Rechargeable Battery Reaction Mechanisms of Tribromotrioxotriangulene Crystal Investigated by Density Functional Theory Calculations

Abstract

Organic materials for lithium-ion battery (LIB) cathodes have attracted considerable attention owing to their high capacity and low cost, but they degrade rapidly owing to their high solubility in organic electrolytes. Electrode deterioration is explained by changes in the crystal structure during charge/discharge; however, the relationship between electrode cycle stability and electronic and crystallographic structure remains unclear because few studies have involved organic systems. Theoretical calculations can facilitate mechanistic understanding by clarifying the electronic structure changes in molecules and crystals. In this study, we used density functional theory to investigate the mechanisms underlying the excellent cycle durability of tribromotrioxotriangulene (Br3TOT). Based on the calculations, the two mechanisms by which Br3TOT crystals interact with guest cations (Li+ or Na+) were determined. In coordination mechanisms, guest cations bridge one-dimensional columns of Br3TOT via intermolecular interactions, enhancing electrode durability. Switching from a coordination mechanism to intercalation results in a high discharge capacity. The same mechanism was confirmed in sodium-ion batteries (SIBs) although Br3TOT exhibits a lower open-circuit voltage and larger volume expansion in SIBs than in LIBs.