Al掺杂的尖晶石型LiMn2O4的结构和电子性质

采用基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)和GGA+U方法下对尖晶石型LiMn₂O₄及其Al掺杂 的尖晶石型LiAl_0.125Mn_1.875O₄晶体的结构和电子性质进行了计算. 结果表明: 采用GGA方法得到尖晶石型LiMn₂O₄是立方晶系结构, 其中的Mn离子为+3.5价, 无法解释它的Jahn-Teller 畸变. 给出的LiMn₂O₄能带结构特征也与实验结果不符. 而采用GGA+U方法得到在低温下的LiMn₂O₄和其掺杂 体系LiAl_0.125Mn_1.875O₄的晶体都是正交结构, 与实验一致. 也能明确地确定Mn的两种价态Mn³⁺/Mn⁴⁺的分布并且能够说明Mn³⁺O₆的z方向有明显的Jahn-Teller 畸变, 而Mn⁴⁺O₆则没有畸变. LiMn₂O₄的能带结构与实验比较也能够符合. 采用GGA+U方法对Al掺杂体系的LiAl_0.125Mn_1.875O₄的研究表明, 用Al替换一个Mn不会明显地改变晶体的电子性质, 但可以有效地消除Al³⁺O₆ 八面体的Jahn-Teller畸变, 从而改善正极材料LiMn₂O₄的性能, 这与电化学实验的观察结果相一致.

Structural and electronic properties of Al-doped spinel LiMn2O4

The structural and electronic properties of spinel LiMn₂O₄ and its Al doping system LiAl_0.125Mn_1.875O₄ are investigated within the density functional theory in both the generalized gradient approximation (GGA) and the GGA with Hubbard U correction (GGA+U). The results from the GGA method suggest that LiMn₂O₄ has a cubic structure and the valences of Mn ions are all +3.5, which is unable to explain the Jahn-Teller distortions in the material. The band structure of LiMn₂O₄ predicted by the GGA method is also inconsistent with experimental result. With the GGA+U method, the low temperature structures of LiMn₂O₄ and its Al doping system LiAl_0.125 Mn_1.875O₄ are shown to be orthogonal, the two different valence states of Mn, i.e., Mn³⁺/Mn⁴⁺ ions, are then determined, which is then able to explain the Jahn-Teller distortion in octahedron Mn³⁺O₆ and the non-existence of distortion in octahedron Mn⁴⁺O₆. These results are in good accordance with experimental data. Their band structures by GGA+U calculations are also consistent with experimental results. The GGA+U calculations on the LiAl_0.125Mn_1.875O₄ indicate that with the replacement of an Mn by Al, the crystal structure and electronic properties are not significantly changed, but the Jahn-Teller distortion in octahedron Al³⁺O₆ can be effectively eliminated, which could improve the performance of the anode materials based on LiMn₂O₄. The phenomenon is in consistent with the electrochemical experiments.