Recently, improving energy efficiency and utilizing waste heat have become critical challenges. This study developed a new thermoelectric material from weathered biotite (WB), a resource that is abundant and nontoxic. Through milling, classification, and treatment of WB with molten salts, crystals that exhibited thermoelectric properties at high temperatures were produced. Notably, the samples with CaCl₂ and KCl additives tended to show thermoelectric performance in the high temperature range from 650 ℃ to 850 ℃ (Fig. 1). The thermoelectric performance properties at high temperatures were attributed to the molten salt process, which transformed the WB into a new structure and promoted conduction, such as ion exchange. Although the current ZT values are lower than those of the existing materials, WB comprises the most abundant elements on Earth, thereby offering advantages in terms of environmental impact and resource efficiency. Future efforts will focus on elucidating the detailed crystal structure and improving ZT values for developing thermoelectric modules.
 This research paves the way for the development of environmentally friendly, low-cost thermoelectric materials with high-temperature stability.

This work was performed in collaboration with Hokkaido University, Wakayama University, and University of Tsukuba supported by JSPS KAKENHI Grant Number JP22K19001.