advances.sciencemag.org/cgi/content/full/6/1/eaay2757/DC1

Supplementary Materials for

Expansion-tolerant architectures for stable cycling of ultrahigh-loading sulfur

cathodes in lithium-sulfur batteries

Mahdokht Shaibani*, Meysam Sharifzadeh Mirshekarloo, Ruhani Singh, Christopher D. Easton, M. C. Dilusha Cooray, Nicolas Eshraghi, Thomas Abendroth, Susanne Dörfler, Holger Althues, Stefan Kaskel,

Anthony F. Hollenkamp, Matthew R. Hill, Mainak Majumder*

*Corresponding author. Email: m.e.shaibani@monash.edu (M.S.); mainak.majumder@monash.edu (M.M.)

Published 3 January 2020, Sci. Adv. 6, eaay2757 (2020) DOI: 10.1126/sciadv.aay2757

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.

Supplementary Figures

Fig. S1. Microstructural study of different cathodes. Top view SEM images (A) Cathode A is formed of

discriminative fine particles. (B) Cathode B is composed of segregated clusters. It is presumed that with the extra water, the polymer binder in cathode B starts to ‘bleed’ back into the fillers, under capillary forces. (C and

D) Cathode C and cathode D, demonstrate very compact microstructures.

Fig. S2. High-resolution SEM images of the cross-linking cathode. (A and B) demonstrating particles

entrapment in the network of polymeric binder.

Fig. S3. Cross-sectional SEM images of cathodes. Cross-sectional SEM images of (A) Cathode A, and (B)

Cathode D provides a clear insight into the morphology of these cathodes.

Fig. S4. Images of ET cathode A and cross-linking cathode C. Photographs and low-magnification SEM

images of (A and C) Cathode C and (B and D) cathode A. It is seen that at high loadings, coating delamination

occurs via a wet mixing approach, as opposed to excellent adhesion of the coating prepared via the dry mixing

approach. Photo Credit: Mahdokht Shaibani, Monash University.

Fig. S5. Peeling test on several different cathodes. (A) ET cathode, (B) cathode B, (C) cathode C, (D) cathode

D, (E) Gum Arabic based cathode at 3-4 mgs cm-2, (F) PVDF-based cathode at at 3-4 mgs cm

-2, (G) PVDF-

based cathode at 1-2 mgs cm-2. It is seen that PVDF is able to make reasonable electrodes at low loadings of

sulfur, which is why it was commonly used in the early years of the emergence of Li-S research where the

importance of high sulfur loading was yet to be realized. Photo Credit: Mahdokht Shaibani and Meysam

Sharifzadeh Mirshekarloo, Monash University.

Fig. S6. Raman spectroscopy. Raman spectra shows dry mixing could result in the homogeneous distribution

of ingredients and reduced coverage of carbon with binder, supporting the microscopy results.

Fig. S7. Photographs of slurries of wet mixing and dry mixing cathodes. It can be visually observed that the

slurry of cathode A, prepared via the un-dissolved binder approach is clearly more viscous. As evident from Fig.

2a. a significant shear thinning behavior was observed for the slurry prepared for fabrication of the ET cathode

(cathode A). The dramatic drop in viscosity with increasing shear rate is attributed to the very high solid content

in the slurry. The shear thinning behavior was less dramatic in slurries of cathodes B and D due to their lower

solid content. On the other hand, the viscosity curve obtained from the slurry prepared for fabrication of cathode

C, appeared to approach a relatively constant Newtonian viscosity at a higher shear rate, suggesting complete

particle dispersion. Photo Credit: Mahdokht Shaibani, Monash University.

Fig. S8. An XPS study: Evaluation of the binder-filler interactions. Representative C 1s and O 1s curve fits

for (A and C) CMC, (B and D) carbon from the electrode after substantial water cleaning and pure carbon

(insets). The analogous XPS experiments with activated carbon suggest rather similar interactions between the

polar groups and defects on the carbon surface and CMC moieties.

Fig. S9. Electrical conductivity data of different cathodes. Electrical conductivity data of different cathodes

demonstrates higher bulk conductivity for cathodes prepared via dry mixing.

Fig. S10. Cycling performance of different high-loading sulfur cathodes. Comparing (A) the rate capability,

long-term cycling performance, (B) Coulombic efficiency, and (C) discharge profile of different cathodes at 0.5

C, clearly demonstrate the superiority of the ET cathode over other cathodes. A direct cycling comparison was

made between cathodes B, C, and D where identical amount of solvents were used in preparation of each slurry.

In spite of the well-defined charge-discharge profile (Fig. 3a), which is due to the facilitated electrolyte

penetration, cathode B showed only average sulfur utilization, which is attributed to a significant portion of

sulfur that is isolated/inaccessible within the large clusters. As expected, in cathode C, the resistive binder layer

coated on the particles, imposed a detrimental effect on the electronic properties, as well as electrolyte uptake,

and resulted in an average capacity at the lower rate (0.1 C) and poor performance at higher rates. Cathode D,

demonstrated an even poorer performance due to the severely limited Li+ diffusion in its very dense cross-linked

microstructure. From these comparisons, it is concluded that dissolved binder, in particular when it forms a

continuous network, adversely affects the ion/electron uptake of the sulfur cathode. It is noticeable that cycling

the ET cathode with an ultra-high loading of 10 mgs cm-2 at a fast rate of 0.5 C (8.34 mA cm-2), demonstrates a

good sulfur utilization (capacity of 600 mAh cm-2). Nonetheless, the release of a relatively large amount of

polysulfides at a short interval increases the resistance of the cell as evident by the V-t profile. This contributes

to the rapid capacity decay at a presumably very viscus environment. When the cycling rate is changed to the

slower 0.2 C, the cell demonstrates very stable cycling performance, demonstrating the retention of mechanical

stability of the cathode even after being exposed to such a high current density. The capacity decay of thick

cathodes at higher rates could be reduced with the use of polysulfide absorbent hosts in the composition of the

cathode. (D) The high-rate cycling performance of ET cathode at 4 mgs cm-2 demonstrates that at medium

loadings of sulfur, stability could be achieved with the use of traditional electrode materials.

Fig. S11. Cycling performance of high- and ultrahigh-loading ET cathodes. (A) At 6 mg cm-2 and 0.2 C rate,

the CE remains close to 100 % after 100 cycles. (B) At 14 mg cm-2 and 0.1 C rate, the CE remains close to 99 %

after 50 cycles. (C) At 15 mg cm-2 and 0.1 C rate, the CE remains close to 98 % after 40 cycles and the capacity

drop is more dramatic.

Fig. S12. Plot of lithium excess percentage versus areal capacity of sulfur cathode. Usually at least 200 μm

thick lithium anodes are used for test cells. A 200 μm thick lithium foil corresponds to 10.7 mg Li cm−2 which

delivers a theoretical areal capacity of 40.5 mAh cm-2. Accordingly, the excess of lithium in a Li-S cell where

the sulfur cathode delivers only 1 mAh cm-2 is in fact more than 4000%. The excess percentage is even higher

when thicker lithium chips or lower loading sulfur cathodes are being used in a test cell. Hence, balancing the

anode and cathode is critical to increase the energy density on a cell level. At 13 mgs cm-2, our cathode delivers

an areal capacity up to 18.7 mAh cm-2, equivalent to only 215 % lithium excess in the cell. Nonetheless, at such

a meaningful balance between the anode and the cathode, the lithium metal is under severe internal stress and

in-situ/ex-situ protection of it seems critical which should be the subject of future studies in the field.

Fig. S13. Cell-level energy metrics in pouch cell configuration. (A) Energy metrics of cathode A in pouch

cell configuration. (B) Photograph of cathode A prepared in large size for pouch cell assembly.

Fig. S14. Charge-discharge profile of the ET cathode in pouch cell configuration. (A) 6 mgs cm-2, (B) 12

mgs cm-2.

Fig. S15. Postmortem analysis of a cycled pouch cell. (A) Inhomogeneous surface of Lithium anode extracted

from a cycled pouch cell, areas with mossy/porous surface alongside areas with planar surface are observed. (B)

Areas with lithium plating on the separator is observed. (C) Swollen pouch cell due to gas formation is another

challenge ahead of realization of Li-S. Photo Credit: Thomas Abendroth, Fraunhofer Institute for Material and

Beam Technology.

Fig. S16. Pore-size distribution and isotherms of the used conductive agents. (A, and C) Pore size

distribution of activated carbon (AC) and Expanded Graphite (Ex-Gr), respectively. (B, and D) N2 physisorption

isotherms of AC and Ex-Gr, respectively indicating a large density of micropores in AC as opposed to almost no

surface area in Ex-Gr.

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

images of a cathode fabricated via the recipe of dry mixing/un-dissolved binder but replacing colloidal sulfur

with the commonly used crystalline sulfur in the composition of the cathode show that achieving a homogenous

distribution of the ingredients and a crack-free microstructure is not trivial due to the coarsening effect of

crystalline sulfur. Particle coarsening, the macroscopic observation of particles increasing in size, is a

combination of processes that increase the overall particle size and affect the distribution of particle sizes(39).

Crystalline sulfur is the ground state, or atomic form of sulfur, S0, and is almost always used as the active

material in the composition of sulfur cathodes. Sulfur atoms have a strong tendency to catenate, resulting in

polymeric forms that can exist as rings or as chains of varying sizes and configurations, but is most stable as an

8-membered ring with a crown-shaped configuration (S8). These rings will quickly aggregate to form very small,

but visible, forms of sulfur that are often in the tens to hundreds of nanometers to few micron size range of

particles. It is then clear that dry mixing in such a system where one of the elements tend to aggregate cannot

result in a uniform distribution of all the ingredients. In addition, bridging bonds might not be strong enough to

bind such big neighboring clusters of particles. (A) The presence of considerably large binder only areas

demonstrates lack of homogeneity and the presence of macro-cracks shows lack of structural integrity - a direct

result of particle coarsening. (B, C, and D) show different binding mechanisms across this cathode: successful

bridging bonds (B and C), networking mechanism to a more extent (C), and unsuccessful bridging bonds (D),

hence the importance of using colloidal sulfur. (E) Inferior cycling performance of the cathode prepared via

crystalline sulfur.

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

dissolved binder. (A and B) We noted that when using PVDF as the binder, electrode fabrication is not trivial

in particular in the presence of relatively large-surface area conductive agents, where very poor adhesion to the

Al foil was observed. Using expanded graphite as the conductive agent, resulted in relatively better coating,

which made it possible to punch a few thick and relatively thick electrodes from the non-uniformly coated

cathode and investigate their performance in coin cell assembly (fig. S19). Nonetheless, from the visual

observation of the coatings and the SEM images it is concluded that in contrast to cellulose, PVDF loses its

adhesive power in the absent of sufficient dissolution in the solvent (NMP). (C and D) Interestingly, very little

agglomeration is seen and colloidal sulfur particles and expanded graphite powders have maintained their

physical characteristics largely.

Fig. S19. Cycling performance of sulfur cathodes with minimally dissolved PVDF binder. (A) and (B) At

4.5 mg cm-2 and 6.1 mg cm-2 and at 0.2 C rate, relatively high capacity and very good capacity retention is

demonstrated and CE remains close to 100 % after 100 cycles. (C) At 10 mg cm-2 and 0.1 C rate, average

capacity and high capacity retention is observed and the CE remains above 99 % after 100 cycles. It is believed

that the extremely open microstructure of this cathode allows for facilitated electrolyte diffusion and

accommodating the cycling stress. However, the average electronic wiring across the electrode (as can be

observed in fig. S18) retards desired sulfur utilization. From a production perspective, for real world

applications such as for use in a pouch cell configuration, homogenous, pinhole-free, and robust coatings on

large-sizes of 2D metallic current collectors are the only industry-suitable solution, which is clearly not

achievable with PVDF. Even so, one can conclude that dry mixing still demonstrates its most important merit in

the case of using PVDF: an open structure where particles are not severely constrained amongst neighbouring

particles.

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

Arabic binder. (A and B) a PVDF-based cathode. (C and D) a Gum Arabic-based cathode.

Fig. S21. High-resolution SEM images of the cycled ET cathode. (A and B) Top view, and (C and D) cross-

sectional SEM images demonstrate the success of the high strength cellulose-based bridging bonds in keeping

the particles together even after intense cycling.

Fig. S22. FIB cross-sectional SEM images of the cycled ET cathode. (A, B, and C) demonstrate the milling

process and more insight into the sample, (D) the presence of bridging bonds throughout the cross-section of the

ET cathode.

Fig. S23. FIB cross-sectional SEM images of the cycled wet mixing cathode. (A and B) microstructural

fragmentation of cathode B is further revealed upon milling.

Supplementary Table

Table S1. Elemental quantification derived from XPS survey spectra. Elemental quantification derived from survey spectra presented as atomic%. Listed are the mean values (± deviation) based on 2 - 3 analyses points.

Sample Na-CMC

Sulfur

AC

Sulfur/Na-CMC AC/Na-CMC

Atomic% Mean Std Mean Std Mean Std Mean Std Mean Std

Na 1s 4.47 0.30 0.00 0.00 0.00 0.00 0.75 0.06 1.69 0.38

N 1s 0.37 0.05 0.00 0.00 0.42 0.02 0.41 0.05 0.47 0.20

C 1s 56.51 0.44 24.96 0.19 89.77 0.37 57.36 0.18 56.30 0.49

Cl 2p 0.16 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

S 2p 0.14 0.04 73.61 0.33 5.52 0.04 2.62 0.39 0.21 0.01

Si 2p 0.00 0.00 0.30 0.11 0.77 0.14 0.08 0.09 0.00 0.00

F 1s 0.00 0.00 0.00 0.00 0.00 0.00 1.18 0.21 0.00 0.00

Ca 2p 0.00 0.00 0.00 0.00 0.00 0.00 0.14 0.04 0.36 0.03

Al 2p 0.00 0.00 0.00 0.00 0.00 0.00 2.65 0.18 1.35 0.25

O total 38.38 0.11 1.14 0.41 3.54 0.23 19.22 1.20 39.61 0.49

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