Spray flame synthesis of the NASICON-structure Na₃Zr₂Si₂PO₁₂ solid electrolyte nanoparticles for solid-state Na⁺ batteries

NASICON structural Na₃Zr₂Si₂PO₁₂ is considered as a promising solid electrolyte for all-solid-state sodium ionic batteries owing to the high ionic conductivity at room temperature, good moisture, thermal and chemical stability, and wide electrochemical window. However, there are still no reliable and scalable synthesis method for such multi-element complicated structural oxides. In this work, NZSP electrolyte particles are successfully synthesized by swirling spray flame synthesis and the solid electrolyte pellets are made by solving the problems of multiple elements sintering and densification. The SSA of the as-synthesized raw NZSP nanopowders with reduced amount of EHA is the largest with size of about 10 nm for the gas-to-particle route and suitable pathway in flame. The raw nanoparticles are of core-shell structure with ZrO₂ cores and amorphous shells of other elements due to the distinctive temperatures for gas-to-solid transformation of different elements. The disk-shape pellets compassed by non-NASICON-structure NZSP raw nanopowders become NASICON structure after reactive sintering and two-step sintering. The NZSP solid electrolyte pellet with stoichiometric P content sintered at 1250/1100 °C for 12 h by two-step sintering method exhibits the highest ionic conductivity of 0.65 mS/cm at room temperature (25 °C) with activation energy of 0.370 eV and highest relative density of 98.6 %. The excellent conductivity and the high-yield production capability of the swirling spray flame synthesis make it a promising synthesizing method for wider application of NASICON structural NZSP solid electrolytes.