The specific energy/energy density of state-of-the-art (SOTA) Liion batteries can be increased by raising the upper charge voltage. However, instability of SOTA cathodes (i. e., Li-Ni y Co x Mn y O 2 ; x + y + z = 1; NCM) triggers electrode crosstalk through enhanced transition metal (TM) dissolution and contributes to severe capacity fade; in the worst case, to a sudden death ("roll-over failure"). Lithium difluorophosphate (LiDFP) as electrolyte additive is able to boost high voltage performance by scavenging dissolved TMs. However, LiDFP is chemically unstable and rapidly decomposes to toxic (oligo)organofluorophosphates (OFPs) at elevated temperatures; a process that can be precisely analyzed by means of high-performance liquid chromatography-high resolution mass spectroscopy. The toxicity of LiDFP can be proven by the wellknown acetylcholinesterase inhibition test. Interestingly, although fluoroethylene carbonate (FEC) is inappropriate for high voltage applications as a single electrolyte additive due to rollover failure, it is able to suppress formation of toxic OFPs. Based on this, a synergistic LiDFP/FEC dual-additive approach is suggested in this work, showing characteristic benefits of both individual additives (good capacity retention at high voltage in the presence of LiDFP and decreased OFP formation/toxicity induced by FEC).
Organofluorophosphates (OFPs) have been reported to pose substantial health hazards due to their structural similarities to pesticides and nerve agents. Formation of OFPs in lithium ion batteries (LIBs) due to hydrolysis of the conducting salt lithium hexafluorophosphate (LiPF6) and the reaction with the organic carbonate solvents has been discussed in the literature. The oxidative formation of OFPs in electrolytes containing fluoroethylene carbonate (FEC) and vinylene carbonate (VC) as film-forming additives is presented here. Furthermore, the impact of potentially reactive positive electrode surfaces is investigated with the layered metal oxide NCM622 which is ascribed to release reactive oxygen species at high voltages and the spinel type LNMO as a typical high-voltage material. Cycling of the self-assembled LIB coin cells (CR2032) at cut-off voltages of 4.8 V gave rise to a number of degradation products including potentially highly toxic OFPs. Here, the presence of the film-forming additive had a massive impact on the amount of OFPs formed during electrochemical cycling experiments, which raises further concerns for the utilization of film-forming additives for high voltage applications. The formation pathway of OFPs through EC-polymerization proposed in literature is evaluated and an alternative mechanism with FEC/ VC as the carbonyl carbon-donor is presented
Invited for this month′s cover is the group of Martin Winter at the University of Münster. The image shows idea of the developed sample treatment method enabling the accumulation of solid electrolyte interphase originating compounds. The Research Article itself is available at 10.1002/cssc.202201912.
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