Highly concentrated LiN(SO2CF3)2/dinitrile electrolytes: Liquid structures, transport properties, and electrochemistry

Abstract

Liquid structures, transport properties, and electrochemical properties of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and dinitrile solvents [succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN)] were investigated. In the LiTFSA/SN and LiTFSA/ADN systems, the stable crystalline solvates of LiTFSA–(SN)1.5 [melting point (Tm): 59 °C] and LiTFSA–(ADN)1.5 (Tm: 50 °C) were formed, respectively. In contrast, the LiTFSA/GN mixtures of a wide range of compositions were found to be glass-forming liquids at room temperature. Raman spectroscopy of LiTFSA/GN liquid mixtures revealed that increasing the LiTFSA concentration results in the formation of the solvent-bridged network structure Li+–GN–Li+. In addition, the considerable formation of contact ion pairs and ionic aggregates was observed in highly concentrated electrolytes. In the liquids, the Li+ ion dynamically exchanged ligands (GN and TFSA) and higher LiTFSA concentrations led to an increase in the ratio of the self-diffusion coefficients of Li+ and TFSA−, DLi/DTFSA, as determined by pulsed field gradient NMR spectroscopy. The Li+ transference number (tLi+) of the [LiTFSA]/[GN] = 1/1.5 electrolyte in an electrochemical cell under anion-blocking conditions was estimated to be as high as 0.74. Furthermore, electrochemical measurements revealed that the reductive stability of the LiTFSA/GN electrolyte increases with increasing LiTFSA concentration. A [LiTFSA]/[GN] = 1/1.5 electrolyte is stable against the Li metal electrode, provided that the polarization is relatively small. Owing to high tLi+, a Li–S battery with the [LiTFSA]/[GN] = 1/1.5 electrolyte showed a high rate discharge capability despite its low ionic conductivity (0.21 mS cm−1) at room temperature.

Solvate electrolytes for Li and Na batteries: structures, transport properties, and electrochemistry

“…In the high salt concentration region with x o 4, the Raman peak position exceeds 744 cm À1 , suggesting the formation of the CIP and AGG solvates in which the TFSA À anions are coordinated to one or more Li + ions. 69,78,79 These results indicate that when the x is lower than 4 (the preferred coordination number of Li + ), anions participate in the coordination of Li + ions to form CIPs and AGGs. Similar ion pairs and ionic aggregates are also formed in the solvent-deficient electrolyte solutions incorporating other anions and alkaline metal ions.…”

Anion effects on Li ion transference number and dynamic ion correlations in glyme–Li salt equimolar mixtures

“…) and potentiostatic polarization method (t PP Li ) were determined for Li-ion conducting electrolytes. 21,39,40 The PFG-NMR method is based on the Nernst-Einstein equation: t NMR Li = D Li /(D Li + D anion ), where D Li and D anion are the self-diffusion coefficients of Li + ions and anions, respectively, and the ions are assumed to be completely dissociated and move independently as ideal dilute electrolyte solutions. Hence, for highly concentrated electrolytes, t NMR Li would not reflect a realistic transference number because of their dynamic ion correlations.…”

Highly Concentrated NaN(SO₂F)₂/3-Methylsulfolane Electrolyte Solution Showing High Na-Ion Transference Number under Anion-Blocking Conditions

“…Detailed theoretical rationales regarding the measurement and its reliability have been reported elsewhere. 19,38 The tNa+ in the [NaFSA]/[MSL] = 1/2 electrolyte under anion-blocking conditions was estimated to be 0.66. The tNa+ in 1 M NaPF6/PC + 0.5 vol% FEC was also evaluated for comparison.…”

“…40 Other high salt-concentration electrolytes have also been reported to show high cation transference numbers. 19,20,22,41…”

“…In addition to these attractive properties, certain highly concentrated electrolytes have been reported to exhibit high cation transference numbers (> 0.5). [16][17][18][19][20] In liquid electrolytes, both cations and anions can migrate. Conventional electrolyte solutions containing 1 M alkali metal salts are known to have low cation-transference numbers (< 0.5) because the hydrodynamic radius of a solvated cation [M(solvent)x] + (M: alkali metal) is larger than that of the anion in solution.…”