A metal-organic compound designed with an extremely stable 3D openframework structure enables the aromatic rings in organic ligand to be activated as Na + storage redox sites. A large number of aromatic structures can be constructed into a 3D metal-organic framework for high-capacity electrode materials.

Although sodium-ion batteries (SIBs) are considered as alternatives to lithium-ion batteries (LIBs), the electrochemical performances, in particular the energy density, are much lower than LIBs. A metal-organic compound, cuprous 7,7,8,8-tetracyanoquinodimethane (CuTCNQ), is presented as a new kind of cathode material for SIBs. It consists of both cationic (Cu ↔Cu ) and anionic (TCNQ ↔TCNQ ↔ TCNQ ) reversible redox reactions, delivering a discharge capacity as high as 255 mAh g at a current density of 20 mA g . The synergistic effect of both redox-active metal cations and organic anions brings an electrochemical transfer of multiple electrons. The transformation of cupric ions to cuprous ions occurs at near 3.80 V vs. Na /Na, while the full reduction of TCNQ to TCNQ happens at 3.00-3.30 V. The remarkably high voltage is attributed to the strong inductive effect of the four cyano groups.

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