Stress-Mediated Enhancement of Ionic Conductivity in Fast-Ion Conductors

Finding solid-state electrolytes with high ionic conductivity near room temperature is an important prerequisite for developing all-solid-state electrochemical batteries. Here, we investigate the effects of point defects (vacancies) and biaxial stress on the superionic properties of fast-ion conductors (represented by the archetypal compounds CaF₂, Li-rich antiperovskite Li₃OCl, and AgI) by using classical molecular dynamics and first-principles simulation methods. We find that the critical superionic temperature of all analyzed families of fast-ion conductors can be reduced by several hundreds of degrees through the application of relatively small biaxial stresses (|σ| ≤ 1 GPa) on slightly defective samples (c_v ∼ 1%). In AgI, we show that superionicity can be triggered at room temperature by applying a moderate compressive biaxial stress of ∼1 GPa. In this case, we reveal the existence of a σ-induced order-disorder phase transition involving sizable displacements of all the ions with respect to the equilibrium lattice that occurs prior to the stabilization of the superionic state. In CaF₂ and Li₃OCl, by contrast, we find that tensile biaxial stress (σ < 0) favors ionic conductivity as due to an effective increase of the volume available to interstitial ions, which lowers the formation energy of Frenkel pair defects. Our findings provide valuable microscopic insight into the behavior of fast-ion conductors under mechanical constraints, showing that biaxial stress (or, conversely, epitaxial strain) can be used as an effective means to enhance ionic conductivity.