Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes

• Dokko, Kaoru

• Watanabe, Daiki

• Ugata, Yosuke

• Thomas, Morgan L.

• Tsuzuki, Seiji

• Shinoda, Wataru

• Hashimoto, Kei

• Ueno, Kazuhide

• Umebayashi, Yasuhiro

• Watanabe, Masayoshi

Abstract

We demonstrate that Li+ hopping conduction, which cannot be explained by conventional models i.e. Onsager’s theory and Stokes’ law, emerges in highly concentrated liquid electrolytes composed of LiBF4 and sulfolane (SL). Self-diffusion coefficients of Li+ (DLi), BF4− (DBF4), and SL (DSL) were measured with pulsed field gradient NMR. In the concentrated electrolytes with molar ratios of SL/LiBF4 ≤ 3, the ratios DSL/DLi and DBF4/DLi become lower than 1, suggesting faster diffusion of Li+ than SL and BF4−, and thus the evolution of Li+ hopping conduction. X-ray crystallographic analysis of the LiBF4: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 BF4− anion also participates in bridging coordination of Li+. Raman spectra of the highly concentrated LiBF4-SL solution suggested that Li+ ions are bridged by SL and BF4− even in the liquid state. Moreover, detailed investigation along with molecular dynamics simulations suggests that Li+ exchanges ligands (SL and BF4−) dynamically in the highly concentrated electrolytes, and Li+ hops from one coordination site to another. The spatial proximity of coordination sites, along with possible domain structure, is assumed to enable Li+ hopping conduction. Finally, we demonstrate that Li+ hopping suppresses concentration polarization in Li batteries, leading to increased limiting current density and improved rate capability compared to the conventional concentration electrolyte. Identification and rationalization of Li+ ion hopping in concentrated SL electrolytes is expected to trigger a new paradigm of understanding for such unconventional electrolyte systems.

Journal

• Journal of Physical Chemistry B

  Journal of Physical Chemistry B 122 (47), 10736-10745, 2018-11-29

  American Chemical Society

Keywords

• Materials Chemistry
• Surfaces, Coatings and Films
• Physical and Theoretical Chemistry

Details

• CRID: 1050845762754665344
• NII Article ID: 120006536971
• ISSN: 15206106
• ISSN: 15205207
• DOI: 10.1021/acs.jpcb.8b09439
• Web Site: http://hdl.handle.net/10131/00012075
• Web Site: http://pubs.acs.org/doi/pdf/10.1021/acs.jpcb.8b09439
• Web Site: https://pubs.acs.org/doi/pdf/10.1021/acs.jpcb.8b09439
• Text Lang: en
• Article Type: journal article
• Data Source:
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