Covalent Organic Framework Bipolar Pseudocapacitive Electrodes in an All-Organic Symmetric Lithium-Ion Battery

Covalent organic frameworks (COFs) have emerged as promising active materials for secondary-ion battery electrodes, owing to their robust porous structure and the flexibility in selecting redox-active building blocks. Here, a novel highly crystalline, electro-active, bipolar-type WTTF-COF, obtained by integrating p-type N,N,N′,N′-tetrakis(4-aminophenyl)-1,4-phenylenediamine (W) and 4,4′,4″,4′″-([2,2'-bi(1,3-dithiolylidene)]-4,4′,5,5′-tetrayl)tetrabenzaldehyde (TTF) molecular building blocks via n-type imine linkages, is reported, serving as a Li-ion battery electrode. In Li-ion half cells, WTTF-COF as a cathode features 12-electron dual-ion redox chemistry per unit cell within a stable, unusually wide potential window of 0.1–3.6 V versus Li/Li⁺, corresponding to a high theoretical capacity of 315 mAh g⁻¹, with an experimental reversible specific capacity of 271 mAh g⁻¹ at 0.1 A g⁻¹. The hybrid redox features coupled with the long-range ordered nanostructure of WTTF-COF enable an efficient pseudo-capacitive charge-storage mechanism. Different diffusion pathways and diffusion coefficients for Li⁺ and PF₆⁻ transport are established through detailed diffusion measurements and theoretical modeling. Among hybrid storage electrodes, WTTF-COF is reported to offer the option to serve as both anode and cathode up to a high rate of 200 mV s⁻¹, as demonstrated in fully organic symmetric cell tests. Summarizing, judiciously designed COFs are suitably established for efficient bipolar electrode applications.