Electric Double\nLayer and Structure of the Li-ion\nBattery Separator/Liquid Electrolyte Interface Detected with the Atomic\nForce Microscope

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