Shielding the electrostatic attraction by design of zwitterionic single ion conducting polymer electrolyte with high dielectric constant

Abstract

According to Coulomb's law, besides the design of bulky anion, increase in dielectric constant can also reduce the electrostatic attraction force, both of which are benefit for ion dissociation. Here, we report a zwitterionic single ion conducting polymer electrolyte by chemically grafting two propanesulfonyl(trifluoromethanesulfonyl)imide groups to the nitrogen of poly(biphenyl piperidium) (PBP-g-Li(PSI)2). One of side-chain groups burdens negative charge to counterbalance the positive charge of quaternary ammonium of main chain, and the other one provides lithium ion to conduct charge. Poly(biphenyl piperidinium) grafted with one lithium propanesulfonyl(trifluoromethanesulfonyl)imide (PBP-g-LiPSI) is synthesized for contrast. It is discovered that the dielectric constant of PBP-g-Li(PSI)2 is 22.3 at 2 MHz which is 3.5 times that of PBP-g-LiPSI, and the ionic conductivity of PBP-g-Li(PSI)2 gel polymer electrolyte membrane is 2.41 × 10−4 S cm−1 at 25 °C, nearly twice that of PBP-g-LiPSI gel polymer electrolyte membrane. The dipole structure of bonded bis(sulfonyl)imide anion and quaternary ammonium cation can shield the electrostatic attraction between lithium cation and immobilized anion as confirmed by Li 1s XPS and 7Li NMR. Zwitterionic polymer electrolyte not only withstands galvanostatic cycling in lithium symmetric cell at ±0.5 mA cm−2@2 mAh cm−2 for 2000 h, but also performs stably in lithium metal secondary battery.