Bifunctional Polyrotaxane‐Based Electrolytes Enable Long‐Cycling and Fire‐Safe All‐Solid‐State Lithium Metal Batteries

The growing demand for sustainable and fire‐safe energy storage underscores the limitations of conventional lithium‐ion batteries, which rely on flammable liquid electrolytes and fossil‐derived polymers. Biomass‐derived solid polymer electrolytes offer a promising alternative by combining renewability, environmental compatibility, and functional versatility. Here, a biobased all‐solid‐state polymer electrolyte (ASPE) comprising polyrotaxane (PLR, polyethylene oxide threaded with biobased α‐cyclodextrin) and lithium phenylphosphonate (PPALi) is presented. The PLR framework suppresses PEO recrystallization, enhances ionic conductivity and mechanical robustness, while PPALi simultaneously stabilizes the electrode–electrolyte interface and imparts intrinsic flame retardancy. Li|LiFePO 4 cells with this ASPE exhibit a threefold longer cycle life than PLR‐based controls, and symmetric Li|Li cells operate stably for 2000 h without short‐circuiting. Its fire safety is markedly improved, with immediate self‐extinguishing behavior in horizontal burning, a 63.8% reduction in peak heat release, and over 30% lower total heat release in cone calorimetry than the reference. XPS, TOF‐SIMS, and Pyrolysis‐GC‐MS confirm the function of PPALi in electrochemical stability and flame retardancy. The electrolyte is also recyclable owing to the reversible interlocking structure and hydrogen bonding within PLR, without impacts on electrochemical performance. This work establishes a sustainable, recyclable, and flame‐retardant biobased electrolyte enabling safe, high‐performance lithium metal batteries.