Tuning Sodium Interfacial Chemistry with Mixed-Anion Ionic Liquid Electrolytes

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• The solid-electrolyte interphase (SEI) layer is crucial for high-performing batteries, influencing electrode stability and internal resistance.
• Traditional lithium-ion battery electrolytes use additives like hexafluorophosphate (PF6) for favorable SEI formation.
• Dicyanamide (DCA) anions in ionic liquids offer a cost-effective alternative but can lead to electrode passivation due to instability.

Purpose of the Study:

• To investigate the use of fluorinated anions to enhance SEI stability in dicyanamide-based ionic liquids (ILs).
• To explore the impact of different sodium salts (NaDCA, NaFSI, NaTFSI, NaFTFSI) on interfacial electrochemistry and SEI formation.
• To rationalize electrochemical observations by understanding anion influence on bulk electrolyte, interfacial structure, and SEI composition.

Main Methods:

• Electrochemical testing using symmetric cells for extended cycling.
• Molecular dynamics simulations to study electrolyte behavior.
• Infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) for surface and bulk analysis.

Main Results:

• Addition of NaFSI to methylpropylpyrrolidinium dicyanamide ([C3mpyr]DCA) IL resulted in a more stable SEI layer, confirmed by extended cycling.
• NaTFSI and NaFTFSI salts led to thicker, highly passivating surfaces, indicating less favorable SEI formation.
• Anion choice significantly influences bulk electrolyte properties, interfacial structure, and the resulting SEI layer composition.

Conclusions:

• Fluorinated anions, particularly FSI, are effective in controlling interfacial electrochemistry and forming stable SEI layers in DCA ILs.
• NaFSI addition provides a promising strategy to improve the long-term stability and performance of batteries utilizing dicyanamide-based electrolytes.
• Understanding anion effects is key to designing advanced electrolytes for stable and efficient energy storage systems.