Various compositions of the series Li 1 + x M 3 +x Zr 2À x (PO 4 ) 3 where M 3 + = Al 3 + , Sc 3 + , Y 3 + were prepared by solution-assisted solidstate reaction, since they could have a higher reduction stability as solid electrolytes in lithium batteries than in germanium-or titanium-containing materials. The influence of substitution on crystallographic parameters, density, and ionic conductivity were investigated. The cation substitution of M 3 + (M = Al, Sc, Y) for Zr 4 + in LiZr 2 (PO 4 ) 3 stabilizes the rhombohedral NaSICON structure (space group R � 3c) at room temperature and increases the ionic conductivity significantly. Here, at 25 °C and with a consistent relative density of 94 %-96 %, an ionic conductivity of 2.7 × 10 À 5 S cm À 1 , 6.7 × 10 À 5 S cm À 1 , and 3.6 × 10 À 6 S cm À 1 was achieved with the compositions Li 1.2 Sc 0.2 Zr 1.8 (PO 4 ) 3 , Li 1.2 Y 0.2 Zr 1.8 (PO 4 ) 3 , and Li 1.2 Al 0.2 Zr 1.8 (PO 4 ) 3 , respectively . In comparison with Li 1 + x Sc x Zr 2À x (PO 4 ) 3 , the Y 3 + substitution in LiZr 2 (PO 4 ) 3 enhanced the ionic conductivity slightly and denoted the maximum Li + ionic conductivity obtained at room temperature. However, substitution with Al 3 + decreased the ionic conductivity. For the first time, this work provides a complete overview of three series of solid Li-ion conductors in the Li 2 O-M 2 O 3 -ZrO 2 -P 2 O 5 system where M = Al, Sc, Y. Noticeable differences in the chemistry of resulting compounds were observed, which likely depend on the ionic radius of the cations being substituted. The series with Sc showed complete miscibility from x = 0 to x = 2 with a continuous change of the NaSICON polymorphs. The series with Y showed a solubility limit at about x = 0.3 and higher substitution levels led to the increasing formation of YPO 4 . The series with Al exhibited continuously decreasing ionic conductivity until x = 1, whereupon the investigation was terminated due to its very low conductivity of about 10 À 10 S cm À 1 .