Effect of Rb and Ta Doping on the Ionic Conductivity and Stability of the Garnet Li_7+2x–y(La_3–xRb_x)(Zr_2–yTa_y)O₁₂ (0 ≤ x ≤ 0.375, 0 ≤ y ≤ 1) Superionic Conductor: A First Principles Investigation

Abstract

In this work, we investigated the effect of Rb and Ta doping on the ionic conductivity and stability of the garnet Li 7+2x−y (La 3−x Rb x )(Zr 2−y Ta y )O 12 (0 ≤ x ≤ 0.375, 0 ≤ y ≤ 1) superionic conductor using first principles calculations. Our results indicate that doping does not greatly alter the topology of the migration pathway, but instead acts primarily to change the lithium concentration. The structure with the lowest activation energy and highest room temperature conductivity is Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (E a = 19 meV, σ 300K = 1 × 10 −2 S cm −1 ). All Ta-doped structures have significantly higher ionic conductivity than the undoped cubic Li 7 La 3 Zr 2 O 12 (c-LLZO, E a = 24 meV, σ 300K = 2 × 10 −3 S cm −1 ). The Rb-doped structure with composition Li 7.25 La 2.875 Rb 0.125 Zr 2 O 12 has a lower activation energy than c-LLZO, but further Rb doping leads to a dramatic decrease in performance. We also examined the effect of changing the lattice parameter at fixed lithium concentration and found that a decrease in the lattice parameter leads to a rapid decline in Li + conductivity, whereas an expanded lattice offers only marginal improvement. This result suggests that doping with larger cations will not provide a significant enhancement in performance. Our results find higher conductivity and lower activation energy than is typically reported in the experimental literature, which suggests that there is room for improving the total conductivity in these promising materials.

Insights into the Lithium-Ion Conduction Mechanism of Garnet-Type Cubic Li5La3Ta2O12 by ab-Initio Calculations

“…These results are consistent with other simulation studies. 25,26,29 In the case of Ga atoms (for x = 0.30), their MSD starts to increase at 1200 K (Figure 5c). This onset temperature for Ga ion diffusion does not vary with respect to Ga content (see Figure S4 in the Supporting Information).…”

“…The trajectory of the x = 0 case is in strong agreement with a recent analysis of neutron powder diffraction data 62 as well as several simulation studies. [25][26][27]29 To make a clear comparison with the x = 0.30 case, we chose an appropriate slab section to expose the full 2D plane view of Li pathway connectivity, such as plane (3 6 4), which is shown in blue in Figure 10a and 10c. As can be seen, pathway features such as junctions formed by Td sites (LiO 4 ) and density lobes formed by Oh sites (LiO 6 ) can be observed in detail (Figure 10b, d).…”

“…28 Miara et al investigated the effect of Rb and Ta doping on the ionic conductivity and stability of the garnet Li 7+2x−y (La 3−x Rb x )-(Zr 2−y Ta y )O 12 (0 ≤ x ≤ 0.375, 0 ≤ y ≤1) using ab initio-based calculations and concluded that (i) Rb or Ta doping does not change the topology of the migration pathways significantly, but instead acts primarily to change the lithium concentration, and that (ii) doping with larger cations will not provide a significant enhancement in performance. 29 Bernstein et al has revealed by DFT and variable cell shape MD simulations that the strong dependence of the tetragonal phase stability on the simultaneous ordering of the Li + ions on the Li sublattice and a volume-preserving tetragonal distortion that relieves internal structural strain. 30 36 and Buschmann et al 37 concluded, based on their 27 Al MAS NMR spectroscopy results, that Al sits mostly in the 24d tetrahedral site.…”

Computational studies of solid-state alkali conduction in rechargeable alkali-ion batteries

“…In general, pentavalent (e.g., Ta 5+ , Nb 5+ ) ion doping on the Zr sites will stabilize the cubic phase and enhance the ionic conductivity. 109,113,114 Trivalent dopants (e.g., Ga 3+ ) on either the Li sublattice or La sites can also stabilize the cubic phase, 114,115 but very limited improvement in ionic conductivity is observed for La site doping. 114 Using AIMD simulations, Miara et al 113 further demonstrated that the ionic conductivity can be maximized by tuning the dopant (and hence Li) concentration in doped LLZO.…”

“…Nevertheless, there have been recent successes in computational dopant optimizations for solid electrolyte materials. 113,130 Despite significant progress, existing computational methods for probing alkali diffusion-ab initio and classical MDs, transition state theory and kinetic Monte Carlo (KMC)-all suffer from various limitations. Ab initio methods, such as NEB and AIMD, tend to be computationally expensive, whereas methods that depend on empirical or fitted potentials, such as classical MD and KMC, while much cheaper computationally, are difficult to transfer between chemistries and structures.…”