Electrochemical properties of Li[CnF2n+1BF3] as electrolyte salts for lithium-ion cells

The fundamental electrochemical properties of lithium perfluoroalkyltrifluoroborates Li[CnF2n+1BF3] (n = 1∼4) were evaluated as electrolyte salts for lithium-ion battery in comparison with LiBF4 and LiPF6. Li[CnF2n+1BF3] showed higher electrolytic conductivities than LiBF4 in aprotic solvents. In these series, the conductivities decreased with the perfluoroalkyl group being longer, and Li[C2F5BF3] exhibited a comparable conductivity to LiPF6. The relationship between the conductivity and the anion size showed that the anion with a moderate size is in favor of obtaining high conductivities. The limiting oxidation potentials determined by linear sweep voltammetry demonstrated that Li[CnF2n+1BF3] were less resistant against oxidation than LiBF4. The HOMO energies and ionization energies of [CnF2n+1BF3]− calculated by ab initio molecular orbital (MO) theory and density functional theory (DFT) supported this observation, however, there was no accuracy to explain the effect of the chain length of perfluoroalkyl groups on the limiting oxidation potentials. The cell performances of a LiC6/Li0.5CoO2 cell using Li[C2F5BF3] were comparable to those using LiPF6 at room temperature, however, it deteriorated at elevated temperature due to the reaction on the cathode.