Achieving Stable Cycling of Li-Metal Anodes in Li-O2 Batteries: Optimizing Solvation Environment in Ionic Liquid/Solvent Blend Formulations

Abstract

The realization of a truly rechargeable, high capacity lithium-oxygen (Li-O2) battery faces serious technological barriers relating to material/intermediate stability, electrochemical reversibility of the cathode discharge processes, and gas supply challenges.1 Furthermore, to maximize energy densities of Li-O2 cells, the utilization of Li-metal anodes is highly desired, thus introducing additional and serious interfacial challenges in the form of electrolyte degradation and susceptibility to dendrite formation on cycling. These factors ultimately create serious safety issues and further contribute to the reduced cyclability of Li-O2 cells that is already limited by the poor reversibility of Li2O2 formation at the cathode. Without introducing artificial anode protection or isolation of Li from a cathoylte compartment, the electrolyte for a high-performance Li-O2 device must exhibit complementary stability at both reactive interfacial regions of the two different electrodes. In many cases, the solvents that mediate good performance at the Li-O2 cathode exhibit poor stability at Li-metal and, consequently, new electrolyte formulations geared towards good stability and performance at both the cathode and the anode are required.In this work, we report on the development of blend electrolytes of ionic liquids (ILs) and molecular solvents for improving cell performance in Li-O2 batteries with considered focus on enhancing stability at the Li-metal anode. Through combined experimental and theoretical investigations, we present how the electrochemical stability of Li-O2-cathode relevant molecular solvents at the Li-metal anode can be regulated by tuning formulation properties. Wide formulation ranges, including dilute to highly concentrated Li-salt mixtures are explored within the ternary formulations of IL, molecular solvent and Li-salt. Good performance in symmetrical Li|electrolyte|Li cells is achieved through improving the Coulombic efficiency of the electroplating/stripping processes, as quantified in asymmetric Li|Cu cells. In lieu of strong SEI forming additives, these observed stability enhancements are found to be highly reliant on Li+-solvent solvation interactions within the liquids. Shifts in vibrational modes associated with changes in coordination, as measured by Raman and IR spectroscopy, provide insights into the nature of these important solvent and salt interactions. These experimental insights are supported by computational modelling of local solvation environments in select exemplary mixtures. Pulsed-field gradient NMR techniques are also utilized to understand what information the diffusional motion of differing species within the IL/solvent blends further reveals about the nature of these stabilizing coordination interactions. The implications of complete formulation design towards achieving mutual stability at anode and cathode interfaces is demonstrated within full cell (Li-O2) testing using composite air-cathodes. 1. D. Aurbach, B. D. McCloskey, L. F. Nazar, and P. G. Bruce, Nat. Energy, 1 16128 (2016).