Journal of Physical Chemistry C
Journal of Physical Chemistry C
5 years ago

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

Role of Solvent Bulkiness on Lithium-Ion Solvation in Fluorinated Alkyl Phosphate-Based Electrolytes: Structural Study for Designing Nonflammable Lithium-Ion Batteries
Yanko M. Todorov, Kenta Fujii, Masayuki Morita, Nobuko Yoshimoto, Michiru Sogawa, Hideyuki Mimura, Daisuke Hirayama
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 [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].

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b06910

DOI: 10.1021/acs.jpcc.7b06910

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