A Novel Optical Diagnostic for In Situ Measurements of Lithium Polysulfides in Battery Electrolytes

Abstract

An optical diagnostic technique to determine the order and concentration of lithium polysulfides in lithium-sulfur (Li-S) battery electrolytes has been developed. One of the major challenges of lithium-sulfur batteries is the problem of polysulfide shuttling between the electrodes, which leads to self-discharge and loss of active material. Here we present an optical diagnostic for quantitative in situ measurements of lithium polysulfides using attenuated total reflection Fourier transform infrared (FT-IR) spectroscopy. Simulated infrared spectra of lithium polysulfide molecules were generated using computational quantum chemistry routines implemented in Gaussian 09. The theoretical spectra served as a starting point for experimental characterization of lithium polysulfide solutions synthesized by the direct reaction of lithium sulfide and sulfur. Attenuated total reflection FT-IR spectroscopy was used to measure absorption spectra. The lower limit of detection with this technique is 0.05 M. Measured spectra revealed trends with respect to polysulfide order and concentration, consistent with theoretical predictions, which were used to develop a set of equations relating the order and concentration of lithium polysulfides in a sample to the position and area of a characteristic infrared absorption band. The diagnostic routine can measure the order and concentration to within 5% and 0.1 M, respectively.

The role of polysulfide dianions and radical anions in the chemical, physical and biological sciences, including sulfur-based batteries

“…120 Instead two other types of reactions by which polysulfide dianions can be interconverted are predicted to be important. The first one is the nucleophilic attack of a small dianion on a larger radical monoanion with formal exchange of sulfur atoms according to eqn (25):…”

“…Such reactions are predicted as generally exothermic and exergonic both in the gas phase and in a polarizable continuum with e = 8 or 78 and both at 298 K and at 600 K. 44 Reactions (25) and (26) allow the observed equilibria between polysulfide dianions of differing chain length to be established in solutions both at room temperature and above. Evidently, the driving force for such reactions is the reduction of the average atomic charges in the products compared to the starting materials (charge equalization).…”

“…15 Seminal work by Rauh and co-workers more than 40 years ago established that the stoichiometric reactions of lithium sulfide with cyclo-S 8 in solvents such as DMSO, DMF or THF are the most efficient synthesis of lithium polysulfides with the nominal composition ''Li 2 S n '' [eqn (1)], 23 and that method is still used in investigations of these materials during the last few years. 22,24,25 However, these products are comprised of an equilibrium mixture, both in the solid state and in solution, and there is no straightforward procedure for the separation of individual polysulfides (Section 3.4).…”

In Situ/Operando Spectroscopic Characterizations Guide the Compositional and Structural Design of Lithium–Sulfur Batteries

“…Infrared (IR) spectroscopy is an adsorption spectroscopy, which has been widely used for distinguishing molecules with different functional groups. Since different chemical functional groups can adsorb the incident infrared light at some specific frequencies, thus causing the specific jumping of the vibrational and rotation energy levels of the target molecules . Specifically, the measured IR spectra could be converted to FTIR spectra by the combination of an interferometer and the mathematic Fourier transform.…”

“…FTIR, a well‐known powerful optical spectroscopic approach that owns the advantages of low cost, short acquisition time, and ease operation, has been adopted to investigate the reaction mechanisms of Li–S batteries. More importantly, FTIR can be applied to identify various polysulfide species based on the SS vibrational modes . For instance, Dillard et al proposed the evolution of polysulfide species and triflate anion coordination states during cycling by using in situ FTIR with attenuated total reflection mode .…”