Poly(Imide) Separator Functionalized by Melamine Phosphonic Acid for Regulating Structural and Thermal Stabilities of Lithium-ion Batteries

As the energy density of lithium-ion batteries (LIBs) continues to increase, various separators are being developed to with the aim of improving the safety performance. Although poly(imide) (PI)-based separators are widely used, it is difficult to control their pore size and distribution, and this may further increase the risk associated. Herein, a melamine phosphonic acid (MP)-coated PI separator that can effectively control the pore structure of the substrate is suggested as a remedy. After the MP material is embedded into the PI separator with a simple one-step casting process, it effectively clogs the large pores of the PI separator, preventing the occurrence of internal short circuits during charging. It is anticipated that the MP material can also suppress rapid thermal runaway upon cycling due to its ability to reduce the internal temperature of the LIB cell caused by the desirable endothermic behavior around 300°C. According to experiments, the MP-coated PI separator not only decreases the thermal shrinkage rate better than commercial poly(ethylene) (PE) separators but also exhibits a desirable Gurley number (109.6 s/100 cc) and electrolyte uptake rate (240%), which is unique. The proposed separator is electrochemically stable in the range 0.0–5.0 V (vs. Li/Li+), which is the typical working potential of conventional electrode materials. In practice, the MP-coated PI separator exhibits stable cycling performance in a graphite–LiNi0.83Co0.10Mn0.07O2 full cell without an internal short circuit (retention: 90.3%).