A Metal-Organic Compound as Cathode Material with Superhigh Capacity Achieved by Reversible Cationic and Anionic Redox Chemistry for High-Energy Sodium-Ion Batteries

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-II <-> Cu-I) and anionic (TCNQ(0)<-> TCNQ(-)<-> TCNQ(2-)) reversible redox reactions, delivering a discharge capacity as high as 255 mAhg(-1) at a current density of 20 mAg(-1). 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(0) 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.