Tracing the Cross-Talk Phenomenon of Vinylethylene Carbonate to Unveil its Counterintuitive Influence as an Electrolyte Additive on High-Voltage Lithium-Ion Batteries (July, 2024)

The formation of effective interphases is crucial to enable high-performance lithium-ion batteries. This can be facilitated by the introduction of electrolyte additives, ensuring improved stability and transport properties. The identification of proper additives requires a comprehensive understanding of the fundamental mechanisms of interfacial reactions governing interphase formation. This study presents a detailed investigation of widely known and less conventional interphase-forming additives in high-voltage LiNi0.6Mn0.2Co0.2O2, NMC622||artificial graphite cells. The electrochemical characterization shows that cells containing vinylethylene carbonate (VEC) significantly outperform all other investigated electrolyte formulations. Surprisingly, gas chromatography-mass spectroscopy measurements of the electrolyte composition after cycling indicate the formation of an ineffective solid-electrolyte interphase (SEI) in the presence of VEC. A thorough analysis of the interfacial composition via operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and surface-enhanced Raman spectroscopy elucidates rather the formation of an effective cathode-electrolyte interphase (CEI). This phenomenon results from the reductive reaction of VEC on the anode, followed by the product transfer and electro-polymerization of reaction products on the cathode. Additionally, focused ion beam secondary ion mass spectrometry (FIB-SIMS) with a time of flight (ToF)-detector is used to analyze the elemental spatial distribution of Li-species and Mn in the respective SEIs.