Lithium Borate Containing Bifunctional Binder To Address Both Ion Transporting and Polysulfide Trapping for High-Performance Li–S Batteries

A slightly cross-linked lithium borate containing single ion-conducting polymer (LBSIP) as a bifunctional binder for lithium sulfur batteries is designed and fabricated via a one-step thiol–ene click reaction. The LBSIP binder exhibits a maximum peeling strength of over 600 mN mm–1 between the sulfur cathode and aluminum foil, together showing a high lithium ion diffusion coefficient of 2.1 × 10–12 cm2 s–1. Owing to the unique electron-donating groups, the binder can provide a good ionic conductive network and a dramatically enhanced polysulfide-trapping feature. The strong interaction between electron-donating groups of LBSIP with Li2S6 was confirmed by the 7Li-NMR analysis and density-functional theory calculation. The cathode using the LBSIP binder exhibited a high Coulombic efficiency of ∼100% and an initial specific capacity of 712 mAh g–1 with a capacity fading rate of 0.06% per cycle after 500 cycles at 0.5 C. Even at a high current rate of 2 C, a reversible capacity of over 500 mAh g–1 was still obtained. However, the capacity of the cathode using the PVDF binder decreased to 331 mAh g–1 after 180 cycles at 0.5 C. This work is very attractive for the rational design of functional binders for Li–S batteries with both ion-transporting and polysulfide-trapping features.