No abstract

Cyclic organic compounds with pentagon rings have been paid less attention for cathodes in lithium-ion batteries as compared with aromatic compounds. In this study, we investigate the Li-binding thermodynamics, redox properties, and theoretical performance for a selected set of heteroatom-containing, pentagon-shaped, organic compounds, namely borole, pyrrole, furan, phosphole, thiophene, and their derivatives to assess their potential for organic cathode materials. This investigation provides us with three important findings. First, the Li-binding thermodynamics and redox properties for the organic compounds would be systematically tailored by the type of the incorporated heteroatom and backbone length, exhibiting both the strongest Li-binding and the highest redox potential for borole. Second, it is highlighted that borole can store up to two Li atoms per molecule exhibiting the exceptionally high charge capacity (839 mA h/g) despite the absence of any well-known redox-active moieties (e.g., carbonyl). Third, dibenzothiophene exhibits weak and comparable Li-binding strengths at multiple feasible binding configurations with an indication of its low Li-storage capability, while the others dominantly bind with Li at their most stable binding configurations. All these findings will provide an insight into the guidelines for the systematic design of promising heterocyclic organic compounds (i.e., borole-based insoluble polymeric forms) for cathodes in secondary batteries.

A fundamental understanding of the effect of incorporating diverse carbonyl functionalities into well-known redox-active compounds, such as anthraquinone, is essential to develop organic cathode materials for rechargeable battery applications. However, there is a lack of studies on the relationship between the local environment of carbonyl functionalities and anthraquinone redox properties. In this study, anthraquinone derivatives bearing carbonyl functionalities with four different primary tails (i.e., −H, −F, −Cl, and −OH) are investigated using an advanced computational protocol. Based on the obtained insights into the impact of these carbonyl functionalities on the redox activity of anthraquinone, the following conclusions are drawn. First, the open-circuit redox potential can be tailored by varying the tail of the carbonyl functionality incorporated into anthraquinone, decreasing in the order of −F, −Cl > −H > −OH. Although this trend is weak, it indicates that a functionality tail with a stronger electron-withdrawing nature can lead to a slight improvement in the redox potential. Second, during the Li-involved discharging process, the Li-storage capability does not rely on the identity of the functionality tail, only on the presence of the redox-active fragment (i.e., carbonyl group). Nevertheless, this observation establishes a crucial design direction for optimizing the theoretical performance of anthraquinone. In particular, the carbonyl functionality with the lowest tail weight (−H) is considered to be optimal for anthraquinone functionalization, providing a theoretical charge capacity of 340 mA h/g and a theoretical energy density of 598 mW h/g. Finally, the observation of correlations with electron affinity and solvation energy implies that these key parameters contribute cooperatively to the electrochemical redox potential. These findings will promote the creation of rational design guidelines for carbonyl-based organic cathode materials for rechargeable batteries.

Replacing conventional inorganic cathode materials with organic compounds is environmentally and economically advantageous. As candidates for organic cathodes in lithium-ion batteries, heteroatom-incorporated crown-based compounds have distinctive structural and electronic properties. Herein, an advanced computational approach reveals that the coincorporation of S and Li into a B-crown compound creates a promising organic cathode with a drastically improved redox potential (4.74 V versus Li/Li+) and theoretical performances (289 mAh/g and 1097 mWh/g). This impressive enhancement originates from heteroatom-induced electron localization, which creates electron-deficient areas. In contrast, Li insertion into F- and Cl-incorporated B-crown compounds with exceptionally high redox potentials (∼5.18 V versus Li/Li+) is predicted to make the compounds electrochemically unsuitable as cathode materials due to the Li-induced cathodic deactivation. Further investigation unveils that this cathodic deactivation is induced by a sudden increase in solvation energy combined with a continuous increase in electron affinity during the discharging process. All of these findings can guide the design of high-performance lithium-ion battery cathodes using nonaromatic organic compounds without well-known redox-active sites.