Lithium propanesulfonyl(trifluoromethylsulfonyl)imide grafted polybenzimidazole as a self-supporting single ion conducting polymer electrolyte membrane for lithium metal secondary batteries

Highlights: • Withstand a 1000 h galvanostatic cycling in lithium symmetric cell. • Demonstrate a single ion conducting LMB with a LFP mass loading of 10.2 mg cm⁻². • Prevent from thermal runaway for lithium metal secondary batteries. • σ = 0.15 mS cm⁻¹ and t₊ = 0.91 of the gel polymer electrolyte at 25 °C. -- Abstract: Dendrite growth and unstable solid electrolyte interphase are two challenges for lithium metal secondary batteries. Immobilization of anions to allow the transportation of cations only, namely single ion conductor, can prevent from concentration polarization and therefore is likely to suppress dendrite growth. In this article, poly(4,4′-(diphenyl ether)-5,5′-bibenzimidazole) (PBI) grafted with lithium propanesulfonyl(trifluoromethylsulfonyl)imide (LiPSTFSI) is found to form a uniform self-supporting single ion conducting polymer electrolyte membrane (PBI-g-LiPSTFSI). The ionic conductivity of the gel polymer electrolyte is 0.15 mS cm⁻¹ at 25 °C. Coupled with a high lithium ion transference number of 0.91, the polymer electrolyte can withstand a 1000 h galvanostatic cycling at 0.5 mA cm⁻² with a Li plating/stripping capacity of 2.5 mAh cm⁻² without short circuit. Even increasing the mass loading of LiFePO₄ up to 10.2 mg cm⁻², the single ion conducting lithium metal secondary battery can deliver 166 mAh g⁻¹ at 0.1 C, 160 mAh g⁻¹ at 0.2 C and 152 mAh g⁻¹ at 0.3 C. Because the rigid polymer chains and strong pi-pi stacking of PBI provide high mechanical strength and excellent thermal stability, PBI-g-LiPSTFSI is a promising single ion conducting polymer electrolyte for safe lithium metal secondary batteries.