(A04 Best Poster Award Winner) Effects of Anion Species on Li Ion Transport and Electrochemical Properties in Highly Concentrated Electrolytes
Authors: Ugata Yosuke , Sasagawa Shohei , Watanabe Daiki , Ueno Kazuhide , Watanabe Masayoshi , Dokko Kaoru 
Published: December 2020

Highly concentrated solutions of Li salt dissolved in polar solvents have various attractive features as battery electrolytes.1 Recently, our group reported that Li+ ion hopping conduction occurs in highly concentrated electrolytes of Li-salt/sulfolane (SL) where Li+ ion diffuses faster than anions and SL.2,3 In the liquids of high concentration Li-salt/SL electrolytes, the polymeric network structures of Li+−SL−Li+ and Li+−anion−Li+ are formed, and Li+ ion hops from one coordination site to another through the ligand exchange. Similar Li+ ion hopping conduction was also observed for highly concentrated electrolytes with certain solvents having multiple coordinating sites, such as keto ester,4 dinitrile,5 and other sulfone solvents.6 We report here the effects of anion species on the transport properties (ionic conductivity (σ) and viscosity (η)) and the battery performance of the highly concentrated electrolytes composed of SL and Li-salts, LiBF4, LiN(SO2CF3)2 (LiTFSA), and LiN(SO2F)2 (LiFSA). The order of molar ionic conductivity (σ/c
Li+) is LiFSA > LiBF4 ⋍ LiTFSA. Li+ ion transference number (t
Li+) under anion-blocking conditions was estimated using a symmetric Li/Li cell. The order of t
Li+ values in the SL-based electrolytes is LiBF4 > LiTFSA > LiFSA, suggesting that the Lewis basicity of the anions and the Li+−anion association affect the Li+ ion transport in the highly concentrated electrolytes. To investigate the effects of anion species on the battery performance, we conducted charge-discharge tests of Li–graphite cells with these electrolytes. The cell using the LiFSA/SL electrolyte, which has the highest value of σ × t
Li+, showed the best rate performance. Although the Li+ ion conductivity σ × t
Li+ of [LiBF4]/[SL] = 1/2 is higher than that of [LiTFSA]/[SL] = 1/2, the rate capability of a Li–graphite cell with [LiTFSA]/[SL] = 1/2 electrolyte is better than one with [LiBF4]/[SL] = 1/2 . To investigate the interfacial resistance (R
int) of graphite electrode/electrolyte, electrochemical impedance measurements were performed. Among the tested electrolytes, the [LiBF4]/[SL] = 1/2 electrolyte showed the highest R
int probably due to the thick SEI layer on the graphite electrode. The higher R
int might lead to the worse rate capability of the Li–graphite cell. Rate performances of Li–LiCoO2 cells with these electrolytes will also be reported.

Acknowledgements

This study was supported in part by JSPS KAKENHI (Grant Nos. 16H06368, 18H03926, and 19H05813) from the Japan Society for the Promotion of Science (JSPS) and the JST ALCA-SPRING Grant Number JPMJAL1301.

References

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2 K. Dokko, D. Watanabe, Y. Ugata, M. L. Thomas, S. Tsuzuki, W. Shinoda, K. Hashimoto, K. Ueno, Y. Umebayashi and M. Watanabe, J. Phys. Chem. B, 2018, 122, 10736–10745.

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4 S. Kondou, M. L. Thomas, T. Mandai, K. Ueno, K. Dokko and M. Watanabe, Phys. Chem. Chem. Phys., 2019, 21, 5097–5105.

5 Y. Ugata, M. L. Thomas, T. Mandai, K. Ueno, K. Dokko and M. Watanabe, Phys. Chem. Chem. Phys., 2019, 21, 9759–9768.

6 S. Sasagawa, Y. Ugata, K. Ueno, K. Dokko, M. Watanabe, 236th ECS Meeting Abstract, Oct. 13-17, 2019, Atlanta, GA.