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)2 ]- (TFSI = bis(trifluoromethanesulfonyl)imide), increase in Li+ activity, resulting in the positive potential shift of LiFePO4 cathode. This allows the development of innovative high energy density Li metal batteries, such as 3.8 V class Li | LiFePO4 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.

Three-Dimensional Porous Frameworks for Li Metal Batteries: Superconformal versus Conformal Li Growth.

Li metal batteries have been considered a promising alternative to Li-ion batteries because of the high theoretical capacity of the Li metal. There have been remarkable improvements in the electrochemical performance of Li metal electrodes, although the current Li metal technology is not sufficiently practical in terms of cycle performance, safety, and volume change during cycling. Herein, the role of pore size distribution in the Li metal plating behavior of porous frameworks is clarified to attain the ideal pore structure of the framework as a Li metal host. The monodisperse pore framework shows the conformal electrodeposition of the Li metal, whereas the pore size gradient framework exhibits the superconformal plating of the Li metal. The conformal and superconformal electrodepositions of the Li metal are elucidated in terms of variations along the pore depth direction in the charge-transfer resistance on the pore walls and the ionic resistance of electrolytes confined in pores. The pore size gradient framework also shows excellent electrochemical performance, such as stable capacity retention over 760 cycles with 0.5 mAh cm-2 at 2 mA cm-2. These findings provide fundamental insights into strategies to improve the electrochemical performance of porous frameworks for Li metal batteries.

Asymmetric behaviour of Li/Li symmetric cells for Li metal batteries.

The failure mechanism of Li metal electrodes has not been fully understood yet. Herein, the asymmetric behavior of Li metal electrodes in Li/Li symmetric cells is demonstrated in terms of electrochemical performance and changes in the morphology of Li metal. This finding sheds light on developing Li metal electrodes.