Lithium-Sulfur Batteries with the Lowest Self- Discharge and the Longest Shelf-Life

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1 Supporting Information Lithium-Sulfur Batteries with the Lowest Self- Discharge and the Longest Shelf-Life Sheng-Heng Chung and Arumugam Manthiram* AUTHOR ADDRESS Dr. Sheng-Heng Chung, Prof. Arumugam Manthiram Materials Science and Engineering Program & Texas Materials Institute The University of Texas at Austin, Austin, TX 78712, USA * (Arumugam Manthiram) S1

2 Section SI. Experimental methods Low self-discharge (LSD) Li-S batteries fabrication: The LSD Li-S batteries were fabricated with pure sulfur cathodes, hierarchical PEG/CNF-coated separators, lithium anodes, and nickelfoam spacers with 34 µl of electrolyte in CR2032 coin cells. The sulfur cathodes used in the LSD batteries contained pure sulfur powder (Alfa Aesar), conductive Super P carbon (TIMCAL), and polyvinyledene fluoride (PVDF, Kureha) in a 70:15:15 wt. ratio. The pure sulfur cathodes had an electrode thickness of 100 µm with a sulfur loading of 3.4 mg cm -2 and a sulfur content of 70 wt.%. The sulfur content values inclusive of the mass of the light-weight mono-layer, duallayer, and triple-layer hierarchical separators were respectively, 68, 67, and 66 wt.%. The detailed fabrication procedures of the hierarchical separators are described in the next section. Hierarchical PEG/CNF-coated separator fabrication: The hierarchical separators were fabricated by coating a PEG/CNF suspension onto polypropylene membranes (Celgard 2500) via a vacuum-filtration process. The PEG/CNF suspension contained a mixture of 10 mg of CNFs (Pyrograf Products, Inc) and PEG (Aldrich) in a 1:1 mass ratio in 20 ml of isopropyl alcohol (IPA). The mixture was stirred overnight and then dispersed in 500 ml of IPA by high-power ultrasonication for 10 min. The suspension was vacuum-filtered onto polypropylene membranes to form evenly distributed, mono-layer PEG/CNF-coated separators. The resulting mono-layer hierarchical separators were dried at 50 C for 24 h in an air-oven. The mono-layer hierarchical separator was also used as the starting substrate for fabricating the hierarchical separators with dual-layer and triple-layer coatings. Additional suspensions in the same volume of PEG/CNF thin-film were simply added onto the mono-layer hierarchical separators either once or twice to create dual-layer and triple-layer hierarchical separators. The design of this layered configuration S2

3 allowed the stack of ultrathin polysulfide-trapping interfaces to effectively reduce the polysulfide diffusion and the thickness of the functional coating layer. The thickness obtained for the hierarchical separators was 5 µm for the mono-layer, 11 µm for the dual-layer and, 17 µm for the triple-layer coatings. The polypropylene membrane has a standard thickness of 25 µm. Nonporous CNFs are selected as the coating material in order to focus on the storage capability and long-term self-discharge of the layered configuration. The CNFs have a low surface area of 26 m 2 g -1 and no micropore absorption behavior, as summarized in Table S3 (Supporting Information). These two factors exclude any supplemental polysulfide-trapping capability contributed by, respectively, the high accessible reaction areas 14,19,21-25 and the microporous polysulfide traps. 14,16,21 Control Li-S batteries fabrication: The control Li-S batteries were assembled in the same manner as the LSD Li-S batteries, while reference cathodes, uncoated separators, and 30 µl of electrolyte were used in CR2032 coin cells. The reference cathodes were fabricated in the same manner as the electrodes above with sulfur powder, CNFs, conductive Super P carbon, and PVDF in a 65:5:15:15 wt. ratio. They had an electrode thickness of 100 µm, sulfur loading of 3.0 mg cm -2, and sulfur content of 65 wt.% and were utilized for an objective performance comparison. The uncoated separators were the commercial polypropylene membrane. Electrochemistry and characterization: The electrolyte used in both the LSD Li-S batteries and the control Li-S batteries had a 1.85 M solution of LiCF 3 SO 3 salt (Acros Organics) dissolved in a 1:1 volume ratio of 1,2-dimethoxyethane (Acros Organics) and 1,3-dioxolane (Acros Organics). The addition of 0.1 M LiNO 3 (Acros Organics) served as a co-salt in electrolyte. S3

4 To gain further insights, the open-circuit voltages (OCV) of freshly made cells were assessed. All examined fresh cells in the final actual measurements were selected to have similar OCV value starting at 2.4 ± 0.03 V before using in the electrochemical analysis. The battery chemistry depicting the OCV values, the discharge/charge profiles, self-discharge behavior, and cycling performance was obtained at room temperature with a battery cycler (Arbin Instruments) at C/10 C/2 rates in the voltage window of V. The batteries after cell assembly were initially rested for an hour before electrochemical tests. The freshly made cells were rested over a time period of 15, 30, 60, 90, 120, 150, 180, and 365 days for the visualization of self-discharge effects in the static cell-resting tests. As a reference, the quantitative analysis of the selfdischarge effect (e.g., capacity-retention rate, capacity-fade rate, and self-discharge constant) was based on the initial discharge capacity (i.e., the remaining charge-storage capacity) of the cells after resting for a certain number of days. The CNF, hierarchical PEG/CNF-coated separators, and cathodes were inspected for morphology by a field emission scanning electron microscopy (FE-SEM) (Quanta 650 SEM, FEI). Energy dispersive X-ray spectroscopy (EDX) was used for detecting elemental signals in regards to the observation of architectural and elemental changes of rested hierarchical PEG/CNF-coated separators, polypropylene membrane, and cathodes. S4

5 Section SII. Supporting Figures Figure S1. Microstructural analysis of CNFs. Figure S2. Microstructural analysis of mono-layer hierarchical coating. Figure S3. Microstructural analysis of dual-layer hierarchical coating. S5

6 Figure S4. Microstructural analysis of triple-layer hierarchical coating. Figures S1 S4 indicate that the vacuum-filtration process enables the fabrication of smooth layered hierarchical coatings onto a polypropylene membrane. The hierarchical coatings have entangled CNFs that build up continuous electron pathways and porous electrolyte channels. As the number of coating layer increases, the functional coatings keep smooth morphology without any uneven surface defects (Figures S2 S4), demonstrating that the use of the layer-by-layer coating technique ensures strong and uniform hierarchical coatings. Figure S5. Self-discharge constant after a 365-day rest period of the cells fabricated with and without the hierarchical separators. S6

7 Figure S6. Microstructural analysis of the rested control cells with a bare polypropylene membrane as the separator: (a) rested polypropylene membranes and (b) rested sulfur cathodes. Figures S6 and 3b,c give the microstructural analysis of the cell components retrieved from different control cells after resting for 150 days. In Figure S6a, the polypropylene membrane shows a layer of surface coating covered on it and many of its micro-sized pores are blocked. In Figure S6b, the sulfur cathode displays many empty pores (SEM image) and low elemental sulfur signals (elemental mapping results). These results demonstrate the severe diffusion of polysulfides out of the cathodes and the irreversible relocation of the diffusing active material. S7

8 Figure S7. Microstructural analysis of the rested LSD Li-S batteries employing the mono-layer hierarchical separator: (a) rested mono-layer hierarchical coating, (b) rested polypropylene membrane, and (c) rested sulfur cathode. S8

9 Figure S8. Microstructural analysis of rested LSD Li-S batteries employing the dual-layer hierarchical separator: (a) rested hierarchical coating, (b) rested polypropylene membrane, and (c) rested sulfur cathode. S9

10 Figure S9. (a) Discharge curves and analysis of (b) upper-discharge plateau and (c) lowerdischarge plateau of the cells employing various hierarchical separators and a polypropylene membrane at C/10 rate. In Figure S9a, the cells fabricated with the hierarchical separators show overlapping discharge curves over 500 cycles. The upper-discharge plateau and the lower-discharge plateau are stable and reversible during long-term cell cycling. Thus, the corresponding upper-plateau discharge capacities (Q H ) and the lower-plateau discharge capacities (Q L ) attain a high utilization rate of 96 and 71% along with a high retention rate of 49 and 40% over 500 cycles. The high utilization and retention rates of Q H and Q L demonstrate the enhanced dynamic polysulfide retention and the redox-reaction capability. In contrast, the control cells fabricated with a polypropylene membrane as the separator display a fast shrinkage of upper-discharge plateau and lowerdischarge plateau along with irreversible Q H and Q L loss, as shown in the rectangles in Figure S9a. S10

11 Figure S10. (a) Discharge curves and analysis of (b) upper-discharge plateau and (c) lowerdischarge plateau of the cells employing various hierarchical separators at C/5 rate. Figure S10 shows the voltage profile and the corresponding Q H and Q L analysis of the cells fabricated with the hierarchical separators at C/5 rate. At a high cycling rate, the cells still achieve high Q H and Q L utilization rates of, respectively, 78 87% and 56 70%. After 500 cycles, the retention rates of Q H and Q L approach 51 and 40%. The results demonstrate the excellent dynamic electrochemical stability and good rate performances brought about by the hierarchical PEG/CNF-coated separators. S11

12 Figure S11. Cell cyclability at (a) C/10, (b) C/5, and (c) C/2 rates. In Figure S11, the peak discharge capacities at C/10 and C/5 rates attain, respectively, 1,329 and 1,241 ma h g -1. The high electrochemical utilization of sulfur approaching 80% is facilitated by the triple-layer PEG/CNF coatings that provide additional conductive pathways for increasing the cell conductivity and improving the reaction kinetics. After 500 cycles, the reversible S12

13 discharge capacities of the cells with the triple-layer hierarchical separators remain at 529 and 542 ma h g -1 at, respectively, C/10 and C/5 rates. The high reversible capacities and long cyclability are direct results of the triple-layer PEG/CNF coatings that provide excellent polysulfide-trapping capability and high redox-reaction accessibility. These advantageous features further allow the hierarchical separators with the triple-layer coatings to enhance the high-rate performances of the corresponding cells. At a C/2 rate, the cells fabricated with the triple-layer PEG/CNF coatings exhibit a high discharge capacity of above 800 ma h g -1 with good cycle stability, approaching 80% retention over 100 cycles. On the other hand, the cells fabricated with mono-layer and dual-layer PEG/CNF coatings need an activation process during the initial cycles to reach a more comprehensive utilization of the active material and then to keep good cycle stability. S13

14 Figure S12. Microstructural analysis of the cycled experimental cells employing the mono-layer hierarchical separator after 500 cycles: (a) cycled mono-layer hierarchical coating, (b) cycled polypropylene membrane, and (c) cycled sulfur cathode. S14

15 Figure S13. Microstructural analysis of the cycled experimental cells employing the dual-layer hierarchical separator after 500 cycles: (a) cycled dual-layer hierarchical coating, (b) cycled polypropylene membrane, and (c) cycled sulfur cathode. S15

16 Figure S14. Microstructural analysis of the cycled experimental cells employing the triple-layer hierarchical separator after 500 cycles: (a) cycled triple-layer hierarchical F coating, (b) cycled polypropylene membrane, and (c) cycled sulfur cathode. Figures S12 S14 show the SEM/EDX inspection of the cells employing, respectively, monolayer, dual-layer, and triple-layer hierarchical separators. We can see, respectively, the cycled S16

17 hierarchical coatings, the polypropylene membrane that was prepared by peeling away the cycled PEG/CNF coatings, and the corresponding cycled sulfur cathodes. The strong sulfur signals detected from the surface of the cycled PEG/CNF coatings provide confirmation of its effective polysulfide-trapping capability. In contrast to the PEG/CNF coatings that exhibit the trapped sulfur-containing species, the bare surface of the polypropylene membranes display very weak elemental sulfur signals. The comparison between these two cycled components shows the sulfur concentration differences, which demonstrate that most of the migrating polysulfides are intercepted by the PEG/CNF coatings and have difficulty in migrating towards the polypropylene membrane and to the anode. The cycled sulfur cathodes with good polysulfide retention maintain strong sulfur signals, which are uniformly distributed within the cathode structure, and even more, in the Super P carbon matrix. The mitigation of the severe polysulfide diffusion demonstrated by the SEM/EDX inspection explains why cells employing hierarchical separators demonstrate improvement in electrochemical stability. S17

18 Dynamic electrochemical characteristics and analysis The good characteristics of the hierarchical separators, as observed in the above dynamic electrochemical and microstructure analyses (Figures S9 S14), lead to all the necessary dynamic electrochemical characteristics (e.g., high charge-storage capacity, low capacity-fade rate, and long cycle life) through two possible mechanisms. First, the hierarchical coatings create a layered polysulfide-trapping interfaces in between the sulfur cathode and the polypropylene membrane. This configuration suppresses free polysulfide diffusion and migration so that it is able to stabilize a high amount of active material within the cathode region of the cell for longterm electrochemical utilization. In the subsequent mechanism, the conductive PEG/CNF coatings facilitate the reutilization of the trapped active material for the rest of the cycles and the cells have the ability to attain an extended cycle life of over 500 cycles. S18

19 Section SIII. Supporting Tables Table S1. Calculation of the defect-free rate of the examined cells Resting time (Days) CNF#1 CNF#2 CNF#3 Celgard 0 5 / 5 (100%) 5 / 5 (100%) 5 / 5 (100%) 5 / 5 (100%) 15 4 / 5 (80%) 5 / 5 (100%) 5 / 6 (83%) 3 / 6 (50%) 30 4 / 5 (80%) 3 / 5 (60%) 5 / 5 (100%) 2 / 15 (13%) 60 3 / 5 (60%) 3 / 5 (60%) 4 / 5 (80%) 2 / 10 (20%) 90 4 / 6 (67%) 3 / 6 (50%) 4 / 6 (67%) 2 / 15 (13%) / 6 (50%) 4 / 6 (67%) 5 / 7 (71%) 2 / 10 (20%) / 6 (50%) 4 / 7 (57%) 5 / 8 (63%) 2 / 15 (13%) / 7 (57%) 3 / 6 (50%) 4 / 6 (67%) 0 / 30 (0%) / 3 (67%) 1 / 2 (50%) 2 / 3 (67%) 0 / 10 (0%) Table S2. Comparative analysis of the cell parameters and the corresponding static battery performances of the LSD Li-S batteries fabricated with the PEG/CNF-coated separators with those in the literature investigating the self-discharge effect Year Modified cell configuration Shelf-life [day] Capacity-fade rate [% per day] Sulfur loading [mg cm -2 ] Sulfur content [wt. %] 2004 High-concentration LiTFSI salt S ~ Metal current collector S TEGDME liquid electrolyte S Ni-foam current collector S Graphene-coated PETE film current collector S S/PEDOT core/shell nanocomposite S6 0 no self-discharge effect in a shortterm measurement BTFE co-solvent (low loading cathode) S BTFE co-solvent (high loading cathode) S S-PPy nanocomposite + 0.4M LiNO 3 electrolyte additive S S-PPy nanocomposite S Super-P-carbon-coated separator S GO-coated separator S Graphite / S-KB nanocomposite cell S S19

20 2015 Li / S-KB nanocomposite cell S High-density LiNO 3 induced passivation layer S Dual-phase electrolyte + Li 2S cathode S13 1 no self-discharge effect in a shortterm measurement 2015 Dual-phase electrolyte + Li 2S cathode S13 2 no self-discharge effect in a shortterm measurement 2015 Dual-phase electrolyte + Li 2S cathode S13 3 no self-discharge effect in a shortterm measurement M LiTFSI in TEGDME/DOL (1:1, v/v) S M LiTFSI in TEGDME/DOL/ETFE (1:1:2, v/v/v) S M LiTFSI in ETFE/DOL (1:1, v/v) S TTE electrolyte S no self-discharge effect in a shortterm measurement 2015 PAN/sulfur/vulcanization accelerator 30 no self-discharge nanocomposite S16 effect PFM binder S PEDOT binder S PVP binder S PVDF binder S AB/S/Au nanocomposites S AB/S nanocomposites S Hybrid separator S Microporous membrane separator S TFTFE co-solvent S TFTFE co-solvent S C/S/C sandwich structure on a PP separator S PP separator S PAN separator S PAN/GO separator S Gel-ceramic multi-layer electrolyte S23 3 no self-discharge effect in a shortterm measurement Ionic liquid electrolyte S AB/CNT/LAGP-coated separator S S/microporous activated carbon nanocomposite S S20

21 2016 S-rich copolymer/graphene nanocomposite S S-rich nanocomposite S Quasi-solid state electrolyte S This work (control cell) This work (CNF#1) This work (CNF#2) This work (CNF#3) Table S3. Analysis of the specific surface area and porosity of CNFs Surface area [m 2 g -1 ] Pore volume [cm 3 g -1 ] Average pore size [nm] Micropore area [m 2 g -1 ] Micropore volume [cm 3 g -1 ] S21

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