Boroxine Crystalline Covalent Organic Frameworks Based Single-ion Quasi-solid-state Conductor in Lithium-ion Battery

Abstract

Solid-state electrolytes are considered to be the vital part of the next-generation solid-state batteries (SSBs), due to their high safety and long operation life span. However, the two major factors that impede the expected performance of batteries are: the easy formation of lithium dendrites due to the concentration gradient of anions, and the low ionic conductivity at room temperature, which prevents reaching ideal electrochemical performance. Single-ion quasi-solid-state electrolytes (SIQSSEs) could provide higher safety and energy density, owing to absence of anion concentration gradient and solvent, as well as good lithium-ion transport ability. The porous covalent organic frameworks (COFs) are beneficial for con-structing appropriate lithium-ion transport pathway, due to the ordered 1D channel. In addition, the boroxine COFs (COF-5) offers strong ability of withdrawing anion part of lithium salt. Last but not the least, boron atom could play the role of coordinate site due to its electron deficiency. These advantages afford an opportunity to obtain a SIQSSE with high ionic conductivity and high lithium transference number (LTN) simultaneously. The COF-5 based SIQSSEs delivered a high ionic conductivity of 6.3×10⁻⁴ S·cm⁻¹, with a high LTN of 0.92 and a wide electrochemical stable window (ESW) of 4.7 V at room temperature. The LiFePO₄ (LFP)/Li cells, which was assembled with COF-5 based SIQSSE, exhibited outstanding long cycle stability, high initial capacity and favorable rate performance. The results indicated COFs could be an ideal material for single-ion solid-state electrolytes in next-generation batteries.