Al掺杂的尖晶石型LiMn_2O_4的结构和电子性质

Abstract

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lIMn_2O_4及其Al掺杂的尖晶石型lIAl_(0.125)Mn_(1.875)O_4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lIMn_2O_4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lIMn_O_4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lIMn_2O_4和其掺杂体系lIAl_(0.125)Mn_(1.875)O_4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Mn--3+/Mn--(4+)的分布并且能够说明Mn--3+O_6的z方向有明显的JAHn-TEllEr畸变,而Mn--(4+)O_6则没有畸变.lIMn_2O_4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_(0.125)Mn_(1.875)O_4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Al--3+O_6八面体的JAHn-TEllEr畸变,从而改善正极材料lIMn_2O_4的性能,这与电化学实验的观察结果相一致.

The structural and electronic properties of spinel LiMn_2O_4 and its Al doping system LiAl_(0.125)Mn_(1.875)O_4 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 LiMn_2O_4 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 LiMn_2O_4 predicted by the GGA method is also inconsistent with experimental result.With the GGA+U method,the low temperature structures of LiMn_2O_4 and its Al doping system LiAl_(0.125)Mn_(1.875)O_4 are shown to be orthogonal,the two different valence states of Mn,i.e.,Mn--(3+)/Mn--(4+) ions,are then determined,which is then able to explain the Jahn-Teller distortion in octahedron Mn--(3+)O_6 and the non-existence of distortion in octahedron Mn--(4+)O_6.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_(0.125)Mn_(1.875)O_4 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 Al--(3+)O_6 can be effectively eliminated,which could improve the performance of the anode materials based on LiMn_2O_4.The phenomenon is in consistent with the electrochemical experiments.