Poly(benzoxazine)‐Based Gel Polymer Electrolytes for Lithium Metal Batteries With Ultralong Lifespans

Gel polymer electrolyte (GPE) is a desirable candidate for high‐safety lithium batteries but is still plagued by the dynamic fluctuations of liquid electrolyte components, which induce localized fluid aggregation or leakage, ultimately leading to performance instability or even degradation. Here, we develop a novel poly(benzoxazine‐propylene‐oxide)‐based GPE, achieving superior electrochemical performance and high safety simultaneously. Through molecular architecture design, the strategic incorporation of long‐chain propylene‐oxide segments and amide functionalities into the benzoxazine backbone endows the polymer matrix with enhanced lithium‐ion transport capability. Catalyst‐free thermal curing triggers oxazine ring‐opening polymerization, constructing three‐dimensional chemically cross‐linked network architecture, generating abundant hydrogen bonds. The synergistic interaction between chemical crosslinking and dynamic hydrogen‐bonding enabled exceptional electrolyte uptake (600% w/w within 5 min) coupled with effective solvent immobilization. The incorporated long‐chain propylene‐oxide segments exhibited synergistic solvation effects with carbonate solvents, enabling superior ionic conductivity (9.62 mS cm−1 at 20 °C). The Li||Li symmetric cells based on PBz‐PO‐GPE 2000 operated for 3000 h at 0.1 mA cm−2, and LiFePO4||Li full cells delivered 140.7 mAh g−1 initial discharge capacity at 2 C rate, near‐unity coulombic efficiency, and 70.5% capacity retention after 1800 cycles. This multiscale design of GPE provides an effective strategy for electrolyte exploration in high‐performance lithium metal batteries.