High-Temperature Discharge Characteristics of Salt-Coated Solid Electrolyte

High-temperature thermal batteries, also called molten-salt batteries, are primary batteries that can be reserved for a long period of up to several decades. The molten salt electrolyte for the thermal battery is a solid insulator that thoroughly prevents self-discharge before activation, but once instantly melted by the pyrotechnic heat source, it becomes an excellent ionic conductor and can supply high-power electricity. Usually, fine MgO powder is added as an inorganic binder to prevent the molten-salt electrolyte from flowing down. Although it is desirable to add a large amount of MgO to hold the liquid electrolyte even under severe acceleration conditions, but if it is excessive, the internal resistance of the cell will increase due to its electrically insulating nature of MgO. Meanwhile, all-solid-state batteries characterized by solid electrolytes have received great attention for their high safety and improved energy density even with high-voltage cathodes. Considering the use for thermal batteries, high-temperature stability and long-term storage properties as well as excellent ionic conductivity of the solid electrolyte are crucially important. The densely sintered body can have high ionic conductivity, however, the poor contact between solids deteriorates the output power capability. In this study, we proposed composite electrolytes that significantly improved the contact area and ionic conductivity by coating the surface of the solid electrolyte with molten salt. It has been shown that the soft salt film coated on the hard surface can also prevent self-discharge and increase the compaction strength. The solid electrolyte used in this study is Li₇La₃Zr₂O₁₂ (LLZO). Discharge performance was tested using disk-shaped LiSi/FeS₂ cells.