High Concentration Electrolytes, or Almost Ionic Liquids, for Advanced Batteries

摘要

For most of the commercial life of lithium ion batteries, the electrolyte of choice has been LiPF_6 salt in mixtures of organic carbonates, such as ethylene carbonate, propylene carbonate, ethyl carbonate, methyl carbonate and ethyl methyl carbonate. In the last several years, however, the electrolyte phase for lithium ion batteries has been given increased scrutiny. One productive area of research has been in the general area of high salt concentration in coordinating organic solvents. In particular, when the mole ratio of electrolyte salt to coordinating solvent becomes less than 1 to 3 or 4, many changes occur in the solution structure. The amount of free (uncoordinated) solvent drops to undetectable levels, as measured, for example by Raman spectroscopy. This means that the "solvent" is completely coordinated to the cation and the solution now resembles an ionic liquid rather than a mixture of coordinated compact ion pairs and free ions in a medium of the excess free solvent molecules like the conventional 1 molar LiPF_6 lithium ion battery electrolyte. The transition to the disappearance of free solvent usually occurs at around 3 to 5 molar salt concentration depending on the salt-solvent combination. It has been known for many years that such transitions occur at high concentrations and models have shown that the degree of association of anion and cation into ion pairs may drop many orders of magnitude. Thus, we view such solutions as ionic liquids where a quasi-lattice exists of cations surrounded by anions and anions by cations.