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

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 Li₂O-inducer, facilitating the formation of a dense Li₂O-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|LiFePO₄ 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.