An amphiphilic chitosan binder regulates SEI microenvironment for highly reversible sodium storage in hard carbon anodes

Hard carbon is one of the most promising anode materials for sodium ion batteries. However, the unstable solid electrolyte interphase film formed on the hard carbon surface causes poor cycling ability and rate performance, which hinders hard carbon's commercial development. Here, an amphoteric supramolecular binder is constructed by protonating the polar functional groups in chitosan and sodium lignosulfonate through electrostatic and hydrogen bonding interactions. Due to the excellent dispersibility, multi-functional groups, and ion conductivity of aqueous binders, the synergistic optimization of interfacial dynamics through ion transport network and amphiphilic groups of the designed binder effectively suppresses excessive electrolyte reduction. Moreover, the internal dipole structure of amphiphilic ions promotes the solvation structure of sodium ions and the interaction between amphoteric functional groups, reducing the desolvation activation energy of sodium ions and increasing the ion migration rate at the interface. Specially, an innovative co-solvent system of citric acid and acetic acid is adopted to dissolve chitosan, thereby enhancing intra-chain mobility and facilitating ion transport via ether bond networks within the binder matrix. This study provides insights into optimizing the interface solvation structure and enhancing ion migration kinetics, and provides information for the development of advanced chitosan-based binder.