Li Ion Hopping Conduction in Highly Concentrated Liquid Electrolytes

• Published in: Meeting abstracts (Electrochemical Society) Vol. MA2019-01; no. 2; p. 259
• Main Authors: Dokko, Kaoru, Watanabe, Daiki, Ugata, Yosuke, Thomas, Morgan L., Ueno, Kazuhide, Watanabe, Masayoshi
• Format: Journal Article
• Language: English
• Published: 01.05.2019
• ISSN: 2151-2043; 2151-2035
• DOI: 10.1149/MA2019-01/2/259

We demonstrate here that Li + ion hopping conduction emerges in highly concentrated liquid electrolytes composed of LiBF 4 and sulfolane (SL). Self-diffusion coefficients of Li + ( D Li ), BF 4 − ( D BF4 ), and SL ( D SL ) were measured with pulsed field gradient (PFG) NMR. In the concentrated electrolytes with molar ratios of SL/LiBF 4 ≤ 3, the ratios of diffusion coefficients, D SL / D Li and D BF4 / D Li , become lower than unity, suggesting faster diffusion of Li + than SL and BF 4 − . X-ray crystallographic analysis of a single crystal of LiBF 4 :SL (1:1) solvate revealed that the two oxygen atoms of the sulfone group are involved in bridging coordination of two different Li + ions. In addition, the BF 4 − anion also participates in bridging coordination of Li + . Raman spectroscopy revealed that Li + ions are bridged by SL and BF 4 − even in the highly concentrated LiBF 4 /SL solution. The fact that D Li is larger than D SL and D BF4 in the highly concentrated electrolytes strongly suggests that Li + exchanges between the coordination sites formed by the ligands (SL and BF 4 − ) and moves forward leaving behind the ligands. The SL and BF 4 − provide closely placed coordination sites in the highly concentrated electrolytes. The spatial proximity of coordination sites is assumed to enable Li + hopping conduction. Furthermore, we found that Li + hopping conduction occurs in the highly concentrated SL-based electrolytes containing other Li salts, such as LiClO 4 and Li(NSO 2 F) 2 . The higher diffusion coefficient of Li + than that of anion results in the higher Li + ion transference number (> 0.5) in the electrolytes. The higher Li + ion transference number was effective in suppressing the concentration polarization in a lithium battery, leading to increased limiting current density and improved rate capability compared to the conventional concentration electrolyte. Acknowledgements This study was supported in part by JSPS KAKENHI (Grant Nos. 16H06368 and 18H03926) from the Japan Society for the Promotion of Science (JSPS) and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). Reference K. Dokko, D. Watanabe, Y. Ugata, M. L. Thomas, S. Tsuzuki, W. Shinoda, K. Hashimoto, K. Ueno, Y. Umebayashi, M. Watanabe, J. Phys. Chem. B , 122 , 10736-10745 (2018).