Mechanically Assisted Li⁺-Conduction in Crown Ether-Covalent Organic Frameworks for Lithium Metal Batteries.

Abstract

Mechanically interlocked molecules (MIMs) enable controlled motions like rotation and shuttling, ideal for molecular machines. Heteroatom-containing MIMs, such as crown ethers, exhibit host-guest interactions, coordinating Li⁺ for transport. Crown ethers are integrated into nitrogen-rich 2D covalent organic frameworks (COFs) to create a high-performance quasi-solid-state electrolyte (Li⁺@Crown-COF) for lithium metal batteries. This electrolyte achieves exceptional ionic conductivity (3.2 × 10^-3 S cm^-1) and a Li⁺ transference number (0.60) at room temperature (r.t.). The mechanically assisted Li⁺ conduction, driven by crown ether motion within the COF's porous framework, enhances ion transport and stabilizes the lithium anode, suppressing dendrite growth. Electrochemical tests show excellent cycling stability, with full cells using an LiFePO₄ cathode retaining 95% capacity after 600 cycles at 0.5C and r.t. At 60 °C and 2C, the cell maintained 85% of its initial capacity after 300 cycles, with 99.99% Coulombic efficiency. Solid-state nuclear magnetic resonance and computational studies confirm mechanical motions and strong Li⁺ binding to COF's nitrogen and oxygen sites. This MIM-COF design, leveraging the chemical novelty of mechanically interlocked systems, paves the way for safe, stable, and high-energy-density LMBs.