Accurately determine Porosity and Surface Area of Battery Separators and Electrodes

Source: ATA Scientific Pty Ltd

Regardless of whether you are working with lithium-ion (Li-ion) batteries, alkaline batteries, lead-acid batteries, fuel cells, MOFs, or any other energy storage device; there are critical component properties that must be characterised for performance and safety considerations. A high energy density, high power density and long cycle life has driven the adoption of Li-ion batteries toward renewable solutions in applications like automotive, grid energy storage and consumer electronics With demand expected to continue to grow, developing domestic battery supply chains, including battery manufacturing capacity is becoming increasingly important.

Micromeritics technologies offer analytical solutions for every step in the Li-ion battery manufacturing process, from precursor consistency to the electrode slurry preparation, coating, drying, and calendaring, and final cell electrolyte filling. Understanding the porosity of the electrodes and separators is important to guarantee the right ion accessibility and safety performance while BET surface area helps to optimise battery capacity and charging. Here we discuss pore structure and surface area determination using two proven techniques, namely mercury intrusion porosimetry and gas physisorption.

The AutoPore V uses mercury porosimetry with intuitive software to characterise Li-ion battery separators and electrodes. This uniquely valuable technique delivers a wide pore measurement range plus speed and accuracy critical to battery safety, energy density, and longer cycle life.

Separators are an important component that mechanically separates the anode and cathode while allowing maximum ionic conductivity of the Li-ion containing electrolyte. Its design and performance directly affect the capacity, cycle life, and safety performance of the battery. The separator must have sufficient porosity to hold liquid electrolyte, but excessive porosity hinders the ability of the pores to close which shuts down an overheated battery. The pore size must be smaller than the particle size of the electrode components, be uniformly distributed, while also having a tortuous structure. This ensures a uniform current distribution helping to suppress the growth of lithium dendrites on the anode – a neural-like network of metallic growth that can form during charging that can short circuit the battery and cause the battery to catch fire.

Mercury Porosimetry method

The porosity of a separator or diaphragm is commonly measured directly by the mercury intrusion method, and the porosity result is generally about 40%-60%. Separators are thin films, less than 100 μm thick and to improve the statistical reliability of the measurement, a test sample consists of several pieces, sized to fit within the sample holder, or penetrometer. However during the test, mercury will be intruded into gaps at low pressure between these sample test pieces and appear to indicate the filling of very large pores that are not characteristic of the material. As the pressure is increased, smaller and smaller sized pores are filled. The Autopore method identifies and eliminates the contribution of the interstitial filling to the porosity analysis of the separator thereby correcting the apparent pore volume distribution, which can be a critical property for the safety and reliability of li-ion batteries. Figures 2a and 2b show the cumulative intrusion and log differential intrusion as a function of applied pressure and pore diameter, respectively, for the separator. This example shows the wide dynamic range of pore volume measurement available, with most at sizes larger than 10,000 nm, due to the filling of interstitial space.

Gas Physisorption

Electrode microstructure resulting from the manufacturing process has a direct influence on energy density, power, lifetime, and reliability of the lithium-ion cell, therefore understanding surface topology is critical. Increasing the surface area of the electrode will facilitate the ion exchange, however it does have limitations due to the degradation interaction of the electrolyte at the surface and resultant capacity loss along with thermal stability. While nanoparticles hold much promise to increase surface area without capacity loss, lower surface area materials are better suited for improved cycling performance of the cell resulting in longer battery life.

Micromeritics offers several physisorption systems like the 3Flex, for the development of cathode and anode materials that impact power and energy density as well as thermal/ chemical stability and enhance battery life and charge cycling.

DFT surface energy reveals surface topology and the level of interactions with an adsorptive gas present on the sample surface. The method uses an experimental isotherm based on the library of model isotherms of non-porous surfaces with different surface energies. The incremental surface area is plotted against the adsorptive potential energy which relates to the isosteric heat of adsorption. The colder the temperature the fewer interactions between the surface and the adsorptive gas and visa-versa. Adsorption potentials ranging 50-60K represent basal planes; below 50K are prismatic surfaces; above 60K are defects, and those near 20K and 100K represent nitrogen condensation and presence of micropores, respectively, so they are unrelated to the surface energy of the material. The DFT surface energy distribution in Figure 3 shows graphene had stronger interactions with nitrogen than the graphite anode sample and graphene oxide exhibited the strongest interactions with the most surface area and defects.

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ATA Scientific Pty Ltd

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Reference:

1. Micromeritics.com/lithium-ion/. CHARACTERIZING LI-ION BATTERY SEPARATORS. [online] Available at: https://www.micromeritics.com/ wp-content/uploads/12.08.21-AutoPore-APP-Note.pdf [Accessed 27 March 2023].

2. Micromeritics.com/lithium-ion/. CHARACTERIZING ADVANCED BATTERY ANODES WITH GAS ADSORPTION BET SURFACE AREA AND DFT SURFACE ENERGY. [online] Available at: https://www. micromeritics.com/Repository/Files/AppNote-202-Anode.pdf [Accessed 27 March 2023].