Stefano Meini

Stability of superoxide radicals in glyme solvents for non-aqueous Li–O2 battery electrolytes

Glyme-based electrolytes were studied for the use in lithium-air batteries because of their greater stability towards oxygen reduction reaction intermediates (e.g., superoxide anion radicals (O2˙(-))) produced upon discharge at the cathode compared to previously employed carbonate-based electrolytes. However, contradictory results of glyme stability tests employing KO2 as an O2˙(-) source were reported in the literature. For clarification, we investigated the reaction of KO2 with glymes of various chain lengths qualitatively using (1)H NMR and FTIR spectroscopy as well as more quantitatively using UV-Vis spectroscopy. During our experiments we found a huge impact of small quantities of impurities on the stability of the solvents. Therefore, we studied further the influence of impurities in the glymes on the cycling behavior of Li-O2 cells, demonstrating the large effect of electrolyte impurities on Li-O2 cell performance.

The Use of Redox Mediators for Enhancing Utilization of Li2S Cathodes for Advanced Li–S Battery Systems

The development of Li2S electrodes is a crucial step toward industrial manufacturing of Li-S batteries, a promising alternative to Li-ion batteries due to their projected two times higher specific capacity. However, the high voltages needed to activate Li2S electrodes, and the consequent electrolyte solution degradation, represent the main challenge. We present a novel concept that could make feasible the widespread application of Li2S electrodes for Li-S cell assembly. In this concept, the addition of redox mediators as additives to the standard electrolyte solution allows us to recover most of Li2S theoretical capacity in the activation cycle at potentials as low as 2.9 VLi, substantially lower than the typical potentials >4 VLi needed with standard electrolyte solution. Those novel additives permit us to preserve the electrolyte solution from being degraded, allowing us to achieve capacity as high as 500 mAhg(-1)Li2S after 150 cycles with no major structural optimization of the electrodes.

Nanosized Carbon‐Supported Manganese Oxide Phases as Lithium–Oxygen Battery Cathode Catalysts

The poor discharge and recharge efficiency demonstrated by lithium–air batteries renders the search for highly active and inexpensive oxygen reduction and evolution catalysts crucial to the development of these energy‐storage and conversion devices. Previous works have shown that manganese oxides are promising lithium–oxygen cathode catalysts, which is in agreement with their remarkable activities for the reduction and evolution of oxygen in aqueous media. Motivated by these resembling catalytic behaviors, we prepared and characterized a number of manganese oxide modifications directly on carbon black and attempted to correlate their oxygen reduction and evolution activities in aprotic and aqueous electrolytes. Although our results cannot confirm this correlation, they provide valuable insight into the reaction mechanisms at play in each medium. More precisely, in 0.1 M potassium hydroxide, the reduction of oxygen is related to the reduction of a manganese(III) intermediate whereas the oxidation of hydrogen peroxide (which was regarded as a mimic of the lithium peroxide produced upon lithium–oxygen battery discharge) correlates with the transition between manganese(II) and manganese(III) phases. In the aprotic medium, manganese oxide cathodes prefilled with lithium peroxide showed a strong catalytic effect but were not active in the oxidation of lithium peroxide produced in the previous discharge. This discrepancy is thought to arise from the stark differences in the sizes and morphologies of the lithium peroxide involved in each test, which implies that the catalytic activity of a material for the oxidation of lithium peroxide prefilled on electrodes is not indicative of its behavior in the charging of a real lithium–oxygen cell.