Electrolyte Composition In Li/O-2 Batteries With Lil Redox Mediators: Solvation Effects On Redox Potentials And Implications For Redox Shuttling

The use of LiI as redox mediator for the charge reaction in nonaqueous Li/O-2 cells has been widely studied recently, as a possible means to fulfill the great promise of the Li/O-2 system as a high energy density "beyond Li-ion" battery. In this work, we highlight the importance of considering the redox potential for both the I-/I-3(-) and I-3(-)/I-2 redox couples and how the electrolyte solvent (here tetraglyme (G4) and dimethyl sulfoxide (DMSO)) and concentration (here 1.0 and 2.8 M) have a profound influence on these potentials. Through a combination of galvanostatic cycling, electrochemical mass spectrometry, and cyclic voltammetry, we thus consider the influence of solvent and electrolyte concentration on both the redox mediation and redox shuttle processes and suggest that this important aspect must be considered for further studies with mediators in Li/O-2 and related systems. We demonstrate that, in our system, 100 mM LiI in 1.0 M Li[TFSA]/DMSO provides the most effective redox mediation among the electrolytes we have studied but conversely exhibits the highest degree of redox shuttling (in the absence of O-2). The balance between effective limitation of redox shuttle and ease of mediator diffusion to discharge products is of great importance and should be considered in any future cell design utilizing a mediator.