Unraveling Three‐Stage Discharging Behaviors of Bio‐Inspired Organic Cathode Materials

Abstract

Despite the creativity in designing materials based on bio-inspired organic compounds and their potential structural diversity, the incorporation of such materials into cathodes has attracted scarce attention, principally due to intrinsically weak redox activities. Herein, a large number of DNA/RNAinspired derivatives are systematically designed, and their electrochemical redox properties are explored with the aim of understanding structurepotential-performance relationship. Four striking conclusions can be drawn from this study. First, charging energy describing the 1st reduction step is a decisive parameter for the open-circuited adiabatic redox potentials of the compounds in the fully charged states, indicating that reorganization energy in the 2nd reduction step has a negligible impact. Second, both the charging and reorganization energies contribute cooperatively to the discharging potentials. Third, the compounds become cathodically inactive at the end of the discharging process owing to a sudden increase in solvation energy; thus, the compounds exhibit "three-stage discharging behavior". Fourth, the charge/energy-storage capability shows a critical dependence on Li binding mechanism, which is in turn correlated with the afore-mentioned core factors, leading to exceptional performance for a guanine derivative (1190 and 1586 mWh g −1 ). These findings will aid in advancing the development of bioinspired cathode materials for high-performance Li-ion batteries.

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

“…The structural distortion of organic compounds experienced by the sequential binding of Na ions becomes generally severe with the increase in the discharged states during the discharging process, resulting in a noticeable electron-induced structural reorganization . On the other hand, the charging energy gradually diminishes as the number of bound Na ions increases because of the Na-induced weakening of reducibility . This behavior enables the electron affinity to be balanced by the two key components: the charging and reorganization energies .…”

“…Similar correlations have been reported in previous studies. [15][16][17]20,21,27,29,44,45 Notably, the charging energy, associated with the first reduction step, is responsible for the linear correlation between the redox potential and electron affinity for these compounds, which implies that the structural reorganization triggered by the insertion of an electron is negligible and has a value close to zero (Figure S7). Interestingly, despite the typical circumstance of the HOMO energy depicting the oxidative ability of an organic compound, the more positive redox potentials strongly correlate with more negative values of the frontier HOMO energy (Pearson correlation coefficient = 0.7894, Figure S6b).…”

“…45 On the other hand, the charging energy gradually diminishes as the number of bound Na ions increases because of the Na-induced weakening of reducibility. 45 This behavior enables the electron affinity to be balanced by the two key components: the charging and reorganization energies. 45 Surprisingly, in this study, the contribution of the reorganization energy to the electron affinity of the TCNQ/F4TCNQ derivatives is consistently negligible throughout sequential Na binding, which contrasts with previous observations (Figure S8).…”

Learn from nature: Bio‐inspired structure design for lithium‐ion batteries

“…In addition, Kim et al designed DNA/RNA inspired derivatives for application as cathode materials in lithium-ion batteries, and then studied their structure-potential-performance relationships from the theoretical perspective. 59…”

“…The battery cells display average potentials of 2.58 V versus Li/Li + and initial discharge capacity of 148 mAh g −1 . In addition, Kim et al designed DNA/RNA inspired derivatives for application as cathode materials in lithium‐ion batteries, and then studied their structure–potential–performance relationships from the theoretical perspective 59 …”