Triple‐Functional Strategy of Anion Anchoring, Radical Quenching, and Flame Retardancy for Stable High‐Voltage Cycling in All‐Solid‐State Lithium Metal Batteries

ABSTRACT Poly(ethylene oxide) (PEO) is a promising solid polymer electrolyte, yet its application in all‐solid‐state lithium metal batteries is limited by low ionic conductivity, a narrow electrochemical window, and flammability. Herein, a triple‐functional molecular engineering strategy integrating anion anchoring, radical quenching, and flame retardancy is realized by incorporating HCCP‐EA (HE), synthesized via one‐step precipitation of hexachlorocyclotriphosphazene (HCCP) and ellagic acid (EA). With only 3 wt.% HE, the crystallinity of 3%HE‐PEO/PE (HE‐PEO/PE) is effectively reduced, delivering an ionic conductivity of 4.80 × 10 −4 S cm −1 and a Li + transference number of 0.40 at 60°C. Owing to radical quenching capability, HE scavenges R─O· and H· generated under high‐voltage, improving the stability of PEO and extending the electrochemical window to 5.02 V. Meanwhile, HE's anion anchoring effect promotes lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissociation, increases free Li + concentration, and lowers TFSI − redox barrier, inducing in situ formation of P/N/F‐rich interface layers. Consequently, the Li//HE‐PEO/PE//Li symmetric cells operate stably for 3000 h (250 µA cm −2 at 60°C), while NCM811//HE‐PEO/PE//Li cells retain 83.6% capacity after 300 cycles at 0.5 C and cycle stably at 4.5 V. This work presents an effective molecular‐engineering strategy for safe, high‐voltage solid‐state lithium batteries.