Self‐Limiting Covalent Ligation Mechanism Enabling Anomalously High Interfacial Compatibility in Organic‐in‐Sulfide All‐Solid‐State Lithium Batteries

ABSTRACT Polymer‐in‐sulfide composite electrolytes have emerged as highly promising candidates for all‐solid‐state lithium batteries (ASSLBs) due to their on‐demand shaping and rapid ion diffusivity. However, a striking paradox arises in the case of ethylene oxide‐tethered polyacrylates (EO‐PAs): their high polarity/strong nucleophilic tendencies constitute a major threat to sulfide stability yet exhibit anomalously high polymer/sulfide compatibility in practice. The underlying mechanism remains a matter of uncertainty. Herein, we first reveal a self‐limiting covalent ligation mechanism that accounts for this compatibility paradox. Central to this principle is the identification of intimate interactions between terminal −CH 3 in EO‐PAs and PS 4 3− units in Li 6 PS 5 Cl, not only suppressing parasitic nucleophilic reactions by EO ligands but also enhancing air stability. The self‐limiting interface was rigorously validated by density functional theory calculations, 31 P solid‐state nuclear magnetic resonance, x‐ray computed tomography, and time of flight secondary ion mass spectrometry. The robust polymer‐in‐sulfide electrolyte achieves dendrite‐free Li plating/stripping for over 1200 h at 3 mA cm −2 and delivers approximately 100% capacity retention over 1000 cycles in NCM811‐based ASSLBs. These findings elucidate the core mechanism of interface regulation and provide pivotal guidance for the development of high‐performance ASSLBs.