Improving the Understanding of the Redox Properties of Fluoranil Derivatives for Cathodes in Sodium‐Ion Batteries

Abstract

Despite the potential of organic cathodes in sodium‐ion batteries, their redox properties still need to be explored. In this study, a density functional theory modeling approach is employed to comprehensively investigate the redox properties and theoretical performance parameters for a selected set of fluoranil derivatives as cathode materials. The redox properties are further correlated with various characteristics including structural variations, electronic properties, and solvation. Three primary conclusions are drawn. First, the incorporation of bulky trifluoromethyl functional group(s) into fluoranil increases its redox potential but significantly decreases its gravimetric charge capacity. This suggests that the trifluoromethyl functional group(s) would be detrimental to the design of high‐performance batteries. Second, fluoranil exhibits significant enhancements in terms of redox properties and theoretical performance compared with its hydrogenated form, benzoquinone, suggesting a desired strategy for designing high‐performance batteries. Third, the redox properties of fluoranil derivatives would strongly rely not only on structural variations (e.g., bulkiness) and electronic properties (e.g., functionality) but also on solvation energy. It is further verified that cathodic deactivation could be completed by solvation energy. The new understanding will provide us with guidelines for an efficient design of promising organic cathode materials.

Electrochemical Characteristics of Cyanoquinones as Organic Cathodes for High-Potential Sodium-Ion Batteries

“…The three isomers with two cyano functional groups at different positions, namely, 2CN-BQ- n where n = 1, 2, and 3, have the redox potentials ranged within 3.61–3.74 V versus Na/Na + , indicating the negligible effect of the configuration of the functional groups on the redox potential. Similar observations have been reported in previous studies. ,− Note that a slightly higher redox potential by 0.13 V is predicted for 2CN-BQ-1, the one with two cyano functional groups at the nearest neighboring position, as compared with the others (Figure b). In a previous study, Kim et al also reported a similar observation: the carbonyl functional groups locally concentrated at the edge of the graphene flake would be more beneficial to the improvement of the redox potential than uniformly distributed carbonyl functional groups …”

“…∑ is the sum of the Boltzmann factors for all the three isomers. 39,48 ■ RESULTS AND DISCUSSION Seven chloranil derivatives with varied numbers of cyano functional groups, namely, tetrachloro-1,4-benzoquinone (BQ), 2-cyano-3,5,6-trichloro-1,4-benzoquinone (1CN-BQ), 2,3-dicyano-5,6-dichloro-1,4-benzoquinone (2CN-BQ-1), 2,6dicyano-3,5-dichloro-1,4-benzoquinone (2CN-BQ-2), 2,5-dicyano-3,6-dichloro-1,4-benzoquinone (2CN-BQ-3), 2,3,5-tricyano-6-chloro-1,4-benzoquinone (3CN-BQ), and tetracyano-1,4-benzoquinone (4CN-BQ), were systematically designed, as shown in Figure 1. Note that quinone derivatives with redoxactive carbonyls have been highlighted to be attractive cathode materials for lithium-ion batteries.…”

“…54 The Boltzmann factor was introduced to further understand the relative thermodynamic stability of the three isomers with two cyano functional groups at different positions. 39,48,51 Figure 3 shows the relative thermodynamic stability derived from the calculated Boltzmann factor. It is clearly seen that 2CN-BQ-1 shows the highest stability among the three isomers, exhibiting the thermodynamic probability approaching 100%.…”

“…Kim et al employed the density functional theory (DFT) modeling approach to comprehensively explore the redox properties and theoretical performances for a series of quinone derivatives for organic cathodes in sodium-ion batteries . They highlighted that the redox potential of a quinone would be cooperatively contributed by electron affinity and solvation. − However, despite these breakthroughs in the cell performance, a significant enhancement in the redox potentials of organic materials is still needed for practical applications.…”

Unexpected Electrochemical Behavior of Crown-Based Organic Compounds for Lithium-Ion Battery Cathodes

“…Our previous studies have highlighted that structure-derived variations in the redox properties would originate from two primary factors: (1) electronic characteristics and (2) solvation. [27][28][29][30]46 Figure 5 shows the correlations between the redox potentials and four main electronic characteristics (electron affinity, the lowest unoccupied molecular orbital (LUMO), the highest occupied molecular orbital (HOMO), and HOMO−LUMO gap).…”