Electric Double Layer and Structure of the Li-ion Battery Separator/Liquid Electrolyte Interface Detected with the Atomic Force Microscope

We have used Atomic Force Microscopy (AFM) as a powerful tool to study the structure of the separator in Li-ion batteries. The measurements were performed in LiTFSI-based electrolytes with different concentrations in two different solvents. We chose propylene carbonate (PC) from the family of carbonate solvents because it has a low vapor pressure at room temperature. The results are compared to measurements in water-based electrolytes. We find that the surface of the separator is always electrically charged, regardless of water or PC solvent. In water-based electrolytes, there is an additional hydrophobic attractive interaction at surface distances below 20 to 60 nm, which is due to the formation of gas microbubbles on hydrophobic surfaces. Adding LiTFSI salt to water reduces the hydrophobic interaction and shields the electric field of the charged polymer surface. For PC-based electrolytes, there is no evidence of hydrophobic attractive interactions, but we observe an electrically charged separator-polymer surface. The addition of LiTFSI salt to PC not only shields the electric field emanating from the charged surface of the polymer but it also has an interesting effect on the morphology of the separator surface. At average LiTFSI salt concentrations of around 0.1 M, we see a swelling of the separator surface. This is confirmed by contact AFM images of the separator surface in the presence of 0.1 M salt, which shows long and curled polymer filaments of 10 nm diameter, forming a random and soft filament mesh on the surface of the separator. Our results show that in situ AFM imaging and force spectroscopy of Li-ion separator–electrolyte interfaces provide valuable information about the structure and electrical properties of the interface.