The most commonly used phase change materials (PCMs), like organic compounds and inorganic salts, were limited in application by their low thermal conductivity. Herein, for the first time, alumina-encapsulated metallic Sn-based PCMs, named Sn@Al2O3, were successfully fabricated with tunable size (60-2000 nm) by a facile process from low-cost chemicals. The robust fabrication process consists of a surfactant-free solvothermal synthesis of SnO2 spheres, boehmite treatment on SnO2 spheres, calcination in the air, and the final hydrogen reduction to transform SnO2 to metallic Sn, endowing the PCMs with high potential for mass production. The as-obtained Sn@Al2O3 showed a core-shell structure, in which a main metallic Sn core located in the center covered with an Al2O3 shell with small Sn nanoparticles distributed inside. The boehmite treatment, in which the penetration of aluminum species into SnO2 spheres played an important role, was found to be responsible for the unique structure formation of final Sn@Al2O3. The understanding of structure formation mechanism gives the possibilities of a new facile way for the synthesis of metal nanoparticles and particle-distributed nanostructures. The obtained Sn@Al2O3 particles not only have high PCM content (92.37 wt %) but also show a stable thermal behavior and morphology during 100 melt freeze cycles in the air atmosphere. Furthermore, the low melting temperature of the PCM core, combined with high thermal conductivity of both core material (Sn, 66.8 W m(-1) K-1) and shell material (Al2O3, 35 W m(-1) K-1), makes Sn@Al2O3 potentially suitable for rapid thermal energy storage in the range 100-300 degrees C.