Copper nanostructures with a high yield are synthesized by a controlled disproportionation of CuCl in oleylamine reaction medium without the involvement of strong reducing agents adopted in conventional approaches. The highest copper yield (50%), a maximum theoretical value, is obtained by optimizing both the initial amount of CuCl added to the reaction medium and the reaction temperature. A potential use of the copper nanostructures in greatly enhancing thermal conductivity of a hydrated CaCl₂·6H₂O salt phase change material (PCM) is further demonstrated. A high thermal conductivity enhancement of the PCM (>50%) is achieved by doping a small amount (<0.2 wt%) of the copper nanostructures. The great enhancement with the use of a very small amount of the copper nanostructures makes the doping cost-effective for practical thermal energy storage applications.