Insights into the lithiation mechanism of CFx by a joint high-resolution 19F NMR, in situ TEM and 7Li NMR approach

Abstract

Li/CF_x cells possess the highest energy density among all lithium primary batteries. Here, the electrochemical lithiation process is elucidated through high-resolution ¹⁹F NMR, in situ⁷Li NMR and TEM methods. Fluorinated carbon CF_L (L is around 0.06 or less) is identified for the first time to be generated in the discharge process of CF_x (x ≥ 0.5) besides LiF and carbon. Moreover, the morphological evolution of the formed LiF, which affects the impedance of the battery during discharge, is illustrated as crystal nuclei of LiF, with a size of around 10 nm, which keep forming and growing into crystal grains of LiF, with a size of around 200 nm, until the surface of the CF_x particles is saturated by LiF and then both crystal nuclei and grains enlarged promptly towards the end of lithiation. This work reveals the electrochemical reaction mechanism of CF_x and provides crucial guidance to enhance the performance of the materials.