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.

针对钠离子电池阴极用有机材料的有限信息已被视为设计具有潜力的候选者时必须克服的关键问题。本研究全面探讨了已知有机分子——芘四酮及其氮掺杂衍生物的氧化还原性质和理论性能。采用密度泛函理论建模方法，不仅研究了完全充电状态下的裸分子，还研究了不同数量和配置的配位Na原子所形成的分子，以描述不同放电状态的水平，从而理解放电过程中氧化还原性质的变化。该研究得出以下四个主要结论。首先，随着电子抽取型氮掺杂剂数量的增加，氧化还原电位升高，表明氮掺杂策略对氧化还原性质的有益影响。其次，随着氮掺杂剂数量的增加，钠存储能力降低，导致电荷容量减少，表明氮掺杂策略对电荷容量的负面影响。第三，通过研究能量密度进一步解释了关于氮掺杂策略效果的争议性结果，能量密度描述了氮掺杂剂对氧化还原电位和电荷容量的综合贡献。特别指出，随着氮掺杂剂数量的增加，能量密度将得到改善，对于含有四个氮掺杂剂的芘四酮，其能量密度表现出显著的高值（661.69 Wh/g）。这表明氮掺杂剂引起的氧化还原电位提高将超过电荷容量的损失。第四，研究还证实，氧化还原电位不仅强烈依赖于结构和电子性质，还依赖于溶剂化作用，强调了维持溶剂化能力在还原驱动下的改善以避免阴极失活的重要性。