First principles study of Li diffusion in ${\text{I-Li}}_2{\text{NiO}}_2$ structure

First principles computations have been used to study Li mobility in the orthorhombic ${\text{Li}}_2{\text{NiO}}_2$ structure with the $Immm$ space group $\left({\text{I-Li}}_2{\text{NiO}}_2\right)$. Understanding Li mobility in ${\text{I-Li}}_2{\text{NiO}}_2$ structure other than the conventional layered structure helps extend our understanding of Li transport in different oxide structures. Our results indicate that ${\text{I-Li}}_2{\text{NiO}}_2$ is a reasonably good lithium ionic conductor with two-dimensional diffusion when the structure is maintained upon lithiation or delithaion. It is predicted that in the orthorhombic cell the activation barriers along the $b$ axis and diagonal direction between $a$ and $b$ axes are fairly low, ensuring the facile lithium diffusion along those directions, while migration along the $a$ axis is unlikely given the very high activation barrier $\left(\sim 2\text{eV}\right)$.

Figure 1

(Color online) The orthorhombic ${\text{Li}}_2{\text{NiO}}_2$ with $Immm$ space group. Ni occupies the center of oxygen rectangle and Li occupies the tetrahedral sites. (Darker big circle: oxygen.)

Figure 2

Calculated energy along the different migration paths in Fig. 1a diffusion path B and Fig. 1b diffusion path D.

Figure 3

(Color online) The tetrahedral Li is shifted away from the Ni due to the electrostatic repulsion. Along path A, the Li goes through the oxygen triangle that faces toward Ni, while along B, it moves through the oxygen triangle that faces away from Ni.