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Stable Formulations for the Lithium and Sodium Metal Interfaces in Alkali Metal-Oxygen Batteries

The high reactivity of both positive and negative interfacial regions in metal-oxygen (M-O2) batteries leads to a variety of parasitic degradation reactions that reduce reversibility on charge/discharge and can lead to rapid cell failure. To reversibly access the high theoretical specific energies these chemistries promise (3456 and 1602 Wh·kg-1 for Li-O2 and Na-O2, respectively), reducing charging overpotentials and the design of new electrode and electrolyte materials with improved stabilities are key targets. Considering the M|electrolyte interface, where decomposition reactions and dendrite formation generates severe safety concerns as well as reduced cycling stabilities, surface protection of the alkali metal anode by pre-treatment techniques or additive formulation has been among the more successful methodologies.1
 
 In this work, we present a formulation strategy targeting enhanced cycling stabilities of the metal anode in M-O2 cells. Initially demonstrated in the Li-O2 chemistry, we report on an in-depth investigation into the electrochemical performance and the spectroscopic and diffusional properties of ternary ionic liquid (IL)-molecular solvent blend electrolytes.2 By introduction of the IL and exploring a wide formulation range (Figure 1a), we attain significant enhancements in the stability of practically-relevant electrolyte materials that otherwise fail rapidly at alkali metal anodes (Figure 1b), achieving the highest Coulombic efficiencies for Li plating/stripping in this solvent. The optimized formulations consequently displayed enhanced performances in Li-O2 full cells and ex situ characterization of Li surfaces also revealed the suppression of dendrite formation and important changes in the solid electrolyte interphase as a result of modulating solvent reactivity. Using Raman spectroscopy and diffusional analyses, we relate this closely to the local solvation environments involving the reactive solvent component in optimized formulations and, therefore, elucidate critical solvent/Li-salt ratios for improved stability.
 By understanding key shifts in Raman spectra relating to solvation effects in concentrated Li formulations, and how this modulates cycling performances, we further demonstrate the applicability of this relationship and formulation strategy to the Na-O2 battery chemistry. Therein, the Na metal interface is comparably more reactive and, therefore, presents greater stability challenges, with decomposition reactions in non-optimized formulations more rapidly causing plating and stripping at the anode to fail. However, we demonstrate the plating/stripping of Na (Figure 1c) and cycle lifetime in Na-O2 cells can be hugely improved by applying the above strategy and understandings of critical solvation interactions to equivalent formulations based on Na-salts.
 
 
 
 Y. Yu, G. Huang, J.-Z. Wang, K. Li, J.-L. Ma, and X.-B. Zhang, Adv. Mater. (Weinheim, Ger.), 
 32 (38), 2004157 (2020).
 
 
 A. R. Neale, R. Sharpe, S. R. Yeandel, C.-H. Yen, K. V. Luzyanin, P. Goddard, E. A. Petrucco, and L. J. Hardwick, Adv. Funct. Mater., 
 31 (27), 2010627 (2021).
 
 
 
 
 Figure 1. (a) Ternary plot highlighting the changes in stability of Li/Na metal anode cycling as a function of the studied formulation ranges and example voltage profiles of symmetric (b) Li|Li cells and (c) Na|Na cells with non-optimized (navy/purple) and optimized (orange) electrolyte formulations.
 
 
 
 
 
 Figure 1
