Effect of Lattice and Dopant-Induced Strain on the Conductivity of Solid Electrolytes: Application of the Elastic Dipole Method

Abstract

Here, we studied the possibility of applying the elastic dipole method (EDM) to predict the response of defect formation and migration energy to an external strain field (∊_ij) in Al-doped cubic Li_6:25Al_0:25La₃Zr₂O₁₂ (Al-LLZO) and Li₁₀GeP₂S₁₂ (LGPS). It is shown that EDM can quantitatively provide accurate values for Li-defect formation energy as a function of ∊_ij. EDM can also predict, qualitatively, how the migration barrier varies with∊_ij. In both Al-LLZO and LGPS systems, the formation energy of Li⁺ vacancy decreases (increases) by applying a tensile (compressive) strain, which is because the lattice parameters tend to expand by formation of a Li⁺ vacancy. An opposite behavior is found for the formation energy of interstitial Li⁺. Furthermore, we found that a compressive strain decreases the diffusion barrier in Al-LLZO, while it increases it in LGPS. The lowering of migration barrier in Al-LLZO is in spite of contraction of bottleneck width of Li diffusion in this system. This finding is in line with a recent experimental study. Analysis of EDM results shows that the lowering (rising) in the migration barrier of Li in Al-LLZO (LGPS) under a compressive strain is due to tendency of the system to contract (expand) Li–O (Li–S) bond lengths in the transition states where Li ions are at the bottlenecks of diffusion pathways. We finally show that the result of Li migration barrier as a function of strain in a non-doped solid electrolyte can be used to predict the global effect of substitution/doping on the conductivity of that system.

Keywords: Stress/Strain, Defect Formation, Li ion Migration, Solid Electrolytes, Elastic Dipole Method