Decoding the Structure-Property-Function Relationships in Covalent Organic Frameworks for Sustainable Battery Design

Ion transport and storage in porous materials play a vital role in numerous energy storage and conversion technologies, such as batteries, capacitors, and fuel cells. In this study, molecular dynamics simulations are used to investigate the structure-property-function relationship of lithium and sodium ion transport and storage in highly porous and π-conjugated Aza-linked covalent organic frameworks (COFs). The simulations reveal that the diffusion coefficients of free lithium ions are significantly higher than those of free sodium ions across all of the COF structures, highlighting the influence of ion size on mobility in the absence of solvent molecules. The presence of nitrogen atoms in the imidazole and phenazine rings of the framework of BCOF-1, referred to throughout this article as Aza-COF, was found to significantly decrease the diffusion coefficients of the metal ions due to the significant electrostatic attraction between the ions and the lone pairs of the nitrogen atoms. Replacing the nitrogen atoms with carbon atoms led to increased diffusion coefficients, suggesting that the lone pairs and π-electrons of the frameworks play critical roles in ion binding. Pore decoration of the frameworks with glycol side chains dramatically reduced ion mobility due to increased electrostatic interactions. This indicates that the ether groups in these side chains create a more restrictive environment for ion movement. Overall, the simulations reveal that the electronic properties and chemical functionality of the pore walls, the free volume of the pores, and the solid-state packing of the COF structure significantly impact the ion transport and storage within the framework. This understanding paves the way for the rational design of new COF materials with tailored ion transport and storage properties for potential use as electrodes and solid-state electrolytes in sustainable batteries.