Low-kV SEM Imaging of Separator Porosity in Lithium-Ion Batteries

Seeing Pores Without Obscuring Them

Battery separators are built to manage ion flow while physically isolating electrodes. This dual role depends on pore structure; size, shape, and distribution all play into ionic transport and mechanical resilience. But imaging these details is not straightforward.

Standard SEM imaging of separators often requires metal coating, which can obscure fine pore dimensions. Alternatively, imaging uncoated samples at low beam energies and currents leads to resolution loss in conventional SEMs. This study demonstrates how ultra-high-resolution SEM enables detailed pore structure analysis without compromising resolution or sample integrity. 

The sample was a commercially available polypropylene (PP) separator, analyzed as received, with no conductive treatment. Imaging was performed with the Tescan CLARA, at 200 eV and using an axial detector specifically chosen for high-contrast topographic response.

Key imaging conditions:

• No coating

• Beam energy: 200 eV

• Magnifications tuned for both pore network overview and fiber detail

Captured images were processed using MIPAR software; automated segmentation tools were applied to:

• Measure pore diameters (range: 20 to 140 nm; average: 59 nm)

• Evaluate fiber thickness (range: 20 to 211 nm; average: 71 nm)

Pore Clarity Without Sample Prep

Even at this low beam energy, the SEM produced crisp images of the membrane surface. Pore edges were well-defined, allowing segmentation and measurement without manual tracing or correction. Fiber width and pore spacing showed significant heterogeneity, emphasizing the importance of direct analysis over assumptions based on material formulation.

The ability to image uncoated polymer samples at nanometer resolution opens new possibilities for separator quality control and materials development, especially when porosity must be matched to electrolyte viscosity or thermal shutdown behavior.

With this method, users gain access to fine-scale features without sacrificing time or damaging delicate samples.