Atomistic Insight into Enhanced Ionic Conductivity in Mixed Halide Sodium Ion Conductor Na₃YBr_6-xCl_x

Exploring solid-state electrolytes with high ionic conductivity and low electronic conductivity is of great importance in realizing all-solid-state batteries.

Here, we provide an atomistic insight into the enhancement of Na ionic conductivity in mixed halides Na₃YCl_xBr_6-x(NYCB_x) by using ab initio calculations and machine-learning interatomic potential-based molecular dynamics simulations.

Here, we define sodium–yttrium mixed halides Na₃YCl_xBr_6-x (NYCBx, x= 0, 1.5, 3, 4.5, 6) as sodium SSEs with the first-principle calculations within the density functional theory (DFT) framework and molecular dynamics (MD) simulations.

Structural optimizations and electronic structure calculations of unit cells are performed, using the Quantum ESPRESSO (QE, version 7.2) package.

Fig. Phonon band centers of total system, Na+ cation and halogen anion sublattice [YCl_xBr_6-x]³-with difference between the Na+ and [YCl_xBr_6-x]³-projected phonon band

To get a deeper atomistic insight into Na ionic mobility and conductivity, we considered the lattice vibrational features with a careful analysis of phonon projected DOS.

We calculated the phonon band centers (PBCs, also called the average vibrational frequencies ω_av) to quantitatively analyze the lattice vibrational feature.

The figure shows the calculated total, Na+ cation-and [Y(Cl_xBr_6-x)]³-anion-projected PBCs, together with the difference between the cation-and anion-projected PBCs.

For all the halides, the total PBC almost coincides with the [YX₆]³-projected PBC, implying that the lattice dynamics is dominated by these anion sublattice vibrations.

In summary, we have investigated the structural distortion, electronic and ionic conductivities and lattice dynamics of mixed halides Na₃YCl_xBr_6-x by using atomistic simulations.

Our calculations revealed that the electronic conductivity decreased gradually as the Cl content x increased, while the Na ionic conductivity reached the highest value with the lowest activation energy in the mixed halide with a middle composition, being ascribed to the largest local distortion and superposition of Na+ cation-and [YX₆]³-anion-sublattice vibrations.

The research results have been published in the "Applied Physics Letters" under the title of "Atomistic insight into enhanced ionic conductivity in mixed halide sodium ion conductor Na₃YCl_xBr_6-x" (https://doi.org/10.1063/5.0257157).