A solid polymer electrolyte based on star-like hyperbranched β-cyclodextrin for all-solid-state sodium batteries

Abstract A star-like hyperbranched β-cyclodextrin is designed and synthesized for the solid polymer electrolyte of all-solid-state sodium batteries by grafting β-cyclodextrin with multiple oligo(methyl methacrylate)- block -oligo(ethylene glycol) methyl ether methacrylate short chains. Complexing this star-like hyperbranched β-cyclodextrin with sodium trifluomethanesulfonate salt leads to a self-standing, transparent, and flexible solid polymer electrolyte film. It is found that this solid polymer electrolyte exhibits superior thermal stability, excellent mechanical properties at the even elevated temperature and good interfacial stability against Na metal electrode. Especially, ascribing to the unique three-dimensional structure of the star-like hyperbranched β-cyclodextrin, a solid polymer electrolyte formed with the optimized star-like hyperbranched β-cyclodextrin containing 69.3 wt.% oligo(ethylene glycol) methyl ether methacrylate shows an ionic conductivity of 1.3 × 10 −4  S cm -1  at 60 °C and a wide electrochemical window of 5.2 V vs. Na + /Na, which are superior to those of the previously reported solid polymer electrolytes. The all-solid-state Na/NaNi 1/3 Fe 1/3 Mn 1/3 O 2 cell shows high first discharge capacity (102.4 mAh g −1 at 0.1C), good reversibility and excellent cycling performance (with 87.8% capacity retention after 80 cycles at 0.1C) at 60 °C. All the results indicate that the star-like hyperbranched β-cyclodextrin is a promising polymer for the solid polymer electrolyte of all-solid-state sodium batteries.