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 D N 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 c Li . Infrared and Raman spectroscopic studies demonstrated that Li ions are solvated by two TFEP molecules and one TFSA anion to form the [Li(TFEP) 2 (TFSA)] complex as a major species in the solutions. This result was in good agreement with the DFT calculations for Li-ion interaction complexes: (1) The interaction energy for the [Li(TFEP) n ] complexes linearly decreased with increasing n up to 2. (2) However, the steric repulsion among the solvated TFEP molecules occurs in complexes with n > 3 due to the molecular bulkiness of TFEP. We found that, in the current system, [Li(TFEP) 2 (TFSA)] is a more energy-stable complex than the solvated complexes [Li(TFEP) n ].