Multi-Shelled Hollow Covalent Organic Framework Nanospheres for Stable Potassium Storage

Multi-shelled hollow covalent organic framework nanospheres (MH-COFs) with at least two shells integrate the merits of the porous crystalline covalent organic framework (COF) matrix with the complex hollow architecture, which can motivate new functions for exceptional performance. However, the fabrication of MH-COFs is still uncultivated and remains a formidable challenge. Herein, we reported a facile template-free protocol for the general synthesis of different MH-COFs by controlling the simultaneous processes of surface crystallization and core etching of the crystalline-inhomogeneity nanospheres of COFs precursor. The crystalline-inhomogeneity solid covalent organic polymer nanospheres (COPs) with robust crystalline surface but vulnerable amorphous core were designed. Subsequently, an acetic acid aqueous solution treatment of crystalline-inhomogeneity COPs not only induced selective etching of the vulnerable cores but also promote the further crystallization of the surface layers, thereby producing hollow COFs. A further step-by-step expansion of seeded growth for inhomogeneous COP layers and then similar acid solution treatment can output the intriguing MH-COFs. An extraordinarily stable K-ion battery anode with high capacity was demonstrated with a MH-COFs/S nanocomposite fabricated by covalently bonding chain sulfur into the MH-COFs matrix. This work opened a simple but powerful avenue in designing complex hollow COFs architectures to boost their potential for applications.