(Invited) Challenges in Reversible Electrodeposition and Stripping of Lithium Peroxide for Li-Air Batteries
Authors: Barde Fanny 
Published: December 2020

Thanks to a high theoretical energy density combined to a relatively high operation voltage around 3 Volts versus Li+/Li, the non-aqueous Li-Air battery is a good candidate for future electric cars with an increased driving range. In principle, this battery relies on the reversible electrochemical reaction between the metallic Li and the gaseous O2 in presence of a non-aqueous electrolyte where Li+ ions and dissolved O2 molecules diffuse and react to form the ideal discharge product lithium peroxide Li2O2. However, this apparently simple reaction process appears to be more complicated when put into practice.
Considering the Li-O2 discharge process, there are two main models proposed in the literature to explain the Oxygen Reduction Reaction which were unified in a single mechanism process where Li2O2 formation goes through a surface reaction or/and a solution reaction1, depending on system parameters such as the applied potential or the solvent properties. Using an electrochemical quartz crystal microbalance (EQCM) we studied the products formed in discharge as well as the reversibility of the reactions from a mass point of view during cycling.2

Regarding the Li-O2 charge process, the Li2O2 decomposition occurring is a rather sluggish and incomplete process. A lot of efforts was made in this area by the research community, and several approaches were proposed to facilitate and fasten the Li2O2 decomposition: the use of solid state catalyst of several kind (doped carbon, non-precious materials...) or soluble catalysts also called redox mediators, such as for example TTF3 (tetrathiafulvalene), TEMPO4 (2,2,6,6-tetramethylpiperidinyloxyl), Cobalt Bis(terpyridine)5, lithium iodide6 (LiI). The mobile redox mediators, electron-hole transfer agents, aid the oxidation of Li2O2 as has been reported previously. In our EQCM study we activated in situ the TTF and confirms its usefulness on the mass reversibility of the deposition/dissolution of Li2O2 during cycling2. Later on, we performed a detailed study of the effect of LiI redox mediator on the formation and decomposition of Li2O2.7 We showed that the solvent could drastically change the mechanism of decomposition during charge8. Finally, we will discuss the parameters able to influence the most the deposition and stripping of Li2O2 in a non-aqueous medium.

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