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Tuning sodium interfacial chemistry with mixed-anion ionic liquid electrolytes

journal contribution
posted on 01.11.2019, 00:00 authored by Maria ForsythMaria Forsyth, Matthias HilderMatthias Hilder, Yafei Zhang, Fangfang ChenFangfang Chen, L Carre, Dmitrii Rakov, M Armand, D R Macfarlane, Cristina Pozo-GonzaloCristina Pozo-Gonzalo, Patrick HowlettPatrick Howlett
The interphase layer that forms on either the anode or the cathode is considered to be one of the critical components of a high performing battery. This solid-electrolyte interphase (SEI) layer determines the stability of the electrode in the presence of a given electrolyte as well as the internal resistance of a battery, and hence the overpotential of a cell. In the case of lithium ion batteries where carbonate based electrolytes are used, additives including hexafluorophosphate (PF6), bis-trifluoromethylsulfonimide (TFSI), (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI), and fluorosulfonimde (FSI) are used to obtain favorable SEI layers. Ionic liquids and salts based on anions containing nitrile groups, including dicyanamide (DCA), offer a less expensive alternative to a fluorinated anion and have also been shown to support stable electrochemistry in lithium and sodium systems. However, longer term cycling leads to the eventual passivation of the electrode, presumed to be due to the instability of the DCA anion. We herein consider the use of a fluorinated anion to control the interfacial electrochemistry and provide a more stable SEI in DCA ILs. We investigate the addition of NaDCA, NaFSI, NaTFSI, and NaFTFSI to the methylpropylpyrrolidinium dicyanamide ([C3mpyr]DCA) ionic liquid. NaFSI was found to generate a more stable SEI layer, as evidenced by extended symmetric cell cycling, while the TFSI and FTFSI salts both lead to thicker, highly passivating surfaces. We use molecular dynamics, infrared spectroscopy and X-ray photoelectron spectroscopy to interrogate and discuss the influence of the anion on the bulk electrolyte, the interfacial electrolyte structure, and the formation of the SEI layer, in order to rationalize the contrasting electrochemical observations.

History

Journal

ACS applied materials and interfaces

Volume

11

Issue

46

Pagination

43093 - 43106

Publisher

ACS Publications

Location

Washington, D.C.

ISSN

1944-8244

eISSN

1944-8252

Language

eng

Publication classification

C1 Refereed article in a scholarly journal