Lithium Difluorophosphate Electrolyte Additive: a Boon for good High Voltage Li Ion Batteries, but a Bane for high Thermal Stability and low Toxicity: Towards a Synergistic Dual-Additive Approach with Fluoroethylene Carbonate to Circumvent this Dilemma

Kubot, Maximilian; Frankenstein, Lars; Muschiol, Elisabeth; Klein, Sven; Esselen, Melanie; Winter, Martin; Nowak, Sascha; Kasnatscheew, Johannes

Abstract

The specific energy/energy density of state-of-the-art (SOTA) Li ion batteries can be increased by raising the upper charge voltage. However, instability of SOTA cathodes, i.e. LiNiyCoxMnyO2 (x + y + z = 1; NCM) triggers electrode crosstalk via enhanced transition metal (TM) dissolution, and leads to severe capacity fade, in worst case, to a sudden death (“roll-over failure”). Lithium difluorophosphate (LiDFP) as electrolyte additive is able to boost high voltage performance via scavenging of dissolved TMs. LiDFP is chemically unstable and rapidly decomposes to toxic (oligo-) organofluorophosphates (OFPs) at elevated temperature; a process which can be precisely analyzed by means of high-performance liquid chromatography – high resolution mass spectroscopy. The toxicity of LiDFP can be proven via the well-known acetylcholinesterase inhibition test. Interestingly, while fluoroethylene carbonate (FEC) is inappropriate for high voltage applications as single electrolyte additive, it is able to suppress OFP formation. In this work, a synergistic LiDFP/FEC dual-additive approach is introduced, showing characteristic benefits of both individual additives, i.e. good capacity retention at high voltage in the presence of LiDFP and decreased OFP formation/toxicity induced by FEC