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...
Li-S batteries are a "beyond Li-ion" technology that delivers large capacity (1672 mAh g -1 ) while also being earth-abundant, lightweight, and low cost. Li-S achieves high capacity via a chemical transformation mechanism rather than Li intercalation as in Li-ion. Elemental sulfur S8 is reduced through a series of soluble chain-like polysulfides (LiPS; Li2Sx, 2≤x≤8) to a final solid discharge product Li2S; this reaction is reversed on charging. However, Li-S suffers from unrealized theoretical capacity and capacity fade due to loss mechanisms that are not well understood. LiPS, while necessary for battery operation, are likely a critical factor in degradation processes; thus, deciphering the speciation and spatial distribution of dissolved LiPS in the electrodes and electrolyte is essential for future battery designs. LiPS are known to dissociate or react with each other to form intermediate species in the electrolyte [1,2]; therefore, operando characterization is necessary to understand the conditions and locations in which LiPS form before additional reactions occur. Operando X-ray absorption spectroscopy has been performed on Li-S in both conventional and cross-sectional geometries [3][4][5], but literature on operando mapping is limited [6,7]. In addition, the solid electrolyte interphase (SEI), a surface film that forms on electrodes, is largely mysterious, especially in newer Li-S chemistries such as those with a polyethylene oxide binder.In this study, ex situ and operando spectromicroscopy at the sulfur K-edge (2472 eV) are used to study the SEI on Li metal and LiPS during cycling. All batteries consisted of Li metal foil with a sulfur/carbon composite thin film electrode (average sulfur loading = 6 mg cm -2 ) on carbon-coated aluminum foil. For ex situ characterization, the conventional electrolyte chemistry of 1 M bis(trifluoromethane)sulfonimide Li salt (LiTFSI) with 0.3 M LiNO3 in a 1:1 mixture by volume of 1,3-dioxolane and 1,2-dimethoxyethane (DOL/DME) was used. For operando measurements, an electrolyte solution of 1 M LiClO4, another common battery salt, with 0.2 M LiNO3 in DOL/DME was used to eliminate the signal from the sulfurcontaining LiTFSI salt. Thus, any detected sulfur signal originated from the sulfur species in the electrode.Ex situ characterization was performed on Li foil removed from a cycled coin cell to determine the composition of the surface films, shown in Figure 1. The energies in (a), (b), and (c) correspond to LiPS, Li2S, and LiTFSI, respectively. These maps demonstrate that even at low current (0.04 mA cm -2 ), active material is lost to deposition of Li2S on the electrode surface.Because the diffusion behavior of the LiPS across the electrolyte is of great interest, a cross-sectional cell (Figure 2) was designed to track LiPS in the separator/electrolyte and cathode region. Figure 3 shows the discharge/charge curve of a cross-sectional battery along with LiPS maps taken at 2470.35 eV [7]. Significant microstructural changes are seen in the electrode, even during the first d...
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