A robust and ion- conductive protein- based binder enabling strong polysulfide anchoring for highenergy lithium- sulfur batteries

Due to the large volume change of sulfur and the diffusion of dissolved polysulfides, the development of a high-performance binder with functionality beyond mechanical adhesion/support for sulfur cathodes is in high demand. This is especially true for the success of high-loading sulfur cathodes. Here, we report a soy-protein-based binder with high mechanical robustness and multiple functionality for high-energy lithium-sulfur batteries. The multi-functional binder (denoted as SP-PAA) is fabricated via facilely incorporating soy protein (SP) with poly(acrylic acid) (PAA) and it has been demonstrated to effectively buffer the large volume change of the sulfur cathode during cycling. More significantly, the abundant polar groups (e.g. amines, carboxyl groups, etc.) of soy protein empower the binder with a strong capability to adsorb polysulfides, as evidenced by density functional theory (DFT) calculations. Meanwhile, the good ion-conduction abilities of soy protein notably promote electrochemical reactions. With the use of this binder, the electrochemical performance (e.g. capacity, cycling stability and rate capability) of the sulfur cathode is substantially enhanced. In addition, owing to the excellent mechanical properties of SP-PAA and its effects on improving electrode microstructures, high-loading sulfur cathodes (>5 mg cm(-2)) with good and stable performance are achieved. The good performance is also attributed to the role that SP-PAA plays in adsorbing polysulfides, sustaining structural stability and promoting electrochemical reactions. This study brings about a cost-effective strategy for the fabrication of superior binders through exploiting natural materials and leads to the realization of high-loading sulfur cathodes for mass production.