Perfluoroalkyl-fluorophosphate anions for high voltage electrolytes in lithium cells: DFT study

Lithium perfluoroalkyl-fluorophosphates (LiFAFP) are almost unexplored organic-inorganic hybrid salts suitable for application in innovative lithium-ion batteries. LiFAFPs are obtained from lithium hexafluorophosphate by replacing one or more fluorine atoms with fluorinated alkyl chains. Among them only lithium tris(pentafluoroethyl)-trifluorophosphate (LiFAP) and lithium bis(trifluoromethyl)-tetrafluorophosphate [LiPF4(CF3)2] have been tested successfully in a lithium cell. In this paper, we present a detailed systematic study by electronic structure calculations of two subfamilies among LiFAFP: the pentafluoroethyl and the trifluoromethyl substituted FAFP [i.e., LiPF6-x(CF3)x with 0 ≤ x ≤ 6 and LiPF6-x(C2F5)x with 1 ≤ x ≤ 4]. In particular, the equilibrium structures, ion pair dissociation energies, and anion ionization potentials have been predicted for the considered chemical species by density functional theory (DFT), also including dispersion effects. Apparently all the evaluated LiFAFPs show a remarkable decrease of the dissociation energies upon perfluoroalklation without suffering any parallel drastic drop in the anion ionization energy. For all FAFP anions with 1-3 perfluoroalkyl substituents, the ionization potentials are predicted above 5 V versus Li+/Li0, a value suitable for application with 5 V cathode materials in lithium-ion cells. LiFAFPs affinity toward water has also been evaluated and the equilibrium constants for the hydrolysis reactions predicted.