Skeletal Nitrogen Functionalization of Isostructural 2D Conjugated MOFs for Enhancement of the Dual-Ion Storage Capacity.

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) are emerging as promising electrode materials for electrochemical energy storage devices. However, a viable path to realize superior dual-ion storage in 2D c-MOFs has remained elusive. Here, we report the synthesis of Cu(N-OHPTP) 2D c-MOFs (x=0,1,2; OHPTP=octahydroxyphenanthrotriphenylene) with precise aromatic carbon-nitrogen arrangements, based on the π-conjugated OHPTP ligand incorporated with one or two nitrogen atoms. The skeletal nitrogen modification in Cu(N-OHPTP) allows the synergistic introduction of additional redox sites, and thus substantially favors the unique dual-ion adsorption capacity. Consequently, the Cu(N-OHPTP) cathode exhibits a largely enhanced electrochemical performance for dual-ion storage (i.e., Li and Cl) with a high specific capacity of 53.8 mAh g, which is twice that of Cu(N-OHPTP) and 1.3 times that of Cu(N-OHPTP). Furthermore, the Cu(N-OHPTP) electrode displays a favorable rate performance of 52 % and good cycling stability of 96 % after 1000 cycles. We identify N-centered redox sites as additional Li adsorption sites by combining ex situ and in situ spectroscopy measurements and theoretical calculations. In addition, calculations underline the synergistic enhancement of the Cl adsorption energy by about 1.0 eV at the more electron-poor CuO linkages after N-incorporation. This work paves the way for the precise design of 2D c-MOFs with superior electrochemical properties, advancing their application in dual-ion storage applications.