A Multifunctional Molecular Modulated Strategy Featuring Novel Li ⁺ Transport Centers and Li ₂ O‐Rich SEI Layer for High‐Performance All‐Solid‐State Lithium Metal Batteries

Abstract The poor ionic conductivity and interfacial instability severely limit the application of polyethylene oxide (PEO)‐based polymer electrolytes. In this work, we introduce a multifunctional molecular modulated strategy using coumarin, which simultaneously boosts the ionic conductivity and interfacial stability of PEO‐coumarin (PLC) membrane. Unlike conventional additives that diminish PEO's crystallinity, coumarin, with its higher Li + adsorption energy and stronger dipole moments, acts as a novel ‘carrier’ for Li + without compromising the mechanical properties of the PEO matrix. Its synergistic effect with PEO creates a more efficient Li + transport pathway to achieve a high ionic conductivity of 1.1 mS cm −1 at 60 °C. Simultaneously, coumarin as a sacrificial agent by utilizing its carbonyl group, preferentially reacts with lithium metal to prevent the decomposition of PEO and lithium salts. Furthermore, coumarin acts as an in situ Li 2 O‐inducer, facilitating the formation of a dense Li 2 O‐rich solid electrolyte interphase (SEI) layer with faster ion diffusion kinetics at the interface. The beneficial effect of the multifunctional molecular engineering design enables the Li|PLC|Li symmetric cell to cycle over 5000 h and allows the Li|PLC|LiFePO 4 battery to deliver a high initial discharge capacity of 161.9 mAh g −1 with a capacity retention ratio of 93 % after 550 cycles at 0.5 C.