Optical Determination of Hydrofluoric Acid Content in Lithium-Based Electrolytes.

Lithium-ion batteries powering electric vehicles can contain up to ∼100 liters of electrolyte solution. This solution typically consists of LiPF6 salt dissolved in a mixture of organic carbonates such as ethylene carbonate and diethyl carbonate. LiPF6 decomposes upon exposure to water vapor or other protic impurities, leading to the formation of up to 3 equivalents of hydrofluoric acid (HF) per molecule of decomposed salt. HF is a deadly and volatile substance that is corrosive to cell and battery components. Here, we describe an all-optical method that can report on the concentration of HF in LiPF6-based electrolytes. The method is based on a specially designed fluorescent dye incorporated in the working electrolyte. The dye response to HF is quantified so that photographic imaging or even visual inspection can be used to indicate the HF concentration in the electrolyte. The method is, moreover, compatible, in principle, with in situ reporting of HF levels during cell assembly and operation.