First-Principles Studies Of Cathode Material LiCoO₂ Of Lithium Ion Batteries

The first-principles PAW pseudopotential method within the generalized gradient approximation (GGA), based on the density functional theory (DFT) is employed to investigate mainly the following two questions:1 , The geometry and electronic structure of bulk cathode material LiCoO₂ have been studied theoretically. The total density of states, partial density of states, band structure, and valance electron charge density contours has been shown. In this paper, three supercell models are compared with each other simply. On one hand, we want to obtain the theory lattice constant and electronic structure in order to comprehend further the whole information of bulk LiCoO₂. On the other hand, the PAW-GGA method will be justified, by comparing with the results attained by other theoretical or experimental methods. It is found that three supercell models are identical, and the results obtained by our calculations are in good agreement with other theoretical results and experimental data.2, Our first-principles calculations, for the first time, give the geometry and electronic structures of LiCoO₂ 2×2 (003) polar surfaces from theory. Furthermore, one possible reconstructed model has been put forward for the LiCoO₂ (003) polar surfaces, which refers to the future studies for the real LiCoO₂ surface shape. That is to say, the free energy of other configuration should be smaller than that of the model mentioned above. Like this, theoretical model will reflect rationally the real shape. In the paper, two ideal, unreconstructed surface models for LiCoO₂ (003) are considered. Our calculation shows that the two surface models have the characteristic of conductor, and that the model with oxygen termination on one side and lkhium termination on the other is the most stable relatively, which explains indirectly the phenomenon that LiCoO₂ coated by MgO possesses the more stable structure [J. Electrochem. Soc., 149 (4) A466-A471 (2002)]. In addition, the reconstructed model possesses the same character of semiconductor as the bulk, and the relaxation of atom geometry is helpful to the motion of Li ions during the intercalation/deintercalation process in Li- ion batteries.