Synergistic Cathode/Anode Interphase Stabilization via Single‐Cosolvent Engineering for Fast‐Charging and Durable Sodium‐Ion Batteries

Sodium‐ion batteries are competitive for grid‐scale energy storage, while face cathode/anode interfacial instability that undermines cycle durability in full cells. Tetrahydrofuran (THF)‐modified carbonate electrolyte is proposed that synergistically stabilizes both interphases through anion‐enriched solvation chemistry and in situ adaptive polymeric film engineering. The weakly coordinated THF facilitates Na+‐anion interaction in solvation sheath, fostering inorganic‐dominated solid–electrolyte‐interphase (SEI). Concurrently, trace water triggers THF's in situ ring‐opening polymerization, generating flexible polymer coatings that mechanically reinforce the fragile SEI at anode side while mitigating transition metal species dissolution and structural degradation at cathode side. Such dual‐interphase stabilization addresses a critical oversight in previous studies emphasizing solely on anode interphase optimization, and enables rapid Na+ migration without compromising ionic conductivity and transference number for fast charging. The optimized full cells achieve 3.8‐fold enhanced capacity retention over 150 cycles versus conventional electrolyte. Remarkably, the capacity is up to 207 mAh g−1 at 5C contrasting complete failure in THF‐free system. Proof‐of‐concept Ah pouch cells show 90% capacity retention upon 200 cycles, which is rarely‐reported via cosolvent engineering in terms of specific energy and cycle life. The work establishes a paradigm shifting electrolyte engineering strategy with synchronized interfacial stabilization toward practical deployment.