ИСТИНА

In the last decades metal-air batteries are broadly studied owing to their highest theoretical energy density and specific capacity. Among them magnesium-air system with aprotic electrolyte provides high theoretical voltage [1] and one of the highest theoretical energy densities due to the high molar charge transferred by the magnesium cation [1]. Another benefit of Mg-O2 batteries is substantially lower cost and better safety in comparison with alkali metalO2 ones. To date little is known about the nature of discharge products in Mg-O2 cells, as well as their ability to decompose electrochemically at charge, that impedes further development of this type of batteries. Here we report the study of ORR/OER on carbon surface in presence of Mg2+ ions in various aprotic electrolytes (DMSO-, diglyme-, acetonitrile-, and tetramethylene sulfonebased) by cyclic voltammetry combined with XPS and EDX to probe the discharge product composition. Finally, we explore an effect of chloride addition to DMSO- and diglyme-based electrolyte solutions on the discharge voltage. By EDX and XRD we demonstrated that in Mg-O2 system on uncatalyzed carbon cathode amorphous magnesium peroxide is the main discharge product in DMSO-based electrolyte. The precipitate also comprises sulfur-containing electrolyte residues, that is consistent with XPS analysis on HOPG electrode. Addition of chloride ions increase Mg-O2 cell discharge voltage in DMSO and diglymebased electrolytes (by decreasing Mg anode dissolution overpotential), although has no effect on the discharge capacity and does not facilitate the cell recharge. It is not possible to discharge Mg-O2 cell with diglyme, TMS, and MeCN-based electrolytes to the appreciable capacity using single compartment cell, due to the Mg anode dissolution issues. We did not observe any faradaic currents related to MgO2 oxidation on glassy carbon disk electrodes even in solvents with a high anodic oxidation limit. However, depassivation of the electrode surface occurs upon electrolyte oxidation in all studied solvents, probably due to disruption of the carbon electrode surface. In DMSO-based electrolyte at partially passivated electrode it was possible to resolve reduction of oxygen to superoxide and superoxide to peroxide. The current of second electron transfer, analogous to Li-O2 system, decreases much faster than the first one, however, MgO2 can be also formed by disproportionation reaction. We conclude that for proper operation of Mg-O2 system the electrolyte composition should be optimized in several ways. DMSO solvent, although providing discharge capacity comparable to Li-O2 system, is unstable during Mg-O2 battery discharge. Therefore, screening of a stable electrolyte enabling good ORR performance and high solubility of magnesium salts is necessary. Additionally, charge mediators for MgO2 oxidation [2] are crucial to enable the recharge process, as well as the electrolyte additives which reduce Mg dissolution overpotential. The work was financed by Russian Science Foundation (project 22-23-20089). 1. Z. Liang, Y. Lu, Mechanistic Understanding of Oxygen Electrodes in Rechargeable Multivalent Metal‐Oxygen Batteries, Batter Supercaps. 4 (2021) 1588–1598. 21 2. Q. Dong, X. Yao, J. Luo, X. Zhang, H. Hwang, D. Wang, Enabling rechargeable nonaqueous Mg–O2 battery operations with dual redox mediators, Chem Commun. 52 (2016) 13753–13756.