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 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 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 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 (0.64 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.