Influence of microstructure and crystalline phases on impedance spectra of sodium conducting glass ceramics produced from glass powder

Crystallization of highly ionic conductive N5 (Na5YSi4O12) phase from melted Na3+3x-1Y1-xPySi3-yO9 parent glass provides an attractive pathway for cost-effective manufacturing of Na-ion conducting thin electrolyte substrates. The temperature-dependent crystallization of parent glass results in several crystalline phases in the microstructure (N3 (Na3YSi2O7), N5 and N8 (Na8.1Y Si6O18) phases) as well as in rest glass phase with temperature dependent viscosity. The electrical properties of dense parent glass and of compositions densified and crystallized at 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C are investigated by impedance spectroscopy and linked to their microstructure and crystalline phase content determined by Rietveld refinement. The parent glass has high isolation resistance and predominantly electrons as charge carriers. For sintering at ≥ 900 °C, sufficient N5 phase content is formed to exceed the percolation limit and form ion-conducting pathways. At the same time, the highest content of crystalline phase and the lowest grain boundary resistance are observed. Further increase of the sintering temperature leads to a decrease of the grain resistance and an increase of grain boundary resistance. The grain boundary resistance increases remarkably for samples sintered at 1100 °C due to softening of the residual glass phase and wetting of the grain boundaries. The conductivity of fully crystallized N5 phase (grain conductivity) is calculated from thorough impedance spectra analysis using its volume content estimated from Rietveld analysis, density measurements and assuming reasonable tortuosity to 2.8 10−3 S cm−1 at room temperature. The excellent conductivity and easy processing demonstrate the great potential for the use of this phase in the preparation of solid-state sodium electrolytes.