Using a Porometer to Characterize Battery Separators

Nickel-Cadmium (NiCd) batteries were first produced industrially in 1946, and are a cheap but low-energy density type of battery. They are also prone to undergo self-discharge. However, their most notable drawback is their toxicity.

As a result, the use of nickel-cadmium batteries has been banned in some countries and applications. Compared to NiCd, Nickel-Metal Hydride (Ni-MH) batteries, which were developed in the 1970s, are cheaper, have better capacity, are not as susceptible to memory effects, and are less toxic. However, Ni-MH batteries are still prone to suffer from high rates of self-discharge and degradation over time.

Newer technologies in battery production have been developed to respond to the need for alternatives with higher power, longer life, lower costs, and with a less harmful impact on the environment. Lithium-based batteries, such as Lithium Ion (Li-Ion) and Lithium Polymer (Li-Polymer) batteries, have answered these needs in the last few years.

Now, Li-based batteries are used in a wide range of everyday products, from home electronics including smartphones, laptops, and photo and video cameras, to hybrid and electric cars and electric storage systems.

What’s in a Battery?

A battery is an electrochemical cell, generating electricity from chemical reactions. A conventional battery consists of two electrodes, a cathode (+), an anode (-), and an electrolyte. The electrodes are electrical conductors that make contact with the non-metallic element of a circuit, which enables the circulation of charge, the electrolyte.

One key element in a battery is the separator. It is a porous membrane that not only acts as a barrier between the two electrodes to prevent short circuits but also permits the transfer of charge in order to close the circuit and enable current circulation.

In early types of batteries, the separators were made of rubber, cellulose, nylon, cellophane or plastic. Now, Li- based batteries are made with a more complex mechanism that requires special characterization tools to make.

In Li-based batteries (Li-ion and Li-polymer), separators are made of materials including polyethylene (PE), polypropylene (PP), a combination of PE and PP, and polyolefin and PVDF, among others, which all have good chemical stability and are cheap to obtain.

Ideally, they should have pore size ranges from 30 to 100 nm to allow circulation of the Li Ions in either direction. During discharge, Li ions move from the anode to the cathode, and from the anode to the cathode during charge. While this happens, the separator also acts as a protective barrier in case of overheating. It does this by melting and blocking the pores as a safety feature.

The structure and properties of the separator are vital, as they greatly affect the battery performance, from its energy and power density to its cycle life and safety features. Because of this, precise characterization of the pore size distribution and the permeability of the separator is central to understanding how it works and to identify routes of improvement. This is where the POROLUX™ 1000 porometer can help.

How Can the POROLUX™ 1000 Help Understanding the Structure of Batteries?

The POROLUX™ porometers, founded on the gas liquid porometry technique, are widely used to measure minimum, maximum (or first bubble point) and mean flow pore sizes, and pore size distribution of the through pores in materials. 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.

Pore size diameter is calculated by using the pressure and the Young-Laplace equation, P=4*γ*cos θ/D, where (P) is the pressure required to displace the liquid from the pore, (γ) the surface tension of the liquid, (θ) the contact angle and (D) is the pore diameter.

For research and development, the POROLUX™ 1000 is the favored model. It is based on the pressure step/stability method to calculate the pore size. A data point is only recorded when the stability algorithms, which are user-defined, are met for both pressure and flow. This is essential when analyzing samples with a complex, porous structure like battery separators, which often have a mixture of pores that are not uniform, and do not all possess the same shape, length, and tortuosity.

For instance, consider tow pores with the same diameter, but one is a straight pore (S) with a pore length of 1, while the other pore is more tortuous pore (T) with a pore length of 1.5. If measurements are taken of these pores with a porometer that increases the pressure continuously (scan system) and measures the resulting flow, both pores will open at a different pressure. Pore T will take more time to open and therefore at a higher pressure, meaning that pore T will be regarded as a pore with a smaller diameter than pore S.

Conversely, in a pressure step/stability porometer, pore S and pore T will open at different times, but because of the stability algorithms, they will still open at the same pressure. This is because the porometer does not record a data point unless the flow at a certain pressure is stable. Therefore, pore S and pore T will be shown as pores with equal diameters.

Characterization of Battery Separators with the POROLUX™ 1000

Different polymeric battery separators recorded here as sample A and sample B respectively, were characterized with the POROLUX™ 1000. Disks of 25 mm diameter and approximately 20 um thickness were measured with Porefil used as a wetting liquid.

An overview of the samples and the results gathered are detailed in the table below.

Material Maximum pore size
Mean flow pore size
Smallest pore size
A 134 31 28
B 61 45 36


Wet, Dry and Half Dry Curves Material A and B

Material B, Wet, Dry and Half Dry Curve

This information has been sourced, reviewed and adapted from materials provided by Particulate Systems.

For more information on this source, please visit Particulate Systems.


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