Although sodium-ion batteries (SIBs) are considered as alternatives to lithium-ion batteries (LIBs), the electrochemical performances,i np articular the energy density,a re muchl ower than LIBs. cuprous 7,7,8,, is presented as an ew kind of cathode material for SIBs.I tc onsists of both cationic (Cu II $Cu I )a nd anionic (TCNQ 0 $TCNQ À $ TCNQ 2À )r eversible redox reactions,d elivering ad ischarge capacity as high as 255 mAh g À1 at ac urrent density of 20 mA g À1 .T he synergistic effect of both redox-active metal cations and organic anions brings an electrochemical transfer of multiple electrons.T he transformation of cupric ions to cuprous ions occurs at near 3.80 Vv s. Na + /Na, while the full reduction of TCNQ 0 to TCNQ À happens at 3.00-3.30 V. The remarkably high voltage is attributed to the strong inductive effect of the four cyano groups.Li-rich layered transition metal oxides, xLi 2 MnO 3 · (1Àx)LiMO 2 (M = transition-metal ions), exhibiting ah igh specific capacity above 250 mAh g À1 ,a re regarded as the potential next-generation cathode materials for LIBs with high energy density. [1][2][3] In contrast to the redox reaction of transition metal ions in conventional layered oxides,the extra capacity of the Li-rich counterpart originates from the partial redox of oxygen anions. [4,5] Theu tilization of more complete oxygen species redox reaction also gives birth to lithium-air batteries with the highest energy density in the Li-ionshuttled batteries. [6,7] Sodium-ion batteries (SIBs) are accepted as low-cost, promising alternatives to LIBs,e specially in the large-scale energy storage applications. [8] How-ever the overall performances of SIBs are inferior to their Li analogues,a nd particularly the energy density,w hich is usually limited by the cathode materials.M any strategies have been proposed to enhance the cathode capacity. [9] However,a ll these efforts are limited by as pecific energy density of 680 Wh kg À1 ,w hich is far from the expectation of practical application. Inspired by the oxygen anion redox reactions in LIBs,w es uppose that taking use of the redox reactions of metallic cations and non-metallic anions simultaneously would realize the transfer reaction of multiple electrons and hence greatly increase the energy density of SIBs.H erein, our attempt of such strategy starts with ac onventional metal organic compound, CuTCNQ,w hich is composed of cuprous ions coordinated by TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) ligands.TCNQ is aredoxactive compound via at wo-electron reversible reaction. [10][11][12] It is expected that the TCNQ À anions would participate in the electron transfer during the electrochemical redox reaction, while Cu ions also show redox activity in some metal-organic frameworks. [13] Recently,m etal-organic compounds have been investigated as cathode materials for LIBs.H owever, those materials suffer from poor electrochemical performance when applied in batteries. [13][14][15] Fortunately,i no ur work, we find that CuTCNQ as SIB cathode undergoes an electro...