Enhancing flexibility and Li ion storage in 3D penta-covalent organic framework via node-site silicon doping: A pathway to high-performance electrochemical applications

Modern society has been transformed by flexible electronics, which have enabled the development of a new generation of power storage devices. Traditional Li-ion batteries (LIBs) use rigid inorganic cathodes, which are brittle, have limited capacity, and can be hazardous to the environment. In this study, we introduce a recently designed 3D penta-COF (unj-2) as a cathode material in LIBs, which is doped with Si atoms (Si-unj-2). DFT simulations indicate that pristine unj-2 is rigid and its structural integrity is compromised when interacting with Li ions, making it unsuitable as a battery electrode. However, when tetrahedral linker sites are substituted with Si, it becomes more mechanically flexible, with anisotropic elastic constants. Dynamic stability was confirmed by the phonon dispersion spectrum. Si-unj-2 can withstand high temperatures, up to 1300 K. The HSE06 band structure reveals a significant narrowing of the band gap of Si-unj-2 to 3.82 eV. The crystal orbital Hamilton population, partial density of states analysis reveals stronger SiC bonding. The material stably adsorbs a Li ion with an adsorption energy of −2.23 eV. A single unit cell of Si-unj-2 shows a theoretical capacity of 575.67 mAh/g. The fully Li-ion-concentrated Si-unj-2 supercell demonstrates that the Si-unj-2 can withstand external stress, highlighting its highly flexible nature. The calculated voltage plateau is 1.10 V, the diffusion barrier is low (0.21 eV), and the analysis of mean-square-displacement indicates that Li diffusion is enhanced at high temperatures. These results show that Si-unj-2 is a promising flexible cathode material for next-generation energy storage systems.