Solvate Structures and Transport Properties in Highly Concentrated Li[FSA]/Succinonitrile Liquid Electrolytes

Abstract

Highly concentrated Li salt electrolyte solutions have attracted attention recently due to the unique physicochemical and electrochemical properties.1 Recently, our group reported that Li+ ion hopping conduction is involved in the ionic conduction in highly concentrated Li salt/sulfolane electrolytes.2 Similar Li+ transport was also observed for the highly concentrated lithium bis(fluorosulfonyl)amide (LiFSA)/keto ester electrolytes.3 We found that the solvent molecules, having two coordinating sites, bridge different Li+ ions, resulting in the formation of the solvent-bridged network structures of Li+−solvent−Li+ in these highly concentrated electrolytes. Therefore, highly concentrated electrolytes with certain solvents having multiple coordinating sites are anticipated to exhibit Li+ ion hopping conduction. We report here the phase behaviors, solvate structures and transport properties of highly concentrated electrolytes composed of LiFSA and succinonitrile (SN) solvent, which has two cyano groups in the molecular structure. LiFSA and SN form a stable complex at 1:2 molar ratio, and the melting point of the solvate of LiFSA-(SN)2 is 63.4 °C. X-ray crystallography for the solvate of LiFSA-(SN)2 revealed that the SN molecule bridges two different Li+ ions in the crystal. At a molar ratio higher than [LiFSA]/[SN] = 1/1, the mixture becomes a glass forming liquid, and the [LiFSA]/[SN] = 1/0.8 mixture maintains a liquid state at room temperature. Raman spectra for [LiFSA]/[SN] = 1/0.8 suggested that solvent-bridged structure of Li+−SN−Li+ is still maintained in the liquid. In addition to the solvent-bridged structure, anion bridged structure of Li+−FSA−−Li+ is also formed in the solvent-deficient mixture of [LiFSA]/[SN] = 1/0.8. The self-diffusion coefficients of Li+, FSA−, and SN measured with pulsed field gradient (PFG) NMR suggested that Li+ ion diffuses faster than anion and SN (Table 1), and the Li+ ion hopping conduction emerges in these liquid electrolytes having the polymeric network structures. The [LiFSA]/[SN] = 1/0.8 electrolyte possesses a wide electrochemical window, and the graphite anode and LiNi1/3Mn1/3Co1/3O2 cathode of Li-ion batteries can be charged and discharged reversibly in this electrolyte. References 1 Y. Yamada and A. Yamada, Review—Superconcentrated Electrolytes for Lithium Batteries, J. Electrochem. Soc., 2015, 162, A2406–A2423. 2 K. Dokko, D. Watanabe, Y. Ugata, M. L. Thomas, S. Tsuzuki, W. Shinoda, K. Hashimoto, K. Ueno, Y. Umebayashi and M. Watanabe, Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes, J. Phys. Chem. B, 2018, 122, 10736–10745. 3 S. Kondou, M. L. Thomas, T. Mandai, K. Ueno, K. Dokko and M. Watanabe, Ionic transport in highly concentrated lithium bis(fluorosulfonyl)amide electrolytes with keto ester solvents: Structural implications for ion hopping conduction in liquid electrolytes, Phys. Chem. Chem. Phys., 2019, 21, 5097–5105. Table caption Table 1. Viscosity (η), density (ρ), concentration of LiFSA (c), ionic conductivity (σ), and self-diffusion coefficients of electrolytes of LiFSA/SN at 30 °C. Figure 1