The staging mechanism of AlCl₄ intercalation in a graphite electrode for an aluminium-ion battery

Abstract

Identifying a suitable electrode material with desirable electrochemical properties remains a primary challenge for rechargeable Al-ion batteries. Recently an ultrafast rechargeable Al-ion battery was reported with high charge/discharge rate, (relatively) high discharge voltage and high capacity that uses a graphite-based cathode. Using calculations from first-principles, we have investigated the staging mechanism of AlCl₄ intercalation into bulk graphite and evaluated the stability, specific capacity and voltage profile of AlCl₄ intercalated compounds. Ab initio molecular dynamics is performed to investigate the thermal stability of AlCl₄ intercalated graphite structures. Our voltage profiles show that the first AlCl₄ intercalation step could be a more sluggish step than the successive intercalation steps. However, the diffusion of AlCl₄ is very fast in the expanded graphite host layers with a diffusion barrier of ∼0.01 eV, which justifies the ultrafast charging rate of a graphite based Al-ion battery. And such an AlCl₄ intercalated battery provides an average voltage of 2.01-2.3 V with a maximum specific capacity of 69.62 mA h g^-1, which is excellent for anion intercalated batteries. Our density of states and Bader charge analysis shows that the AlCl₄ intercalation into the bulk graphite is a charging process. Hence, we believe that our present study will be helpful in understanding the staging mechanism of AlCl₄ intercalation into graphite-like layered electrodes for Al-ion batteries, thus encouraging further experimental work.