Thermodynamics of the Hydrolysis of Lithium Salts: Pathways to the Precipitation of Inorganic SEI Components in Li-Ion Batteries

Here we discuss the thermodynamics of the hydrolysis of four fluorinated lithium salts widely used in aprotic electrolytes for lithium-ion batteries: lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethylsulfoyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI) and the hybrid salt lithium fluorosulfonyl-trifluorofulfonyl imide (LiFTFSI). We performed density functional theory calculations at the B3LYP/6-31++G** level of theory using an implicit solvation model, to derive the electronic structures, vibrational properties and the thermodynamic stability of reagents, reaction intermediates and hydrolysis products. We combined ab initio thermodynamic evaluations in solution with thermochemical cycles to derive the variation of the Gibbs energy of heterogeneous reactions, where the precipitation of solid products is considered. Our analysis has been performed at three different temperatures to investigate the possible influence of the temperature on the formation of the reaction products. Overall, the use of imide-based anions limits the spontaneous hydrolysis, by hindering the formation of POF3 differently from LiPF6. All FSI- TFSI- and FTFSI-hydrolyses lead to the precipitation of solid LiOH and, to a lesser extent, of solid LiF. In comparison with other salts, LiTFSI apparently holds down more efficiently the precipitation of solid LiF and limits the onset of the self-feeding salt degradation mechanisms.