Ionic Conductivity of Ca²⁺ Doped NaI–LiI Solid Electrolyte and Its Stability with Li Metal

Abstract

The effects of Ca²⁺ doping on the ionic conductivity and the stability with a Li of NaI–LiI solid electrolyte were investigated. Evidence of the Ca²⁺ doping was found in a Rietveld analysis of the synchrotron X-ray diffraction pattern. The conductivity of 9(9NaI·CaI₂)·LiI was measured to be 2 × 10⁻⁵ S/cm at 60°C, which was higher than the Ca²⁺-free sample at the same temperature (1 × 10⁻⁵ S/cm). The short-circuit of the Li | 9(9NaI·CaI₂)·LiI | Li cell was observed after keeping the cell at the open-circuit. The reductive decomposition of the solid electrolyte to LiI and Ca (or Li–Ca alloy) was suggested by contacting with Li. The short-circuit was attributed to the continuous growth of the Li–Ca alloy. The phase separation at Li/9NaI·LiI interphase was not observed. These results suggest that while Li⁺ conductors stabilized with Li can be developed based on Na compounds, Ca²⁺ doping is not an effective strategy to enhance conductivity.