Synthesis and characterization of divalent ion conductors with NASICON-type structures

Divalent ion-conducting solid electrolytes with NASICON-type three-dimensional network structures (MxHf1−x)4/(4−2x)Nb(PO4)3 (M = Ni, Mg, Ca, Sr) were successfully developed by introducing M2+ cations into HfNb(PO4)3 solids. The presence of high-valence cations such as Hf4+, Nb5+, and P5+ in the structures effectively reduced the electrostatic interaction between the conducting M2+ cations and the surrounding oxide anions, enabling the M2+ cations to migrate smoothly in the rigid crystal lattice. The relationship between the cation conductivity in NASICON-type solids and the ionic radius of the migrating divalent cation species was also clarified by taking into account the relative sizes of the conduction pathways in the structures.