Conductivity and self-diffusivity measurements on molten lithium electrolytes for battery applications.

摘要

Several lithium salt systems, classified in this work as solvent-free and solvent containing, have been investigated with the purpose of determining their qualities as potential electrolytes for applications in lithium batteries. With this objective, their thermal. properties, of which the glassforming ability was considered of fundamental importance, and their experimental conductivities and lithium self-diffusivities were determined to build a body of information that could be considered complete insofar as the evaluation of the material was concerned.; Mixtures of lithium salts with fluorine-based anions, including LiCF ₃SO₃, LiBF₄ and LiN(SO₂CF₃) ₂ (or LiIm), were studied as part of a search for chemically and electrochemically stable glassforming lithium salts. Although the observation of the glassforming ability of some binary and ternary systems was considered a partial success, the high glass transition temperatures recorded and their inability to avoid crystallization discouraged the author from any attempt of using these materials as practical electrolytes. Attention was then placed on a family of tetrahaloaluminate lithium salts among which LiAlCl₄, although non-glassforming when pure, can be easely vitrified upon the addition of small amounts of a second component or plasticizing agent. By extrapolation to zero content of plasticizing agent it was found that T_g for this salt is −35°C, the lowest value recorded for an ionic system.; Although the LiAlCl₄-based systems obtained by introducing LiIm, LiIm-AlCl₃ or LiAl(SO₃Cl)₄ as second components produce room temperature, non-crystallizing liquids, they unfortunately fail in providing conductivities with values acceptable for the applications intended. In the case of the system LiAlC1₄-LiAl(SO₃Cl) ₄, lithium self-diffusivity measurements are compared via the Nernst-Einstein relation to the conductivity values in order to obtain insight on lithium-ion transport properties.; Solvent-containing electrolytes which include the new boron-based esther of a glycol (BEG-1) solvent have also been studied. The combination of conductivity data and pulsed field gradient nuclear magnetic resonance (PFG NMR) studies of self-diffusivity of cations and anions allowed the determination of the transport numbers for lithium ion and ion dissociation coefficients. Enough results are presented to prove the important role of the BEG-1 molecule on the improved properties of this new kind of electrolyte.