NASICON

NASICON, short for Sodium Super Ionic CONductor, refers to a family of ceramic solid electrolytes characterized by a three-dimensional framework that facilitates rapid Na+ transport. The archetypal composition Na1+xZr2P3O12 forms a network built from ZrO6 octahedra linked by PO4 tetrahedra, creating spacious channels that host mobile sodium ions. The result is a solid electrolyte with relatively high room-temperature ionic conductivity for a ceramic material and a wide electrochemical stability window.

A common variant involves substituting phosphate with silicate to yield Na1+xZr2P3−xSixO12 (NASICON-type). Silicon substitution modifies lattice dimensions and Na+ sites, often improving conductivity and stability. Doping level x tunes the balance between Na content and framework rigidity, enabling conductivities in the 10−4 to 10−3 S cm−1 range at room temperature for optimized compositions; activation energies are typically in the 0.2–0.4 eV range. The materials maintain chemical compatibility with common sodium cathodes and with sodium metal in certain configurations.

Synthesis typically employs solid-state reactions, sol–gel processing, or solution-based methods, followed by high-temperature sintering to form dense ceramics. Densification and grain boundary control are important for reducing interfacial resistance and achieving mechanical robustness for device integration.

Applications and challenges: NASICON-type solid electrolytes are among the leading candidates for all-solid-state sodium batteries, offering good ionic conductivity, chemical stability, and a relatively wide electrochemical window. Challenges include optimizing dense microstructure to lower grain-boundary resistance, mitigating moisture sensitivity for some compositions, and ensuring stable interfaces with electrodes.