3D diagram of lithium battery separator pores: 13% of the pores in the separator are non-through pores, marked in purple in the diagram, which will reduce the battery charge and discharge rate.
To obtain high-resolution images of the internal structure of lithium-ion battery separators, Swiss scientists devised an innovative method of soaking batteries in butter. The butter flows into the battery materials, providing an excellent way to understand the structure of the battery's internal pores and potentially improving battery performance.
Battery separators are composed of porous polymer films and are used to keep the positive and negative electrodes separate while allowing lithium ions to pass through. Ideally, lithium ions can pass through freely and quickly. Wood believes there is significant room for improvement in separators because separator materials have hardly changed in the past 50 years.
Careful examination of the membrane's microstructure reveals that approximately 70% of the battery's interior is solid, while the remaining 30% is a porous structure filled with electrolyte. Wood's team removed the electrolyte, immersed the membrane in melted butter (available commercially), and then allowed it to solidify.
The advantage of butter is that it can flow at relatively low temperatures and solidify at room temperature. Wood said that whether in solid or liquid form, the volume of butter hardly changes, so it does not damage the microstructure of the membrane. Then, the membrane filled with butter was cut using a graft ion beam, with the membrane layers removed layer by layer in 10μm units. A scanning electron microscope was used to record the surface of the newly exposed membrane. Through repeated cutting and scanning, the 3D microstructure of the membrane's interior was finally formed.
Wood's team discovered that 13% of the membrane pores were closed. These closed pores do not facilitate the flow of lithium ions during the charging and discharging phases.
We've discovered a key factor limiting the application of separators. Billy Wu, a battery researcher at University College London, says that achieving fast charging isn't simply about increasing the separator's porosity; it's crucial to understand whether the pores are open-cell. We aim to accelerate the charging of electric vehicles, increasing start-up speed. To do this, we need to optimize the material's structure, allowing lithium ions to more easily move between the positive and negative electrodes. The next challenge will be manufacturing separators with high porosity.
