Graphdiyne and Hydrogen-Substituted Graphdiyne as Potential Cathode Materials for High-Capacity Aluminum-Ion Batteries

In chloroaluminate ion based aluminum-ion batteries, a graphite cathode shows promising electrochemical performance. However, its low capacity and high volume expansion is a major hindrance. This is because of the limited intercalation of the AlCl4– ion into the graphite layers. As a possible solution to this, graphdiyne (GDY) and hydrogen-substituted graphdiyne (HsGDY), two-dimensional carbon allotropes with large pores, are tested here as potential cathode materials by density functional theory calculations. We find that both materials exhibit better performance than graphite, in terms of easiness of AlCl4 intercalation, voltage profile, and storage capacity. To ensure AlCl4 intercalation during charging, the cathode materials must expand to accommodate the AlCl4. The energy required for the expansion is defined as the distortion energy. Both GDY and HsGDY require a lower distortion energy (0.006 and 0.003 eV/Å2, respectively) than graphite (0.014 eV/Å2) meaning it will be easier for the AlCl4 to intercalate into the cathode during charging. The average voltage for the AlCl4 intercalated single layer of GDY and HsGDY is around 2.15 V, which is higher than that of graphite (2 V). Finally, the storage capacity of the bilayer system of GDY and HsGDY based on stage-1 storage (AlCl4 intercalation between every layer of the cathode material) is 124 and 456 mA h g–1, respectively; whereas a bilayer graphene system has a capacity of 155 mA h g–1. Therefore, HsGDY has a significantly higher storage capacity than graphite.