Direct Evidence of Reversible Changes in Electrolyte and its Interplay with LiO2 Intermediate in Li-O2 Batteries.

Lithium-oxygen batteries show promising energy storage potential with high theoretical energy density; however, further investigation of chemical reactions is required. In this study, experimental Raman and theoretical analyzes are performed for a Li-O2 battery with LiClO4/dimethyl sulfoxide (DMSO) electrolyte and carbon cathode to understand the role of intermediate species in the reactional mechanism of the cell using a high donor number solvent. Operando Raman results reveal reversible changes in the DMSO bands, in addition to the formation and decomposition of Li2O2. On discharge, a decrease in DMSO polarizability is observed and bands of DMSO-Li+-anion interactions are evidenced and supported by ab initio density functional theory (DFT) calculations. Molecular dynamics (MD) force field simulations and operando Raman show that DMSO interacts with LiO2(sol), highlighting the stability of the electrolyte compared to the interaction with reactive O 2 - ${\rm O}_2^{-}$ . On charging, the presence of Li+ indicates the formation of a lithium-deficient phase, followed by the release of Li+ and oxygen. Therefore, this study contributes to understanding the discharge/charge chemistry of a Li-O2 cell, employing a common carbon cathode and DMSO electrolyte. The combination of a simple characterization technique in operando mode and theoretical studies provides essential information on the mechanism of Li-O2 system.

Operando Synchrotron XRD of Bromide Mediated Li-O2 Battery.

Li-O2 battery technology offers large theoretical energy density, considered a promising alternative energy storage technology for a variety of applications. One of the main advances made in recent years is the use of soluble catalysts, known as redox mediators (RM), decreasing the charge overpotential and improving cyclability. Despite its potential, much is still unknown regarding its dynamic, especially over higher loading electrodes, where mass transport may be an issue and the interplay with common impurities in the electrolyte, like residual water. Here we perform for the first time an operando XRD characterization of a DMSO-based LiBr mediated Li-O2 battery with a high loading electrode based on CNTs aiming to reveal these dynamics and track chemical changes in the electrode. Our results show that, depending on the electrode architecture, the system's issue can move from catalytic to a mass transfer. We also assess the effect of residual water in the system to better understand the reaction routes. As a result, we observed that with DMSO, the system is even more sensitive to water contamination compared to glyme-based studies reported in the literature. Despite the activity of LiBr on the Li-peroxide oxidation and its contribution to cyclability, with the system and electrode configuration used in this study, we verified that a mass transfer limitation caused a cell "sudden death" caused by clogging after cycling.