LiF Protective Layer on a Li Anode: Toward Improving the Performance of Li-O2 Batteries with a Redox Mediator.

The high charging overpotential and insulating/insoluble Li2O2 discharge products have seriously hindered the development of Li-O2 batteries. Here, we report a highly concentrated 5.0 M lithium nitrate (LiNO3) in N,N-dimethylacetamide (DMA) with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a redox mediator (RM) and with the LiF layer by adding N,N-dimethyltrifluoroacetamide (DMTFA) to the electrolyte, which reduces the charge voltage (3.6 V over 65 cycles) and allows stable cycling for 100 cycles without noticeable fading in capacity. The Li plating/stripping test and electrochemical impedance of the Li/Li symmetric cell results reveal that a Li/Li symmetric cell with DMTFA is stable due to the formation of a LiF protective layer on the Li metal, which suppresses the RM shuttle effect, improving the interface stability of Li and the electrolyte and also restraining the growth of dendrite during cell cycling. This work may provide a novel strategy for designing a protective layer for Li anodes in Li-O2 batteries when using an RM.