Tailoring Electrochemical Redox Properties of Trioxotriangulene Organic Cathodes by a Heteroatom-Doped Strategy

Emerging organic electrodes have been regarded as one of the most prospective alternatives to the inorganic ones due to their advantages of environmentally friendliness, higher theoretical capacity, and fast redox kinetics. However, their lower redox potentials compared with those of inorganic materials plague their performance for practical lithium-ion batteries (LIBs) systems. Herein, to address this a problem, we designed a series of boron- and pyridine-type nitrogen-doped cathode materials based on two trioxotriangulene (TOT) radical derivatives with relatively high redox potentials: a tri-tert-butylated TOT derivative [(t-Bu)3TOT] and a tribrominated derivative (Br3TOT). Their electrochemical redox properties and theoretical performance parameters are explored by using density functional theory calculations. These comprehensive calculations reveal that two-boron- or nitrogen-doped strategies are helpful for improving the adiabatic redox potentials compared with the pristine (t-Bu)3TOT and Br3TOT in the fully charged states. Among the stably doped TOT derivatives in thermodynamics, Br3TOT-2B5 exhibits the highest specific capacity of 582 mAh g–1 and specific energy density of 792 mWh g–1. During the discharging process, the adiabatic redox potentials of the doped derivatives gradually decrease with increasing number of bound lithium atoms, which weakens their reductive ability. Further, the adiabatic redox potentials of these doped derivatives are strongly correlated with the lowest unoccupied molecular orbital and electron affinity in the fully charged states and the positive-potential range during the discharging process. The sudden increase in the electron affinity and solvation energy synergistically results in the loss of cathodic activity at the end of the discharging process. The charging energy plays a crucial role in the adiabatic redox potentials during the discharging process, while the reorganization energy only has a negligible influence. In addition, these derivatives exhibit poor solubility in the electrolyte solvent. We believe that the present results can provide a comprehensive rationale for systematically designing promising TOT-based cathode materials for LIBs.