Solvent and electrolyte instability during Oxygen Reduction Reaction in Li-O2 battery cathodes

ALVARO Y. TESIO; TORRES, WALTER R.; WILLIAMS, FEDERICO J.; SANTIAGO E. HERRERA; FLORENCIA MARCHINI; MOZHZHUKHINA, NATALIIA; CALVO, ERNESTO J.

Buenos Aires

Congreso; 20th Topical Meeting of the International Society of Electrochemistry; 2017

International Society of Electrochemistry

The importance of lithium metal and lithium salts has rapidly increased due to its multiple applications in diverse fields. Particularly, lithium is the key component in a number of electrochemical devices for energy storage and conversion; some of them constitute nowadays a mature technology (the well-known lithium-ion batteries) whereas some others are still under development (which is the case of Li-O2 high energy density batteries for electric vehicles). As for the latter, during the discharge reaction (ORR) the O2 from the air is reduced to form lithium peroxide at the cathode, and the reverse reaction (OER) takes place during charging in the ideal Li-O2 battery: 2Li (s) + O2 (g) ↔ Li2O2 (s)It is precisely in the reversibility of this reaction where the success of the device relies, as the occurrence of "parasitic" reactions results in a loss of charge and a greater over potential of recharge, in case the products of these reactions are deposited on the surface of the electrode. Therefore, one of the major obstacles to overcome is the choice of a suitable solvent/electrolyte pair, as it must exhibit complete stability against the highly reactive species generated during the discharge process. In this context, the stability towards the species arising from ORR and further OER of different solvent/electrolyte pairs (combinations of DMSO or acetonitrile with LiPF6 or LiBF4) has been tested. For this purpose, a vast number of different techniques have been used in order to study these systems from different and complementary points of view: the electrode surface, the liquid phase and even the gas phase, leading us to the conclusion that all solvent/electrolyte pairs tested were unstable at some extent.As for the understanding of the surface chemistry taking place at the electrode-electrolyte interface, X-ray and UV-ray photoelectron spectroscopies (XPS and UPS, respectively) have been performed. These experiments were carried out in an electrochemical cell coupled to an ultra high vacuum line (UHV) equipped with a transfer system built in our laboratory that allows easy, rapid and controlled transfer of the sample between the UHV environment and the ambient pressure (Argon) liquid electrochemical environment. This transfer system serves the purpose to perform ex-situ electron spectroscopic measurements on samples that are initially clean in UHV and are not exposed to the laboratory atmosphere before or after electrochemical measurements, ensuring optimal clean conditions for the experiments. Surface studies were also complemented with atomic force microscopy (AFM) imaging. The electrolyte solution was monitored using IR spectroscopy, while the composition of the gas mixture released during ORR and OER was examined with differential electrochemical mass spectroscopy (DEMS). All these results will be presented as pieces of the same puzzle that, when put together, give the whole picture for a deep understanding on the unfeasibility of the solvents and electrolytes tested to be used in a future Li-O2 battery. References1.F. Marchini, S. Herrera, W. Torres, A.Y. Tesio, F.J. Williams, E.J. Calvo. Langmuir, 31 (33), July 2015. 2.F. Marchini, S. Herrera, E.J. Calvo, F.J. Williams. Surface Science, (646) 154 ? 159, August 2015.3.Nataliia Mozhzhukhina, Lucila P. Méndez De Leo, Ernesto Julio Calvo. The Journal of Physical Chemistry C, 117(36):18375?18380, September 2013.4.Walter Torres, Alvaro Yamil Tesio, Nataliia Mozhzhukhina, Ernesto J. Calvo. Journal of The Electrochemical Society 161(14), October 2014.