Impact of the Binder Nature on the Morphological Change of Sulfur Electrodes upon Cycling Investigated By in Situ Characterization Methods
Authors: Lemarie Quentin , Idrissi Hassane , Maire Eric , Thivel Pierre-Xavier , Alloin Fannie , Roue Lionel 
Published: December 2020

Even though the lithium-sulfur (Li/S) battery system is one of the most promising candidates to become the successor of the current Li-ion battery, its insufficient cycle life needs to be resolved before envisioning their commercial use. Several phenomena are reported to be involved in the capacity fading of the Li/S cells upon cycling, among which important structural changes in the sulfur electrode. The binder material plays a crucial role in maintaining the integrity of the electrode through cycling, and the use of innovative in situ characterization techniques enables to get a better understanding of its complex mechanisms. In the present study, operando acoustic emission (AE) and dilatometry techniques, and in situ synchrotron X-ray tomography are used to study the degradation mechanism of sulfur-based electrodes depending on their binder formulation [1-3]. In particular, the impact of using a polyelectrolyte binder (poly(diallyldimethylammonium) bis(trifluromethane sulfonyl)imide), previously known for its polysulfide-retaining and ionic conductive abilities [4], on the morphological degradation of the sulfur electrode is evaluated and compared to more conventional binders (poly(vinylidene difluoride) (PVdF) and carboxymethyl cellulose (CMC)) in the present study
To date, AE technique has been used to characterize the degradation mechanisms (crack growth, friction, delamination, matrix cracking, corrosion, etc) of various materials subjected to a stress by mechanical, pressure or thermal means, and has been used in our previous work to monitor the mechanical degradation (e.g. cracking, collapsing) of electrodes upon cycling. The operando electrochemical dilatometry technique allows to measure thickness variation of an electrode material undergoing electrochemical reactions and has been used to study various battery electrode materials such as graphite or Si anodes. Finally, X-Ray computed tomography (XRCT) is certainly one of the most powerful analytical tools enabling non-destructive 3D imaging of objects with complex and porous morphologies such as S-based electrodes. Using appropriate image processing, segmentation and analysis procedures, quantitative parameters can be extracted such as the volume fraction, the size distribution, the connectivity and the geometrical tortuosity of the constitutive phases of the sulfur electrodes. A spatial resolution of a few tens of nm can be reached with a synchrotron X-ray source.

Operando dilatometry is used for one the very first times on sulfur-based electrodes and shows that during the initial sulfur dissolution process, the polyelectrolyte-based electrode displays a lower irreversible thickness contraction of ~16% compared to ~22% and ~31% for CMC and PVdF, respectively. This is confirmed by the XRT measurements showing a reduced thickness variation for the polyelectrolyte electrode compared to the CMC electrode. The same trend is found in the AE results, where a lower acoustic activity attributed to the rupture of the binder/carbon/sulfur network is detected during the 1st discharge plateau for the polyelectrolyte electrode. All these results confirm the major role of the binder for the Li/S system as well as the highly promising performance of the present polyelectrolyte binder. Thanks to its multifunctionality, it impacts both the diffusion of the active material outside the electrode and the electrode integrity and therefore the conduction paths and accessible active surface for electrochemical processes.
[1] Q. Lemarié, H. Idrissi,E. Maire, P.X. Thivel, F. Alloin, L. Roué. Impact of the binder nature on the morphological change of sulfur electrodes upon cycling investigated by in-situ characterization methods, J. Power Sources 477 (2020) 228374.
[2] Q. Lemarié, E. Maire, H. Idrissi, P.X. Thivel, F. Alloin, L. Roué,Sulfur-based electrode using a polyelectrolyte binder studied via coupled in situ synchrotron X-ray diffraction and tomography, ACS-Applied Energy Materials 3 (2020) 2422-2431.
[3] Q. Lemarié, F. Alloin, P.X. Thivel, H. Idrissi, L. Roué, Study of sulfur-based electrodes by operando acoustic emission, Electrochim. Acta 299 (2019) 415-422
[4] L. Li, T.A. Pascal, J.G. Connell, F.Y. Fan, S.M. Meckler, L. Ma, Y.-M. Chiang, D. Prendergast, B.A. Helms, Molecular understanding of polyelectrolyte binders that actively regulate ion transport in sulfur cathodes, Nat. Commun. 8 (2017) 1–10.

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