Operando Spectromicroscopy of Sulfur Species in Lithium-Sulfur Batteries

Abstract

In this study, a novel cross-sectional battery cell was developed to characterize lithium-sulfur batteries using X-ray spectromicroscopy. Chemically sensitive X-ray maps were collected operando at energies relevant to the expected sulfur species and were used to correlate changes in sulfur species with electrochemistry. Significant changes in the sulfur/carbon composite electrode were observed from cycle to cycle including rearrangement of the elemental sulfur matrix and PEO 10 LiTFSI binder. Polysulfide concentration and area of spatial diffusion increased with cycling, indicating that some polysulfide dissolution is irreversible, leading to polysulfide shuttle. Fitting of the maps using standard sulfur and polysulfide XANES spectra indicated that upon subsequent discharge/charge cycles, the initial sulfur concentration was not fully recovered; polysulfides and lithium sulfide remained at the cathodes with higher order polysulfides as the primary species in the region of interest. Quantification of the polysulfide concentration across the electrolyte and electrode interfaces shows that the polysulfide concentration before the first discharge and after the third charge is constant within the electrolyte, but while cycling, a significant increase in polysulfides and a gradient toward the lithium metal anode forms. This chemically and spatially sensitive characterization and analysis provides a foundation for further operando spectromicroscopy of lithium-sulfur batteries. New "beyond lithium-ion" battery chemistries are essential to meet the increasing demand for long-lasting, high capacity energy storage in portable electronics and electric vehicles. Lithium-sulfur (Li-S) batteries are an attractive Li-ion alternative that provides large capacity (1672 mAh g −1 ) and energy density (2500 Wh kg −1 ) while also being low cost, earth-abundant, and lightweight.1-3 Li-S batteries have a unique chemistry that achieves high capacity via chemical transformation rather than Li intercalation. Elemental sulfur (S 8 ) is reduced through a series of soluble Li polysulfides (Li 2 S x , 2 ≤ x ≤ 8) to a final solid discharge product (Li 2 S), and the process is reversed upon charging. [4][5][6] However, Li-S suffers from unrealized theoretical capacity and rapid capacity fade due to loss processes that are not well-understood. 2,7 While many advances in electrode and electrolyte engineering have been made in an attempt mitigate these performance issues, 8-14 a gap remains in the mechanistic understanding of Li-S battery operation. Several mechanisms for reduced capacity and capacity fade have been proposed. Lithium polysulfides, which are necessary for operation, dissolve into the liquid electrolyte, remain dissolved, and "shuttle" between the electrodes rather than participating in the electrochemical reactions, decreasing the amount of active material available. 3,[15][16][17][18] Low sulfur utilization results in initially low capacity that then continues to decrease with subsequent cycles. [19][20][21] Batteries are spatiall...

Rate Constants of Electrochemical Reactions in a Lithium-Sulfur Cell Determined by Operando X-ray Absorption Spectroscopy

“…For example, Barchasz et al 21 The subject of reaction mechanisms in sulfur cathode is of considerable current interest. 21,27,[51][52][53][54][55][56][57]28,31,32,35,38,42,49,50 Our detection of Li 2 S at the very early stage of discharge (as seen in Figure 8c) is consistent with the findings of Waluś et al 20,58 , Cuisinier et al 33 , and Conder et al 52 . Similarly, the formation and subsequent consumption of Li 2 S 4 up to 500 mAh/g of discharge in Figure 9 is similar to the findings of Dominko et al 36 , Zhang et al 55 and Zheng et al 51 .…”

“…X-ray diffraction (XRD) can be used to detect the presence of crystalline species such as Li 2 S and S 8 but it is insensitive to the presence of amorphous polysulfides. 19,20 Uv-vis, [21][22][23] Raman, 24,25 NMR 26,27 and X-ray absorption spectroscopy (XAS) [28][29][30][31][32][33][34][35][36][37][38][39] can, in principal be used to detect Cell Assembly and Cycling. A pouch cell was prepared according to the method described in the work by Wujcik et al 37 The electrolyte film was placed on the cathode.…”

A Lithium–Sulfur Cell Based on Reversible Lithium Deposition from a Li2S Cathode Host onto a Hostless‐Anode Substrate

“…In this case, the exact composition and concentration of the in-situ generated polysulfides in the electrolyte will be unknown and expected to be highly variable with cycling, state of charge, sulfur loading, and electrolyte amount. [14] Studies on the SEI formed on the lithium-metal chip in Li-S batteries are more precise, but the results are skewed by the excess lithium present in the system. [15] Even attempts at improving the reversibility of lithium-metal anode with the use of electrolyte additives or surface coatings still use excess lithium in the form of a lithium chip.…”

Characterizing Batteries by In Situ Electrochemical Atomic Force Microscopy: A Critical Review

“…[ 254 ] 2) X‐ray absorption spectroscopy (XAS) can reveal the existence of and Li2S2 and Li2S intermediates, and spatially resolved SoC of LiS. [ 255,256 ] 3) In situ and operando XRD is able to detect some of the intermediate sulfur phases during the redox process. [ 257 ] However, these methods are unable to detect all species and morphological changes associated with the SEI, thereby significantly limiting the field of view with respect to all parasitic reactions taking place.…”