First-Principle Studies Of Cathode Materials LiNi_0.5Mn_1.5O₄of Lithium Ion Batteries

Magnetic properties and crystal structures of LiMn₂O₄are investigated using thefirst-principles density functional theory taken into account with the on-site Coulombinteraction within the GGA+U scheme. Results indicate that the magnetic groundstate of LiMn₂O₄corresponds to an antiferromagnetic order with Mn chains along anyone of [110] direction. Jahn-Teller-type local atomic distortion of LiMn₂O₄is found tobe a type of mixed-valent compound containing a1:1mixture of Mn³⁺and Mn4+,which is in good agreement with the experimental conclusion.Magnetic properties and crystal structure of LiNi_0.5Mn_1.5O₄have beeninvestigated using the first-principles density functional theory within the GGA+Uscheme. Calculations indicate that the magnetic ground state of LiNi_0.5Mn_1.5O₄（Fd-3m and P4332） corresponds to a ferrimagnetic order. It is also demonstrated thatnonstoichiometric LiNi_0.5Mn_1.5O₄δhas the space group of Fd-3m and stoichiometricLiNi_0.5Mn_1.5O₄has the space group of P4332. In addition, we well explain thatLiNi_0.5Mn_1.5O₄δhaving the space group of Fd-3m shows higher electrochemicalconductivity than LiNi_0.5Mn_1.5O₄with the space group of P4332due to the existenceof O-vacancy in the former.Migration of small polarons in LiMn₂O₄and LiNi_0.5Mn_1.5O₄（Fd-3m and P4332）have been investigated using the first-principles density functional theory within theGGA+U scheme. Based on the initial and final states of small polarons, we calculatethe respective migration energy barriers of small polaron migration in LiMn₂O₄andLiNi_0.5Mn_1.5O₄. Results indicate that small polarons in LiMn₂O₄migrate betweenMn³⁺and Mn4+ions, whereas small polarons in LiNi_0.5Mn_1.5O₄only migrate betweenNi2+and Ni3+ions.Defect behaviors in LiMn₂O₄and LiNi_0.5Mn_1.5O₄（Fd-3m and P4332） have beeninvestigated using the first-principles density functional theory within the GGA+Uscheme. Results indicate the most energetically favorable defect in LiMn₂O₄is excess Li located in empty16c position and substituting for Mn in16d position, whereas inLiNi_0.5Mn_1.5O₄that is Li located in empty16c position and substituting for Ni in16dposition.