The effects of lattice volume and carrier concentration on the conductivity of NASICON-type Li X In 0.5 Z 0.5 (PO 4 ) 3 ( X = Ti, Zr; Z = Nb, Ta) oxides

Novel NASICON-type Li X In 0.5 Z 0.5 (PO 4 ) 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 Li X 2 (PO 4 ) 3 , the conductivities of all samples were improved by approximately an order of magnitude, with LiTiIn 0.5 Ta 0.5 (PO 4 ) 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 (In 3+ , Nb 5+ , or Ta 5+ ). 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 (Zr 4+ , In 3+ , or Nb 5+ /Ta 5+ ) in a smaller unit cell volume, thus forming a weakened Li-O bond.