Mechanism for the Stable Performance of Sulfur-Copolymer Cathode in Lithium Sulfur Battery Studied by Solid-State NMR Spectroscopy

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1 Mechanism for the Stable Performance of Sulfur-Copolymer Cathode in Lithium Sulfur Battery Studied by Solid-State NMR Spectroscopy Alexander Hoefling, Dan Thien Nguyen, Pouya Partovi-Azar, Daniel Sebastiani, Patrick Theato, Seung- Wan Song *, and Young Joo Lee * Table of Contents 1. Deconvolution of 13 C{ 1 H} CP MAS NMR spectra of poly(s-co-dib) copolymers and T 1 ( 13 C) relaxation time 2. Calculation of the 13 C NMR chemical shifts of C q carbons in systems bound to two polysulfide chains 3. Stability of poly(s-co-dib) cathode materials in electrolyte 4. Optimizing critical factors affecting the cycling ability of poly(s-co-dib) cathodes 5. Deconvolution of 13 C{ 1 H} CP MAS NMR spectra of S-DIB-50 and S-DIB-10 cathodes at various SOD C{ 1 H} CP MAS NMR spectra of the discharged electrodes after washing 7. Surface characterization of the cycled S-DIB10-CF cathode: ex situ X-ray photoelectron spectroscopy (XPS) 1

2 1. Deconvolution of 13 C{ 1 H} CP MAS NMR spectra of poly(s-co-dib) copolymers and T 1 ( 13 C) relaxation time The 13 C spin lattice relaxation time, T 1( 13 C), provides valuable information on the molecular structure and motion, and thus, helps distinguishing between different species. Therefore, T 1 relaxation time of the individual carbon species was measured utilizing CP-T1 pulse sequence under MAS. 1 The signals assigned to the carbons linked to long-chain polysulfides exhibit longer T 1( 13 C) values than those bound to short-chain sulfides (For S-DIB-30: T 1( 13 C) was 17 and 9 s for the signals at 59 and 55 ppm, respectively, and 60 and 34 s for the signals at 140 and 144 ppm, respectively), indicating that the long-chain polysulfide linkages provide lower segmental motion than the linkages of short-chain sulfides. This reduction in chain mobility as the polysulfide chain lengthens has been attributed to the delocalization of unpaired electron spin density on sulfur atoms via empty d orbitals, which leads to an increased π characters of the S S bonds. 2 Figure S1. Deconvolution of 13 C{ 1 H} CP MAS NMR spectra of poly(s-co-dib) copolymers with varying ratio of DIB vs S shown in Figure 1. 2

3 2. Calculation of the 13 C NMR chemical shifts of C q carbons in systems bound to two polysulfide chains In order to have a more realistic model of the poly(s-co-dib) copolymers, we have calculated the 13 C NMR chemical shifts of quaternary carbons of the same molecule with an additional polysulfide chain of the same length attached, i.e. phenyl C q(ch 3)(CH 2 S x CH 3) S x CH 3 (Figure S2a). Both polysulfide chains are assumed to be fully extended. This extended structure is expected to resemble the microstructure of poly(s-co-dib) copolymers more closely because real poly(s-co-dib) copolymers contain macrocycles where polysulfide chains are branched with DIB units and, as a result, the degree of freedom and mobility of polysulfide chains are greatly reduced. The computed chemical shifts of C q carbons as a function of polysulfide chain length in these systems are shown in Figure S2b. The chemical shifts are referenced internally with respect to that of the system possessing two sulfide chains of length one, at 0 ppm. Similar to the case of the molecules with a single polysulfide chain, the 13 C NMR chemical shifts have positive values, which increase gradually as the number of sulfur atoms in the polysulfide chain increases and converge to about 8 ppm when x > 4. Figure S2. (a) Optimized structure of phenyl C q(ch 3)(CH 2 S x CH 3) S x CH 3 connected to two polysulfide chains of length 4. The sulfur atoms in the polysulfide chains (S x) are represented as yellow spheres; the polysulfide chains are terminated with a methyl group for the simplicity. (b) Computed isotropic 13 C NMR chemical shifts for quaternary carbon (C q) as a function of the lengths of the polysulfide chains. The chemical shifts are referenced internally to the system with the shortest sulfide chains of length one for both linkages. 3. Stability of poly(s-co-dib) cathode materials in electrolyte In order to test the chemical stability of poly(s-co-dib) cathode active materials in the electrolyte, we immersed 10 mg of S-DIB-10 and S- DIB-50 reddish powders in 600 µl of liquid electrolyte composed of 2M LiTFSI M LiNO 3/DME:DOL (1:1 volume ratio) for hours, which amounted to the E/S ratio (i.e., the ratio of electrolyte volume to the weight of sulfur) of 60 µl mg 1 (E/S). We then took the photos reproduced in Figure S3. While no dissolution of S-DIB-10 occurs, S-DIB-50 is partly dissolved in the electrolyte, resulting in a color change of the electrolyte to yellow. This phenomenon demonstrates the chemical stability of S-DIB-10 cathode material in ether-based electrolytes. Figure S3. Photos for the chemical stability of S-DIB-10 and S-DIB-50 cathode active materials in the electrolyte of 2M LiTFSI+0.31M LiNO 3/DOL:DME. 3

4 4. Optimizing critical factors affecting the cycling ability of poly(s-co-dib) cathodes Lithium salt concentration in the electrolyte, the ratio of electrolyte to sulfur, and the type and amount of binder materials and carbon conductor used for cathode preparation have been recognized to be altogether critical factors affecting the cycling ability of Li S cells with elemental sulfur cathodes. We, therefore, carried out the optimization of these factors. First, in order to optimize the binder materials for the preparation of S-DIB-10 cathodes, we tested polyvinylidene difluoride (PVdF, MW 534,000, Aldrich) in N-methyl-2-pyrrolidone (NMP, Aldrich) solvent as non-aqueous binder, and a mixture of sodium carboxymethyl cellulose (CMC, MW 250,000, Aldrich) and polyacrylic acid (PAA, MW 450,000, Aldrich) in water, as aqueous binder 3. The cathode compositions were fixed to 70 wt% active material, 25 wt% carbon black and 5 wt% binder. Since the polysulfide chains of rubbery poly(sco-dib) are chemically bound to the DIB units of the polymeric framework, neither dissolution nor side reaction with water occurred during slurry preparation and cathode coating, when using the CMC-PAA aqueous binder. Binder-dependent cycling ability of the poly(s-co-dib) cathodes in lithium cells is depicted in Figure S4a. Cathodes prepared with the CMC-PAA binder exhibited superior cycling ability to those prepared with PVdF. CMC-PAA was thus designated as the optimal binder for the preparation of the poly(s-co-dib) cathodes. Second, various concentrations of the LiTFSI salt (from 0.38M to 2M) in the electrolyte were tested to determine the best salt concentration for improved performance. It has been suggested that the use of concentrated lithium salt can be effective in lowering the solubility of polysulfides by the so-called common ion effect. Furthermore, high salt concentrations can enhance ionic conductivity, despite an increase in viscosity. 4 Salt concentration-dependent cycling ability data on S-DIB-10 cathodes are compared in Figure S4b. Capacity values vary in the following order: 2M >0.38M >1M LiTFSI. Capacity retention at the 20th cycle follows the same order: 2M (77%) >0.38M (75%) >1M (74%). Thus, the use of 2M LiTFSI is found to offer improved cycling ability, compared to other concentrations. No noticeable improvement was observed at concentration higher than 2M (data not shown). A similar trend was observed for S-DIB-50 cathodes (data not shown). Third, various ratios of electrolyte volume to sulfur weight (E/S; from 20 to 100 µl mg 1 in a 2032 coin cell) were tested for S-DIB-10 cathodes. The resulting cycling abilities are reported for comparison in Figure S4c. Upon increasing the E/S ratio, the cycling ability fades more rapidly. Thus, the E/S ratio was first roughly optimized to near 20 µl mg 1. After further detailed optimization, we determined an E/S ratio of 25 µl mg 1 as the optimized ratio and used it for testing the cycling performance of Li//poly(S-co-DIB) coin cells as displayed in Figure 3c f. Fourth, the content and type of carbon were optimized (Figure S4d). Initially, the poly(s-co-dib) cathodes, S-DIB-50 and S-DIB-10, were prepared with a slurry containing S-DIB-x active material (70 wt%), carbon black (CB; Super-P, 20 wt%) and binder (10 wt%). As shown in Figure 3 and Figure S5, good cycling performances were obtained for both cathodes. The discharge curve of the S-DIB-50 cathode (Figure 3c) exhibits a negligible plateau at 2.3 V and a long plateau at 2.1 V. The initial discharge and charge capacities of S- DIB-50 are 758 and 720 ma h g 1, respectively, corresponding to an initial coulombic efficiency of 105%. During 50 cycles, capacity retention of 65% and coulombic efficiencies in the % range were delivered, while the plateau at 2.1 V was significantly shortened. S-DIB-10 cathode (Figure 3d) yields a remarkable increase in the initial discharge and charge capacities to 953 and 958 ma h g 1, respectively, with high initial coulombic efficiencies to 99.5%. A stable cycling pattern was attained after the 10th cycle, whereby a capacity retention of 73% at the 50th cycle and a high coulombic efficiency of >99% were measured. However, in the experiment conducted with S-DIB-10 as cathode, an initial discharge plateau appeared at lower voltage than the counterparts observed using elemental sulfur or S-DIB-50. Polarization (i.e., the voltage gap between main discharge and charge plateaus), possibly caused by the low conductivity of the S-enriched S-DIB-10 cathode and the high fraction of insulating Li 2S formed during cycling, seems to be the origin of the lower discharge voltage. Interestingly, the polarization is more prominent at the discharge plateau characterized by a lower voltage ( V), which is related to the production of short-chain sulfides. This observation implies a slowing down of the production of shortchain sulfides and Li 2S in the case of S-DIB-10 cathode. This polarization is reduced with prolonged cycling. After 10 cycles, the discharge plateau reaches the same voltage known for elemental sulfur. Although, in principle, polarization can be reduced by enhancing the conductivity through an increase in the amount of conducting carbons, a simple increase in the amount of carbon black (CB) to 25 wt% did not improve performance (Figure S4d). We, therefore, partially replaced CB with carbon fibers (CF), to attain a composition of 20 wt% CB and 5 wt% CF. By interweaving with and surrounding S-DIB-10 particles, CF can offer the intimate contact to S-DIB-10 particles and the connectivity between particles, thus enhancing the electronic conductivity as well as the structural integrity of the cathodes. Furthermore, CF can play a role in accommodating the volume change of S-DIB-10 upon cycling. Consequently, in S-DIB-10 cathodes containing CF (S-DIB-10-CF) polarization was dramatically reduced (Figure 3e). The S-DIB-10-CF cathode (Figure 3e f) delivers discharge capacities between 845 and 684 ma h g 1 over 120 cycles and a remarkably improved (with respect to S-DIB-10 without CF) capacity retention of 91% at the 50th cycle and of 81% at the 120th cycle. For this system, the Initial coulombic efficiency was 101%, and the efficiency quickly reached a value of ~100% after the 3 rd cycle. 4

5 Figure S4. Optimization of the factors affecting the cycling performance of S-DIB-10 cathodes. (a) (c) The cells were cycled with the cathode compositions of 70 wt% active material, 25 wt% carbon black and 5 wt% binder and with the electrolyte consisting of xm LiTFSI+0.31M LiNO 3/DME:DOL in the voltage region of V at the rate of 0.05C (80 ma g 1 ). (a) The influence of the binder, PVdF vs. CMC-PAA: the cells were tested with 2M LiTFSI and the E/S ratio of 20 µl mg 1. (b) The effects of LiTFSI salt concentration in the electrolyte: the salt concentration was varied from 0.38 to 2M and the E/S ratio was fixed to 20 µlmg.1. (c) The influence of E/S ratio: the cells were cycled with varying E/S ratio from 20 to 100 µl mg 1, 2M LiTFSI and cathodes prepared utilizing CMC-PAA binder. (d) The effects of the carbon on the cycling ability of S-DIB-10 cathode: the cathode compositions include 20 wt% CB+10 wt% binder, 25 wt% CB + 5 wt% binder and 20 wt% CB + 5 wt% CF + 5 wt% binder, respectively and the cells were cycled between 1.5 and 2.6 V at 0.1 C. Figure S5. Comparison of (a) discharge capacity and (b) coulombic efficiency of S-DIB-50 and S-DIB-10 cathodes that consist of 70 wt% active material, 20 wt% carbon black and 10 wt% binder between V at the rate of 0.1 C (162 ma g 1 ). These are deduced from Figure 3c and d. 5

6 5. Deconvolution of 13 C{ 1 H} CP MAS NMR spectra of S-DIB-50 and S-DIB-10 cathodes at various SOD. Figure S6. Deconvolution of 13 C{ 1 H} CP MAS NMR spectra of S-DIB-50 cathodes at various SOD. 6

7 Figure S7. Deconvolution of 13 C{ 1 H} CP MAS NMR spectra of S-DIB-10 cathodes at various SOD. 7

8 6. 13 C{ 1 H} CP MAS NMR spectra of the discharged electrodes after washing In order to identify soluble and insoluble components, 7 Li and 13 C{ 1 H} CP MAS NMR spectra of cycled electrodes, before and after being washed with the electrolyte solvent, were compared. First, after disassembly of the cells, the extracted cathodes were dried under vacuum to evaporate the electrolyte solvent. They were then packed in the MAS rotors for the subsequent NMR measurements. These NMR spectra will display resonance signals due to both soluble and insoluble components formed during electrochemical cycling. Second, after extraction from the cells, the cathodes were rinsed with DOL three times and dried under vacuum. The NMR spectra of these washed electrodes display mainly the resonance peaks of insoluble components. The 13 C{ 1 H} CP MAS NMR spectra of the cathodes before and after washing step were compared, as shown in Figure S8. For S- DIB-50, the C q signal associated with short-chain sulfides decreases in intensity after the washing step, suggesting that this component is partly soluble in the electrolyte. It is likely that a small fraction of S-DIB-50 cathodes is fully reduced to monomeric units during discharge, and that those units then dissolve into the electrolyte. By contrast, no noticeable differences were seen in the case of S-DIB-10 cathodes before and after the washing step, indicating that the reduction products are linked to the polymeric network and remain in the solid phase. This evidence is in good agreement with the 7 Li NMR results and with the different solubility of S-DIB-50 and S-DIB-10 as inferred through the photo shown in Figure S3. Note that when sulfur is fully utilized to the theoretical level, S-DIB-10 cathode materials will also transform into monomeric units similar to S-DIB-50. Nonetheless, the unique microstructure of S-DIB-10, which is composed of long-chain polysulfide loops and DIB as linker, prevents and retards the dissolution of cathode materials into the electrolyte, providing structural integrity and long-term cycling stability. Figure S8. 13 C{ 1 H} CP MAS NMR spectra of the cathodes discharged to various states before and after washing with DOL solvent. (a) S-DIB-50 at SOD = 60%, (b) S- DIB-50 at SOD = 100%, (c) S-DIB-10 at SOD = 60%, (d) S-DIB-10 at SOD = 100%. 8

9 7. Surface characterization of the cycled S-DIB10-CF cathode: ex situ X-ray photoelectron spectroscopy (XPS) XPS, a powerful tool for probing surface compound, was utilized to determine ex situ the surface composition of the S-DIB10-CF cathode after electrochemical cycles. After 120 cycles in 2M LiTFSI M LiNO 3/DOL:DME, the cycling was ended in the charged (delithiated) state. The cycled cathode was washed with DOL in an Ar-filled glove box in order to remove any residual electrolyte. It was then dried at room temperature in vacuum for 1 day. XPS spectra were collected using an X-ray photoelectron spectrometer (Thermo, Multilab 2000) with Al Kα X-ray source at 15 kv. Note that XPS detects the species present just at the top layer of the cathode. A comparative S 2p spectral analysis of the S-DIB-10-CF cathode before (Figure S9-i) and after cycling (Figure S9-ii) was carried out to monitor the changes in surface composition of S-DIB-10. The cathode consisted of 70 wt% active material, 20 wt% CB, 5 wt% CF, and 5 wt% CMC-PAA binder. The pristine cathode exhibits a broad peak in the ev range due to sulfur in the S 2p spectrum. This broad signal includes multiple peaks attributable to S S and S C (S DIB) bonds. After cycling, the same peaks for S S and S C bonds observed in the pristine cathode are still present, indicating that the S DIB linkages are maintained and that a reduction and oxidation reaction of poly(s-co-dib) cathode is reversible. However, significantly weakened S S/S C peak upon cycling (Figure S9-ii) is related to the coverage of the cathode surface with newly formed surface species containing S O functionality. In particular, the absence of Li 2S and the marginal presence of Li 2S 2 are associated with the complete transformation of Li 2S into poly(s-co-dib)copolymer, 5 implying the prevention or suppression of the dissolution of cathode material into the electrolyte and demonstrating a reaction reversibility of our S- DIB-10 cathode. In addition, the signatures of new sulfur-containing and oxygen-containing surface species are observed in the S 2p spectrum of the cycled cathode, which are attributable to electrolyte decomposition products along with the residual traces of electrolyte. Figure S9. XPS S 2p spectral comparison of S-DIB10-CF cathode (i) before and (ii) after 120 cycles in the electrolyte of 2M LiTFSI+0.31M LiNO 3/DOL:DME, followed by washing with DOL and drying in Ar-filled glove box for 1 day. References (1) Pines, A.; Gibby, M. G.; Waugh, J. S. Proton Enhanced NMR of Dilute Spins in Solids. J. Chem. Phys. 1973, 59 (2), (2) Ramakrishnan, L.; Sivaprakasam, K. Synthesis, Characterization, Thermal Degradation, and Comparative Chain Dynamics Studies of Weak-Link Polysulfide Polymers. J. Polym. Res. 2009, 16 (6), (3) Nara, H.; Yokoshima, T.; Mikuriya, H.; Tsuda, S.; Momma, T.; Osaka, T. The Potential for the Creation of a High Areal Capacity Lithium-Sulfur Battery Using a Metal Foam Current Collector. J. Electrochem. Soc. 2017, 164 (1), A5026 A5030. (4) Chang, D. R.; Lee, S. H.; Kim, S. W.; Kim, H. T. Binary Electrolyte Based on Tetra(ethylene Glycol) Dimethyl Ether and 1,3-Dioxolane for Lithium-Sulfur Battery. J. Power Sources 2002, 112 (2), (5) Fantauzzi, M.; Elsener, B.; Atzei, D.; Rigoldi, A.; Rossi, A. Exploiting XPS for the Identification of Sulfides and Polysulfides. RSC Adv. 2015, 5 (93),