Stability of Glyme-Solvate Ionic Liquid Electrolyte in the Presence of Oxygen Reduction Product

The lithium-oxygen (Li-O₂) battery continues to attract much attention as one of the more promising future energy storage technologies, due in part to its high theoretical gravimetric energy density.[1] In earlier work, we reported the use of a solvate ionic liquid (SIL) as electrolyte for the Li-O₂ system, and demonstrated reversible electrochemical reaction could be achieved via cyclic voltammetry.[2] In the current work, we report on our further study of the SIL system, applied to a functioning Li-O₂ battery and here present an investigation probing one of the key challenges for the Li-O₂battery, namely the reactivity of the electrolyte.

SILs are exemplified by the equimolar binary mixture of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4), shown in Fig. 1(a), which forms a room temperature liquid comprised of a solvate cation and counter-anion, [Li(G4)₁][TFSA]. The intriguing thermal and physicochemical properties of such SILs, including negligible vapor pressure, high ionicity and a low proportion of free (un-coordinated) solvent, have been previously reported.[3] SILs also possess attractive characteristics as electrolytes for electrochemical devices including high molar Li ion concentration and high transference numbers.[4] We have previously demonstrated the use of SILs for battery systems, utilizing both conventional Li intercalation electrodes, and metallic lithium, notably the lithium-sulfur battery.

Fig 1(b) shows a typical discharge-recharge curve for the application of [Li(G4)₁][TFSA] in a Li-O₂battery. The high charge over-potential observed is a widely reported characteristic of this cell chemistry, and not only reduces the overall energy efficiency of the cell, but also provides an oxidative potential sufficient to decompose typical dilute glyme-based electrolytes, contributing to low cycle-ability. As we have previously demonstrated, the formation of a SIL results in an improved oxidative stability, due to the lowering of the HOMO level of the glyme upon coordination with Li,[4,5] and thus the use of a SIL electrolyte provides a means to ensure electrolyte stability at the high oxidative potential observed during recharge.

However, the reactivity of the oxygen reduction intermediate (O₂^- radical) and product (Li₂O₂) with electrolyte solvent is another widely reported challenge, and has not yet been clarified. Here we report on our study of the stability of the SIL compared to the dilute solutions, and comment on these findings in light of recent reports of (i) a decreased reaction barrier for simulated C-O bond breaking of G4 coordinating with Li⁺,[6] and (ii) the switch in the principal contributor to reductive stability (LUMO) from solvent to anion for highly concentrated acetonitrile/TFSA solutions.[7]

References
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Acknowledgements
This study was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST) and by the Technology Research Grant Program of the New Energy and Industrial Technology Development Organization (NEDO) of Japan.