The effects of lattice volume and carrier concentration on the conductivity of NASICON-type LiXIn0.5Z0.5(PO4)3 (X = Ti, Zr; Z = Nb, Ta) oxides.

Novel NASICON-type LiXIn_0.5Z_0.5(PO₄)₃ (X = Ti, Zr; Z = Nb, Ta) oxides were successfully synthesized by solid-state reaction, and resultant crystal structures (space group R-3c) and ionic conductivities were investigated based on a scheme of changed unit cell volume but an unchanged nominal carrier concentration (or constant Li⁺ amount). Relative to unsubstituted LiX₂(PO₄)₃, the conductivities of all samples were improved by approximately an order of magnitude, with LiTiIn_0.5Ta_0.5(PO₄)₃ sample showing the highest value of 2.5×10^-6 S cm^-1 at room temperature. The improvement in ionic conductivity of Ti-containing samples is considered due to an increase in unit cell volume caused by the larger size of substituted ions (In³⁺, Nb⁵⁺, or Ta⁵⁺). Conversely, the improvement in ionic conductivity of Zr-containing samples may be attributed to an increase in lithium ion migration caused by a repulsion between the lithium ion and its surrounding cations (Zr⁴⁺, In³⁺, or Nb⁵⁺/Ta⁵⁺) in a smaller unit cell volume, thus forming a weakened Li-O bond. [ABSTRACT FROM AUTHOR]