Non-equilibrium kinetics for improving ionic conductivity in garnet solid electrolyte

Solid-state electrolytes (SSEs), as an essential component of all solid-state batteries, exhibit limited ionic conductivity. The fractional occupancy of Li⁺ ions, regulated by the doping of hetero-valent transition metals, is an important characteristic to enable high Li⁺ conductivity. However, the structural and kinetic mechanism of this is still unclear, preventing the rational design of higher-conductivity SSEs. Here, taking the typical garnet SSE Li_7-La₃Zr_2-TaO₁₂ (0≤ x ≤0.625) as an example, we revealed that a Ta⁵⁺-doping concentration of x = 0.25 leads to a high amount of non-equilibrium Li⁺ configurations in the form of [LiO₆]-[LiO₄]-[V_LiO₆]. Non-equilibrium configurations induce high off-center shifts and high electrostatic energies of Li⁺ ions, reducing the activation energy of Li⁺-ionic transport. As a result, the doping of hetero-valent ions has a great effect on Li⁺-ionic conductivity through controlling the amount of non-equilibrium Li⁺ ions. These findings provide important insight into the understanding of ionic transport and pave the way towards optimizing Li⁺ distribution to improve ionic conductivity.