High-Voltage Instability of Vinylene Carbonate (VC): Impact of Formed Poly-VC on Interphases and Toxicity

Full exhaustion in specific energy/energy density of state-of-the-art LiNixCoyMnzO2 (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. Illustration of the experiment to investigate possible organofluorophosphates-(OFP) interphase incorporation due to oxidative degradation of vinylene carbonate (VC)-containing electrolytes by means of different mass spectrometric methods. The findings illustrate in operando formation of poly-VC at high-voltages for the first time and also support the thesis of OFP incorporation into the solid electrode interphase during battery cell operation.image