Electrochemical Energy Storage Capability of Pyrenetetrone Derivatives Tailored by Nitrogen Dopants

Limited information on organic materials for cathodes in sodium-ion batteries has been addressed as a critical issue to be overcome for designing promising candidates. Herein, we comprehensively investigate the redox properties and theoretical performances for a well-known organic molecule, pyrenetetrone, and its nitrogen-doped derivatives. The density functional theory modeling approach is employed to study not only bare molecules at fully charged states but also molecules with different numbers and configurations of bound Na atoms, which describe different levels of discharged states, to understand the change in the redox properties during the discharging process. This investigation draws four primary conclusions. First, the redox potential increases with the increasing number of electron-withdrawing nitrogen dopants, indicating the beneficial effect of the nitrogen-doping strategy on the redox properties. Second, the Na storage capability decreases with the increasing number of nitrogen dopants, leading to the reduction in the charge capacity and indicating the negative effect of the nitrogen-doping strategy on the charge capacity. Third, this controversial result on the effect of the nitrogen-doping strategy is further explained by the investigation of the energy density, which describes a combined contribution of the nitrogen dopants to redox potential and charge capacity. It is highlighted that the energy density would be improved with the number of nitrogen dopants, exhibiting the remarkably high value (661.69 W h/g) for pyrenetetrone with four nitrogen dopants. This suggests that the nitrogen dopant-induced improvement of redox potential would overcompensate for the penalty in charge capacity. Fourth, it is also verified that redox potential would strongly rely not only on the structural and electronic properties but also on solvation, emphasizing the importance of sustaining the reduction-driven improvement of solvation capability to avoid the cathodic deactivation.