(644d) Electrolyte Salt Considerations Toward a High-Capacity, Reversible Li-Air (Li-O2) Battery

The nonaqueous lithium-oxygen (Li-O₂) electrochemistry has garnered significant attention due to its high theoretical specific energy compared to the state-of-the-art lithium-ion battery. Commonly consisting of a lithium negative electrode, an oxygen-saturated porous carbon positive electrode, and an anhydrous ether-based electrolyte, the nonaqueous Li-O₂ battery operates via the electrochemical formation (discharge) and decomposition (charge) of lithium peroxide, Li₂O₂. Unfortunately, lithium peroxide (Li₂O₂) is an insulator and is insoluble in ether-based electrolytes, causing charge transport issues at even small depths of discharge. Additionally, Li₂O₂ participates in irreversible parasitic reactions with both the ether-based electrolyte and carbon positive electrode, inhibiting the reversibility of the desired oxygen electrochemistry.¹

In this work, we present recent insights into electrolyte considerations toward mitigating these challenges. We report on our recent results that appropriately selecting the salt anion in the electrolyte solution can circumvent passivation related to Li₂O₂ deposition, resulting in a fourfold increase in Li-O₂ cell capacity without adversely affecting electrochemical stability.² This improvement is a result of enhanced Li⁺ stability in solution imparted by anions with high Lewis basicity. Enhanced Li⁺ stability in solution induces solubility of the lithium superoxide (LiO₂) intermediate to Li₂O₂ formation, leading to Li₂O₂ formation in aggregated toroid structures, leaving carbon sites open for further reaction. Our anion selection was confirmed to have no effect on electrochemical stability via differential electrochemical mass spectrometry and Li₂O₂ titrations. This anion selection strategy could be useful in any electrochemical system where adjusting intermediate stability in solution could induce desired mechanisms of product formation, and it corroborates the development of a molten salt electrolyte of entirely LiNO₃ and KNO₃ toward high Li₂O₂ solubility and the absence of organic solvent decomposition.³

References:

(1) McCloskey, B. D.; Burke, C. M.; Nichols, J. E.; Renfrew, S. E. Chem Commun 2015, 51, 12701.

(2) Burke, C. M.; Pande, V.; Khetan, A.; Viswanathan, V.; McCloskey, B. D. Proceedings of the National Academy of Sciences 2015, 112, 9293.

(3) Giordani, V.; Tozier, D.; Tan, H.; Burke, C. M.; Gallant, B. M.; Uddin, J.; Greer, J. R.; McCloskey, B. D.; Chase, G. V.; Addison, D. Journal of the American Chemical Society 2016, 138, 2656.