Multipaths Li + Migration and In Situ Interfacial Alloying of Composite Solid‐State Electrolyte Enables High‐Performance All‐Solid‐State Lithium Metal Batteries

ABSTRACT Polyethylene oxide (PEO)‐based electrolytes face critical challenges of interfacial instability and lithium dendrites in ASSLMBs. Herein, porous BiF 3 nanoparticles with channel structures and reactivity were synthesized via a one‐step precipitation method and introduced into PEO to construct a novel composite solid‐state electrolyte (CPE) with enhanced interfacial stability and high ionic conductivity. Density functional theory (DFT) calculations verify that BiF 3 nanoparticle promotes lithium salt dissociation, thereby increasing the mobility of free Li + , while its channel architecture establishes more paths for Li + transport. Furthermore, BiF 3 undergoes an in situ alloying reaction with the lithium anode to form Li x Bi and LiF, so as to build a gradient composite solid electrolyte interphase (SEI), which demonstrates exceptional interfacial stability and rapid Li + transport kinetics, effectively inhibiting lithium dendrite propagation. As a result, Li|Li symmetrical cell with PEO‐5%BiF 3 CPE achieves stable cycling over 6000 h at 0.1 mA cm −2 without short‐circuiting, and its Li|LFP full cell exhibits exceptional electrochemical performance across a wide temperature range (45–90°C). Moreover, it also demonstrates excellent cycling stability and capacity retention in Li|NCM811 system. Notably, the excellent electrochemical performance and safety of Li|PEO‐5%BiF 3 CPE|LFP pouch cell demonstrate good application potential.