APPLICATION NOTE CHARACTERISATION OF BATTERY SEPARATORS WITH THE POROLUX™ 1000
Nickel-Cadmium (NiCd) batteries (first industrially produced in 1946) are cheap but they have low energy density and they undergo self discharge. However, the biggest concern is their toxicity.. In consequence, its use has been banned in some countries and applications. Compared to NiCd, Nickel- Metal Hydride (Ni-MH) batteries (developed in the 1970s) are cheaper, have superior capacity, are less prone to memory effects and are less toxic. Nevertheless, they still suffer from high self discharge rates, and degradation over time.
Motivated by the need of more environmentally friendly alternatives with higher power, longer life and lower costs, newer technologies Lithium based batteries, such as Lithium Ion (Li-Ion) and Lithium Polymer (Li-Polymer) batteries, have proliferated in the last years. Nowadays Li-based batteries can be found in multiple everyday products, such as home electronics (smartphones, laptops, photo and video cameras), hybrid and electric cars and in electric storage systems.
WHAT’S IN A BATTERY?
A battery is an electrochemical cell, i.e. a device generates electricity from chemical reactions. A typical battery consists of two electrodes, a cathode (+) and an anode (-), and an electrolyte. The electrodes are electrical conductors and make contact with the non-metallic part of a circuit which allows for the circulation of charge, the electrolyte.
One of the key parts in a battery is the separator. It is a porous membrane which, on the one hand acts as a barrier between the two electrodes to prevent short circuits, and on the other hand it also permits the transfer of charge (to close the circuit and enable current circulation).
The separators used in early batteries were made of rubber, cellulose, nylon, cellophane or plastic.
Li- based batteries are based on a more complex mechanism than traditional batteries, for which special characterisation tools are required. In Li-based batteries (Li-ion, Li-polymer), separators are based on materials such as polyethylene (PE), polypropylene (PP), combinations of these, polyolefin and PVDF, amongst others. These materials are cheap and have good chemical stability. Ideally they should have pore size ranges from 30 to 100 nm 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.
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The structure and properties of the separator are very important because they affect the battery performance (energy and power density, cycle life and safety) to great extent. Therefore, an accurate characterisation of the pore size distribution and the permeability of the separator is fundamental to understand how it works and to identify routes of improvement. This is how our POROLUX™ 1000 porometer can help.
HOW CAN THE POROLUX™ 1000 HELP UNDERSTANDING THE STRUCTURE OF BATTERIES?
of the through pores in materials. The principle of measurement is the displacement of a wetting liquid calculate the pore size 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) the surface tension of the liquid, (θ) the contact angle and (D) is the pore diameter.
The POROLUX™ 1000 is the preferred model for research and development. It is based on the pressure step/stability method to determine the pore size: a data point is only recorded when the stability algorithms a complex porous structure such as battery separators, which often have a mixture of pores where not all of them have the same shape and/or length and tortuosity.
For instance, let’s take the example of tow pores with the same diameter, but one is a straight pore (S) with a pore length of 1, the other one is more tortuous pore (T) with a pore length of 1.5. If we measure these pores with a porometer which increases the pressure pores will open at a different pressure (pore T will open later in time, thus at a higher pressure) and therefore pore T will be regarded as a pore with a smaller diameter than pore S. On the contrary, in a pressure step/stability porometer, pore S and pore T will open at a different certain pressure is stable before recording a data point). Therefore pore S and pore T will be shown as pores with the same diameter.
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CHARACTERISATION OF BATTERY SEPARATORS WITH THE POROLUX™ 1000
Different polymeric battery separators (for confidentiality reasons we will refer to sample A andB respectively) were characterised with the POROLUX™ 1000. Disks of 25 mm diameter and approximately 20um thickness ere measured and Porefil was used as wetting liquid. An overview of the samples and the obtained results are shown in the table below:
Material Maximum pore size Mean flow pore size Smallest pore size (nm) (nm) (nm)
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
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