One possible improvement on these would be the use of feolite, a sort of brick of iron oxides. It has a heat capacity far above that of normal brick and is use

One possible improvement on these would be the use of feolite, a sort of brick of iron oxides. It has a heat capacity far above that of normal brick and is used in electric heat storage systems. The heat capacity may be in excess of what was listed on Wikipedia, I did some digging in hopes of finding the exact composition only to find that some research projects on this material indicated an even greater heat capacity than the official numbers. And its volumetric heat capacity is staggering.