中文摘要：

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Li1-xNaxMnSiO4（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.

英文摘要：

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 Li2-xNaxMnSiO4（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 α-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 Li2-xNaxMnSiO4（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 Li2-xNaxMnSiO4（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.