Redox-Active Planar Ge(IV)O₄ Linkers in Covalent Organic Frameworks for Enhanced Anodic Na⁺ Storage

Covalent organic frameworks (COFs) for ion storage usually suffer from the employment of inactive linking units and intrinsically lower conductivity than 10^-6 S cm^-1, resulting in significant specific capacity loss. Developing COFs with redox-active linkers in addition to the functional building blocks and highly intra-layer conjugated electronic structure for enhanced conductivity is therefore crucial toward enhancing ion storage capacity. Herein, two dimensional (2D) phthalocyanine-based (Pc-based) COFs, GeO₄-MPc-COFs (M = Co, Ni, and Zn), with redox-active Ge(IV)O₄ linkers and multiple active sites in the functional Pc building blocks were fabricated from octahydroxylphthalocyaninato metal complexes MPc(OH)₈ and GeO₂. The planar arrangement of Ge(IV)O₄ moieties induces significantly p-π interaction between Ge(IV)O₄ moieties and Pc macrocycles, facilitating the delocalization of π electrons throughout the 2D networks of GeO₄-MPc-COFs and resulting in an impressive conductivity of 0.14-0.36 × 10^-2 S cm^-1. This, in combination with the reversible redox activity of Ge(IV)O₄ linkers and N-rich Pc building blocks in the GeO₄-MPc-COFs, leads to outstanding anodic Na⁺ storage performance with a high reversible specific capacity (607 mA h g^-1 at 100 mA g^-1) and excellent cycling stability (only 0.00057% capacity decay per cycle during 4,000 charge-discharge cycles at 5 A g^-1), representing the thus far reported best performance.