In situ TEM visualization of LiF nanosheet formation on the cathode-electrolyte interphase (CEI) in liquid-electrolyte lithium-ion batteries

The formation behaviors of LiF nanocrystals have been studied on anodes in Li-ion batteries; there is no research work reporting LiF crystal growth on cathodes. Here, we report a real-time LiF crystal formation on positively charged titanium (Ti) electrodes in an electrochemical liquid cell with 150-nm-thick Li hexafluorophosphate (LiPF6)/propylene carbonate (PC) liquid electrolyte loaded. The LiF nanocrystals show two-dimensional (2D) morphologies on the electrode surface, which can serve as a cathode electrolyte interface (CEI). Furthermore, the merging of LiF nanosheets was also observed, which may underlie the self-healing ability of LiF-based CEIs. Theoretical modeling indicates that there are two types of LiF formation paths on positive voltage-biased Ti electrodes. This work shows the remarkable morphing mobility and self-healing ability of LiF nanosheets and sheds light on strategies of modulating LiF nanocrystals and cathode surface chemistry for improving battery performance and cycle life.