Synthesis and characterisation of Li₄(OH)₃Br for thermal energy storage

The peritectic compound Li₄(OH)₃Br has been suggested as a candidate material for latent heat thermal energy storage due to its high calculated melting enthalpy of 804 joules per gram around 300 degrees Celsius. However, experimental reports have obtained much lower values, at or below 250 joules per gram. In this work, we demonstrate that the crystal structure established for Li₄(OH)₃Br in the literature corresponds to a metastable hydrated compound. Instead, we propose that the thermodynamically stable phase belongs to the Pmmn space group. The hydrated phase dehydrates at approximately 175 degrees Celsius, rendering the exceptional previous predictions inapplicable. An experimentally measured melting enthalpy of 263 joules per gram is reported for high-purity Li₄(OH)₃Br. Theoretical modeling suggests a crystal structure for Li₄(OH)₃Br, from which a melting enthalpy of 260 joules per gram is calculated. This result aligns well with the experimental findings and supports that, despite lower-than-expected values, Li₄(OH)₃Br can still offer an impressive storage capacity at around 290 degrees Celsius.