Manu U. M. Patel

X-ray Absorption Near-Edge Structure and Nuclear Magnetic Resonance Study of the Lithium–Sulfur Battery and its Components

Understanding the mechanism(s) of polysulfide formation and knowledge about the interactions of sulfur and polysulfides with a host matrix and electrolyte are essential for the development of long-cycle-life lithium-sulfur (Li-S) batteries. To achieve this goal, new analytical tools need to be developed. Herein, sulfur K-edge X-ray absorption near-edge structure (XANES) and (6,7) Li magic-angle spinning (MAS) NMR studies on a Li-S battery and its sulfur components are reported. The characterization of different stoichiometric mixtures of sulfur and lithium compounds (polysulfides), synthesized through a chemical route with all-sulfur-based components in the Li-S battery (sulfur and electrolyte), enables the understanding of changes in the batteries measured in postmortem mode and in operando mode. A detailed XANES analysis is performed on different battery components (cathode composite and separator). The relative amounts of each sulfur compound in the cathode and separator are determined precisely, according to the linear combination fit of the XANES spectra, by using reference compounds. Complementary information about the lithium species within the cathode are obtained by using (7) Li MAS NMR spectroscopy. The setup for the in operando XANES measurements can be viewed as a valuable analytical tool that can aid the understanding of the sulfur environment in Li-S batteries.

Application of In Operando UV/Vis Spectroscopy in Lithium–Sulfur Batteries

Application of UV/Vis spectroscopy for the qualitative and quantitative determination of differences in the mechanism of lithium-sulfur battery behavior is presented. With the help of catholytes prepared from chemically synthesized stoichiometric mixtures of lithium and sulfur, calibration curves for two different types of electrolyte can be constructed. First-order derivatives of UV/Vis spectra show five typical derivative peak positions in both electrolytes. In operando measurements show a smooth change in the UV/Vis spectra in the wavelength region between λ=650 and 400 nm. Derivatives are in agreement with derivative peak positions observed with catholytes. Recalculation of normalized reflections of UV/Vis spectra obtained in operando mode enable the formation of polysulfides and their concentrations to be followed. In such a way, it is possible to distinguish differences in the mechanism of polysulfide shuttling between two electrolytes and to correlate differences in capacity fading.

A Cross-Linked Soft Matter Polymer Electrolyte for Rechargeable Lithium-Ion Batteries

Over the last few decades, there has been an upsurge in research activity to develop alternatives for conventional liquid electrolytes (e.g. , lithium hexafluorophosphate (LiPF6) in 1:1 volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC)). One such direction has been to synthesize soft matter electrolytes based on polymers, 4] ionic liquids, 6] and plastic crystalline materials. 8] This has facilitated the idea of development of an all solid-state lithium ion battery. Solid polymer electrolytes (SPEs) are the most widely studied soft matter electrolyte comprising essentially of a salt (say lithium) solvated in a polyether matrix. The major drawback of SPE is low ambient temperature ionic conductivity (ca. 10 6 W 1 cm ). Several attempts have been made to overcome this drawback. Notable among them are the gel electrolytes (non-aqueous liquid solvent + polymer electrolyte) and composite polymer electrolytes (heterogeneous doping with oxide filler). However, both approaches have failed to generate materials with optimized ionic conductivity, mechanical, and electrochemical properties simultaneously. It has been proposed that special polymer architecture, copolymerization may additionally aid in optimization of physical properties such as ionic conductivity and mechanical strength of polymer and hence generate superior polymer electrolytes. We demonstrate here for the first time, a soft matter electrolyte obtained from free-radical polymerization of vinyl monomers in a liquid solution matrix comprising of dinitrile adiponitrile (N C (CH2 CH2)2 C N, ADPN) and lithium salt [bis(trifluoromethanesulfonimide), LiTFSI] . ADPN was chosen as it is a colorless, moderately viscous (ca. 10 2 Pa) liquid with a dielectric constant (e) of 30. It readily dissolves a wide variety of ionic lithium salts and demonstrated to be non-corrosive towards metallic current collector in Ref. [16] We present here the electrical, thermal, mechanical, and electrochemical properties of the cross-linked polymer electrolytes obtained from the new methodology. For acrylonitrile (AN)/ADPN >0.20 (w/w), free radical polymerization in LiTFSI-ADPN solution resulted in gel-like solid electrolytes (Figure 1D,E). For all concentration regimes, no phase separation was observed and electrolyte samples were homogeneous and mechanically stable. Figure 2 shows the thermogravimetric analysis (TGA) for ADPN and AN/ADPN (0.25 and 0.31). The TGA trace for ADPN showed a 100% weight loss in a single step between 120 8C (onset) and 230 8C. The gel electrolytes on the other hand showed a two step weight loss of total 83% in the temperature range between 100 8C to 450 8C. The initial loss of approximately 65% (2–4% below 100 8C and 61% within 100-200 8C) corresponds to the decomposition of unpolymerized monomers and adiponitrile present in the polymer-gel electrolyte and the final step of 18% (200– 450 8C) correspond to the cross-linked polymer network. Thus, formation of a polymer network in the LiTFSI–ADPN leads to an electrolyte with higher thermal stability. Mechanical properties of the neat and polymer-gel electrolytes were studied using static and dynamic rheology (Figure 3a,b). Figure 3a shows the variation of viscosity (h) as a function of shear rate (ġ) for LiTFSI–ADPN and AN/ADPN >0.20. For the liquid electrolyte (i.e. , 0 wt% AN), the viscosity is nearly independent of the shear rate, indicating a Newtonian behavior. On the other hand, the viscosity decreases with an increase in shear rate for

Stable Crystalline Forms of Na Polysulfides: Experiment versus Ab Initio Computational Prediction.

For the design of light-metal-sulfur batteries and for the understanding of their performance, knowledge on the stable crystalline polysulfides is very important. We confronted experimental and ab initio crystal structure prediction studies on the stability of Na polysulfides. The selected evolutionary-based structure-prediction algorithm was able to quickly and correctly predict the thermodynamically stable crystalline forms of Na polysulfides with small unit cells. For Na polysulfides with large unit cells, the algorithm correctly proposed short unbranched polysulfide chains to be energetically favorite structural motifs, but could not find proper three-dimensional structures in the limited number of generations. Experimentally, the polysulfides were studied by X-ray diffraction and 23 Na solid-state NMR spectroscopy. Complemented by calculations of the isotropic chemical shifts and quadrupolar coupling constants, NMR spectroscopy proved to be an excellent tool for the examination of Na polysulfides, because it allowed easy distinction and quantification of components in the samples.