Unraveling and Mitigating the Storage Instability of Fluoroethylene Carbonate-Containing LiPF₆ Electrolytes To Stabilize Lithium Metal Anodes for High-Temperature Rechargeable Batteries

Implementing Li metal anodes provides the potential of substantially boosting the energy density of current Li-ion battery technology. However, it suffers greatly from fast performance fading largely due to substantial volume change during cycling and the poor stability of the solid electrolyte interphase (SEI). Fluoroethylene carbonate (FEC) is widely acknowledged as an effective electrolyte additive for improving the cycling performance of batteries consisting of electrode materials that undergo large volume changes during cycling such as Li metal. In this study, we report that while FEC can form a robust SEI on the electrode, it also deteriorates the shelf life of electrolytes containing LiPF6. The degradation mechanism of LiPF6 in FEC solutions is unraveled by liquid- and solid-state NMR. Specifically, traces of water residues induce the hydrolysis of LiPF6, releasing HF and PF5 which further trigger ring-opening of FEC and its subsequent polymerization. These reactions are significantly accelerated at elevated temperatures leading to the formation of a three-dimensional fluorinated solid polymer network. Moisture scavenger additives, such as lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI), can delay the degradation reaction as well as improve the cycling stability of LiNi1/3Mn1/3Co1/3O2/Li metal batteries at 55 °C. This work highlights the poor shelf life of electrolytes containing FEC in combination with LiPF6 and thereby the great importance of developing proper storage methods as well as optimizing the content of FEC in practical cells.