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

Abstract

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.

Elucidating lithium-ion and proton dynamics in anti-perovskite solid electrolytes

“…[7][8][9][10][11] Although the high Li-ion conductivities of Li-rich antiperovskite materials have been known for decades, 12,13 it is only recently that significant interest has been generated for solid electrolyte applications. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Zhao and Daemen 14 reported high ionic conductivities (410 À3 S cm À1 at room temperature) and low activation energies (0.18-0.26 eV) for Li 3 OX-based compositions, where X = Cl or Br. However, subsequent reports have noted reduced Li-ion conductivities and increased activation barriers for these materials.…”

Large barocaloric effects in thermoelectric superionic materials

“…The β phase exhibits the well-known cubic fluorite structure found in many binary fast-ion conductors (e.g., CaF 2 and UO 2 ) [25,36], in which the Se ions are arranged according to a face-centered cubic lattice (space group F m3m) ( Fig.1a). Copper ions in the β phase diffuse throughout the crystal by hopping between tetrahedral sites and off-centered octahedral interstitial positions [16,37,38] (Fig.1b-c). The critical temperature of the α → β phase transition can be modified in practice with alloying and hydrostatic pressure as well [31].…”

Novel mechanocaloric materials for solid-state cooling applications

“…The normal to superionic phase transition in FIC has associated a very large change of entropy (~100-200 JK -1 kg -1 ) and can be modulated by external fields [90,91]. In particular, application of mechanical stresses has been proved as a very effective means to significantly lower or raise the superionic transition temperature in several archetypal FIC [92]. The physical causes behind these effects are the σ-induced lowering or increase in the kinetic energy barriers and formation energy of defects (e.g., Frenkel pairs) that intervene on ionic migration ( Figure 5) [92].…”

“…In particular, application of mechanical stresses has been proved as a very effective means to significantly lower or raise the superionic transition temperature in several archetypal FIC [92]. The physical causes behind these effects are the σ-induced lowering or increase in the kinetic energy barriers and formation energy of defects (e.g., Frenkel pairs) that intervene on ionic migration ( Figure 5) [92]. As we explain next, such a mechanical control over the ionic conductivity, and simultaneously over the entropy, in superionic materials presents great prospects in the context of mechanocaloric solid-state cooling ( Table 2).…”