Studies of hexafluorophosphate, tetrafluoroborate, and perchlorate electro-intercalation into graphitic carbon.

摘要

There has been some interest in using carbon materials as both working electrodes in electrochemical cells and rechargeable batteries [1--6]. This would result in the intercalation of not only of lithium ions into one carbon electrode but the anion component of the lithium salt, such as PF 6-, into the other carbon electrode. The intercalation of the anion component of the salt into carbon electrodes has not been studied extensively and it is not completely understood. The work presented here will expand on this rarely touched subject through electrochemical cycling as well as in-situ and ex-situ X-ray diffraction experiments. The anions that will be studied are: PF6- , BF4- and ClO4 -. It will be shown that anion intercalation occurs for various types of soft carbons and that the process can be greatly affected by the amount of turbostratic disorder present in the carbon material as well as by the specific anion used.; It was discovered that using ethyl methyl sulfone, EMS, as the solvent component of the electrolyte resulted in more stable electrochemical cells than ethylene carbonate/diethyl carbonate, a more common solvent, at the high potentials required for anion intercalation. It was also discovered that PF 6 and BF4 formed staged phases during electrochemical cycling whereas ClO4 did not. The amount of disorder present in the carbon electrode did affect the intercalation of the anion. The samples with a greater amount of disorder present had a larger amount of capacity loss between charge and discharge capacities. It was also found that purer and more distinct staged phases occurred in the more ordered carbon samples. The turbostratically disordered carbon layers may rotate to accommodate PF6 and therefore become slightly more ordered.; X-ray diffraction evidence suggests that intercalated PF6 molecules may be free-rotating between the carbon layers. However, the orientation of BF4 molecules between the carbon layers could not be determined. There may also be some co-intercalation of the solvent, mainly with ClO 4 and to a lesser extent BF4 and PF6. It is thought that a large amount of solvent co-intercalation occurs with ClO4 and this is the most probable reason why staged phases were not observed.; An unfortunate aspect of this study is that dual carbon cells are not at all viable as commercial cells. The energy densities of dual carbon cells are much lower than the currently available lithium-ion cells. For dual carbon cells to become viable new inexpensive salts and solvents that can operate at high potentials and high concentrations must be discovered. With further investigation, combinations of different anions and solvents may result in higher specific capacities that would also make dual carbon cells more viable.