Survey of Zirconium-Containing NaSICON-type Solid-State Li+ Ion Conductors with the Aim of Increasing Reduction Stability by Partial Cation Substitution

Abstract

Various compositions of the series Li1+xM3+xZr2−x(PO4)3 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 (Formula presented.)) 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 2.7×10−5 S cm−1, 6.7×10−5 S cm−1, and 3.6×10−6 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 comparison 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-P2O5 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.