Open-Framework Na-Rich Fluoride Solid Electrolytes Reinforced by Heterointerface Construction and Grain-Boundary Decoration for Na Metal Batteries

Solid-state Na metal batteries are among the promising next-generation electrochemical energy storage systems due to their abundant resources and high energy density. It is of great significance to develop novel Na-based solid electrolyte prototypes and customized synthesis strategies. Fluoride Na+ solid electrolytes are expected to have the best air stability and the widest electrochemical stable window, but their low ionic conductivities (often 10–6 to 10–7 S/cm at room temperature) are still a shortcoming. In this work, we report a low-temperature ionic liquid (IL) method for the synthesis of nanostructured binary-phase sodium-rich fluoride solid electrolytes Na3AlF6/Na5Al3F4, both with open-framework structures. This composition electrolyte is dually reinforced by heterointerface construction and grain-boundary decoration. The solidified IL derivatives can serve as an in situ binder surrounding fluoride nanoparticles, which can strengthen the entire electrolyte pellet and reduce its tendency to crack and pulverize during electrochemical cycling. The heterogeneous Na3AlF6/Na5Al3F14 nanodomains are in contact with each other in electrolyte particles, and the interfaces between Na3AlF6 and Na5Al3F14 enable the faster Na+ transport with lower diffusion energy barrier. Na3AlF6/Na5Al3F4 achieves a high ionic conductivity (in a level of 10–4 S/cm at 40 °C), which is among the highest levels currently achieved by sodium fluoride solid electrolytes. This fluoride solid electrolyte has excellent electrochemical stability toward Na metal, and there is no dendrite formation on the surface of the Na anode after cycling, benefiting from the formation of uniform NaF, NaCl, and Na3N as Na+ conductive components at the interface between the anode and electrolyte. The solid-state Na/Na3AlF6/Na5Al3F14/Na3V2(PO4)3 batteries can deliver a capacity of 109 mAh/g and successfully run for at least 140 cycles at 60 °C. The multiscale in situ modifications of heterogeneous interfaces and particle boundaries for accelerated ion migration provide solutions to high-conductivity nanostructured electrolytes for sustainable solid-state batteries.