Stacking disorder in layered covalent-organic frameworks Ju Huang 1 , Matthias J. Golomb 1 ,Seung-Jae Shin 3 , Kasper Tolborg 1 , Seán R. Kavanagh 1,2 , Alex M. Ganose 1 , Gabriel Krenzer 1 , Aron Walsh 1,3 1 Thomas Young Centre, Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, 2 Thomas Young Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK, 3 Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea Layered covalent organic frameworks (2D COFs) exhibit a high degree of structural flexibility and chemical stability, making them promising candidates in energy storage and conversion applications. However, their weak interlayer interactions mean stacking faults are common. These imperfections can significantly affect their performance in energy technologies arising from variations in crystallinity, porosity, and electrical conductivity. To date, many studies have reported ideal eclipsed stacking from powder X-ray diffraction (PXRD) measurements, however, there is growing evidence that 2D COFs are disordered with offsets between layers. Herein, we address this issue by studying the displacive instabilities in two prototypical COFs, Tp-Azo and DAAQ-TFP that have been used as high-performance Li-ion battery electrodes. We demonstrate the existence of an unusual “sombrero” potential energy surface (PES) for layer displacements from density functional theory (DFT) calculations. The “sombrero” potential energy surface exhibits a striking preference for slipped structures with horizontal offsets between layers ranging from 1.7 Å to 3.5 Å in a potential energy minimum that forms a low energy ring. Furthermore, we elucidate the effects of interlayer π-πinteractions on the electronic band structures and band gaps in eclipsed and slipped stacking sequences. We reveal a pronounced band gap opening of 0.8-1.4 eV in slipped arrangements that arises from subtle changes in the interlayer πorbital overlap. We further develop and apply on-the-fly machine learning force field (MLFF) to efficiently model the COF dynamic structures over longer length and time scales at room temperature. We find that an initially eclipsed stacking mode spontaneously distorts to form a zigzag configuration that lowers the free energy of the crystal, while the inclined configurations remain in the inclined mode, showing that both are locally stable states at ambient conditions. However, due to the larger entropic contribution to the zigzag stacking, we expect this to be the favoured structure. The predicted diffraction patterns of the “random” zigzag mode from MLFF exhibit much better agreement with the reported experimental PXRD than the perfectly eclipsed or inclined stacking patterns, highlighting the importance of sampling the dynamic structures. Treatment of stacking disorder is critical for screening COFs for applications in energy storage and conversion systems where electrochemical and photochemical descriptors are significantly altered including accessible voltage ranges for batteries, stability windows for electrocatalysis, and visible light absorption for photoelectrochemical systems. References 1. Huang, J., Golomb, M.J., Kavanagh, S.R., Tolborg, K., Ganose, A.M. and Walsh, A., 2022. Band gap opening from displacive instabilities in layered covalent-organic frameworks $^\dag$.J. Mater. Chem. A, 2022,10, 13500-13507.
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