Anion Coordination Characteristics of Ion-pair Complexes in Highly Concentrated Aqueous Lithium Bis(trifluoromethanesulfonyl)amide Electrolytes

Abstract

We report on the structures of Li-ion complexes in salt-concentrated aqueous electrolytes based on lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), particularly focusing on the anion coordination behavior of the ion-pair complexes in the high concentration region c_Li > 3.0 mol dm⁻³. Quantitative data analysis of the Raman spectra revealed the following. (1) Li ions do not coordinate with TFSA anions at lower c_Li (<3.0 mol dm⁻³) to exist as ion pair-free ions. (2) In the concentrated region (c_Li = 3.0 – 4.0 mol dm⁻³), the TFSA anions coordinate as monodentate ligands (mono-TFSA) with Li ions to form ion-pair complexes and coexist with free TFSA in the bulk. (3) Further increasing the c_Li (4.0 – 5.2 mol dm⁻³) results in both monodentate and bidentate coordination (bi-TFSA) modes of TFSA anions to Li ions, yielding complicated ion-pair complexes in the first coordination sphere. The Walden plots, based on ionic conductivity and viscosity data, implied that the ion-conducting mechanism in the highly salt-concentrated region was considerably different from that in the dilute region (i.e., vehicle mechanism).