Electronic Supplementary Information. Polysulfide Speciation and Electrolyte Interactions in Lithium-Sulfur Batteries with In Situ

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1 Electronic Supplementary Information Polysulfide Speciation and Electrolyte Interactions in Lithium-Sulfur Batteries with In Situ Infrared Spectroelectrochemistry Caitlin Dillard, Arvinder Singh and Vibha Kalra* Chemical and Biological Engineering Department, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA * The depth of penetration (dp) of the IR beam into the sample depends on several factors including wavelength of IR beam (λ), incident angle (θ), refractive index of ATR crystal (η1) and refractive index of the sample (η2) as given below: λ d p = 2π(n 2 1 sin 2 θ n 2 2) For diamond crystal, η1 is ~2.4 and angle of incident θ for our ATR FT-IR is 45⁰. We have analyzed two IR regions in FT-IR spectra near to 500 and 1000 cm -1. Our CNFs provide large macropores (pockets under IR observation) which mostly contains electrolyte (for most organic substances, refractive index (η2) is ~1.5) and sulfur (η2 ~1.0). Although, sample in contact with diamond crystal contains electrolyte (DME, DOL, LiCF3SO3, LiNO3), S-CNFs cathode, and in situ formed polysulfides, it is difficult to choose refractive index value. However, for approx. value, we used η2 ~1, which gives penetration depth between μm for 500 and 1000 cm -1. Due to the presence of electrolyte, some of the energy of evanescent wave will be absorbed which can compromise the penetration depth (<3 µm for the present case). However, in the present case, IR mainly probed electrolyte pockets (liquid), which provides good contact with the diamond crystal. Therefore, the path length < 3 µm was good enough to observe IR bands in the spectra of interest with good signal to noise ratio. S1

2 Figure S1 shows the spectra acquired during the charge process from 1.6 to 2.8 V at the slow scan rate of 0.05 mv s -1. The ion coordination state (Figure S1 a) showed an abrupt shift to 1050 cm -1 between 2.3 and 2.45 V corresponding to the single oxidation peak in CV. In addition, a slight polysulfide peak can be observed at 2.4 and 2.45 V, shown in Figure S1 b. During the charge process, the kinetics are slower and the solid reduction products react at the electrode-electrolyte interface rather than in the bulk of the electrolyte pockets, thus the polysulfide IR bands are weak. Figure S1. In situ FT-IR spectra showing the dynamic changes of the (a) ion coordination and (b) polysulfide regions during CV during the anodic (charge) potential sweep at 0.05 mv s -1 scan rate. The intervals between collected spectra are 50 mv. Figure S2. Example of FT-IR spectrum deconvolution of S-S band in the cm -1 range. The raw spectrum was taken at 2.8 V during a slow CV scan rate of 0.05 mv s -1. S2

3 As mentioned in the manuscript, we synthesized Li 2S8 (Li2S:S 1:7), Li 2S6 (Li 2S:S 1:5) and Li 2S2 (Li2S:S 1:1) reference solutions (concentration: 10 mmol) by adding sulfur and Li 2S in a stoichiometric ratio and subsequently stirring and heating at ~60 C for 96 h. The collected ATR FT-IR spectra for these reference solutions are shown in Figure S3. Figure S3. (a) and (b) FT-IR spectra for Li2S8 (1:7), Li2S6 (1:5) and Li2S2 (1:1) reference polysulfide solutions (c) a calibration curve showing linear fit for S-S peak positions collected for reference polysulfide solutions and observed in situ during Li-S operation. As we can see from the Figure S3 that S-S peak position (equilibrium) for Li 2S8 (Li 2S: S; 1:7) is centered at ~505.8 cm -1, which is shifted to ~497.5 cm -1 for Li 2S6 (Li2S: S; 1:5) solution. For Li 2S2 (Li2S: S; 1:1) solution, the equilibrium is further shifted to l ower wavenumber and a broad peak centered at ~472.5 cm -1 is appeared. Similar linear trends were observed by Saqib et al. 22 for the reference polysulfide solutions prepared in TEGDME and DMSO solvents. Further, we estimated S-S peak positions for various polysulfides from the linear fit of the observed data points (for ref. solutions). The comparison between S -S peak S3

4 positions observed in situ during Li-S cell operation (assigned peaks in the manuscript) and estimated values from the fitted data (ref. solutions) are given in the table below: Table S1: S-S peak positions for reference polysulfide solutions, obtained from calibration curve and observed in situ during Li-S cell operation for various polysulfide order (x values) Li 2S x x value Observed for prepared ref. solutions cm -1 ) Estimated from linear fitting of ref. solutions data (cm -1 ) Y = 5.65x Reported in the manuscript (cm -1 ) 2 ~ ~ ~ It is clear from these values that assigned S-S peak positions for various polysulfides are very close to those estimated from the calibration curve (linear fit of S-S values for ref. solutions). To quantify the relative concentrations of different polysulfide chain length species, each spectrum was deconvoluted by peak fitting analysis with Omnic software (Thermo Fisher Scientific) with Omnic Macros to fit seven Gaussian peaks in the cm -1 wavenumber range, shown in Figure S2. Prior to deconvolution, the spectra were first normalized to the 517 cm -1 peak and then divided by the blank electrolyte spectrum (a lso normalized to the 517 cm -1 peak), resulting in distinct peaks. The relative concentrations were calculated based on the diagnostic developed by Saqib et al. 22 As mentioned by Saqib et al, the molar concentration of polysulfide for a given polysulfide order is directly proportional to a rea (A) under the observed S-S peak for that polysulfide as given in equation below. 22 A C = ( ) a a 2 O + b 3 + b 3 2 We calculated area under the S-S peak for a given polysulfide order (for example 2 ; Li 2S2) at different potentials during discharge cycle of the Li-S cell starting from 2.8 V (fully charged state) to 1.6 V (full discharge state). These calculated areas at different potentials give us a trend showing how area under S-S peak for order-2 polysulfide varies when potential reaches S4

5 from 2.8 to 1.6 V. Since area under the curve is directly proportional to concentration for a given polysulfide order and electrolyte system, we can get same trend for concentration showing how concentration for order -2 polysulfide varies when potential reaches from 2.8 to 1.6 V. In order to calculate concentrations, we used parameters a 2, a3, b2, and b3 from Saqib s paper 22 for TEGDME solvent with a multiplication factor ~0.55. The linear fitting equation for our observed in situ S -S peak positions is Y (S-S peak position) = *X (polysulfide order) X = (Y/4.74) + ( /4.74) X = *Y + ( ) So, a1 and b1 for our in situ observed data are ~ and which are ~0.55 times of Saqib s a 1 and b1 parameters for TEGDME solvent. Our electrolyte was prepared in DME:DOL solvents which are very similar- DME/DOL falls within the range of many properties between DMSO and TEGDME. Therefore, we also used other parameters of TEGDME reported by Saqib et al. 22 with a multiplication factor of 0.55 to estimate concentrations. Moreover, we are reporting relative concentrations instead of absolute values. A slight deviation from the values of parameters used in our case of DME:DOL system would be minimized for relative concentrations and we can still understand the changes in concentration during cycling. Therefore, for concentrations calculation parameters were adapted from Saqib s TEGDME data 22 (multiplied by a factor of ~ 0.55) such tha t our values were (approximately): a1 = , b1 = , a2 = , b2 = , a3 = 0.564, and b3 = Figure S4. Observed FT-IR spectra for reference solutions. S5

6 To quantify the relative concentrations /areas of triflate coordination states, each spectrum was deconvoluted by peak fitti ng analysis with Origin Pro 8 software to fit three Gaussian peaks in the cm -1 wavenumber range, shown in Figure S5. For spectra with higher intensity peaks at higher wavenumbers, a new baseline was set before peak fitting to estimate the 1050 cm -1 peak more accurately. Figure S5. Deconvolution of FT-IR spectra in the cm -1 range at (a) 2.8V and (b) 2.35V at a scan rate of 0.05 mv s -1. Figure S6 shows the spectra collected during CV at 0.05 mv s -1 scan rate for a blank cell containing no sulfur. This cell contained every other component (pristine) CNF, electrolyte, separator, and lithium anode. The resulting spectra demonstrate there are no changes in the triflate anion coordination state (Figure S6 a) and no absorption bands appear in the 500 cm -1 polysulfide region (Figure S6 b). Figure S6. In situ FT-IR spectra showing no change (a) triflate anion coordination state or (b) polysulfide bands at various potentials during discharge for a blank cell with pure CNF as the cathode (no active sulfur). The intervals between collected spectra are 50 mv. The CV scan rate is 0.05 mv s -1. S6