Thermal batteries have been used for many military applications such as missiles and ordnance devices. As the thermal battery is composed of anode, cathode, MgO based-molten salt electrolyte and additional heat source, its specific energy density is lower than those of commercial batteries.

   By means of substituting MgO based-electrolyte to increase energy density of thermal batteries, we have indentified various ionic solid electrolytes such as LLT(La_0.5Li_0.5TiO₃), LSTZ(Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃) and LLZ(Li₇La₃Zr₂O₁₂). 

   As a ruslt, we found out that the LLZ with additivies, Ca and Nb, presents not only excellent ionic conductivity but also stable discharge characteristics at high temperature(≈ 300^oC). Additionally, we have verified that the LLZ solid electrolyte is suitable for the full scale of thermal batteries with high energy density.