Exploring Ionic Liquid-Solvent Blend Formulations for the Stable Cycling of Li-Metal Anodes in Li-O2 Batteries
Authors: Neale Alex Ryan , Sharpe Ryan , Yeandel Stephen , Goddard Pooja , Petrucco Enrico , Hardwick Laurence J. 
Published: February 2020

The scope of non-aqueous lithium-oxygen (Li-O2) battery chemistries as valuable “beyond Li-ion” energy storage technologies relies on complex cathode processes for the reversible generation of Li-oxide species.1 As such, the majority of Li-O2 battery research justifiably focuses on the (electro)chemistry of the cathode and the additional complications of applying Li-metal as the full-cell anode in Li-O2 cells have received less attention. It is highly desirable to utilize Li-metal anodes to maximize the attainable energy density of practical devices but the high reactivity of Li0, coupled with the tendency for dendrite formation, leads to severe cyclability and safety issues.2 In Li-O2 batteries, this is further complicated by the reactive nature of the cathode reaction intermediates and products. Therefore, in lieu of anode protection or separation, the electrolyte media needs to exhibit mutual stability/performance at the two contrasting interfacial environments to achieve a highly rechargeable system. In this regard, many interesting solvents utilized in Li-O2 battery investigations decompose readily in contact with Li-metal (e.g. DMSO, amides) and there has been some recent electrolyte literature showing methods for stabilizing Li plating/stripping in these solvents.3, 4 Furthermore, to also impede electrolyte evaporation via the cathode under dynamic gas-flow of an operating Li-O2 cell, candidate electrolytes must exhibit low vapor pressures.

In this work, we report on the formulation and testing of liquid blend electrolytes based on Li-O2-cathode relevant components for the stable electroplating/stripping cycling at the Li-metal anode. Blends were prepared and studied through iterative formulations with selected molecular solvents, ionic liquids (ILs) and lithium salts. Therein, low boiling solvents (e.g. DME and acetonitrile) were immediately excluded, along with solvents known to decompose during O2 reduction at the cathode (e.g. organic carbonates). Galvanostatic cycling in symmetrical Li|electrolyte|Li cells was utilized under low current/capacity regimes to screen the electrolyte/electrode interfacial stability and under higher currents/capacities to compare performances with leading electrolytes for Li-anode application (generally not concerning Li-O2 cathode relevant materials). Furthermore, the application of asymmetric Li|electrolyte|Cu cells was used for the accurate determination of Coulombic efficiency of the Li plating/stripping process.5 Simple introduction of stable ILs into binary electrolyte formulations in rational quantities effectively stabilized otherwise reactive molecular solvents and the importance of Li+-cation solvation on solvent stabilization was further explored spectroscopically. Through further iterative improvements of more complex electrolyte formulations, over 3000 h of continued low current Li-plating/stripping cycling has been achieved (with >97% Coulombic efficiency of Li plating/stripping measured in Li|Cu cells). Higher Coulombic efficiencies and longer cycle lifetimes must be achieved under industrially relevant current densities to enable practical devices, but these results are notable given the electrolytes contain the highly reactive components considered useful at the Li-O2 cathode.

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