Solvation Structure of Li⁺ in Concentrated Acetonitrile and N,N‑Dimethylformamide Solutions Studied by Neutron Diffraction with ⁶Li/⁷Li Isotopic Substitution Methods

Neutron diffraction measurements on ⁶Li/⁷Li isotopically substituted 10 and 33 mol % *LiTFSA (lithium bis­(trifluoromethylsulfonyl)­amide)-AN-d₃ (acetonitrile-d₃) and 10 and 33 mol % *LiTFSA–DMF-d₇(N,N-dimethylformamide-d₇) solutions have been carried out in order to obtain structural insights on the first solvation shell of Li⁺ in highly concentrated organic solutions. Structural parameters concerning the local structure around Li⁺ have been determined from the least squares fitting analysis of the first-order difference function derived from the difference between carefully normalized scattering cross sections observed for ⁶Li-enriched and natural abundance solutions. In 10 mol % LiTFSA-AN-d₃ solution, 3.25 ± 0.04 AN molecules are coordinated to Li⁺ with a intermolecular Li⁺···N­(AN) distance of 2.051 ± 0.007 Å. It has been revealed that 1.67 ± 0.07 AN molecules and 2.00 ± 0.01 TFSA^– are involved in the first solvation shell of Li⁺ in the 33 mol % LiTFSA-AN solution. The nearest neighbor Li⁺···N_AN and Li⁺···O_TFSA^– distances are obtained to be r(Li⁺···N) = 2.09 ± 0.01 Å and r(Li⁺···O) = 1.88 ± 0.01 Å, respectively. The first solvation shell of Li⁺ in the 10 mol % LiTFSA-DMF-d₇ solutions contains 3.4 ± 0.1 DMF molecules with an intermolecular Li⁺···O_DMF distance of 1.95 ± 0.02 Å. In highly concentrated 33 mol % LiTFSA-DMF-d₇ solutions, there are 1.3 ± 0.2 DMF molecules and 3.2 ± 0.2 TFSA^– in the first solvation shell of Li⁺ with intermolecular distances of r(Li⁺···O_DMF) = 1.90 ± 0.02 Å and r(Li⁺···O_TFSA^–) = 2.01 ± 0.01 Å, respectively. The Li⁺···TFSA^– contact ion pair stably exists in highly concentrated 33 mol % LiTFSA-AN and -DMF solutions.