Enhancing Na-ion conducting capacity of NASICON ceramic electrolyte Na_3.4Zr₂Si_2.4P_0.6O₁₂ by NaF sintering aid

NASICON-type material Na_1+xZr₂Si_xP_3−xO₁₂ (0 ≤ x ≤ 3) is considered to be one of the most promising solid-state electrolytes that can be applied in all-solid-state sodium batteries (SSNBs). However, further increasing the ionic conductivity of Na_1+xZr₂Si_xP_3−xO₁₂ (0 ≤ x ≤ 3) to obtain higher performance electrolyte materials is still a challenge for its practical application. Herein, the Na_3.4Zr₂Si_2.4P_0.6O₁₂ (NZSP) powder is prepared by the solution-assisted solid-state reaction (SA-SSR), and solid-state electrolytes with excellent electrochemical properties are obtained by doping with NaF as a sintering aid. With the increase of NaF doping, the crystal structure of NZSP is changed as well as a slight excess of Na⁺ may increase the bottleneck size of the crystal, so that it enhances the migration ability of Na⁺ in the crystal. Also, NaF as a sintering aid lowers the sintering temperature, increases the densities and improves the effective contact between crystals, which reduces the grain boundary impedance of NZSP. However, the excessive NaF dopant leads to the generation of microcracks decreasing the Na⁺ conducting and mechanical properties of solid-state electrolytes. Using this simple and low-cost strategy of NaF doping, NZSP-0.3NaF achieves an excellent conductivity of 4.98 mS cm⁻¹ with unexpectedly low activation energy of 0.227 eV. Therefore, this composition is one of the few electrolytes with high conductivity reported so far and is a competitive option for solid-state sodium batteries.