Ab initio modeling of realistic tetrachloroaluminate intercalation into van der Waals electrode materials

Abstract

Aluminum-ion batteries (AIBs) have recently attracted much attention due to their very fast charging and discharging speeds and excellent cycle stability. Unlike lithium-ion batteries, AIBs can be designed using a variety of ions, and the operating mechanism of ion intercalation in actual battery systems has not been fully clarified. Among them, the AIB based on tetrachloroaluminate(AlCl<sub>4</sub><sup>−</sup>) ion has the fastest charging and discharging speed and more than 8,000 cycles of cycle stability. However, due to the large size of the AlCl<sub>4</sub><sup>−</sup> ion (∼ 5.28 Å), there has been controversy over its actual intercalation/deintercalation behavior and it is difficult to explain the characteristics of AIBs with very fast ion exchange behavior and excellent cycle stability. Theoretical studies using first-principles calculations have reported that the most stable structure when AlCl<sub>4</sub><sup>−</sup> ion is inserted into graphite is that the ion intercalation gallery of graphite should be extended to∼9 Å. However, this is in contradiction with the experimental observation results (∼5.7 Å). In this paper, we solved this discrepancy between theory and experiment, and proposed a first-principles calculation-based computational simulation model that considers more realistic ion intercalation conditions. We found that the operating mechanism of AlCl<sub>4</sub><sup>−</sup> ion intercalation can vary depending on the range of expansion of the out-of-plane lattice constant of the graphite structure. In other words, when the distance between the ion insertion galleries is physically constrained and a deformation force exists, AlCl<sub>4</sub><sup>−</sup> ion is stabilized to have a flat planar shape, which is in good agreement with the experimental observation results. We also applied the computational simulation model that considers this behavior to explain the AlCl<sub>4</sub><sup>−</sup> ion intercalation mechanism in 2D van der Waals electrode materials with confined out-of-plane lattice lengths, and predicted the battery performance of aluminum-ion battery electrodes in hetero-structures where different 2D materials form interfaces.