Battery Materials Analysis: From Cathode Coatings to Copper Foil

Australia is one of the world’s largest lithium producers, and there is growing interest in doing more of the value chain here: refining, precursor and cathode materials, cell research and recycling. Every step depends on knowing what is in a material and how it is arranged, often at length scales from millimetres down to nanometres.

A lithium-ion cell is a layered stack: a cathode coated on aluminium foil, a separator, and an anode, usually graphite, coated on copper foil, all soaked in electrolyte. Performance and lifetime depend on details at every scale: particle chemistry and coatings, how evenly the electrode is mixed, the microstructure of the current collectors, and the contaminants that sneak in during manufacture. This article looks at how three electron-microscope techniques address those questions, with examples from Bruker’s application notes.

Nanometre coatings on cathode particles: EDS in TEM and SEM

Nickel-rich cathode materials such as NCM (lithium nickel cobalt manganese oxide) give high energy density, but their particle surfaces react with the electrolyte, and capacity fades with cycling. A thin protective coating, often applied by atomic layer deposition (ALD), helps, but only if it covers the particles evenly. The coating is too thin and too similar in contrast to see reliably in an image, so it has to be mapped chemically.

Bruker’s note on EDS analysis of a coated Li-ion cathode particle looked at NCM particles with an ALD titanium coating. A cross-section was cut with a FIB-SEM and then mapped two ways: by STEM EDS in a TEM, and by EDS of the thin section in an SEM (T-SEM). Both showed the titanium coating, 20–30 nm thick, and allowed its uniformity to be checked.

The note also compared detectors. In the SEM, an annular XFlash FlatQUAD detector, which sits above the sample and collects X-rays over a much larger solid angle, produced much better data than a conventional detector in the same time. Bruker estimates it reaches the same data quality about 15 times faster. For thin, beam-sensitive battery samples, collecting more signal at lower dose is a real advantage.

Relevant systems: QUANTAX EDS for TEM for STEM work, and QUANTAX EDS for SEM, which includes the FlatQUAD option.

Copper current collectors: EBSD

The copper foil under the anode has to stay intact through thousands of charge cycles while being as thin as possible. Its strength, ductility and conductivity depend on grain size, crystal texture and twin boundaries, which manufacturers control through electrodeposition or rolling and annealing. Texture matters in particular: a <111> orientation is generally preferred for electrochemical stability, while <110> is associated with lower ductility and a greater risk of failure during cycling.

In Bruker’s note on EBSD of copper foil for Li-ion batteries, a very-smooth-profile foil was gently polished and mapped with the eWARP direct-detection EBSD detector at 10 kV and 12 nA. More than three million points were collected in about 18 minutes, and a finer 30 nm step resolved individual twin domains. The results:

• around 9,500 grains measured, averaging about 7 µm and up to 18 µm;
• very low misorientation within grains, so little residual plastic deformation, apart from around pores and scratches;
• twin boundaries (mainly Σ3, with a few Σ9) made up more than 70% of all boundaries;
• <111> and <110> textured areas covered roughly 15% and 17% of the map, with no strong overall preferred orientation.

The authors concluded that the foil’s strength is governed mainly by its twin boundaries. Measurements like these let a foil maker link process changes to microstructure, and a cell maker compare suppliers objectively. See QUANTAX EBSD, and our article on ultra-fast EBSD for more on the detector.

Whole electrodes and contaminants: micro-XRF on the SEM

Some battery questions need a much larger field of view: how evenly the active material is spread across an electrode, whether a cross-section is uniform, or where a metallic contaminant particle is hiding. Metal particles such as iron, copper or zinc that end up in a cell can cause internal short circuits, so finding them matters.

Micro-XRF on the SEM uses a focused X-ray beam, collected by the same EDS detector, to map areas several centimetres across with trace-level sensitivity and no coating needed. Once a feature is found, the electron beam takes over for high-resolution EDS on the same spot. Bruker demonstrates this approach in a video on correlating SEM micro-XRF and SEM EDS analysis of a Li-ion battery. See QUANTAX Micro-XRF, and our article on micro-XRF and EDS for gold exploration for the same workflow applied to ore.

Sample preparation for battery materials

Battery materials are among the hardest samples to prepare. Electrodes are porous composites of hard oxide particles, soft binder and carbon. Many components, lithium metal and charged electrodes especially, react with air and moisture within seconds. Mechanical polishing smears soft phases and pulls out particles.

Broad-beam argon ion milling cuts clean cross-sections through these mixed materials without mechanical contact. The COXEM IP-10K handles metals, ceramics and battery components, with two options of particular interest here: Peltier cooling to protect heat-sensitive binders and polymers, and an air-protection module that carries air-sensitive samples from the glove box to the polisher and on to the SEM without exposure.

A note on lithium

Lithium itself is very difficult to detect with EDS. Its characteristic X-rays have extremely low energy and are produced weakly. Analysis of battery materials in the electron microscope therefore concentrates on the transition metals, coating elements, current collectors and contaminants, and lithium distribution is usually studied with other techniques.

Whether your work is on cathode chemistry, current collectors, cell failure analysis or recycling, we can help you choose the right combination of detectors and sample preparation. Contact us to start the conversation.

Application examples summarised from Bruker’s published notes, linked above. Figures are reproduced as Bruker published them, with their credits.