NaSICON-type solid-state Li+ ion conductors with partial polyanionic substitution of phosphate with silicate

Abstract

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.