Lithium-ion batteries (LIBs) have attracted considerable attention for use in larger power sources like electric vehicles and energy storage system because of their high energy densities. In the LIB, carbon materials, such as highly oriented pyrolytic graphite (HOPG), are used as negative electrode. During the charge and discharge cycle, Li-ions are intercalated to the graphite. For the Li- ions intercalation into the layered graphite, phase transition between different staging structures occurs. At first, Li-ions are intercalated randomly to whole layers, called dilute stage1. Next, stage transformation between dilute stage 1 and stage 4, stage 4 and stage 2, stage 2 and stage 1 proceed. This stage transformation plays an important role for the LIB performance, especially the maximum capacity and charge discharge rate of batteries. Regarding atomic scale mechanism of the stage transformation, Daumas and Hérold model was proposed. In the model, fully occupied and empty region of Li-ions in the layers exist, but the layers can be distorted to localize the Li-ion occupied region. This layer distortion is the key for the stage transformation. To clarify this mechanism from the view point of the first-principles study, large-scale calculations is required.

In this study, we investigate the atomic and electronic structures of Li-intercalated graphite by using our large-scale DFT program CONQUEST [1]. The CONQUEST code uses real-space local orbital functions to express density matrices instead of the plane-wave basis. Especially, recently-developed “multi-site method” [2] reduces the number of local orbital basis functions to be minimal size keeping high accuracy, which reduces the computational cost dramatically and enables us to treat large systems containing 10,000+ atoms. The comparison of the geometries and electronic structures of Li-intercalated graphite at several stage structures with several thousand atoms (as in figure 1) will be discussed in the presentation.

[1] http://www.order-n.org/

[2] A. Nakata, D. R. Bowler, T. Miyazaki, Phys. Chem. Chem. Phys. 17, 31427 (2015).

Figure 1. Large-scale simulation model of Li-intercalated graphite (stage1).