RESUMO
Full exhaustion in specific energy/energy density of state-of-the-art LiNix Coy Mnz O2 (NCM)-based Li-ion batteries (LIB) is currently limited for reasons of NCM stability by upper cut-off voltages (UCV) below 4.3 V. At higher UCV, structural decomposition triggers electrode crosstalk in the course of enhanced transition metal dissolution and leads to severe specific capacity/energy fade; in the worst case to a sudden death phenomenon (roll-over failure). The additive lithium difluorophosphate (LiDFP) is known to suppress this by scavenging dissolved metals, but at the cost of enhanced toxicity due to the formation of organofluorophosphates (OFPs). Addition of film-forming electrolyte additives like vinylene carbonate (VC) can intrinsically decrease OFP formation in thermally aged LiDFP-containing electrolytes, though the benefit of this dual-additive approach can be questioned at higher UCVs. In this work, VC is shown to decrease the formation of potentially toxic OFPs within the electrolyte during cycling at conventional UCVs but triggers OFP formation at higher UCVs. The electrolyte contains soluble VC-polymerization products. These products are formed at the cathode during VC oxidation (and are found within the cathode electrolyte interphase (CEI), suggesting an OFP electrode crosstalk of VC decomposition species, as the OFP-precursor molecules are shown to be formed at the anode.
RESUMO
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.
RESUMO
Despite large research efforts in the fields of lithium ion and lithium metal batteries, there are still unanswered questions. One of them is the formation of the solid-electrolyte interphase (SEI) in lithium-metal-anode-based battery systems. Until now, a compound profile analysis of the SEI on lithium metal was challenging as the amounts of many compounds after simple contact of lithium metal and the electrolyte were too low for detection with analytical methods. This study presents a novel approach on unravelling the SEI compound profile through accumulation in the gas, liquid electrolyte, and solid phase. The method uses the intrinsic behavior of lithium metal to spontaneously react with the liquid electrolyte. In combination with complementary, state-of-the-art analytical instrumentation and methods, this approach provides qualitative and quantitative results on all three phases revealing the vast variety of compounds formed in carbonate-based electrolytes.
RESUMO
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., LiNiy Cox Mny O2 ; 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 well-known 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).