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The increasing demand for safe energy storage has led to intensive investigations of solid-state Li+-ion conductors in the Li2O-M2O3–ZrO2–SiO2–P2O5 system. As a continuation of the cation substitution in this system, which we reported on very recently, a study of the impact of polyanionic substitutions on ionic conductivity was carried out here in two series, Li3+xSc2SixP3-xO12 (0 ≤ x ≤ 0.6) and Li1.2+xSc0.2Zr1.8SixP3-xO12 (0.3 ≤ x ≤ 2.8), with the aim of increasing ionic conductivity, determing the phase stability, and optimizing the processing conditions – especially decreasing the sintering temperatures – depending on the level of substitution.The polyanionic substitution, i.e. the substitution of (PO4)3- with (SiO4)4-, in the Li2O–Sc2O3–ZrO2–SiO2–P2O5 system revealed that a) the sintering temperature can effectively be reduced, b) the presence of zirconium can limit the evaporation of lithium species even at high sintering temperatures, c) the purity of the NaSICON materials has a strong influence on the grain boundary resistance, and therefore on the ionic conductivity, and d) the silicate substitution in Li3+xSc2SixP3-xO12 (0 ≤ x ≤ 0.6) stabilized the monoclinic polymorph (space group P21/n) with an enhanced total ionic conductivity at 25 °C from 6.5 × 10−7 S cm−1 to 1.2 × 10−5 S cm−1 for x = 0 to x = 0.15, respectively, exhibiting the highest ionic conductivity at 25 °C among the compositions investigated.

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 R3‾c ${R\bar{3}c}$) 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. [ABSTRACT FROM AUTHOR]