Research ArticlePHYSICAL SCIENCES

Expansion-tolerant architectures for stable cycling of ultrahigh-loading sulfur cathodes in lithium-sulfur batteries

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Science Advances  03 Jan 2020:
Vol. 6, no. 1, eaay2757
DOI: 10.1126/sciadv.aay2757
  • Fig. 1 Morphological study, tomographic reconstruction, and schematic illustration of cathodes with identical composition and different slurry preparation methods.

    Top-view SEM images and schematic illustration of the architecture in (A to C) cathode A, demonstrating evenly distributed web-like bridging bonds holding the fine particles together. The arrows in (C) illustrate the high degree of freedom for expansion and the easily accessible surfaces in ET cathode. (D to F) Cathode B demonstrating ribbon-like bridging bonds holding large clusters together. (G to I) Cathode C demonstrating a cohesive network of agglomerated particles, heavily coated by the binder. (J to L) Cathode D demonstrating a very compact microstructure with particles being tapped in the network of the binder. More images can be found in figs. S1 and S2. (M and N) X-ray tomography. Grayscale 2D view and 3D rendering of a portion of the (M) ET cathode and (N) cathode C, demonstrating the noticeably finer morphology of the ET cathode. 3D gray shape visualizes all phases in the specimen. Pink shape enables the visualization of weakly absorbing features such as the polymeric binder and binder-coated fillers, the fraction of which is larger in cathode C in agreement with SEM observations. Yellow shape represents more strongly absorbing sample components such as exposed sulfur, the fraction of which is larger in ET cathode. Scale bars, 50 μm (2D image) and 10 μm (3D image).

  • Fig. 2 Cross-sectional SEM images and elemental mapping of cathodes with identical composition and different slurry preparation methods.

    Providing further insight to top-view SEM images: (A and B) Cathode A demonstrates a segregated structure and the presence of bridging bonds across the thickness of the cathode. (D and E) Cathode B demonstrates large clusters in an open structure. (G and H) Cathode C demonstrates compactness throughout the thickness. (J and K) Cathode D demonstrates areas where cross-linking has resulted in the formation of binder films. More images can be found in fig. S3. Elemental mapping images demonstrate (C) excellent sulfur exposure in cathode A, as opposed to (F) nonuniformity in cathode B, and (I and L) heavy coverage with binder in cathodes C and D, respectively.

  • Fig. 3 Evaluation of the binder-filler interactions.

    (A) The viscosity data demonstrate marked difference in rheological behavior of slurries with identical composition yet different preparation method; inset photos are available in larger scale in fig. S7. (B) Elemental energy-dispersive x-ray mapping across the cross section of the cathode prepared via dry mixing illustrates excellent distribution of ingredients and high sulfur exposure. (C) Representative C 1s and O 1s curve fits for CMC and sulfur from the electrode after substantial water cleaning and pure sulfur (insets). Despite the extensive purification of the S powders collected from the electrodes, the powders retain substantial content of cellulose residue on the surface as evident from the comparison of the C 1s and O 1s spectra of DI water–cleaned sulfur electrode (S-electrode) powder and unmodified S powder. It is seen that that the S-electrode powder presents a surface with a significant amount of O relative to the unmodified S powder confirmed by the elemental quantification derived from survey spectra (table S1) and high-resolution C 1s and O 1s spectra similar to pure CMC. Using the pure CMC C 1s spectral data as a fit component for the S-electrode powder spectrum, this component accounts for most of the intensity, highlighting the amount of Na-CMC observed on the surface of the S-electrode powder. The remainder of the intensity is accounted for using standard Gaussian-Lorentzian components where most of the intensity is assigned to C─C and C─H groups (C1) and C─O group (C2) (please refer to Materials and Methods for full details). It is believed that such an extensive particle coverage with the binder limits the high sulfur utilization due to the considerably reduced free reaction surfaces of the particles—an inevitable consequence of using predissolved binder solutions to fabricate the sulfur cathode.

  • Fig. 4 Cycling performance of high-loading sulfur cathodes.

    (A) Comparing the discharge profile of different cathodes demonstrates the superiority of cathode A over other cathodes; long-term cycling, CE, and 0.5 C discharge profile comparison is available in fig. S10. (B and C) Electrochemical impedance spectroscopy: the evolution of cell impedance for the four cathodes, demonstrating the importance of the ET architecture in maintaining good contact with the current collector and excellent electron/ion conductivity upon cycling. (D) Excellent cyclability of ultrahigh-loading cathodes prepared via the undissolved binder approach in terms of gravimetric capacity, areal capacity, and CE at 0.1 C rates. (E) Excellent long-term cycling performance of a sulfur cathode prepared via undissolved CMC binder and expanded graphite as the conductive agent at 0.2 C; CE remains close to 100% after 200 cycles. (F) The plot of areal and gravimetric capacity as a function of sulfur loading demonstrates that the optimum performance happens at 13 mg cm−2. (G) Performance of cathode A in a pouch cell configuration demonstrates excellent correlation with coin cell data; energy metrics are calculated on the basis of the whole package and are shown in fig. S13. (H) Comparing the areal capacity and total gravimetric capacity of the cathodes after 50 cycles (calculated on the basis of the mass of S/binder/carbon) in various references demonstrates unique metrics for our cathodes.

  • Fig. 5 Evaluating the cathode integrity after an intense cycling regime.

    Top-view SEM images of (A and B) cycled ET cathode demonstrating the success of the high strength cellulose-based bridging bonds in keeping the particles together, leading to the strongest cathode microstructure. More images can be found in fig. S17. (C and D) Cathode B demonstrating a relatively well-preserved structure via ribbon-like bridging bonds. (E and F) Cathode C demonstrating disintegration of the cathode and formation of isolated colonies of fillers. (G and H) Cathode D demonstrating damaged binder nets but intact microstructure due to the low internal stress experienced in this low-capacity cathode.

  • Fig. 6 Detailed cross-sectional observation of the four cathodes after an intense cycling regime.

    Cross-sectional SEM images of (A to C) cycled ET cathode demonstrating integrity. (D to F) Cathode B demonstrating areas with the strong presence of binder. (G to I) Cathode C demonstrating the evolution of large cracks. (J to L) Cathode D demonstrating binder films and torn binder nets.

  • Fig. 7 Cross-sectional SEM images of FIB milled cycled cathodes.

    (A to C) Cycled ET cathode demonstrating structural integrity throughout the thickness of the cathode and hence the effectiveness of our dry mixing approach in accommodating the cycling stress. (D to F) Cathode B demonstrating the evolution of large cracks proving that the commonly used wet mixing approach, although most useful for low-capacity electrodes such as those of LIB, could be problematic for cycling of high-capacity thick electrodes that are prone to volume expansion.

  • Table 1 Description of cathodes with identical compositions but different slurry preparation protocols.

    CathodeMixing methodSolventSolvent content per gram
    of electrode
    ingredients (ml)
    Viscosity (Pa·s) at
    0.01 s−1 shear rate
    A (our work)Dry mixing of all ingredientsDI water1.545,100.0
    B (our work)Dry mixing of all ingredientsDI water5.0379.0
    C (common practice)Wet mixing of S/C mixture in a predissolved CMC solutionDI water5.00.8
    D (cross-linking)Wet mixing of S/C mixture in
    a predissolved CMC solution
    Cross-linking solution5.0117.0

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/1/eaay2757/DC1

    Fig. S1. Microstructural study of different cathodes.

    Fig. S2. High-resolution SEM images of the cross-linking cathode.

    Fig. S3. Cross-sectional SEM images of cathodes.

    Fig. S4. Images of ET cathode A and cross-linking cathode C.

    Fig. S5. Peeling test on several different cathodes.

    Fig. S6. Raman spectroscopy.

    Fig. S7. Photographs of slurries of wet mixing and dry mixing cathodes.

    Fig. S8. An XPS study: Evaluation of the binder-filler interactions.

    Fig. S9. Electrical conductivity data of different cathodes.

    Fig. S10. Cycling performance of different high-loading sulfur cathodes.

    Fig. S11. Cycling performance of high- and ultrahigh-loading ET cathodes.

    Fig. S12. Plot of lithium excess percentage versus areal capacity of sulfur cathode.

    Fig. S13. Cell-level energy metrics in pouch cell configuration.

    Fig. S14. Charge-discharge profile of the ET cathode in pouch cell configuration.

    Fig. S15. Postmortem analysis of a cycled pouch cell.

    Fig. S16. Pore-size distribution and isotherms of the used conductive agents.

    Fig. S17. Characterization and cycling performance of a cathode fabricated with crystalline sulfur.

    Fig. S18. SEM images of a PVDF-based cathode fabricated via the recipe of dry mixing/minimally dissolved binder.

    Fig. S19. Cycling performance of sulfur cathodes with minimally dissolved PVDF binder.

    Fig. S20. SEM images of cathodes fabricated via traditional wet mixing method using PVDF and Gum Arabic binder.

    Fig. S21. High-resolution SEM images of the cycled ET cathode.

    Fig. S22. FIB cross-sectional SEM images of the cycled ET cathode.

    Fig. S23. FIB cross-sectional SEM images of the cycled wet mixing cathode.

    Table S1. Elemental quantification derived from XPS survey spectra.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Microstructural study of different cathodes.
    • Fig. S2. High-resolution SEM images of the cross-linking cathode.
    • Fig. S3. Cross-sectional SEM images of cathodes.
    • Fig. S4. Images of ET cathode A and cross-linking cathode C.
    • Fig. S5. Peeling test on several different cathodes.
    • Fig. S6. Raman spectroscopy.
    • Fig. S7. Photographs of slurries of wet mixing and dry mixing cathodes.
    • Fig. S8. An XPS study: Evaluation of the binder-filler interactions.
    • Fig. S9. Electrical conductivity data of different cathodes.
    • Fig. S10. Cycling performance of different high-loading sulfur cathodes.
    • Fig. S11. Cycling performance of high- and ultrahigh-loading ET cathodes.
    • Fig. S12. Plot of lithium excess percentage versus areal capacity of sulfur cathode.
    • Fig. S13. Cell-level energy metrics in pouch cell configuration.
    • Fig. S14. Charge-discharge profile of the ET cathode in pouch cell configuration.
    • Fig. S15. Postmortem analysis of a cycled pouch cell.
    • Fig. S16. Pore-size distribution and isotherms of the used conductive agents.
    • Fig. S17. Characterization and cycling performance of a cathode fabricated with crystalline sulfur.
    • Fig. S18. SEM images of a PVDF-based cathode fabricated via the recipe of dry mixing/minimally dissolved binder.
    • Fig. S19. Cycling performance of sulfur cathodes with minimally dissolved PVDF binder.
    • Fig. S20. SEM images of cathodes fabricated via traditional wet mixing method using PVDF and Gum Arabic binder.
    • Fig. S21. High-resolution SEM images of the cycled ET cathode.
    • Fig. S22. FIB cross-sectional SEM images of the cycled ET cathode.
    • Fig. S23. FIB cross-sectional SEM images of the cycled wet mixing cathode.
    • Table S1. Elemental quantification derived from XPS survey spectra.

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