Abstract
The element fluorine is omnipresent in both secondary and primary batteries, in salt or electrolyte additives, separators, electrode formulation binders or anode or cathode materials. By way of example, LiPF6 and NaPF6 are used as salt, fluoroethylene carbonate as additive, and fluoropolymers as separators for liquid electrolyte. To mention just two electrode materials, fluorinated carbons are emerging as the most effective cathode materials for primary lithium batteries (BPLi) and Na3V2(PO4)2F3 is establishing itself as a cathode in sodium ion batteries. The heterogeneous gas/solid fluorination reaction enables this element to be incorporated into electrode materials. This well-controlled process can be limited to the surface of the material or, on the contrary, to its entire volume. The high reactivity of the molecular fluorine F2 generally used makes perfect control of the operating parameters essential. This is due to the low dissociation energy of F2 and the high exothermicity of the reaction.
After outlining the experimental difficulties involved in handling F2 gas, several examples of spatially localized gas/solid fluorination will be discussed, highlighting the gain in electrochemical performance. When fluorination is limited to the surface, LTO cyclability is improved. When fluorinated carbons are considered as BPLi cathodes, excluding certain carbons from fluorination and maintaining their electronic conductivity, according to the concept of sub-fluorination, makes up for their main shortcoming, namely low power density. When the precursor is a transition metal oxide or oxidized graphite, a homogeneous distribution of fluorine and oxygen atoms can be achieved by suitable methodologies. Syntheses of manganese oxyfluorides and graphites will be presented to illustrate the adaptations needed to employ molecular fluorine. The aim of these numerous examples is to demonstrate the potential of spatially localized gas/solid fluorination.
