Marc DUBOIS

Abstract:

Although primary lithium batteries have shown promising performances as a cathode, only a few works have been devoted to graphite oxyfluorides [1]. For such applications, most of the studies about covalent Graphite Interclation Compounds (GICs) mainly concern graphite oxides (GO) [2] and graphite fluorides (GF) [3-10]. Nevertheless, the rare works on oxyfluorides suggest high potentialities, such as an energy density of 1347 Wh.Kg^-1 for a sample obtained by a two-step synthesis combining fluorination and oxidation and even 2265 Wh.Kg^-1, thus exceeding graphite fluorides [11]. Fluorinated oxides were highly capacitive while oxidized fluorides have highest discharge potential despite having the same graphite precursor. Such data demonstrated that engineering the synthesis enables to modulate the properties. Different graphite oxyfluorides were synthesized via Hummer's oxidation of sub-fluorinated graphites in order to maintain sp² carbon atoms available for the oxidation, C-F bonds being non-reactive. In comparison with the graphite fluoride precursors, significant improvement of the energy density in primary lithium battery is achieved when the graphite oxyfluorides are used as cathode. When Hummer's oxidation was carried out on graphite fluoride with both the CF_0.60 composition and a homogenous dispersion of non-fluorinated regions into fluorinated lattice, oxidation focused on the remaining sp² carbon atoms and decomposed them. Defected graphite fluorides were then synthesized. The highest ever measured energy density in the primary lithium battery with fluorinated carbons as cathode, i.e. 2825 Wh.Kg^-1, was reached with this particular sample. Solid state NMR allowed the functional groups C-F, COC, COH, COOH and sp² C to be quantified in graphite oxyfluorides and fluorides and their role in electrochemical processes to be highlighted.

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