there are indeed some commercial solutions for your range based on salts, please take a look at https://www.pcmproducts.net/Phase-Change-Material-Solutions.htm at the data sheets of the high temperature salt PCMs (PlusICE PCM TEMPERATURE (H) RANGE).
Microencapsulated phase change materials with high heat capacity and high cyclic durability for high-temperature thermal energy storage and transportation
Types, methods, techniques, and applications for microencapsulated phase change materials (MPCM): a review