Pyrogallic acid coated polypropylene membranes as separators for lithium-ion batteries

To solve the wetting capability issue of commercial polypropylene (PP) separators for lithium-ion batteries (LIBs), we developed a simple and new dipping surface coating method. In this method, pyrogallic acid (PA) is used as the sole coating precursor, and the PA coatings are formed spontaneously on the PP separator surfaces. Attenuated total reflection-infrared (ATR-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements show that PA has been successfully coated on the surface of the PP separator. The PA coatings make the PP surfaces hydrophilic, while the micro-porous structure of the separators remains intact. The improved LIB performance including discharge specific capacity, cycling performance and rate capability is obtained by using these PA coated PP separators due to their better wetting capability, higher electrolyte uptake and ionic conductivity. XPS measurements indicate that the PA coatings show good stability and strong adhesion to the PP separators after the cycling test in LIBs. This study provides an effective and cheap way to achieve hydrophilic modification of commercial polyolefin separators for LIB applications, and has potential application to solve similar surface property issues of other membranes.