Ion diffusion mechanism in Pn NaxLi2−xMnSiO4

Abstract

Based on first-principles calculations, we investigated ion diffusion in Na_xLi_2−xMnSiO₄ (x = 2, 1, 0) compounds with a Pn space group. The ion diffusion in the Na₂MnSiO₄ and Li₂MnSiO₄ structures was studied and compared with each other. In Na₂MnSiO₄, the Na⁺ ions migrate through 3D diffusion channels, similar to the Li⁺ migration in Li₂MnSiO₄, but with relatively lower activation barriers. The enhanced Na⁺ conductivity could be attributed to the wider ionic diffusion channels in Na₂MnSiO₄ than in Li₂MnSiO₄. In the NaLiMnSiO₄ structure, the Na⁺ and Li⁺ ions are both demonstrated to be mobile. Thus, the ionic conduction in NaLiMnSiO₄ is thought to be governed by a mixed Li⁺/Na⁺ diffusion mechanism. We found that the mixed Li⁺/Na⁺ diffusion in NaLiMnSiO₄ can also be fast, even faster than the Li⁺ diffusion in Li₂MnSiO₄.