Zwitterion-doped self-supporting single-ion conducting polymer electrolyte membrane for dendrite-free lithium metal secondary batteries

Exploring self-supporting polymer electrolyte membranes that exhibit both high ionic conductivity and cation transference poses a challenge for lithium metal secondary batteries. In this research, we utilized poly(4,4'-(diphenyl ether)-5,5'-bibenzimidazole) (O-PBI) and grafted it with lithium propanesulfonyl(trifluoromethyl sulfonyl)imide (PBIg-LiPSI) through a nucleophilic substitution reaction involving sodium 3-iodopropanesulfonyl(trifluoromethyl sulfonyl)imide (NaIPSI) and pre-activated O-PBI with lithium hydride. This process was followed by a lithium-ion exchange. The excellent leaving ability of the iodine substituent in NaIPSI allowed for a grafting ratio close to 100%, resulting in the highest possible lithium content. Moreover, the strong pi-pi interaction among the aromatic polybenzimidazole facilitated the formation of a self-supporting polymer electrolyte membrane, even when doped with zwitterionic 1-propanesulfonyl (trifluoromethyl sulfonyl)imide-3-methylimidazolium (MeImPSI). The ionic conductivities and Li-7 nuclear magnetic resonance chemical shifts of MeImPSI-doped PBI-g-LiPSIs showed a linear increase with the doping mass fraction of zwitterion. This finding confirmed that the zwitterion can act as a dipole, reducing the electrostatic attraction between the lithium cation and immobilized bis(sulfonyl)imide anion. Among different doping ratios, a 25-wt% MeImPSI-doped PBI-g-LiPSI exhibited the highest ionic conductivity of 0.68 mS cm(-1) at room temperature, along with a lithium transference number of 0.95. To assess the performance of the electrolyte as well as separator, a lithium symmetric cell was assembed using the 25-wt% MeImPSI-doped PBI-g-LiPSI. The cell exhibited stable performance during galvanostatic cycling at +/- 0.5 mA cm(-2) with a charge-discharge capacity of 2 mA h cm(-2), for an impressive duration of 2100 h. Additionally, we successfully demonstrated the application of single-ion conducting lithium metal secondary batteries. The film-forming property of PBI, combined with the enhanced ionic mobility provided by the zwitterion contributed to the overall excellent performance of the single-ion conducting polymer electrolyte system.