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

Polyethylene oxide (PEO)-based electrolytes face critical challenges of interfacial instability and lithium dendrites in ASSLMBs. Herein, porous BiF₃ 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 BiF₃ nanoparticle promotes lithium salt dissociation, thereby increasing the mobility of free Li⁺, while its channel architecture establishes more paths for Li⁺ transport. Furthermore, BiF₃ undergoes an in situ alloying reaction with the lithium anode to form Li_xBi 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%BiF₃ 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%BiF₃ CPE|LFP pouch cell demonstrate good application potential.