Probing the heterogeneous nature of LiF in solid-electrolyte interphases

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• The electrolyte-electrode interface is critical for battery performance.
• The solid-electrolyte interphase (SEI) is essential for rechargeable lithium-ion battery (LIB) reversibility.
• Accurate characterization of the SEI's chemical composition is challenging due to its low crystallinity and sensitivity.

Purpose of the Study:

• To investigate the precise chemical composition of lithium fluoride (LiF) within the SEI.
• To explore the implications of SEI composition on battery performance, particularly in lithium-metal batteries.
• To provide new insights for designing advanced electrode-electrolyte interfaces.

Main Methods:

• Utilized 19F solid-state nuclear magnetic resonance (NMR) spectroscopy to analyze LiF in the SEI.
• Employed 6Li isotope NMR, synchrotron X-ray diffraction, and cryo-electron microscopy (cryo-EM) for validation.
• Characterized SEI formed in various electrolytes to determine dominant phases.

Main Results:

• Identified LiF in the SEI (LiFSEI) as LiF-LiH solid solutions, comprising H-rich (LiH1-yFy) and F-rich (LiF1-xHx) phases.
• Confirmed the dominance of the LiH1-yFy phase in high-coulombic-efficiency electrolytes.
• Demonstrated that LiH1-yFy-rich coatings offer superior performance in lithium-metal batteries compared to LiF-rich coatings.

Conclusions:

• The SEI component LiF exists as a LiF-LiH solid solution, not pure LiF.
• The LiF-LiH solid solution exhibits improved ionic conductivity over pure LiF, explaining its prevalence in efficient electrolytes.
• Understanding the heterogeneous nature of SEI components is crucial for optimizing electrode-electrolyte interface design in next-generation batteries.