Fluorinated Cyclic Phosphorus(III)-Based Electrolyte Additives for High Voltage Application in Lithium-Ion Batteries: Impact of Structure-Reactivity Relationships on CEI Formation and Cell Performance

Two selected and designed fluorinated cyclic phosphorus(III)-based compounds, namely 2-(2,2,3,3,3-pentafluoropropoxy)-1,3,2-dioxaphospholane (PFPOEPi) and 2-(2,2,3,3,3-pentafluoro-propoxy)-4-(trifluormethyl)-1,3,2-dioxaphospholane (PFPOEPi-1CF₃), were synthesized and comprehensively characterized for high voltage application in lithium-ion batteries (LIBs). Cyclic voltammetry (CV) and constant current cycling were conducted, followed by post mortem analysis of the NMC111 electrode surface via scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). To support and complement obtained experimental results, density functional theory (DFT) calculations and molecular dynamics (MD) simulations were performed. Theoretical and experimental findings show that the considered phospholane molecule class enables high voltage LIB application by sacrificial decomposition on the cathode surface and involvement in the formation of a cathode electrode interphase (CEI) via polymerization reaction. In addition, obtained results point out that the introduction of the CF₃ group has a significant influence on the formation and dynamics of the CEI as well as on the overall cell performance, as the cell impedance as well as the thickness of the CEI is increased compared to the cells containing PFPOEPi, which results in a decreased cycling performance. This systematic approach allows researchers to understand the structure-reactivity relationship of the newly synthesized compounds and helps to further tailor the vital physicochemical properties of functional electrolyte additives relevant for high voltage LIB application.