Multi-Li⁺ Coordination for Superior Room-Temperature Ionic Conductivity in PEO Solid Electrolytes

Ion transport in poly(ethylene oxide) (PEO) electrolytes is severely constrained by sluggish Li⁺ mobility arising from tight chelation of ether oxygens, and by insufficient lithium salt dissociation. Inspired by polycarboxylate ether (PCE) superplasticizers in cement, we demonstrate phosphonate-functionalized PCE (P-PCE) could overcome both limitations. The electron-rich -PO₃ ^2- groups could liberate Li⁺ from the strong Li⁺-PEO chelation and promote efficient salt dissociation without trapping Li⁺. Instead, they facilitate rapid Li⁺ migration through a unique multi-Li⁺ coordination mechanism, where the electrostatic repulsion among multiple Li⁺ ions coordinated to a single -PO₃ ^2- destabilizes solvation and promotes rapid Li⁺ hopping between adjacent sites. Meanwhile, P-PCE suppresses PEO crystallization and, through multi-Li⁺ coordination, induces a more extended conformation of PEO, which fosters high-entropy Li⁺ coordination environments involving TFSI^-, ether oxygens, and -PO₃ ^2- in transient combinations, promoting long-range Li⁺ transport. The simultaneously enhanced Li⁺ concentration and mobility lead to a high ionic conductivity (1.7 × 10^-4 S cm^-1, 30°C) without adding liquid plasticizers. Correspondingly, Li||LiFePO₄ cells deliver high capacities of 140.6 mAh g^-1 at 0.5 C and 119.7 mAh g^-1 at 1 C (30°C). This work provides a simple yet effective strategy for developing high-performance solid-state batteries and molecular-level insights for rational polymer electrolyte designs.