Highly-Loaded Sulfur Electrode upon Cycling in Li/S Batteries: Application of Electrochemical Impedance Spectroscopy and In Situ X-ray Diffraction

Lithium/Sulfur batteries, due to their high theoretical energy density values (2500 Wh.kg^-1) became one of the most popular candidates for next-generation energy storage system¹. However, the practical discharge capacity and cycle life of Li/S cells are still below expectation. Therefore, it is necessary to better understand the working mechanism of this system, in order to help improving the electrochemical performances. Several different techniques can be used to analyze the components of Li/S batteries. Nevertheless, characterization methods are very often applied via an ex situ methodology, where post treatments of the samples are required. In our work we were studying the behavior of the Li/S cell upon cycling by combining two techniques: X-Ray Diffraction and Electrochemical Impedance Spectroscopy both applied via an in situ and operando mode. The system which we focused on was composed of a sulfur electrode (S/carbon/binder = 80/10/10 wt%), an ether-based liquid electrolyte and metallic Li°. The use of nonwoven carbon tissue as current collector allowed us to obtain highly loaded sulfur electrodes (5÷7 mg_Sulfur/cm²) without sacrificing good cyclability and practical capacity (initially 1200 mAh/g).

XRD could be successfully applied to Li/S system, since the active material is changing its form upon cycling between solid (S₈ and Li₂S/Li₂S₂) and liquid (lithium polysulfides: Li₂S_n, 2 < n ≤ 8) phases². Our experiments were performed in two Synchrotron facilities. Originally designed pouch cells (Figure 1) mounted on movable sample holder allowed us to monitor the signal coming from each electrode separately. Thanks to the high quality and large number of recorded diffractograms we were able to precisely indicate the moments of: (i) complete S₈ reduction and crystalline Li₂S formation during discharge, (ii) reversible Li₂S oxidation and solid S₈ creation during charge. We also found out that sulfur after recrystallization appeared in another allotropic form: monoclinic β-S₈. It was the first time we reported such unusual formation of β-S₈ in Li/S system.³ Influence of cycling rate (C/20 and C/8) on formation/disappearance of solid phases was taken into consideration. Moreover, further cycles (and not only the initial one, which is very often the case of in situ studies) were monitored as well.
By applying EIS measurements in Li/S system, other important information concerning the transfer reactions, the electrolyte conductivity evolution, the electrode morphology and the passivation layer formation (Li₂S and S₈) could be obtained.
The impedance results of complete Li/S cell (CR2032 coin cells) as a function of both state of charge and cycle number will be presented. In addition, the influence of the current collector nature (classical aluminum foil or nonwoven carbon tissue) on the impedance of positive electrode will be discussed.

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(2) C. Barchasz, F. Molton, C. Duboc, J. -C. Leprêtre, S. Patoux, F. Alloin, Anal. Chem.84 (2012) 3973.
(3) S. Waluś, C. Barchasz, J. -F. Colin, J. -F. Martin, E. Elkaïm, J. -C. Leprêtre, F. Alloin, Chem. Commun. 49 (2013) 7899.