Pushing the Theoretical Limit of Lithium/Carbone Fluoride Batteries Using Fluorinated Nanostructured Carbon Nanodiscs

Abstract

The Li/CFx primary battery is the first lithium battery developed since the 1970th, and it possesses the highest energy-density among all primary lithium batteries (theoretically 2180 W h kg-1).When fluorinated carbons (denoted CFx) are used as electrode material in primary lithium battery, the C-F bonds are broken during the electrochemical discharge and carbon and LiF particles are formed. Then, the higher the fluorine content, the higher the theoretical capacity, the maximal value being 865 mAh/g for a CF1 composition. Although this limitation, we found higher capacities (Cexp) than the theoretical ones (Ctheo) for several optimized fluorinated nanocarbons. The faradic yield, defined as 100xCexp/Ctheo, was then higher than 100% and reached even 170%. This means that a second electrochemical process takes place after the defluorination. In order to understand such additional mechanism, a set of carbonaceous nanomaterials, namely carbon nanodiscs, graphitized carbon blacks, double- and multi-walled were fluorinated using either pure F2 gas or a solid fluorinating agent (TbF4) and their electrochemical performances were investigated. After a deep characterization of the material (SEM, TEM, XRD, 19F solid state NMR, EPR), the discharge mechanisms were investigated using the same complementary techniques and compared to conventional graphite fluorides. This systematic study of about ten different CFx underlined that the key point for the extra-capacities lies in both the maintaining of some unfluorinated parts and the ability of the fluorinated carbon to reform its raw structure after electrochemical defluorination are necessary for extra-capacity. LiF particles, which are formed during this process, are then located outside the new carbonaceous matrix and may participate to the second mechanism, similar to the intercalation into the freshly formed carbon matrix. These two processes, electrochemical defluorination and intercalation into the defluorinated carbon are well exemplified by the case of carbon nanodiscs. This sample was fluorinated with TbF4 in order to homogenously locate the fluorine atoms in the whole volume of the discs, except in the central discs, less or not fluorinated. The deep characterization of the fluorinated discs allows this conclusion to be proposed in accordance with the properties of the starting materials. After the defluorination, a shell of agglomerated LiF particles covered the discs. Any exfoliation of the discs has been observed after the defluoration contrary to the studied conventional graphite fluoride. The process similar to an intercalation into the reformed discs may take place thanks to the maintaining of the discotic geometry. The extra-capacity comes from this additional process, which occurs through the LiF shell. In other words, the less or not fluorinated central discs act as a reinforcement that allows the rebuilding of the carbon matrix. Both the unfluorinated carbon and the LiF covering, which is formed outside the carbon lattice during the discharge mechanism, play a key role for the achievement of the extracapacity by the consumption of Li+ to form Li2F+ species stabilized by the carbon host structure formed after the electrochemical defluorination. The necessity of a reinforcement was then confirmed for the cases of fluorinated double and low diameter multi-walled carbon nanotubes, for which the fluorination with F2 gas has been conducted in order to avoid the fluorination of the inner tube, which allows the reinforcement and the carbon rebuilding. The higher faradic yield (170%) was obtained for those samples.