Correlation between Redox Potential and Solvation Structure in Biphasic Electrolytes for Li Metal Batteries.

The activity of lithium ions in electrolytes depends on their solvation structures. However, the understanding of changes in Li activity is still elusive in terms of interactions between lithium ions and solvent molecules. Herein, the chelating effect of lithium ion by forming [Li(15C5)] gives rise to a decrease in Li activity, leading to the negative potential shift of Li metal anode. Moreover, weakly solvating lithium ions in ionic liquids, such as [Li(TFSI) ] (TFSI = bis(trifluoromethanesulfonyl)imide), increase in Li activity, resulting in the positive potential shift of LiFePO cathode. This allows the development of innovative high energy density Li metal batteries, such as 3.8 V class Li | LiFePO cells, along with introducing stable biphasic electrolytes. In addition, correlation between Li activity, cell potential shift, and Li solvation structure is investigated by comparing solvated Li ions with carbonate solvents, chelated Li ions with cyclic and linear ethers, and weakly solvating Li ions in ionic liquids. These findings elucidate a broader understanding of the complex origin of Li activity and provide an opportunity to achieve high energy density lithium metal batteries.