Enhancing the understanding of the redox properties of lithium-inserted anthraquinone derivatives by regulating molecular structure

Organic compounds, such as quinone compounds, are promising renewable electrode materials for lithium ion batteries (LIBs) with redox stability and structural diversity. However, quinone electrode materials (QEMs) have low redox potentials as cathode and high solubility. In particular, low redox potentials result in low energy density and power density of QEMs. In this work, the substituent influence on anthraquinone (AQ) are studied by density-functional theory calculations to predict redox properties of potential positive materials for LIBs. The calculated results indicate a positive correlation between the redox potentials and the number of substituent, and the full substitution groups with electron-withdrawing groups (EWGs) gain the highest redox potentials, which is borne out by lowest unoccupied molecular orbital (LUMO). Mono-substitution, however, bring the highest specific capacity. Also, the different substituent sites on AQ can influence the redox potentials of AQ. Moreover, the calculation of molecular electrostatic potential (MESP) reveal carbonyl groups with local minima of MESP easily tend to bind lithium. AQ with Li binding carbonyl groups is more stable than the bare AQ, which is confirmed by the calculation of nucleus-independent chemical shift. At the same time, lithium atoms also easily bind electronegative atoms of EWGs and form intramolecular lithium bonds, which improve the thermodynamic stabilization of the lithiation AQ. The present study provide the new understanding with guidelines to design potential organic cathode materials with efficiency for LIBs.