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

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 LiCo1/16Mn15/16O4 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 Co3.5+ 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 LiCo1/16Mn15/16O4. Our computational results demonstrate the importance of the effect of charge distribution on the ion jump.