Functional Electrolyte: Highly-Safe LIB Using Branched Carboxylic Acid Ester as Electrolyte Additive

Abstract

The electrolyte in current automotive lithium-ion batteries is a mixture of ethylene carbonate (EC), which has a high dielectric constant, and ethyl methyl carbonate or dimethyl carbonate, which have a low viscosity. However, the flash point of these mixed solvents is as low as 25 °C, so safety precautions must be taken. On the other hand, if only EC or propylene carbonate with a high flash point is used, the flash point will be 120 °C or higher. However, the high dielectric constant solvent cannot wet the hydrophobic separator whose material is polyethylene and/or polypropylene. Therefore, there is a problem that the battery does not work. In 1999, we proposed a “functional electrolyte” in which a small amount of an additive with a new function is added to the electrolyte, and many additives have since been commercialized. In recent work, we focused on a low viscosity linear carboxylic acid esters, designed an electrolyte with a flash point above 120 °C that provides wettability to the separator even in an electrolyte with a high dielectric constant solvent.