Renewable energy systems will need large-scale energy storage to ensure reliability and provide power when and where it is needed. Even though lithium-ion batteries are increasingly used for large-scale storage, their costs and competition in terms of materials needed for electric vehicles are driving the need for alternative batteries for stationary energy storage systems. Molten sodium-ion batteries that operate at intermediate temperatures, approximately 150 °C or less, offer an abundant and cost-effective solution to our energy storage issues. This review paper highlights the materials, enhancements, and performance of molten Na–ion batteries that operate at temperatures at or below 150 °C for use in energy storage systems as well as an outlook on future improvements for this energy storage system.