Quaternary Ammonium Salts Modified Polyolefin Separators for High-Performance Lithium Metal Batteries

Lithium dendrites form in lithium–metal batteries due to the inhomogeneous electrodeposition of lithium ions on the anode surface during the charging process. These dendrites have the capacity to penetrate conventional separators, which can result in a short circuit within the battery. Consequently, the proliferation of lithium dendrites can be curtailed by modifying the separator, thereby ensuring the stability of the battery. The solution to this issue involves surface modification by introducing positively charged quaternary ammonium salt functional groups onto the surface of commercial polypropylene (PP) separators, thereby inhibiting lithium dendrite growth. First, the self-polymerizing nature of dopamine was employed to introduce a large number of polar groups on the PP surface, thereby providing active sites for subsequent reactions. Subsequently, epoxy propyl dimethyl dodecyl ammonium chloride (EPDMDAC) was prepared by hydrothermal synthesis, and then the synthesized EPDMDAC was configured into an aqueous solution, which was then subjected to a ring-opening reaction with the reactive groups on the PDA coating to achieve grafting. The findings demonstrated that the modified separator exhibited superior wettability and ionic conductivity. The positive charge on the surface of the separator attracts the anions in the electrolyte and promotes Li+ transport, thus obtaining a lithium-ion mobility as high as 0.71. This effectively alleviates the concentration polarization and mitigates the growth of lithium dendrites. The QPP separator demonstrates excellent cycling stability and high Coulombic efficiency. LiFePO4||Li button batteries are discharged at 1C, and the specific capacity of QPP-0.06 was as high as 161 mAh g–1. After 600 stable cycles of cycling, the Coulombic efficiency of QPP-0.06 is maintained at approximately 98%, which is much higher than that of commercial PP separators. This study proposes a straightforward yet efficacious design concept for the fabrication of advanced lithium-ion battery (LIB) separators.