Freeze Cast NASICON Solid Electrolytes: Towards High-Performance Sodium Batteries

Solid-state sodium batteries have received great interest recently as sustainable and viable electrochemical energy storage systems. To bolster the energy density as well as the safety of sodium batteries, it is crucial to develop solid-state electrolytes with high ionic conductivity and electrochemical stability. Among others, NASICON (Na⁺ super ionic conductor) materials are promising candidates as solid electrolytes due to their high ionic conductivity and thermal and electrochemical stability. Herein, we employed a freeze-casting technique to develop porous NASICON solid electrolytes based on sodium aluminum titanium phosphate and sodium zirconium silicophosphate materials with high ionic conductivities. By controlling the slurry composition and sintering conditions, we optimized the porous solid electrolyte. We investigated the uniform distribution of parallel channels in the bulk of the freeze-cast solid electrolyte via X-ray tomography technique. Additionally, the effect of wetting agents including ionic liquid (sodium trifluoromethanesulfonimide in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) and carbonate electrolytes (sodium perchlorate in propylene carbonate) on the electrochemical performance was explored. Furthermore, a prototype solid-state cell was assembled with the infiltration of sodium vanadium phosphate cathode material into the porous solid electrolyte. The resulting cell delivered a good capacity of nearly 100 mAh g^-1_cathode and good cycle life with an energy density of nearly 330 Wh kg^-1_cathode.The improved electrochemical performance could be attributed to the low tortuosity pathways which are uniformly distributed parallel to the thickness of the solid electrolyte, facilitating uniform and efficient sodium ion conduction. The study offers new insights into the optimization of ion conduction in solid electrolytes for sodium batteries.