.5
-2 100 rpm -3 400 rpm -4 900 rpm -5 -6
0.0
1600 rpm
1.5
2.0
2.5
3.0
3.5
+
+
Potential vs Li/Li (VLi/Li )
Potential vs Ag/Ag (VAg/Ag )
+
+
Figure S3. (a) Potential versus Ag/Ag+; (b) Potential versus Li/Li+ Potential versus Capacitively, non-Ohmically corrected ring disk currents recorded at 50 mV/s in Ar saturated 4 mM S8 – 0.2 M LiClO4 DMSO (Blue) and 4 mM S8 – 1.0 M LiTFSI DOL:DME(1:1) (Red) at rotation rates between 100 and 1600 rpm and continuously holding the Au ring electrode at 0.5 VAg for DMSO and at 0.6 VAg for DOL:DME(1:1).
6
Figure S4. Levich−Koutecky plots derived from the negative-going scan disc current values various potentials (specified in the parentheses VAg and VLi) in (a) DMSO and (b) DOL:DME. The numbers in the parentheses denote the fitted LK slopes.
4-
-6 Diffusion coefficient (x10 cm/s) "S8,DOL:DME"(0.86)
1.2 1.0
S8,DOL:DME (2.6) 4-
"S8, DMSO"(4.2)
Ts (s)
0.8
2-
S8,DMSO (4.4) S8,DMSO (6.5)
0.6 0.4 0.2 0.001
0.002
0.003
(rpm
0.004
0.005
-1
)
Figure S5. Relation between the inverse of the rotation speed and the transient time (Ts) in potential-stepping experiments obtained for S8 and polysulfides performed in DMSO (circle) or DOL:DME (square). The values represented the fitted diffusion coefficient. Note that the fitted lines were forced to pass through the origin of the axes (0,0).
7
Figure S6. Images of chemically made “Li2S8” dissolved in DOL (dielectric constant = 7.13), THF (7.39), DME (7.075), CAN (35.95), DMSO (46.5), DMF (36.71) and N,Ndimethylacetamide (DMA, 37.78). Dielectric constant of solvent is quoted from Ref.4.
Figure S7. Capacitively, non-Ohmically corrected disk currents recorded at 50 mV/s in Ar saturated 4 mM S8 – 1.0 M LiTFSI DOL:DME (Red) and 4 mM S8 – 1.0 M TBATFSI DOL:DME (Blue) at rotation rates between 100 and 900 rpm.
8
Table S1. Solvent properties, calculated diffusion-limiting current and calculated Levich Koutecky (LK) slopes assuming 2e-/S8. The formula used to calculate the LK slope is 1/(1000*0.62*n*96485*(DS8)0.666*(-1/6)*CS8).
DMSO [cps] #1
2.2
DS8[cm2/s] #2
6.5E‐6
4) 3 2 3 2 [g/cm ] v [cm /s] c S8 [mol/cm ] i100rpm,2e [mA/cm ] LK slope 2e-
#1
0.02
1.10
4.00E‐06
1.04
3.10
DOL:DME [cps] #1
1.4
DS8[cm2/s] #2
2.6E‐6
4) 3 2 3 2 [g/cm ] v [cm /s] c S8 [mol/cm ] i100rpm,2e [mA/cm ] LK slope 2e-
#1
0.013
1.10
4.00E‐06
0.61
5.29
Note: #1: Measured in this study. #2: Fitted diffusion coefficient via RRDE-steeping experiment in this study (Table S2)
Table S2. Transient time measured using potential-stepping experiment. The fitted slope in Fig. S5 (K(/D)1/3is used to calculate diffusion coefficients. Rotation Speed (rpm)
Transient time (s)
Transient time (s)
DMSO 200 300 400 600 900 Diffusion coeficient
S8 0.74 0.50 0.38 0.27 0.20
S82‐ 0.84 0.57 0.43 0.29 0.22
"S84‐" 0.86 0.57 0.45 0.31 0.21
6.5
4.4
4.2
DOL:DME (1:1) S8 "S84‐" 0.84 1.21 0.59 0.85 0.47 0.67 0.33 0.47 0.24 0.33 2.6
0.86
(x10‐6 cm/s)
Supporting Information References. 1. Herranz, J.; Garsuch, A.; Gasteiger, H. A. Using Rotating Ring Disc Electrode Voltammetry to Quantify the Superoxide Radical Stability of Aprotic Li-Air Battery Electrolytes. J. Phys. Chem. C 2012, 116 (36), 19084-19094. 2. Gan, F.; Chin, D. T. Determination of diffusivity and solubility of oxygen in phosphoric acid using a transit time on a rotating ring-disc electrode. J Appl. Electrochem. 1993, 23 (5), 452-455. 3. Rauh, R. D.; Shuker, F. S.; Marston, J. M.; Brummer, S. B. Formation of lithium polysulfides in aprotic media. J. Inorg. Nucl. Chem. 1977, 39 (10), 1761-1766. 4. Aurbach, D.; Weissman, I. Nonaqueous Electrochemistry: An Overview. In Nonaqueous Electrochemistry, Aurbach, D., Ed. CRC Press: 1999.
9
Probing the Lithium-Sulfur Redox Reactions: A Rotating-Ring Disk Electrode Study Yi-Chun Lu †,‡,* Qi He † and Hubert A. Gasteiger† †
Institute of Technical Electrochemistry,
Technische Universität München, Lichtenbergstr. 4, 22 D-85748, Garching, Germany. ‡
Department of Mechanical and Automation Engineering, Shun Hing Institute of Advanced
Engineering, The Chinese University of Hong Kong, Shatin. N.T., Hong Kong SAR, China *E-mail: [email protected]
1
Supporting Experimental Details. Rotating-Ring Disk Electrode Measurement. The nonaqueous rotating-ring disk electrode configuration used in this study was adopted from that reported by Herranz et al.1 The working electrode consisted of a PTFE embedded glassy carbon disc of 5.0 mm in diameter surrounded by a gold ring with an internal diameter of 6.5 mm and an external diameter of 7.5 mm (Pine Research Instrumentation, Durham, NC). Prior to its use, the ring disc electrode was polished with a 0.05 μm alumina suspension (Buehler, Düsseldorf, Germany), cleaned by sonication in ultrapure water, screwed onto a PEEK shaft that was fed through a stopper equipped with a ceramic ball-bearing seal (Pine Research Instrumentation), and then dried for 1 h in an oven at 70 °C. The counter electrode was a platinum wire (99.99+%, Advent, Oxford, England) sealed with a glass fitting and immersed in a glass tube partially filled with electrolyte and terminated by a medium-porosity frit that prevented the diffusion of the species evolved at the Pt counter electrode into the main electrolyte compartment. The reference electrode consisted of a glass tube filled with 0.1 M AgNO3 (99.9999% metals basis, Sigma Aldrich) in acetonitrile (99.8% anhydrous, Sigma Aldrich; also predried over zeolites and containing ≤4 ppm H2O) and sealed with a Vycor 7930 frit (Advanced Glass & Ceramics, Holden, MA) at its tip; the silver wire was embedded in a plastic cap, which sealed the reference compartment against the ambient. The reference electrode was assembled in the glovebox at least 30 min before putting together the rest of the electrochemical cell; once ready, it was partially immersed into a beaker containing the electrolyte of interest, along with a piece of Li foil (99.9%, Chemetall, Frankfurt, Germany) connected to a Ni wire (99.98%, Advent). The potential difference between both electrodes was subsequently measured for a minimum of 60 min, and all potentials in this work are corrected using this potential difference (3.75 and 3.24 V in DMSO and DOL:DME, respectively) and are referred to as “volts vs Li/Li+” or VLi. 4 mM of S8 was dissolved in pure DMSO and DOL:DME and stirred for overnight. The RRDE working electrode, the Pt wire counter electrode, the reference electrode, and a glass bubbler allowing for direct flow of gas into the solution or blanketing atop the electrolyte were all assembled inside the glovebox. Once assembled, the electrochemical cell was taken outside of the glovebox, the working electrode rod was mounted onto the rotator, and the bubbler was connected to a gas line constantly fed with argon (6.0 quality, Westfalen-AG, Münster, Germany). This allowed for a constant overpressure inside the cell and prevented contamination from the atmosphere. All subsequent electrochemical measurements were performed using an AFCBP1 bipotentiostat (Pine Research Instrumentation) controlled with Aftermath software. Prior to 2
the RRDE measurements, ac impedance measurements to determine the Ohmic drop between working and reference electrodes were recorded with a VMP3 multichannel potentiostat (BioLogic, Grenoble, France), applying a 10 mV voltage perturbation (1 MHz to 100 mHz) at open circuit. In the resulting Nyquist plots, the high-frequency intersection with the real axis was taken as the uncompensated solution resistance; the values obtained for 0.2 M LiClO4 in DMSO and 1.0 M LiTFSI in DOL:DME were 185 and 67 Ω, respectively. Measurements of Diffusion Coefficient of Polysulfide. Diffusion coefficient of S8, S82-, and “S84-“ in DMSO and DOL:DME are determined using a method established previously for oxygen and oxygen radicals.1 Briefly, the diffusion coefficient of S8 and polysulfides are determined by fitting a relation between the inverse of the rotation speed and the transient time in potential-stepping experiments described below. Note that in all potential-stepping experiments, the disk potential was stepped from a defined potential that is close to OCV (no reaction happening) but not too far away from where it is stepping to (i.e., -0.8 VAg for DMSO and 0 VAg for DOL:DME). In DMSO: 1. The diffusion coefficient of S8 is measured with a constant ring voltage that consumes S8 on the ring (i.e., -1.5 VAg) and a stepping disk voltage that consumes S8 on the disk (i.e., 1.5 VAg). As soon as the steeping disk voltage is applied, the ring will sense a sudden drop of S8 concentration due to S-reduction at the disk. 2. The diffusion coefficient of S82- is measured with a stepping disk voltage that generating S82- (i.e., -1.5 VAg) and a constant ring voltage that oxidizes S82- generated by the disk (i.e., 0.5 VAg). As soon as the steeping disk voltage is applied, the ring will oxidize the S82- sent from the disk. 3. The diffusion coefficient of “S84-“ is measured with a stepping disk voltage that generating “S84-“ (i.e., -2.25 VAg) and a constant ring voltage that oxidizes “S84-“ generated by the disk (i.e., 0.5 VAg). As soon as the steeping disk voltage is applied, the ring will oxidize the “S84“sent from the disk. In DOL:DME: 1. The diffusion coefficient of S8 is measured with a constant ring voltage that consumes S8 on the ring (i.e., -1.3 VAg) and a stepping disk voltage that consumes S8 on the disk (i.e., 1.3 VAg). As soon as the steeping disk voltage is applied, the ring will sense a sudden drop of S8 concentration due to S-reduction at the disk. 2. The diffusion coefficient of polysulfide is measured with a stepping disk voltage that generating “S84-“ (i.e., -1.8 VAg) and a constant ring voltage that oxidizes “S84-“ generated by 3
the disk (i.e., 0.6 VAg). As soon as the steeping disk voltage is applied, the ring will oxidize the “S84-“sent from the disk. The relationship between the rotation speed and the transient time can be described as Ts = K(/D)1/3-1 where K is a proportionality constant depending on the RRDE’s geometry: K = 43.1[log(r2/r1)]2/3 (for Ts reported in seconds and in rpm).2 For the RRDE used here, r1 = 2.5 mm, r2 = 3.25 mm, the theoretical K is 10.1 rpm-s. We used optical microscopy to measure the actual dimensions and K is determined to be 10.34 (r1 = 2.485 mm, r2 = 3.257 mm). is viscosity and is rotation speed. The transient times measured and the diffusion coefficient calculated from the fitted slope (i.e., (K(/D)1/3, the fitted lines were forced to pass through the origin of the axes (0,0)) are listed in Table S2. The relation between the inverse of the rotation speed and the transient time (Ts) in potential-stepping experiments obtained for S8 and polysulfides performed in DMSO (circle) or DOL:DME (square) is shown in Figure S5. Preparation of Dissolved Polysulfide. A polysulfide sample with nominal composition “Li2S8” is prepared following the method developed by Rauh et al.3 Briefly, lithium sulphide (Sigma Aldrich, 99.98 %, stored in an Ar-glovebox upon received.) and sulfur were mixed with magnetic stirring in various solvents described in Fig. S6 following: Li2S + (n-1)/8S8 Li2S8 where n = 8 in an Ar-filled glovebox to yield Li2S8 concentration between 0.05 mM to 0.5 mM.
4
Supporting Figures.
Figure S1. CVs of GC electrode in (a) 4 mM S8-0.2 M LiClO4 DMSO and (b) 4 mM S8-1.0 M LiTFSI DOL:DME(1:1) at 50 mV/s.
2
Current Density (mA/cm )
1.0
DOL:DME
0.5
0.0
DMSO
DMSO DOL:DME
-0.5
Ec,p -0.0360 -0.0058 Ea,p 0.0489 0.0925 1/2(Ec+Ea) 0.0065 0.0434 difference 0.037
-1.0 -0.4
-0.2
0.0
0.2
0.4
0.6
+
Potential / V vs. Ag/Ag
Figure S2. CVs of GC electrode in 5 mM ferrocene, 4 mM S8-0.2 M LiClO4 in DMSO (blue) and 5 mM ferrocene, 4 mM S8-1.0 M LiTFSI in DOL:DME(1:1) at 50 mV/s. The redox potential (i.e., 1/2*(Ec+Ea)) is 6mV for DMSO and 43mV in DOL:DME.
5
300
(a)
300
Ering = 0.6 VAg
250 Ring current (A)
Ring current (A)
250
Ering = 0.5 VAg
200 150 100
(b)
Ering = 3.84 VLi Ering = 4.25 VLi
200 150 100 50
50 0 0
DOL:DME
DOL:DME
100 rpm
DMSO
-1
2
Disk current (mA/cmdisk)
-1
0
DMSO
2
Disk current (mA/cmdisk)
0
-2
400 rpm
-3 900 rpm
-4 -5
1600 rpm
50 mV/s
-6 -2.5
-2.0
-1.5
-1.0
-0.5
-2 100 rpm -3 400 rpm -4 900 rpm -5 -6
0.0
1600 rpm
1.5
2.0
2.5
3.0
3.5
+
+
Potential vs Li/Li (VLi/Li )
Potential vs Ag/Ag (VAg/Ag )
+
+
Figure S3. (a) Potential versus Ag/Ag+; (b) Potential versus Li/Li+ Potential versus Capacitively, non-Ohmically corrected ring disk currents recorded at 50 mV/s in Ar saturated 4 mM S8 – 0.2 M LiClO4 DMSO (Blue) and 4 mM S8 – 1.0 M LiTFSI DOL:DME(1:1) (Red) at rotation rates between 100 and 1600 rpm and continuously holding the Au ring electrode at 0.5 VAg for DMSO and at 0.6 VAg for DOL:DME(1:1).
6
Figure S4. Levich−Koutecky plots derived from the negative-going scan disc current values various potentials (specified in the parentheses VAg and VLi) in (a) DMSO and (b) DOL:DME. The numbers in the parentheses denote the fitted LK slopes.
4-
-6 Diffusion coefficient (x10 cm/s) "S8,DOL:DME"(0.86)
1.2 1.0
S8,DOL:DME (2.6) 4-
"S8, DMSO"(4.2)
Ts (s)
0.8
2-
S8,DMSO (4.4) S8,DMSO (6.5)
0.6 0.4 0.2 0.001
0.002
0.003
(rpm
0.004
0.005
-1
)
Figure S5. Relation between the inverse of the rotation speed and the transient time (Ts) in potential-stepping experiments obtained for S8 and polysulfides performed in DMSO (circle) or DOL:DME (square). The values represented the fitted diffusion coefficient. Note that the fitted lines were forced to pass through the origin of the axes (0,0).
7
Figure S6. Images of chemically made “Li2S8” dissolved in DOL (dielectric constant = 7.13), THF (7.39), DME (7.075), CAN (35.95), DMSO (46.5), DMF (36.71) and N,Ndimethylacetamide (DMA, 37.78). Dielectric constant of solvent is quoted from Ref.4.
Figure S7. Capacitively, non-Ohmically corrected disk currents recorded at 50 mV/s in Ar saturated 4 mM S8 – 1.0 M LiTFSI DOL:DME (Red) and 4 mM S8 – 1.0 M TBATFSI DOL:DME (Blue) at rotation rates between 100 and 900 rpm.
8
Table S1. Solvent properties, calculated diffusion-limiting current and calculated Levich Koutecky (LK) slopes assuming 2e-/S8. The formula used to calculate the LK slope is 1/(1000*0.62*n*96485*(DS8)0.666*(-1/6)*CS8).
DMSO [cps] #1
2.2
DS8[cm2/s] #2
6.5E‐6
4) 3 2 3 2 [g/cm ] v [cm /s] c S8 [mol/cm ] i100rpm,2e [mA/cm ] LK slope 2e-
#1
0.02
1.10
4.00E‐06
1.04
3.10
DOL:DME [cps] #1
1.4
DS8[cm2/s] #2
2.6E‐6
4) 3 2 3 2 [g/cm ] v [cm /s] c S8 [mol/cm ] i100rpm,2e [mA/cm ] LK slope 2e-
#1
0.013
1.10
4.00E‐06
0.61
5.29
Note: #1: Measured in this study. #2: Fitted diffusion coefficient via RRDE-steeping experiment in this study (Table S2)
Table S2. Transient time measured using potential-stepping experiment. The fitted slope in Fig. S5 (K(/D)1/3is used to calculate diffusion coefficients. Rotation Speed (rpm)
Transient time (s)
Transient time (s)
DMSO 200 300 400 600 900 Diffusion coeficient
S8 0.74 0.50 0.38 0.27 0.20
S82‐ 0.84 0.57 0.43 0.29 0.22
"S84‐" 0.86 0.57 0.45 0.31 0.21
6.5
4.4
4.2
DOL:DME (1:1) S8 "S84‐" 0.84 1.21 0.59 0.85 0.47 0.67 0.33 0.47 0.24 0.33 2.6
0.86
(x10‐6 cm/s)
Supporting Information References. 1. Herranz, J.; Garsuch, A.; Gasteiger, H. A. Using Rotating Ring Disc Electrode Voltammetry to Quantify the Superoxide Radical Stability of Aprotic Li-Air Battery Electrolytes. J. Phys. Chem. C 2012, 116 (36), 19084-19094. 2. Gan, F.; Chin, D. T. Determination of diffusivity and solubility of oxygen in phosphoric acid using a transit time on a rotating ring-disc electrode. J Appl. Electrochem. 1993, 23 (5), 452-455. 3. Rauh, R. D.; Shuker, F. S.; Marston, J. M.; Brummer, S. B. Formation of lithium polysulfides in aprotic media. J. Inorg. Nucl. Chem. 1977, 39 (10), 1761-1766. 4. Aurbach, D.; Weissman, I. Nonaqueous Electrochemistry: An Overview. In Nonaqueous Electrochemistry, Aurbach, D., Ed. CRC Press: 1999.
9
