Formation and Suppression of Toxic Organofluorophosphates in Lithium Ion Batteries: Making the High-Voltage Additive Lithium Difluorophosphate Viable for Commercial Applications

Abstract

Organofluorophosphates (OFPs) are one of the major concerns regarding the safety of Lithium Ion Batteries (LIBs) due to their neurotoxic properties. The formation through the hydrolysis of the conducting salt LiPF6 and subsequent reactions (reductive/oxidative) with the cyclic and linear carbonates is a major drawback in terms of commercial application of LIBs. The additive Lithium Difluorophosphate, which is literature known and beneficial for high-voltage LIBs due to its transition metal (TM) capturing abilities, accelerates the formation of toxic OFPs even further as it also is a hydrolysis product of LiPF6. The suppression of the formation of toxic OFPs through a synergetic dual additive approach with fluoroethylene carbonate (FEC) is presented in this study. Here, the thermal- and electrochemical formation of OFPs are successfully inhibited with the addition of 2 wt.% of FEC to the electrolyte containing LiDFP (1 wt.%). Characterization of OFPs is conducted via high performance liquid chromatography mass spectrometry and the toxicological capabilities of the aged electrolyte is assed through an inhibition-evaluation of Acetylcholinesterase lyophilizate. Combined, these two methods illustrate the successful inhibition of OFP formation in FEC/LiDFP dual-additive electrolytes. Further, this dual-additive approach does not deteriorate the performance of LiDFP as single additive making the FEC/LiDFP additive combination a possible new benchmark for high-voltage LIBs.