Na+替位掺杂对Li2MnSiO4的电子结构以及Li+迁移过程的影响

在锂二次电池中, 硅酸锰锂作为正极材料得到广泛研究, 但其固有的电子和离子电导率较低, 直接影响着电池的功率密度和充放电速率. 本文建立了不同浓度的Na⁺离子替位掺杂Li⁺离子形成的Li_1-xNa_xMnSiO₄(x=0, 0.125, 0.25, 0.5)结构, 采用第一性原理的方法, 研究了掺杂前后硅酸锰锂的电子结构以及Li⁺离子的跃迁势垒. 发现在Li⁺位替代掺杂Na⁺, 导带底的能级向低能方向发生移动, 降低了Li₂MnSiO₄ 材料的禁带宽度, 有利于提升材料的电子导电性能. 随着掺杂浓度的升高, 禁带宽度逐渐变窄. CI-NEB结果表明, 在Li₂MnSiO₄体系中具有两条有效的Li⁺离子迁移通道, 掺杂Na⁺以后扩大了Li⁺ 离子在100]晶向上的迁移通道, Li⁺离子的跃迁势垒由0.64 eV降低为0.48, 0.52和0.55 eV. 掺杂浓度为 x=0.125时, 离子迁移效果最佳. 研究表明Na⁺掺杂有利于提高Li₂MnSiO₄材料的离子和电子电导率.

Effect of Na substitution on the electronic structure and ion diffusion in Li2MnSiO4

With the developments of electric vehicles, the portable electronics and the large-scale storage systems, the research of the Li-ion rechargeable battery has focused on its high gravimetric and volumetric capacity. As a potential cathode, the Li₂MnSiO₄ structure has been intensively studied, in which two lithium ions of per formula unit (f.u.) can be extracted, and it exhibits a high theoretical capacity of about 330 mAh/g. However the low intrinsic electron conductivity and the slow lithium diffusion prevent its further development. In this paper, we build three structures with different Na⁺ doping concentrations in Pmn21 symmetric Li₂MnSiO₄, the electronic properties and Li⁺ ion diffusion behavior are studied by using the first principle and considering the transition barrier of the Mn-3d. Within the GGA+U scheme, the pure Li₂MnSiO₄ structure is semiconducting with a large band gap (3.28 eV), which is primarily derived from Mn-3d and O-2p states. Because lithium and sodium ions in the same main group have similar chemical properties, all the doped Li_2-xNa_xMnSiO₄ (x= 0.125, 0.25, 0.5) are still semiconducting with the analogous densities of state (DOSs) to the pure Li₂MnSiO₄, however the band gaps reduce to 3.23 eV, 3.19 eV and 3.08 eV, respectively. Thus Na⁺ substitution can improve the electron conductivity. In Li₂MnSiO₄, the Li⁺ ions have two major diffusion channels predicted by the climbing image-nudged elastic band (CI-NEB) method. Channel A is along the a-direction 100], and channel B is in the bc plane with a zigzag trajectory. In the migration process, each of all the structures has only one migration pathway of Li ions. In the doped structures, the volumes of the crystal structures are increased by 1.40%, 2.65% and 5.25% for Li_2-xNa_xMnSiO₄ (x= 0.125, 0.25, 0.5), and thus enlarge the hopping distances. Along channel A, the longer Li-O bond makes the ionic diffusion channel wider, therefore Li_2-xNa_xMnSiO₄ (x= 0.125, 0.25, 0.5) have lower activation barriers of 0.48, 0.52 and 0.55 eV than the pure Li₂MnSiO₄ (0.64 eV). However, in channel B, the strong Li-O bonds increase the activation barriers of Li ion migration. When the doping concentration is x=0.125, the Li⁺ ion migration effect is strongest. For the Li⁺ ion migration pathways, it is easier for Li ion to hop into the site near Na ion. It means that the crystal structures are stabler at the short Li-O bond site. Therefore, doping Na⁺ ions would be a feasible method to improve the electron conductivity and Li⁺ ion migration rate in Li₂MnSiO₄ of Pmn21 phase.