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

Abstract

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.