A lack of efficient crystallization methods and the metastability of supercooled liquids have limited the use of supercooling phase change materials (PCMs) in long-term thermal energy storage (LIES). Here, we propose a new supercooling PCM-polymer mixture that is not prone to spontaneous crystallization during the storage period. The material releases the stored thermal energy by cold-crystallization on re-heating. The PCM-polymer mixture is composed of supercooling polyol (erythritol or D-mannitol) dispersed in a cross-linked sodium polyacrylate (PAANa) matrix. PAANa efficiently prevents the conventional cooling crystallization of the PCM. Instead of crystallization, the melt polyol supercools and eventually vitrifies as an amorphous solid. Deeply supercooled or vitrified polyol is stable against crystallization below its cold-crystallization temperature. The heat charged and discharged remained similar in 100 repeated melting-crystallization cycles. Its good long-term performance and the stability of the supercooled polyol-PAANa against crystallization at low temperatures make the new material extremely promising for LIES.