Ion–ion interactions of LiPF 6 and LiBF 4 in propylene carbonate solutions

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Self-diffusion coefficients of Li + D Li + , PF 6 − D PF 6 − and solvent propylene carbonate (PC) D PC in LiPF 6−PC solutions were determined at 298 K by the pulse gradient spin echo (PGSE) NMR technique over the salt concentration range of 0.1–3.0 M (M = mol dm – 3 ). The order of the diffusion coefficients was found to be D Li + < D PF 6 − < D PC over the concentration range examined, and they were monotonically decreased with increasing the salt concentration. Haven ratio Λ/ Λ NMR, where Λ and Λ NMR represent the ionic conductivity measured electrochemically and that estimated via the Nernst-Einstein equation using the diffusion coefficient, respectively, was evaluated as the measure of the ion–ion interaction in the LiPF 6–PC solutions. Though Λ/ Λ NMR values for LiPF 6-solutions decrease with increasing the salt concentration, they were greater than those for LiBF 4–PC solutions over the whole concentration range examined, which indicates that the ion pair formation ability of PF 6 – ion is weaker than that of the BF 4 – ion. The smaller value of the ionic conductivity for the highly concentrated LiPF 6–PC solution (above 2.0 M) than that of the LiBF 4-solutions can be attributed to the more rapidly increased viscosity relative to the LiBF 4-solution. Classic molecular dynamics (MD) simulations for the respective LiPF 6 and LiBF 4-solution of 0.5 and 1.0 M were also carried out based on the effective pair potentials. Diffusion coefficients, ionic conductivity and Haven ratio for these solutions were calculated from MD trajectories, and they qualitatively agree with those evaluated by experiments. Pair correlation functions g LiO( r) (for Li +–O (PC) pair) and g LiPF 6 ( r) (for Li +–PF 6 – pair) or g LiBF 4 ( r) (for Li +–BF 4 – pair) revealed that the lithium ion weakly forms the contact ion pairs with PF 6 –, whilst strongly with BF 4 –, which supports the present experimental results. Moreover, the simulation results show that both anions in the contact ion pairs predominantly take the monodentate form, which is in contrast to the multidentate coordination predicted by ab initio calculation in gas phase.