Heat storage material for the high-temperature range and method for the production thereof

A process for producing halogen salts of an alkali metal or alkaline earth metal with a content of 90% by weight or more of pure halogen salt, comprising, heating a hydrated halogen salt of alkali metals or alkaline earth metals from room temperature to the melting temperature of an anhydrous halogen salt corresponding to the hydrated halogen salt with a heating rate of from 0.1 K/min to 10 K/min in the presence of at least one additive as a reducing agent, wherein the additive is ammonium chloride (NH4Cl) or magnesium, and wherein the heating is effected under vacuum or in a protective atmosphere of inert gas, wherein the halogen salt is selected from bromine, fluorine, chlorine and their mixtures, the alkali metal is selected from lithium, sodium, potassium and their mixtures and the alkaline earth metal is selected from magnesium, calcium, strontium, barium and their mixtures.