posted on 2019-11-08, 01:29authored byMaria Forsyth, Matthias Hilder, Yafei Zhang, Fangfang Chen, Ludovic Carre, Dmitrii A. Rakov, Michel Armand, Douglas R. Macfarlane, Cristina Pozo-Gonzalo, Patrick C. 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.