Fluorophosphate-Based Nonflammable Concentrated Electrolytes with a Designed Lithium-Ion-Ordered Structure:...

Area of Science:

• Electrochemistry
• Materials Science
• Chemical Engineering

Background:

• Developing safer lithium-ion batteries (LIBs) is critical for widespread adoption.
• Nonflammable electrolytes are essential for enhancing LIB safety.
• Concentrated electrolytes offer potential safety benefits but can hinder electrode reactions.

Purpose of the Study:

• To design molecular structures in nonflammable concentrated electrolytes for improved Li-ion battery (LIB) electrode reactions.
• To investigate the role of molecular additives in enabling electrode reactions in concentrated electrolytes.
• To elucidate the mechanism of solid electrolyte interphase (SEI) formation for safer LIBs.

Main Methods:

• High-energy X-ray total scattering experiments.
• All-atom molecular dynamics simulations.
• In situ surface-enhanced infrared absorption spectroscopy (SEIRAS).

Main Results:

• Concentrated Li bis(fluorosulfonyl)amide (FSA)/tris(2,2,2-trifluoroethyl) phosphate (TFEP) electrolytes initially showed no electrode reaction due to Li-ion mononuclear complex formation.
• Addition of acetonitrile (AN) as a small molecular additive reduced steric effects, promoting Li-ion-ordered structures linked by FSA anions.
• These ordered complexes stabilized the lowest unoccupied molecular orbital (LUMO) of FSA, facilitating anion-derived SEI formation on the graphite electrode.
• Reversible charge/discharge behavior was achieved with the AN additive.

Conclusions:

• Molecular design of Li-ion-ordered complexes in nonflammable concentrated electrolytes is a viable strategy for safer LIBs.
• Acetonitrile acts as a crucial additive to overcome steric limitations and enable electrode reactions in TFEP-based electrolytes.
• The formation of specific Li-ion-ordered structures is key to achieving stable SEI layers and improved battery performance.