Modulation of Backbone Architecture to Design Structurally Durable Tetracyanoquinodimethane Derivatives with High Redox Activity

To improve the structural durability of macromolecular sodium-ion battery cathode materials, selected tetracyanoquinodimethane (TCNQ) and perfluorinated TCNQ (F4TCNQ) derivatives are designed with aromatic and cumulene backbones bridging the redox-active cyanide moieties. The electrochemical characteristics of these materials show that introduction of the bridging skeletons is an effective means of introducing structural bulkiness beneficial to cyclic stability without diminishing the electrochemical properties. The electrochemical and structural characteristics of TCNQ/F4TCNQ derivatives with bridging cumulenes are further highlighted to depend on whether the cumulene number is odd or even. The inductive effect increases with increasing bridging chain length only for even-numbered cumulenes, which leads to an increase in the intrinsic redox potential. However, the presence of electronically delocalized odd-numbered cumulene skeletons enables the preference of the sequential Na binding near the redox-active cyanides. Thus, the structural integrity of the bridging chains is sustained, thereby enhancing cyclic stability. These findings enable us to design a structurally durable [9]F4TCNQ unit, with an exceptionally high electrochemical performance resulting from the synergism between the electron-withdrawing characteristics of fluorine and the intrinsic nature of the odd-numbered cumulene skeleton.

Automated summary

In this study, TCNQ and F4TCNQ derivatives with aromatic and cumulene backbones were designed to improve the structural durability and electrochemical properties of macromolecular sodium-ion battery cathode materials. The presence of odd and even cumulene chains affected the electrochemical and structural characteristics. For even cumulenes, the inductive effect increased with increasing chain length, leading to an increase in the intrinsic redox potential. In the case of odd-numbered cumulene skeletons, the sequential Na binding near the redox-active cyanides was preferred, thereby enhancing cyclic stability. A structurally durable [9]F4TCNQ unit with exceptional electrochemical performance was designed by combining the electron-withdrawing characteristics of fluorine and the intrinsic nature of the odd-numbered cumulene skeleton.