First-principles study of lithium ion migration in lithium transition metal oxides with spinel structure

Abstract

The migration of lithium (Li) ions in electrode materials is an important factor affecting the rate performance of rechargeable Li ion batteries. We have examined Li migration in spinels LiMn₂O₄, LiCo₂O₄, and LiCo_1/16Mn_15/16O₄ by means of first-principles calculations based on density functional theory (DFT). The results showed that the trajectory of the Li jump was straight between the two adjacent Li ions for all of the three spinel compounds. However, there were significant differences in the energy profiles and the Li jump path for LiMn₂O₄ and LiCo₂O₄. For LiMn₂O₄ the highest energy barrier was in the middle of the two tetrahedral sites, or in the octahedral vacancy (16c). For LiCo₂O₄ the lowest energy was around the octahedral 16c site and the energy barrier was located at the bottleneck sites. The difference in the energy profile for LiCo₂O₄ stemmed from the charge disproportion of Co^3.5+ to Co³⁺/Co⁴⁺ caused by a Li vacancy forming and jumping, which was not observed for LiMn₂O₄. Charge disproportion successfully accounted for the faster Li migration mechanism observed in LiCo_1/16Mn_15/16O₄. Our computational results demonstrate the importance of the effect of charge distribution on the ion jump.