Researchers from the School of Engineering at The Hong Kong University of Science and Technology (HKUST) have pioneered a mechanical bond strategy to create quasi-solid-state electrolytes (QSSEs) for lithium-metal batteries (LMBs). This marks the first use of mechanically interlocked molecules (MIMs) in covalent organic frameworks (COFs) to achieve high-performance battery operation, harnessing the unique chemistry of interlocked systems to enable safe, stable, and high-energy-density LMBs.
Traditional flammable liquid electrolytes pose risks like unstable lithium anodes, dendrite growth, and unwanted interphase layers. Solid-state electrolytes offer a safer alternative, with ether-based Poly(ethylene oxide) (PEO) commonly used for Li+ coordination and conduction. However, these polymers suffer from low ionic conductivity due to entangled networks and undefined pathways, necessitating innovative strategies. MIMs, known for applications in molecular machines like shuttles, have been underexplored in energy storage. Crown ethers, a key macrocycle in MIMs, exhibit strong host-guest interactions with Li+ and mobility when complexed. By integrating these into highly crystalline, porous COFs, researchers can exploit their properties for efficient Li+ transport and anode stabilization.
Building on this principle, led by Prof. KIM Yoonseob, Associate Professor of the Department of Chemical and Biological Engineering at HKUST, the team designed a MIM-COF QSSE that leverages mechanical bonds responsive to forces or coordination. These bonds serve as functional units, while COFs amplify their dynamics into macroscopic properties, advancing MIM integration in porous frameworks for energy devices.
The resulting MIM-COF QSSE boasts exceptional room-temperature ionic conductivity (3.20 × 10⁻³ S cm⁻¹) and Li+ transference number (0.60). Computational studies revealed crown ether dynamics and Li+ binding sites, supporting experimental findings and guiding future electrolyte designs. In practical tests, an LMB full cell with this QSSE and a LiFePO₄ composite cathode delivered an initial discharge capacity of 113 mAh g⁻¹ at 0.5C and room temperature, retaining 95% capacity over 600 cycles. At 60°C and 2C, it maintained 85% initial capacity after 300 cycles with 99.99% Coulombic efficiency, demonstrating the potential of the QSSE to boost battery stability and longevity.
Prof. Kim noted, "Our study on crown ether motions in batteries, building on established MIM research, can inspire broader utilization of interlocked components. We aim to functionalize these macrocycles further for advanced battery materials."
This collaborative effort involved HKUST, Shanghai Jiao Tong University, The Hong Kong Polytechnic University, and the University of Hong Kong. The findings, titled "Mechanically Assisted Li⁺-Conduction in Crown Ether-Covalent Organic Frameworks for Lithium Metal Batteries", were published in Advanced Materials.
