Electron microscopy is a key technique in understanding battery materials but simple outer surface observation provides insufficient insight. The creation of high quality cross-sections is important to develop a correct understanding as it reveals the true structure and distribution of particles, pores and the various components.
Techniques for cross-sectioning
Mechanical techniques such as resin-embedding and mechanical polishing, cutting with a precision saw or cleaving are widely used, but are difficult to use for porous composite materials. Pores may get filled with polishing debris, hard particles may get pulled out, soft materials are smeared and layers are easily delaminated.
Focused Ion Beam (FIB) instruments using a liquid metal ion gun (Ga+) or plasma source (typically Xe+) provide a solution for high precision, site-specific cross-sectioning, but are generally not well suited for milling wide areas due to limited speed and field of view.
Broad Ion Beam Milling (BIB) is a technique where a low energy Ar+ ion beam is used to quickly cross section large areas with limited sample damage. This technique offers high quality sectioning over the length-scales which are most meaningful for battery materials.
Anodes
Cross-sectional studies give insights into cluster sizes, binder distribution, compaction ratio, film uniformity, film thickness and adhesion to the electrode.
Left image: Anode cross section
Right image: Detail of anode cross section showing adhesion to the Cu foil
Cathodes
Typical sintered metal powder clusters for cathodes need to be studied in cross-section to reveal the internal grain or pore structure which is critical for performance. Properties such as particle size distribution, pore size, binder distribution and particle cracking can be studied and quantified.
Following the formation and aging process, or following battery cycling, electrode properties such as further swelling or cracking, degradation and SEI layer formation can be studied and correlated with practical battery performance.
Left image: NMC cathode active material particle
Right image: Electrode of Lithium Sulphur battery
Separator foils
Although the separator foil is not an active part of the cell it plays a key role in ion transport and has a strong influence on cell performance, life time and safety. Cross sectioning can help in understanding pore size, structure of the ceramic coating layer or possible residues after cycling which may impact long term performance.
Left image: Cross section of separator membrane
Right image: Separator imaged at 300V showing residues after cycling
Conclusion
Electron microscopy and high quality cross sections from broad beam ion milling are invaluable to understand and develop solutions for energy storage. Revealing and studying the real structure in this visual way offers much deeper insight than alternative “numerical only” techniques such as a particle size distribution chart from laser diffraction or a measurement of gas adsorption using BET.
Contact us to find out how broad beam ion milling can be applied to your materials.
