Understanding the lithium–sulfur battery redox reactions via operando confocal Raman microscopy

Abstract

The complex interplay and only partial understanding of the multi-step phase transitions and reaction kinetics of redox processes in lithium–sulfur batteries are the main stumbling blocks that hinder the advancement and broad deployment of this electrochemical energy storage system. To better understand these aspects, here we report operando confocal Raman microscopy measurements to investigate the reaction kinetics of Li–S redox processes and provide mechanistic insights into polysulfide generation/evolution and sulfur deposition. Operando visualization and quantification of the reactants and intermediates enabled the characterization of potential-dependent rates during Li–S redox and the linking of the electronic conductivity of the sulfur-based electrode and concentrations of polysulfides to the cell performance. We also report the visualization of the interfacial evolution and diffusion processes of different polysulfides that demonstrate stepwise discharge and parallel recharge mechanisms during cell operation. These results provide fundamental insights into the mechanisms and kinetics of Li–S redox reactions.

Kinetically Favorable Li–S Battery Electrolytes

“…It is well established that the introduction of kinetically favorable Li–S battery electrolytes might give rise to the formation of new reaction intermediates, which need to be unraveled in order to gain fundamental insights into the underlying mechanisms of complicated sulfur chemistry. To this end, advanced operando characterizations are worthy of further development to achieve the real-time and precise detection of various sulfur species along with other electrolyte components during the electrochemical cycling process. − For instance, NMR, as a critical toolbox to investigate the ion coordination structure, has been widely employed to uncover the chemical interaction and identify the reaction intermediates. In a broader context, in situ NMR techniques deserve further exploration, making it possible to inspect the chemical reaction and species evolution under the regulation of kinetically favorable Li–S battery electrolytes.…”

Turning on Lithium–Sulfur Full Batteries at −10 °C

“…In the case of the baseline electrolyte, characteristic Raman spectra of soluble polysulfides, S n 2– ( n ≥ 4), were not detected in the composite cathode during the first charging process, indicating direct conversion of Li 2 S to S 8 (Figure a–c). At the beginning of charging, the weak but distinguishable signals at 395 and 448 cm –1 corresponding to S 8 n – and S 4 2– , − respectively, appeared and continued up to the 20% charging process (Figure d–f). In the following charging up to 35%, those peaks corresponding to LiPSs gradually disappeared, and peaks corresponding to solid-phase S 8 were observed, simultaneously.…”

Competing oxygen evolution reaction mechanisms revealed by high-speed compressive Raman imaging

“…The unique liquid and solid phase chemical sensitivity and potential to be quantitative (Raman intensity is proportional to concentration), means such methods will find great use in probing other electrochemical systems, e.g. batteries, where intercalation mechanisms, solvent/electrolyte polarisation gradients and interfacial reactions may all be studied 40 . This is particularly in contrast to other low-cost optical techniques such as scattering/reflection microscopy 24,41 which are limited to solids and can also be challenging to interpret/make quantitative.…”