Asymmetric Cooperative Solvation Structure Enables Entropy‐Enthalpy Compensation for Ultralow Temperature Sodium‐Metal and Sodium–Ion Pouch Cells

ABSTRACT

Sodium‐metal batteries are promising for low‐temperature energy storage, yet conventional electrolytes suffer from an intrinsic trade‐off between thermodynamic stability and kinetic transport, causing salt precipitation and solvation collapse. Herein, an asymmetric cooperative solvation structure (ACSS) is introduced to spatially decouple these conflicting requirements via coordination competition between a strong solvating solvent (diethylene glycol dimethyl ether, DME) and a weak solvating (diethylene glycol diethyl ether, DEE). The DME‐rich inner core forms compact ternary coordination clusters with Na ⁺ and anions (PF ₆ ⁻ /BF ₄ ⁻ ), ensuring enthalpic stability and high salt solubility via geometric ion confinement, while the DEE‐rich outer shell sustains entropic mobility through rapid ligand exchange, facilitating hopping transport and lowering desolvation barriers. This enthalpy–entropy compensation preserves ionic conductivity and structural integrity below −40°C. High anion occupancy within the stable clusters promotes preferential anion reduction, yielding an inorganic‐rich SEI with high mechanical modulus and low Na ⁺ migration energy, effectively suppressing dendrite growth and interfacial polarization. Consequently, Na||Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ pouch cell achieves a capacity retention of 94% at −40°C. Scaled‐up 1.2 Ah hard carbon||Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ and 1.2 Ah hard carbon||NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ pouch cells sustain 400 and 550 cycles at −30°C and −20°C, respectively, demonstrating the promising practical applicability of this electrolyte design.