Investigating the effect of binder for stabilizing a sulfurized polyacrylonitrile cathode in sodium–sulfur batteries

Abstract

Replacing elemental sulfur to sulfurized polyacrylonitrile (SPAN) as cathode is an effective strategy to alleviate the “shuttle effect”, a problem in sodium-sulfur (Na-S) batteries because sulfur atoms are covalently bonded to a conductive carbon backbone in SPAN, eliminating the dissolution of Na-polysulfides in the electrolyte solution during cycling. Nevertheless, the use of SPAN cathode with a conventional poly(vinylidene fluoride) (PVdF) binder in Na-S batteries suffers from mechanical deformation due to weak adhesion strength and environmental concerns (the use of polyfluoroalkyl substance and N–Methyl–2–Pyrrolidone solvent). To replace PVdF binder, herein, poly(acrylic acid) (PAA) and sodium alginate (Na–Alg.) are introduced as water–soluble eco–friendly binders with strong adhesive strength and systematically evaluated for their physicochemical and electrochemical compatibility with the SPAN cathode in Na–S batteries. Compared to PVdF and Na–Alg. binders, the strong interaction between the carboxyl functional group (–COOH) in PAA and the polar C=N bonds in SPAN facilitates the uniform stress distribution and enhances the structural integrity of the SPAN cathode. As a result, Na–S batteries using the SPAN cathode and PAA binder achieved the high capacity of 2.6 mAh cm⁻² with a long lifespan over 200 cycles at 0.5C.