Is Messier Better? Mechanistic Insights toward Ultracryogenic Metal-Ion Batteries

Aqueous zinc ion batteries (ZIBs) are compelling candidates for grid energy storage due to their safety and cost-effectiveness. High-entropy electrolyte design is widely applied to improve the antifreezing capability and interfacial stability of ZIB; however, the neglected ion-solvent interactions jeopardize its practical adaptability as a guideline. Here, we propose the ion synergistic polarization index (SPI) as an effective descriptor, which exhibits a decisive correlation with electrolyte solidification behavior, outperforming traditional parameters. The SPI metric works by striking a critical balance between cationic perturbation strength with anionic polarization capability. Guided by this principle, we develop a high-SPI electrolyte with different cations. Integrated theoretical simulations and experimental characterizations confirm that the high-SPI electrolyte effectively inhibits water crystallization, maintaining a noncrystalline state even at a record-low temperature of -136.67 °C while delivering rapid ionic transport (an ionic conductivity of 2.4 mS cm^-1 at -60 °C) and robust interfacial stability. When tested at -60 °C, a Zn∥Zn symmetric cell enables ultrastable cycling for over 4800 h. Meanwhile a Zn∥polyaniline full cell retains 70% of its room-temperature capacity. These findings establish a generic framework for reconciling the trade-off between cryogenic operation and electrochemical stability hurdles in next-generation ZIBs and beyond (e.g., lithium-ion batteries).