The future changes to our transportation systems and societal consumption patterns demand the development and improvement of reliable and safe high-energy storage systems for both mobile and stationary applications. In recent years, O2ǁLi cell chemistries have been under investigation as promising beyond-lithium ion candidates in mobile applications, mainly due to their superior theoretical specific energy densities. Conventional aprotic organic solvent-based electrolytes are often problematic for the use in such cells, as many of them lack chemical or electrochemical stability on either cathode- or anode-side. Moreover, additional drawbacks in terms of longevity and safety are posed by the volatility, toxicity and flammability of these solvents.
In this work, non-volatile electrolytes based on the ionic liquids (IL) Pyr14TFSI, Pyr13TFSI, Pip14TFSI and N1114TFSI were systematically investigated for their potential application in O2ǁLi cells. The degradation of IL-electrolytes at the current collector on the Lithium-anode side was found to be a detrimental limiting factor towards the cycle life of the investigated cells. Headspace-GC-MS and NMR-spectroscopy were used to identify degradation products of the electrolytes at four different current collector materials. By this, strong hints towards the occurrence of reductive side-chain eliminations of the IL-cations were gathered, thus indicating that the galvanic corrosion reactions take place upon the formation of galvanic couples, as no adequate passivation layer is formed on the current collector. These claims could further be supported by zero resistance ammetry and morphological surface investigations via SEM in a systematical comparison between the different IL and current collector materials. The findings are consistent with the data from electrochemical dissolution and deposition experiments at different temperatures. It is implied by the results from this work, that the contact of IL-electrolyte towards the current collector must be minimized either chemically by the formation of a passivation layer or spatially by the use of sophisticated cell designs to ensure high cycle lives of the investigated cells.
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