Tailoring binder–cathode interactions for long-life room-temperature sodium–sulfur batteries†
Room-temperature sodium–sulfur batteries (NaSBs) are well poised as candidates for next-generation battery applications. However, two important limitations must first be overcome: irreversible capacity loss from long-chain polysulfide dissolution and cathode pulverization from severe volume expansion. Although covalent-sulfur composites like sulfurized polyacrylonitrile (S-PAN) prevent polysulfide dissolution, they do not address the latter issue of sustaining the cathode structure during the sodiation reaction. In this work, we demonstrate that the unique interactions between polar binders and insoluble short-chain sulfur species can be exploited as a strategy to solve both challenges concurrently. Our hypothesis is that specific polar groups, like the carboxyl moiety, interact strongly with sodium sulfide and short-chain polysulfides, as compared to traditional fluoropolymer binders employed in most sulfur-based cathodes. Binder–cathode interactions were first predicted for sodium–sulfur batteries using theoretical calculations, and then confirmed experimentally using a polyacrylic acid (PAA) binder, in combination with a S-PAN cathode. This strategy can be further generalized to other carboxyl binder systems, as demonstrated using two additional binders derived from natural products. Compared to conventional polyvinylidene difluoride-based cathodes experiencing large initial capacity losses, the PAA-based S-PAN cathode delivered a long 1000 cycle lifetime with initial and final discharge capacities of 1195 and 1000 mA h g(S)−1 respectively, representing a low capacity loss of 0.016% per cycle. Rational design of NaSBs based on the synergistic interactions between insoluble sulfur species and carboxyl-binders allows us to overcome key challenges to their practical development.