Role of Solvent Bulkiness on Lithium-Ion Solvation in Fluorinated Alkyl Phosphate-Based Electrolytes: Structural Study for Designing Nonflammable Lithium-Ion Batteries

We report the characteristics of nonflammable tris(2,2,2-trifluoroethyl) phosphate (TFEP) as a nonaqueous solvent for lithium (Li)-ion battery electrolytes at a molecular level, particularly focusing on the solvent parameters, such as the electron pair-donating ability and molecular bulkiness of TFEP, and the solvation behavior around Li ions. The binding energy for the interactions between a Li ion and several solvents using density functional theory (DFT) calculations indicated that TFEP can be categorized as a nonaqueous solvent with weak electron pair-donating ability, resulting in the prediction of a small Gutmann’s donor number (the DN value is approximately 12.9). The Walden plots based on ionic conductivity and viscosity suggested that Li ions tend to exist as contact ion pairs (Li+···TFSA–) in the TFEP-based solutions, irrespective of the Li salt concentration cLi. Infrared and Raman spectroscopic studies demonstrated that Li ions are solvated by two TFEP molecules and one TFSA anion to form the ...