Local Structure of Li⁺ in Concentrated Ethylene Carbonate Solutions Studied by Low-Frequency Raman Scattering and Neutron Diffraction with ⁶Li/⁷Li Isotopic Substitution Methods

Abstract

Isotropic Raman scattering and time-of-flight neutron diffraction measurements were carried out for concentrated LiTFSA-EC solutions to obtain structural insight on solvated Li as well as the structure of contact ion pair, Li···TFSA, formed in highly concentrated EC solutions. Symmetrical stretching vibrational mode of solvated Li and solvated Li···TFSA ion pair were observed at ν = 168-177 and 202-224 cm, respectively. Detailed structural properties of solvated Li and Li···TFSA contact ion pair were derived from the least-squares fitting analysis of first-order difference function, Δ(Q), between neutron scattering cross sections observed for Li/Li isotopically substituted 10 and 25 mol % *LiTFSA-ECd solutions. It has been revealed that Li in the 10 mol % LiTFSA solution is fully solvated by ca. 4 EC molecules. The nearest neighbor Li···O(EC) distance and Li···O(EC)═C(EC) bond angle are determined to be 1.90 ± 0.01 Å and 141 ± 1°, respectively. In highly concentrated 25 mol % LiTFSA-EC solution, the average solvation number of Li decreases to ca. 3 and ca. 1.5. TFSA are directly contacted to Li. These results agree well with the results of band decomposition analyses of isotropic Raman spectra for intramolecular vibrational modes of both EC and TFSA.

Effect of Solvation Shell Structure and Composition on Ion Pair Formation: The Case Study of LiTDI in Organic Carbonates

“…This result could be interpreted as a change in the oscillator strength of TDI − and the different ion pairs, but DFT computations show that the oscillator strength is similar for TDI − as a free ion, SSIP, and CIPs with different carbonates (see the Supporting Information). Thus, the appearance of the high-frequency band shows that the structure of the organic carbonate affects the formation of contact ion pairs, which is in agreement with previous studies. , In particular, it shows that CIP species are more prevalent in linear carbonates, which differs with other anions such as hexafluorophosphate (PF 6 – ) ions and bis­(trifluoromethanesulfonyl)­imide (TFSI – ) ions at the same concentrations. ,,, Moreover, it has been previously shown that the charge delocalization of the anion is not sufficient to avoid a strong interaction with the cation. For example, the thiocyanate anion (SCN – ) also shows a strong interaction with cations of different sizes and charge density in various environments, even though it has a relatively delocalized charge and it is on the extreme of the Hofmeister series. ,,, …”

“…While the preferential solvation by organic carbonates is an extensively studied topic, − the role of the mixed solvation shell in the speciation of Li + is a topic vastly unexplored. So far, most studies have focused on either lithium hexafluorophosphate (LiPF 6 ) or lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI), ,,, but the lack of simple vibrational modes in PF 6 − and TFSI − has complicated their analysis and interpretation. ,,, Thus, this study focuses on the effect of carbonate structure and composition on the speciation of lithium 4,5-dicyano-2-(trifluoromethyl)­imidazole salt (LiTDI, Scheme ).…”

Mechanistic Insights on Concentrated Lithium Salt/Nitroalkane Electrolyte Based on Analogy with Fluorinated Alcohols

“…These values seem slightly shifted to higher wavenumber region compared to previously reported Raman shifts of LiFSI in other solvents, presumably due to the low donating nature of NM. Raman spectra of LiTFSI/NM were successfully deconvoluted by assuming two peaks assigned to bound species (749 cm –1 ) and SSIP (742 cm –1 ), which are in good agreement with reported data …”

“…The in solution structure of inorganic Li salts has been studied extensively in the field of lithium ion batteries. Raman spectroscopy is specifically useful for this purpose, and many researchers have proposed that lithium salts change their form from a solvent‐separated ion pair (SSIP) to a contacted ion pair (CIP) and an aggregate (AGG) species at higher concentrations (Figure ) . SSIP is also referred to as free species, whereas CIP and AGG are referred to as bound species (Figure ).…”

“…The concentration‐dependent Raman spectra for LiFSI and LiTFSI in NM and their deconvolution results are presented in Figure S1, 2. Peak around 700–800 cm –1 derives from S–N–S vibration mode, which is commonly chosen to estimate the in solution structure of LiFSI and LiTFSI . The Raman spectra for LiFSI/NM were well‐fitted with three distinct peaks, assigned to SSIP (727 cm –1 ), CIP (737 cm –1 ) and AGG (751 cm –1 ).…”