Behavior of Li in LiMn₂O₄ Structure: Molecular Dynamics Study

Abstract

Molecular dynamics (MD) was used to study ionic conduction processes in mixed valence compound LiMn₂O₄. The evolution of three different Mn valence distribution models was examined. For two models, I and II, the ideal spinel structure was adopted with randomly assigned valences. For model II the Mn valences were periodically randomly altered while preserving overall charge neutrality. In the third simulation, III, a charge ordered Mn valence model resembling the observed low temperature LiMn₂O₄ phase was adopted. Model I reproduces structural characteristics of the ideal spinel well, exhibiting modest Li ion self-diffusion. In model II the Li diffusion was greatly enhanced and in the charge ordered model III almost no diffusion was observed at all. Model II therefore reproduces the observed physical properties of the high temperature cubic spinel phase best, strongly supporting a Mn valence electron hopping model. Structural distortion indices were also calculated confirming that Li diffusion from the ideal tetrahedral LiO₄ environment is strongly associated with tetrahedral distortion.