Achieving Pristine Cross Sections of Battery Samples for Scanning Electron Microscopy
Lithium-ion batteries present unique challenges for sample preparation prior to imaging and analysis with the Scanning Electron Microscope (SEM)
. Batteries consist of layers of thin films that form multiple solid−solid interfaces and are sensitive to air when the inner structures are exposed. The ultrathin layers of cathode materials, separators, anode materials, metal current collectors, and electrolyte – all in the form of powders, sheets, and fluids – are each of different compositions and hardnesses. This layered composite of varying and sensitive materials requires special preparation techniques.
Broad beam ion milling is a robust method for obtaining pristine cross sections of such a complex system. Other, more traditional mechanical polishing tools introduce various artifacts, such as scratches and embedded polishing media, that obscure the original microstructure, crystallographic information and precise layer thickness measurements.
Figure 1. Scanning Electron Microscope image of Cross sectioned battery layers.
The JEOL Ion Beam Cross Section Polisher (CP)
is widely used for preparing pristine samples prior to high resolution imaging and elemental analysis with the SEM. It produces pristine cross sections of samples – hard, soft, or composites – without smearing, crumbling, distorting, or contaminating them in any way.
The JEOL CP uses an Argon ion beam to mill cross sections or polish virtually any material that is affixed to the continuously rotating sample holder. During milling, the sample is rocked automatically to reduce creating beam striations on the cross-sectioned surface.
Figure 2. JEOL Ion Beam Cryo Cross Section Polisher
With the emergence of increased research on batteries, JEOL developed a special Cooling Cross Section Polisher (CCP)
for preparation and polishing of materials that are sensitive to exposure to air or thermal damage. This new configuration is designed for cryo-preparation (down to LN2 temperature) and air-isolated transfer for atmosphere sensitive specimens The CCP allows long cooling periods while conserving liquid nitrogen.
The CCP is integrated into JEOL’s air-isolated workflow using a sample transfer vessel for air and heat-sensitive samples. The sample is initially prepared in an inert gas environment (such as in a glove box) then safely transferred to the Cryo Cross-section Polisher, and subsequently into the SEM (or Focused Ion Beam preparation and Transmission Electron Microscope) using the special air-isolated sample holder.
Figure 3. JEOL air-isolated workflow for battery sample preparation and imaging in the SEM.
Using this technique, it is possible to examine morphological (surface) details and compositional characterization of battery materials without any air exposure. Every layer of detail can be examined clearly in the SEM. Ultra-low voltage imaging combined with signal filtering in the SEM allows direct imaging and analysis of battery constituents (anode and cathode) with nanometer resolution.
Figure 4. Scanning Electron Microscope image showing cross section of Lithium Ion Battery cathode. Larger image is magnified at 7500X for details of precise cross sectioning.
Figure 5. Scanning Electron Microscope image showing cross section of Lithium Ion Battery separator.