Meaning
This analytical method measures the pore size distribution and gas permeability of porous battery separators by utilizing liquid displacement. By saturating the membrane with a wetting liquid and then gradually displacing it with a pressurized gas, the instrument calculates pore diameters. Capillary flow porometry identifies the largest pore, known as the bubble point, as well as the mean flow pore size.
The test is critical for qualifying separator membranes used in liquid-electrolyte lithium-ion batteries. It is limited to through-pores that extend from one side of the membrane to the other, completely ignoring closed or blind pores.
Experimental Protocol
The separator sample is securely clamped into a test chamber where it is completely saturated with a low-surface-tension fluid. Pressurized nitrogen or dry air is introduced behind the membrane, and the pressure is slowly increased to force the liquid out. The first gas bubble emerges through the largest pore at a specific threshold pressure, which relates to the pore diameter via the Washburn equation.
As pressure continues to rise, smaller pores empty, and the gas flow rate increases until the membrane is dry. Comparing the dry and wet flow curves yields the full distribution of active pores across the separator sample.
Supply Qualification
Sourcing engineers utilize this porometry data to verify the quality of separator shipments from different membrane manufacturers. The average pore diameter directly influences the internal resistance and the short-circuit risk of the finished cell. Having a consistent pore size prevents the localized current concentrations that trigger dendritic growth and subsequent thermal runaway.
Purchasing agreements stipulate that the mean flow pore size must remain within a narrow, defined window to ensure uniform electrochemical performance. This testing standard allows buyers to compare different separator options without relying on the manufacturers own proprietary datasheets.
Analytical Limits
The technique cannot measure blind pores that do not contribute to ionic conductivity, as these do not allow gas flow. High gas pressures can mechanically deform highly flexible polymer membranes, artificially enlarging the pores during the measurement process. The selection of the wetting fluid is critical because any chemical interaction with the polymer will alter the surface tension and distort the results.
The method does not measure tortuosity directly, requiring secondary electrochemical tests to determine how the pore network behaves when filled with electrolyte. These factors necessitate careful calibration and standard reference materials to ensure comparable results across different test laboratories.
