Spectroscopic insights into the thermal aging process of lithium-ion battery electrolytes

The thermal aging process of lithium-ion battery electrolyte was studied using Raman spectroscopy and fluorescence spectroscopy. The system studied is specifically a carbonate-based electrolyte containing lithium hexafluorophosphate (LiPF₆). Raman spectroscopy results show that LiPF₆ will decompose and produce substances with significant fluorescence effects under the thermal aging. The fluorescence spectrum results also show that the concentration of this fluorescent substance has a significant positive correlation with the intensity of thermal aging. The mechanism behind the above experimental phenomena was explained and verified by Ab Initio Molecular Dynamics (AIMD) and Density Functional Theory (DFT). AIMD confirmed that LiPF₆ can decompose and polymerize into polyfluorophos-phoric acid (PFPA) with different degrees of polymerization at elevated temperature. Then, the theoretical fluorescence spectra and excited state transition of PFPA based on DFT prove that the source of the fluorescence effect is the PFPA molecule, and specifically comes from the transition of π electrons and lone pair electrons on oxygen atoms.