Visualization and Chemical Characterization of the Cathode Electrolyte Interphase Using He-Ion Microscopy and In Situ...

Area of Science:

• Materials Science
• Electrochemistry
• Battery Technology

Background:

• Unstable cathode electrolyte interphase (CEI) formation is a key factor limiting the performance and lifespan of high-voltage lithium-ion batteries.
• Current characterization techniques often fail to correlate nanoscale CEI properties with macroscale microstructure, hindering understanding of degradation mechanisms.

Purpose of the Study:

• To investigate the process of CEI formation in the high-voltage cathode material LiCoPO4.
• To correlate the morphology and chemistry of the CEI layer with the underlying cathode microstructure using advanced imaging and spectroscopy.

Main Methods:

• Utilized helium ion microscopy (HIM) for high-resolution imaging of CEI morphology.
• Employed in situ time-of-flight (ToF) secondary ion mass spectrometry (SIMS) for chemical analysis of the CEI layer.
• Correlated nanoscale CEI features with local cathode microstructure (position, thickness, chemistry) on LiCoPO4.

Main Results:

• HIM revealed partial dissolution of the CEI layer during discharge, leading to inhomogeneous coverage on LiCoPO4 agglomerates.
• ToF-SIMS detected oxyfluorophosphates (from electrolyte attack) and a Li-rich surface region.
• Variable CEI thickness and inactive Li accumulation were linked to severe electrode degradation within 10 cycles.

Conclusions:

• The combined HIM-SIMS technique effectively links CEI morphology and chemistry to cathode microstructure.
• Understanding these structure-property relationships is crucial for mitigating degradation in high-voltage Li-ion batteries.
• This approach offers potential for future development of improved cathode and anode materials.