The importance of effective and reliable storage of energy has been considered for the ubiquitous use of a wide range of devices, from small electronic devices to electric cars. As the safety issues such as an explosion or a fire continuously arises, the solid electrolyte based battery systems have attracted much interests to solve the safety limitations of the currently used organic liquid electrolyte based battery systems. Compared to highly flammable organic liquid electrolytes, the solid electrolyte completely separates the electrodes physically, thus exhibiting excellent safety by preventing circuit failure due to local shorts. Among the various solid electrolytes, sodium superionic conductor (NASICON) has been studied as the most promising sodium ion solid electrolyte due to its high sodium ion conductivity and isotropic properties. In this study, we have investigated several factors on the phase formation and the sinterability of NASICON to enhance the ionic conductivity. In order to solve the phase formation and densification problems on NASICON, we have proposed a reaction sintering process which is used for sintering of low-sinterable multicomponent compounds. To evaluate the reaction sintering conditions, the properties of powders calcined at various temperatures were investigated by TG-DTA, XRD, and SEM. Powders calcined at various temperatures were sintered at the same temperature to compare the microstructure and the measured density. Based on the observed reaction sintering conditions, the effect of rare-earth ion doping on the ionic conductivity of NASICON was also investigated via EIS analysis performed at from room temperature to $300 ^\circ C$. In conclusion, we obtained well-densified NASICON and the highest conductivity was measured at $1.05×10^{-3}$ S / cm in the La doped sample. The ionic conductivity measured in this study was higher than that of other ion-doped NASICONs reported recently.
고체 전해질 세라믹스는 현재 사용되는 액체 유기 전해질과는 달리 발화성이 없기 때문에, 최근 화두로 떠오른 배터리의 안정성 문제의 해결책으로 주목 받고 있다. 나시콘 고체 전해질은 높은 소듐 이온 전도도를 가지며, 등방적인 특성을 나타내기 때문에 가장 유력한 소듐 이온 고체 전해질로 연구되고 있다. 본 연구에서는, 나시콘의 이온 전도성 향상을 위해 상 형성 및 소결성에 영향을 미치는 요인들을 연구하였다. 나시콘의 낮은 소결성 문제를 해결하기 위한 방법으로 난소결성 다성분계 화합물의 소결에 이용되는 반응 소결 공정을 제안하였다. 제안한 반응 소결 공정을 이용하여 치밀화된 나시콘을 얻을 수 있었으며 교류 임피던스 분광법을 이용하여 상온에서 300도까지의 온도에서 이온 전도도를 측정하였다. 그 결과, 상온에서 $10^{-3}$ S/cm의 높은 이온 전도도를 가지는 시편을 얻을 수 있었으며, 본 연구에서 제안한 방법으로 만들어진 시편들은 최근 보고된 논문들의 이온 전도도보다 높은 값을 가졌다.
