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

Yosuke Ugata; Ryoichi Tatara; Kazuhide Ueno; Kaoru Dokko; Masayoshi Watanabe

doi:10.1063/1.5145340

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

J Chem Phys. 2020 Mar 14;152(10):104502. doi: 10.1063/1.5145340.

Authors

Yosuke Ugata¹, Ryoichi Tatara¹, Kazuhide Ueno¹, Kaoru Dokko¹, Masayoshi Watanabe¹

Affiliation

¹ Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan.

PMID: 32171228
DOI: 10.1063/1.5145340

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 (T_m): 59 °C] and LiTFSA-(ADN)_1.5 (T_m: 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^-, D_Li/D_TFSA, as determined by pulsed field gradient NMR spectroscopy. The Li⁺ transference number (t_Li⁺ ) 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 t_Li⁺ , 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.