Survey of zirconium‐containing NaSICON‐type solid‐state Li+ ion conductors with the aim of increasing reduction stability by partial cation substitution

Various compositions of the series Li1+xMx3+Zr2-x(PO4), where M3+ = Al3+, Sc3+, Y3+ were prepared by solution-assisted solid-state 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 M3+ (M = Al, Sc, Y) for Zr4+ in LiZr2 (PO4)(3) stabilizes the rhombohedral NaSICON structure (space group R (3) over barc) at room temperature and increases the ionic conductivity significantly. Here, at 25 degrees C and with a consistent relative density of 94%-96%, an ionic conductivity of 2.7 x 10(-5) S cm(-1), 6.7 x 10(-5) S cm(-1) , and 3.6 x 10(-6) S cm(-1) was achieved with the compositions Li1.2Sc0.2Zr1.8(PO4)(3), LI1.2Y0.2Zr1.8(PO4)(3), and Li1.2Al0.2Zr1.8(PO4)(3), respectively In compar- ison with Li1+xScxZr2-x(PO4)(3), the Y3+ substitution in LiZr2(PO4)(3) enhanced the ionic conductivity slightly and denoted the maximum Li+ ionic conductivity obtained at room temperature. However, substitution with Al3+ decreased the ionic conductivity. For the first time, this work provides a complete overview of three series of solid Li-ion conductors in the Li2O-M2O3-ZrO2-P2O, 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 YPO4. 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(-10) S cm(-1).