Optimization of Compositions of Solid Electrolytes Na0.5–xR0.5+xF2+2x on Ionic Conductivity and Thermal Stability

Optimization of the compositions of nonstoichiometric fluorite phases Na0.5–xR0.5+xF2+2x and Na0.5−x(R,R′)0.5+xF2+2x (R, R′ are rare-earth elements Tb−Lu, Y; 0.08 ≤ x ≤ 0.13) on ionic conductivity and thermal stability was performed based on temperature measurements of the electrical conductivity of single crystals in dependence of the ionic radii of rare-earth cations (R) and the content of RF3 trifluorides (x) and a comparative analysis of the phase diagrams of condensed NaF–RF3 systems. It has been shown that the ionic conductivity (at 500 K) of 15 out of 27 studied compositions of Na0.5−xR0.5+xF2+2x and Na0.5−x(R,R′)0.5+xF2+2x solid solutions exceeds the conventional limit of 10−5 S/cm, below which the use of solid electrolytes in electrochemical devices is considered undesirable because of insufficient electrical conductivity. The fluorite phase Na0.5–xLu0.5+xF2+2x has the highest thermal stability, and its composition Na0.4Lu0.6F2.2 has the maximum fluorine-ion conductivity, equal to 1.4 × 10−4 S/cm.