Intrinsic Defects in LiMn2O4: First-Principles Calculations

Abstract

Spinel LiMn₂O₄ has attracted wide attention due to its advantages of a high-voltage plateau, good capacity, environmental friendliness, and low cost. Due to different experimental synthesis methods and conditions, there are many intrinsic point defects in LiMn₂O₄. By means of first-principles calculations based on a reasonable magnetic configuration, we studied the formation energies, local structures, and charge compensation mechanism of intrinsic point defects in LiMn₂O₄. The formation energies of defects under the assumed O-rich equilibrium conditions were examined. It was found that O, Li, and Mn vacancies, Mn and Li antisites, and Li interstitial could appear in the lattice at some equilibrium conditions, but Mn interstitial is hard to form. The charge was compensated mainly by adjusting the oxidation state of Mn around the defect, except for the defects at the 8a Wyckoff site. The binding energies between point defects were calculated to shed light on the clustering of point defects. Furthermore, the diffusion of Li ions around the defects was discussed. Cation antisites led to a decrease of the Li diffusion barrier but O vacancy caused an increase of the barrier. This study provides theoretical support for understanding point defects in spinel LiMn₂O₄.

Figure 1

(a) Crystal and magnetic configurations of spinel-type LiMn₂O₄. Green spheres are Li, red spheres are O, yellow and purple spheres are Mn³⁺ and Mn⁴⁺, respectively. The blue arrows indicate the direction of the magnetic moment of Mn. The Mn ions are with AFM configuration in the Mn³⁺ layer and with FM configuration in the Mn⁴⁺ layer. (b) The structure of Mn⁴⁺-O₆ and Mn³⁺-O₆ octahedrons and two paths for Li migration that consist of different oxidation states of Mn ions. (c) Schematic representation of electron distribution in the 3d orbital of Mn ions.

Figure 2

Projected density of states (DOS) of Mn-3d and O-2p. The solid and dashed lines in the upper panel represent Mn-e_g and Mn-t_2g orbitals, respectively. The O-p_z (solid lines) and O-p_x,y orbitals (dashed lines) are shown in the lower panel. The zero of the energy is set to the Fermi level.

Figure 3

DOS of the e_g orbital of Mn around some point defects. The Fermi energy is set at zero.

Figure 4

Calculated diffusion barriers of Li along different paths. (a) Li diffusion in pristine LiMn₂O₄; (b) Li diffusion in LiMn₂O₄ with a Mn_Li defect; (c) Li diffusion in LiMn₂O₄ with a Li_Mn defect; and (d) Li diffusion in LiMn₂O₄ with V_O. The inset shows the six-membered ring in the diffusion paths. Orange, purple, and green balls represent Mn³⁺, Mn⁴⁺, and Li, respectively.