A Phosphine-Amine-Linked Covalent Organic Framework with Staggered Stacking Structure for Lithium-Ion Conduction

In-plane ionic conduction over two-dimensional (2D) materials is desirable for flexible electronics. Exfoliating 2D covalent organic frameworks (COFs) towards a few layers is highly anticipated, whereas most examples remain robust via pi-stacking against the interlayered dislocation. Herein, we synthesize a phosphine-amine-linked 2D COF by a nucleophilic substitution reaction of phosphazene with amines. The synthesized COF is crystalline, and stacks in an AB-staggered fashion, wherein the AB dual layers are interlocked by embedding P-Cl bonds from one to another layer, and the non-interlocked layers are readily delaminated. Therefore, in situ post-quaternization over phosphazene can improve the ionization of backbones, accompanied by layered exfoliation. The ultrathin nanosheets can decouple lithium salts for fast solid-state ion transport, achieving a high conductivity and low activation energy. Our findings explore the P-N substitution reaction for COF crystallization and demonstrate that the staggered stacking 2D COFs are readily exfoliated for designing solid electrolytes. A phosphine-amine-linked two-dimensional COF crystallizes via a nucleophilic substitution reaction of phosphazene with amines, featuring an opposite staggered stacking of the interlocked dual layer in a unit cell. Post-quaternization on phosphazene moieties leads to the exfoliation of the staggered stacking COF into the few-layer ionized nanosheets, which can decouple lithium salts for rapid solid-state ion transport.+image

Automated summary

In this study, we synthesized a phosphine-amine-linked two-dimensional covalent organic framework (COF) through a phosphine-amine substitution reaction. The COF exhibited a staggered stacking structure and could be easily delaminated into few-layer ionized nanosheets for solid electrolyte design.