Molten Alkali Metal Batteries Based on Solid Electrolytes

The development of energy storage technologies with high safety, low cost, and high energy densities is essential for the widespread use of renewable energy sources. Battery technology is one of the most promising candidates because of its pollution-free operation, high round-trip efficiency, flexible power and energy characteristics, long cycle life, and low maintenance cost. Although most batteries operate at room temperature, high-temperature systems are expected to perform better owing to improved electrolyte conductivity, faster reaction kinetics, and reduced interface impedance. The reported high-temperature batteries can be classified into liquid and solid electrolyte-based systems, with the latter having the potential to achieve higher energy densities while avoiding self-discharge effects. This review summarizes solid electrolyte-based liquid sodium and lithium batteries (SELS and SELL batteries). SELS batteries primarily use beta-Al2O3 and NASICON electrolytes, while SELL battery systems use garnet electrolytes. Because the microstructures and compositions of ceramic electrolytes significantly affect their conductivity and stability, novel manufacturing and element doping methods are being intensively investigated. Surface modification technology is also a major research focus to improve the wetting properties of molten alkali metals on the ceramic electrolyte, which assists to decrease interfacial resistance and increase the rate performance and power density of the battery. In addition, the selection of the cathode materials of SELS and SELL batteries has a significant impact on the energy and power densities, cycling stability, material cost, and application scenarios of the real devices. Until now, lead alloys, metal chlorides, sulfur, selenium, and iodine have been reported as potential choices. We describe in detail the reaction mechanisms, existing problems, and the latest research progress of these battery systems, with their electrochemical performance and raw material costs systematically summarized and compared. It is worth noting that the SELS and SELL batteries have different levels of technological maturity. In 2019, an energy storage system using SELS batteries with a capacity of 108 MW/648 MWh was built, whereas SELL battery research is a relatively emerging field. However, SELL batteries demonstrate promising application prospects because of their higher energy densities, lower operating temperatures, and competitive raw material costs. In addition, we believe that several research advancements and technical achievements related to these two types of batteries can be shared, with the future research directions listed in the conclusion section.