Low-cost and easily scaled-up acidification is a potential synthesis approach to produce CaO-based composites derived from steel slag for thermal energy storage/release via multicyclic calcination/carbonation. Acid concentration, acidification temperature, and acidification duration were comparatively investigated. The cyclic thermal energy storage/release properties of such composites are closely related to the acidification parameters, mainly attributed to the differences in the composition of the synthetic composites under different acidification conditions. The synergistic effect of the increased Ca and Mg content as well as the reduced content of Si and Fe within the composites contributes to improve their cyclic thermal energy storage/release performance.