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

Li/CFx cells possess the highest energy density among all lithium primary batteries. Here, the electrochemical lithiation process is elucidated through high-resolution F-19 NMR, in situ(7)Li NMR and TEM methods. Fluorinated carbon CFL (L is around 0.06 or less) is identified for the first time to be generated in the discharge process of CFx (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 CFx 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 CFx and provides crucial guidance to enhance the performance of the materials.