Pore Size Measurement of Battery Separators

A typical battery consists of two electrodes, a cathode (+) and an anode (-), and an electrolyte. The electrodes are electrical conductors that contact with the electrolyte allowing for the circulation of charge.

One of the key parts in a battery is the separator.  It is a porous membrane that acts as a barrier between the two electrodes to prevent short circuits but permits the transfer of charge enabling current circulation. In Li-based batteries (Li-ion, Li-polymer), separators are made of polymeric materials such as polyethylene (PE), polypropylene (PP), and others. These materials are low cost and have good chemical stability. Ideally, the separator should have the pore size of tens of nanometers to allow circulation of the Li ions in either direction. Li ions move from the anode to the cathode during discharge and from the anode to the cathode during charge. At the same time the separator also acts as protective barrier in case of overheating, by melting and blocking the pores as a safety feature.

The structure and properties of the separator are very important to the battery performance. Therefore, an accurate characterization of the pore size distribution and the permeability of the separator is fundamental to understand how a battery works and to identify routes of performance improvement. Among the current technologies, the gas-liquid porometry is the most suitable one to characterize the pore structure of battery separators.

By using a porometer, one can measure minimum, maximum (or the first bubble point) and mean flow pores sizes of the through pores in the separators. The principle of measurement is the displacement of a wetting liquid from the pores of the samples by applying a gas flow at increasing pressure. The pressure is used to calculate the pore diameter with the Young-Laplace equation, P=4*γ*cos θ/d, where P is the pressure required to displace the liquid from the pore, γ is the surface tension of the liquid, θ is the contact angle and d is the pore diameter.

There are two modes in porometer measurements: the pressure step/stability mode where a data point is only recorded when both pressure and flow stabilities are achieved and the pressure scan mode where a continuous gas pressure increment is applied. Very accurate measurement of pore sizes and calculation of real pore size distribution can be achieved even for complex porous structures by using the former and fast and very reproducible results, suitable for QC work, can be obtained by using the latter.

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