Interfacial Strain Effects on Lithium Diffusion Pathways in the Spinel Solid Electrolyte Li-Doped MgAl₂O₄

The (Li,Al)-codoped magnesium spinel (LixMg1-2xAl2+xO4) is a solid lithium-ion electrolyte with potential use in all-solid-state lithium-ion batteries. The spinel structure means that interfaces with spinel electrodes, such as LiyMn2O4 and Li4+3zTi5O12, may be lattice matched, with potentially low interfacial resistances. Small lattice parameter differences across a lattice-matched interface are unavoidable, causing residual epitaxial strain. This strain potentially modifies lithium diffusion near the electrolyte-electrode interface, contributing to interfacial resistance. Here, we report a density functional theory study of strain effects on lithium diffusion pathways for (Li,Al)-codoped magnesium spinel, for xLi=0.25 and xLi=0.5. We have calculated diffusion profiles for the unstrained materials, and for isotropic and biaxial tensile strains of up to 6%, corresponding to 100 epitaxial interfaces with LiyMn2O4 and Li4+3zTi5O12. We find that isotropic tensile strain reduces lithium diffusion barriers by as much as 0.32eV, with typical barriers reduced by ∼0.1 eV. This effect is associated with increased volumes of transitional octahedral sites, and broadly follows qualitative changes in local electrostatic potentials. For biaxial (epitaxial) strain, which more closely approximates strain at a lattice-matched electrolyte-electrode interface, changes in octahedral site volumes and in lithium diffusion barriers are much smaller than under isotropic strain. Typical barriers are reduced by only ∼0.05 eV. Individual effects, however, depend on the pathway considered and the relative strain orientation. These results predict that isotropic strain strongly affects ionic conductivities in (Li,Al)-codoped magnesium spinel electrolytes, and that tensile strain is a potential route to enhanced lithium transport. For a lattice-matched interface with candidate spinel-structured electrodes, however, epitaxial strain has a small, but complex, effect on lithium diffusion barriers.