In Situ Polymerized Localized High‐Concentration Electrolytes for Ultrahigh‐Rate Sodium Metal Batteries

Abstract Sodium metal batteries (SMBs) offer a promising alternative to lithium‐ion systems due to the natural abundance of sodium. Nevertheless, their practical application is hindered by challenges of sodium dendrite growth and unstable electrolyte/electrode interfaces in conventional liquid electrolytes. Here, an in situ polymerized localized high‐concentration gel electrolyte (IS‐LHCE) is presented and engineered through strategic integration of 1,3‐dioxolane as both diluent and polymer precursor. Unlike conventional approaches using inert diluents, the design establishes a polymer‐confined solvation structure that simultaneously achieves anion coordination regulation and ion transport decoupling. This unique configuration reduces Na + activation energy to 0.0379 eV, enabling exceptional ionic conductivity of 6.07 × 10 −4 S cm −1 and a wide electrochemical stability window (≈4.56 V). The in situ formed polymer network of IS‐LHCE promotes preferential anion decomposition, forming a gradient inorganic‐rich solid electrolyte interphase dominated by NaF/Na 2 S phases, which enables Na||Na symmetric batteries to achieve unprecedented cycling stability of over 1,200 h at 0.1 mA cm −2 . The Na 3 V 2 (PO 4 ) 3 ||Na full batteries demonstrate record‐breaking longevity, with 90.8% capacity retention after 3,000 cycles at 10 C rate. This work presents a new paradigm in polymer electrolyte design, fundamentally resolving the longstanding trade‐offs between interfacial instability and ion transport, avoiding dendrite formation, and advances practical high‐energy‐density SMBs.