NaSn₂F₅ nanocluster composed of nanoparticles with matched lattices induced by dislocations: Accelerated sodium-ion transport via in situ oxidation in solid-state sodium metal battery

Na metal batteries using inorganic solid-state electrolytes (SSEs) have attracted extensive attention due to their superior safety and high energy density. However, their development is plagued by the unclear structural/volumetric evolution of SSEs and the corresponding Na⁺ migration mechanisms. In this work, NaSn₂F₅ (NSF) clusters are composed of nanoparticles (NPs) with matched lattices induced by dislocations, which can mitigate the volume swelling/shrinkage of the NPs. NSF behaves like a single ion conductor with a high Na⁺ transference number (t_Na+) of 0.79. Specially, the ionic conductivity (σ) of NSF is increased from 7.64 × 10^-6 to 5.42 × 10^-5 S cm^-1 after partial irreversible oxidation of Sn²⁺ (0.118 Å) → Sn⁴⁺ (0.069 Å) with the shrunk ionic radius during the charge process, giving more spaces for Na⁺ migration. Furthermore, a poly(acrylonitrile)-NaSn₂F₅-NaPF₆ composite polymer electrolyte (NSF CPE) was fabricated with a σ of 4.13 × 10^-4 S cm^-1 and a t_Na+ of 0.60. The NSF CPE-based symmetric cell can operate over 3000 h due to the couplings between the different components in NSF CPE, which is beneficial for ion transfer and the construction of stable solid electrolyte interface. And the quasi-solid-state Na|NSF CPE|Na₃V₂(PO₄)₃ full cell displays excellent electrochemical performance.