Tuning Reaction Kinetics of Fluorinated Molecules to Construct Robust Solid Electrolyte Interphases on SiO_x Anode

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• Silicon anodes offer high capacity but suffer from volume changes during cycling.
• Fluorinated electrolyte additives create a lithium fluoride (LiF)-rich solid-electrolyte interphase (SEI) to stabilize silicon anodes.
• Current research often overlooks the decomposition kinetics of these additives, focusing instead on thermodynamics.

Purpose of the Study:

• To investigate the correlation between the molecular structure of fluorinated electrolyte additives and their decomposition kinetics.
• To understand how additive structure influences the formation and stability of the SEI layer on silicon anodes.
• To enhance the cycling stability of silicon-based anodes in lithium-ion batteries.

Main Methods:

• Design and synthesis of two fluorinated ester additives: diethyl fluoromalonate (F1DEM) and diethyl 2,2-difluoromalonate (F2DEM).
• Utilized a combination of advanced characterization techniques and computational simulations.
• Evaluated battery performance using 5 Ah 21700 cylindrical cells under practical testing conditions.

Main Results:

• Despite similar reduction thermodynamics, F1DEM exhibited more favorable reduction kinetics than F2DEM.
• The superior kinetics of F1DEM facilitated the early formation of a LiF-rich SEI layer.
• This resulted in a more robust SEI on SiOₓ anodes, leading to improved cycling stability.
• Achieved 84% capacity retention after 1000 cycles in 5 Ah 21700 batteries.

Conclusions:

• The decomposition kinetics of fluorinated electrolyte additives play a critical role in SEI formation and anode stability.
• Diethyl fluoromalonate (F1DEM) is a promising additive for constructing stable SEI layers on silicon anodes.
• This research highlights the importance of considering reaction kinetics for designing effective electrolyte additives for advanced batteries.