Seasonal energy storage based on phase-change materials (PCMs), long-chain alkylimidazolium bromide ionic liquids [C16MIM]Br and [C16MMIM]Br, are investigated in this paper. The structures of ionic liquids are measured by infrared (IR) spectra. Thermodynamic properties of ionic liquids, such as melting/freezing temperature and endothermic/exothermic heat, are measured by differential scanning calorimetry (DSC): The active use of supercooling of ionic liquids is proposed, which means that the thermal energy is stored in a supercooled liquid state and released by nucleating agents when needed. Copper powder and graphite powder, used as nucleating agents, are added with different mass fractions (0-25 wt %), and their effects on the thermodynamic properties of two ionic liquids are investigated. The results show that the nucleating agents have little effect on the thermodynamic properties of ionic liquids. The degree of supercooling of [C16MIM]Br is maintained at about 20 K when copper powder or graphite powder is added. The proper melting point and stable supercooling state of [C16MIM]Br indicates that it is more suitable for seasonal energy storage.