NaSbF4 nanoparticles as solid-state electrolytes in sodium-iodine batteries: sodium ion transport via conversion reaction and shuttle effect retarded by fluoride

Na-I2 batteries have emerged as an appealing energy storage technology owing to high energy density and cost-effectiveness. However, the practical application of Na-I2 batteries faces the dissolution of iodide/polyiodide in the electrolyte during the charge/discharge process. Herein, we immobilize I2 by utilization of NaSbF4 (NSF)-based solid-state electrolyte (SSE). Impressively, the Na+ transport in NSF nanoparticles (NPs) is via the conversion reaction of NaSbF4 + NaF ↔ Na2SbF5, which is different from the acknowledged Na+ diffusion mechanism via vacancies in inorganic SSEs. To improve the interfacial compatibility, a composite polymer electrolyte (CPE) is fabricated based on NSF, poly (vinylidene fluoride - hexafuoro -propylene) (PVDF-HFP) and NaClO4, giving rise to stable Na stripping/plating in Na|Na symmetric cell. Especially, the quasi-solid-state Na|NSF CPE |I2 full cell shows an outstanding electrochemical behavior, which is associated with the partial reduction of ClO4− → Cl− and the stabilization of I+ by F− or Cl− in the charge/discharge process. The resultant I+-F- or I+-Cl- intermediate can promote the multi-electron transfer of 2I− → I2 → 2I+, leading to enhanced capacity and mitigated shuttle effect of iodide. Meanwhile, the formation of I+-F- in turn weakens the interaction of Na⋯F in NSF, which is beneficial for Na+ transport.