Asymmetric Ionic Liquid Modulated Anion-Reinforced Electric Double Layer for Advanced Durable Lithium Batteries

Area of Science:

• Materials Science
• Electrochemistry
• Chemical Engineering

Background:

• The electric double layer (EDL) and solid electrolyte interphase (SEI) are critical for battery performance, but electrolyte design often overlooks interfacial phenomena.
• Current electrolyte design strategies primarily rely on bulk solvation properties, limiting optimization of interfacial processes.

Purpose of the Study:

• To engineer an electrolyte that promotes an anion-reinforced EDL for improved SEI formation and battery performance.
• To investigate the role of molecular asymmetry in ionic liquids for interfacial control.

Main Methods:

• Design and synthesis of an asymmetric phosphonium ionic liquid, (2-methoxyethoxy)methyl phosphonium hexafluorophosphate (PMEP).
• Utilized molecular dynamics (MD) simulations and density functional theory (DFT) calculations to study interfacial solvation structures.
• Fabricated and tested Li|LiFePO4 batteries and Graphite|LiFePO4 cylindrical cells.

Main Results:

• PMEP's molecular asymmetry facilitates PF6- participation in Li+-centered interfacial solvation clusters.
• Formation of a stable, dual organic/inorganic SEI layer (including LiF and Li2O) due to anion reinforcement.
• Reduced interfacial impedance and activation energy for Li+ transfer, enabling uniform lithium deposition.
• Demonstrated excellent cycling stability and capacity retention in Li|LiFePO4 batteries and Graphite|LiFePO4 cells.

Conclusions:

• Shifting electrolyte design from bulk properties to interfacial solvation structure engineering is crucial for next-generation batteries.
• Anion-reinforced EDLs can significantly enhance SEI properties and battery longevity.
• The developed PMEP ionic liquid offers a promising pathway for high-performance lithium-ion batteries.