First-principles study of the lithium interaction with polycyclic aromatichydrocarbons

We have performed first-principles calculations in order to understand the binding mechanism of Li atoms in disordered carbon materials that are used for negative electrodes of rechargeable lithium batteries. We used pyrene, anthracene, and phenanthrene molecules as parts of disordered carbon. We ex amined several binding sites for two Li atoms in these aromatics and found that they are bound with substantial negative binding energies. The most ne gative one was - 142.8 kJ/mol for Li-containing pyrenes, -211.0 kJ/mol for anthracenes, and -146.2 kJ/mol for phenanthrenes at the B3LYP/6-31G*//HF/6- 31G* level of calculation. Li atoms are bound to interstitial (ring-over) a nd edge sites. In addition to these binding mechanisms, we found that Li at oms could be bound, forming a Li dimer in anthracene and phenanthrene. Thei r binding energies are -200.5 and -146.2 kJ/mol, respectively, being larger in magnitude than Li-2 dissociation energy. These aromatics lose their pla narity when they accommodate Li atoms. We found that larger distortion brin gs more strong interaction between the aromatics and Li atoms. The amount o f energy required for the distortion increases in the order the interstitia l, edge, and Li-dimerized sites. The highest occupied molecular orbital ene rgy, which is closely related to the electrode potential during discharge p rocess, decreases in that order. This energy lowering may be related to the origin of the hysteresis observed during the charge/discharge cycles.