Ionic covalent organic framework based quasi-solid-state electrolyte for high-performance lithium metal battery

Abstract

Lithium metal solid-state batteries are promising as rechargeable energy storage devices due to their non-combustible nature, resistance to high temperatures, and non-corrosive properties. However, their widespread application is hindered by low lithium-ion conductivity and poor compatibility at the electrode/electrolyte interface. To address these challenges, two covalent organic frameworks (COFs), one with functional imidazolium groups (Dha-COF_im) and one without (Dha-COF), were synthesized. Ionic liquids (ILs) were then incorporated into these COFs to create quasi-solid-state electrolytes (Dha-COF_im-IL and Dha-COF-IL). The Dha-COF_im, with its ordered porous structure, forms interconnected ion channels that enable fast lithium-ion transport and enhance lithium salt dissociation, achieving excellent thermal stability, high ionic conductivity (1.74 × 10⁻³ S cm⁻¹), and a wide electrochemical window at room temperature. Density functional theory (DFT) calculations showed that the fixed imidazolium groups in Dha-COF_im enhance interactions with TFSI⁻ anions, improving lithium salt dissociation. This allows free lithium ions to move quickly through the channels with minimal energy loss. Additionally, the formation of a stable SEI layer rich in LiF and Li₃N at the lithium metal/electrolyte interface accelerates Li⁺transport, ensuring uniform lithium deposition and superior battery performance. When combined with a LiFePO₄ cathode, the LiFePO₄‖Dha-COFim-IL‖Li cell delivers high discharge capacity and excellent cycling stability, providing a new strategy for designing quasi-solid-state electrolytes for high-energy-density lithium batteries.