Quaternary ammonium cationic polymer as a superior bifunctional binder for lithium-sulfur batteries and effects of counter anion

Bifunctional polymer binders featured with both strong binding and superior polysulfide trapping properties are highly desired for the fabrication of sulfur cathodes with suppressed polysulfide shuttling in Li-S batteries. In this paper, we have explored the potential of a quaternary ammonium cationic polymer, polydiallyldimethylammonium (PDADMA-X; X = T, B, P, and Cl) with different counter anions (TFSI-, BF4−, PF6−, and Cl−, respectively) as the bifunctional binder. We have also revealed the dramatic effects of the counter anion on the performance of the cationic polymer binder. PDADMA-X's containing the former three weakly associating anions have been demonstrated to show polysulfide adsorption capability. In particular, PDADMA-T having the largest, least interacting TFSI- anion shows the optimum performance, with strong binding strength and the best polysulfide adsorption capability. Relative to commercial PVDF and PDADMA-X's of other counter anions, it offers sulfur cathodes with lowered polarization, higher discharge capacity, significantly better capacity retention, and improved cycling stability. With its convenient synthesis from commercially available PDADMA-Cl, cationic PDADMA-T having the TFSI- anion is a promising bifunctioal binder for sulfur cathodes in practical Li-sulfur batteries.