Exploring the Physicochemical Properties and Electrochemical Behavior of Phosphonium-Based Ionic Liquids as Additives in Carbonate Electrolytes for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are widely utilized for energy storage in various technologies. Conventional organic electrolytes used in LIBs face challenges, such as safety risks, limited cycling performance, and narrow electrochemical windows. In this study, several organic ionic liquid electrolytes are investigated. These electrolytes are composed of a mixture of EC/EMC/DEC (1:1:1 wt) as the organic solvent, combined with various phosphonium-based ionic liquids (ILs) possessing different alkyl chains on the phosphonium cation, namely, trimethyl(isobutyl) phosphonium bis(fluorosulfonyl)imide (P_111i4FSI), tri-n-butyl(methyl) phosphonium bis(fluorosulfonyl)imide (P₁₄₄₄FSI), and tri-n-butyl(decyl) phosphonium bis(fluorosulfonyl)imide (P₄₄₄₁₀FSI), serving as additives. Subsequently, the physicochemical properties and electrochemical behavior of all electrolytes are investigated. The results indicate that the compositions of EC/EMC/DEC 90% + P_111i4FSI 10% and EC/EMC/DEC 50% + P_111i4FSI 50% (wt %) have higher ion conductivity and thermal stability at 25 and 50 °C, respectively, compared to the organic electrolyte. Although all organic and hybrid electrolytes can support Li electrochemistry, making them promising candidates for various anodes, the addition of 10% and 50% P_111i4FSI into the organic electrolyte leads to an increased oxidation limit from 1.88 to 1.94 and 2.03 V (vs Fc/Fc⁺), respectively.