Ionic conductivity of cation electrolytes from KMC simulations

Methods

Electronic structure calculations based on density functional theory are applied to obtain parameters such as migration barriers and interactions, which are important for ion transport on the atomic level. These parameters are applied in Kinetic Monte Carlo simulations to predict the ionic conductivity of the respective materials as a function of composition and temperature.

Results

Subproject 1: Proton conductivity in Y-doped barium cerate
Literature reported energy data was complemented by our own calculations and the parameters were used in Monte Carlo simulations of proton conductivity. Doped barium cerate adopts a distorted perovskite structure with long and short translation paths with different migration barriers. In addition, interactions between dopant and proton depend on the dopant radius. For the case of yttrium, weak trapping of the protons and a slight reduction of the migration barrier is found. The simulated activation energies are almost independent of the dopant concentration and are in line with previous simulation data for Gd-doping. In contrast, the experimental data shows a slight increase of the activation energy with increasing dopant concentration.

Subproject 2: Migration in in layered Na₂X₂TeO₆
Site energies, Na⁺-Na⁺ interactions and Na⁺ migration energies were calculated in Na₂X2TeO6 with X=Mg,Zn,Ni,Co. By using a multilinear regression model, energy parameters were determined for subsequent Kinetic Monte Carlo simulations. The simulations reveal that the ionic conductivity increases in the order Ni < Co < Mg < Zn. This effect is directly connected to the interlayer spacing that results in more space for the Na⁺-ions to move.

Subproject 3: Ionic conductivity in doped Na_0.5Bi_0.5TiO₃
Test simulations were performed for the ionic conductivity of Na0.5Bi0.5TiO3. However, the structure turned out to be complicated resulting in a high computational effort regarding both density functional theory calculations and Monte Carlo simulations. Therefore, this subproject was no longer pursued.

Discussion

Subproject 1 is finalized. The simulations were successful considering the limitations of the simulation model. Results are in line with previous simulations for a similar composition. The direct comparison to experimental values remains difficult due to the scattering of the latter ones. Energy profiles and site energies were successfully obtained in subproject 2. The simulated conductivities reflect the trend of the experimental findings depending on the nature of the X-cation. Nevertheless, the values are underestimated by the simulations. This could be due to concerted migrations steps that were not considered in the simulations and will be investigated in further work. In addition, molecular dynamics simulations will be performed for comparison.
Subproject 3 will no longer be pursued in this form. Nevertheless, alternative methods like molecular dynamics might be considered.

Additional Project Information

DFG classification: 302-02 Physical Chemistry of Solids and Surfaces, Material Characterisation
Software: VASP, Mocassin
Cluster: CLAIX

Publications

Giulia Winterhoff, Steffen Neitzel-Grieshammer, A review of proton migration and interaction energies in doped barium zirconate, Solid State Ionics, 2023, 397, 116231.