Molecular Dynamics Study for Lithium Ion Diffusion in Layered LixCoO2 (x=0.5~1.0)

Diffusion constant of lithium ion is one of the most important properties in cathode materials for lithium ion battery. Various experimental and theoretical approaches have been studied to investigate the diffusion constant in LiCoO₂ [1-6]. Lithium vacancy ordering is essential phenomenon to determine the lithium ion dynamics [7, 8] and it is important to understand the relationship between the ordered phase generation and the diffusion constant. In theoretical approach, ab initio calculations using density functional theory (DFT) have been established to evaluate the lithium ion diffusivity in cathodes from lithium hopping model based on activation barrier [4-6]. However, DFT is not appropriate for large simulation including various vacancy configurations because of computational cost explosion. Alternatively, molecular dynamics (MD) is more suitable approach to simulate over ten thousand atoms and MD can directly simulate the lithium diffusion with effective force field. However, generic force field cannot support for variable lithium ion number in LiCoO₂ [9-12], because the force field development is difficult under the drastic electronic state changing of cobalt by lithium ion extraction/injection processes.

Here, we have improved the force field using embedded atom method to modify the total energies in accordance with amount of neighboring lithium ion. The force field parameters have been optimized to reproduce potential energy surfaces of several lithium diffusion paths estimated by DFT calculations. We also fit vacancy energies and elastic constants at several lithium configurations. In addition, ab initio MD results are used as a reference to reproduce finite temperature fluctuation. For validation, MD simulations in several temperatures were performed. The diffusion constants calculated from mean square displacement of lithium ion were good agreement with experimental results [1, 2] qualitatively. We will talk about microscopic picture of lithium vacancy ordering on that day.

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