Divalent Ni2+ cation conduction in NASICON-type solid

Divalent Ni2+ cation-conducting solid electrolytes, (NixHf1−x)4/(4−2x)Nb(PO4)3, were successfully developed by introducing Ni2+ cations into a HfNb(PO4)3 solid exhibiting a three-dimensional NASICON-type structure. Although the HfNb(PO4)3 solid has been reported to be purely a Hf4+ ion conductor, the conducting species was changed to the divalent Ni2+ cation upon Ni2+ doping, due to the facile migration of lower-valence ions in the rigid crystal lattice. In addition, by selecting the well-ordered NASICON-type structure as the mother crystal, the (NixHf1−x)4/(4−2x)Nb(PO4)3 solid electrolytes exhibited lower activation energies and higher Ni2+ cation conductivities than a previously reported NiZr4(PO4)6 solid with the β-Fe2(SO4)3-type structure. Among the (NixHf1−x)4/(4−2x)Nb(PO4)3 solids, (Ni0.06Hf0.94)4/3.88Nb(PO4)3 (x = 0.06) showed the highest Ni2+ ion conductivity, of 2.27 × 10−4 S·cm−1 at 600 °C, which is ca. 23 times higher than that of the NiZr4(PO4)6 solid.