First-Principles Study Of Graphene For Potential Anode Materials Of Lithium Ion Batteries

Mechanims on lithium intercalated graphite and lithium storage on graphene for potential anode material of lithium ion battery have been investigated using the first-principles calculations. They mainly include the geometry structure and potential platform of lithium intercalated graphite, the variation of electronic structures of the lithium-graphene system, the behavior of lithium storage in graphene cells with different sizes, the evolution of the stacking structure of bilayer graphene system change associated with the intercalation of Li, and the effect of a carbon defect on bilayer graphene on lithium storage. Conclusions can be summarized as follows:1. Detailed studies for the geometry structure and potential platform of lithium intercalated graphite for anode material of lithium ion battery have been carried out using the first-principles calculations based on density functional theory. Results show that the intercalation configuration with lithium in the center of hexagon ring is energetically favorable, only causing the interlayer separation of graphite. Calculated potential platforms of lithium intercalated graphite (LIC) in first and second stages are 0.081V and 0.138V, which are in reasonable agreement with the experimental values of 0.085 V and 0.100 V.2. Detailed studies for lithium adsorption on graphene have been carried out using the first-principles calculations. Results show that the Li-2s states are degenerate upon lithium adsorption on graphene and the emptyπ*-bands of graphene become occupied and eventually get distorted, suggesting that charge is transferred from the adsorbed lithium atom to the graphene substrate and occupation of distortedπ*-bands gives rise to metallization of semimetallic graphene sheets. Simultaneously, it is found that lithium adsorption can be affected by the cell sizes of graphene. By comparing the lithium adsorption behaviors for the ( 3×3), (2×2) and (4×4) patterns, we find that the (2×2) pattern is the most favorable for the Li storage and release.3. Detailed studies for lithium storage on bilayer graphene, which involve the the respective effects of lithium intercalation on structure of perfect bilayer graphene and an isolated C atom defect of bilayer graphene on lithium storage capacity, have been carried out using the first-principles calculations. Results show that there exist AB and AA stacking sequences for bilayer graphene in which the latter is more favorable for the lithium storage and the former will be evolved into the latter with the intercalation of lithium ions. The relationship between the interlayer distance of two graphene sheets and the intercalated capacity of lithium ions is discussed. As for defective bilayer graphene, an isolated C atom defect can capture six lithium ions, thus enhancing the lithium storage capacity.