Researchers developed a multiscale simulation framework that maps lithium-ion transport across polycrystalline solid electrolytes, connecting atomic hopping at grain boundaries with continuum-scale percolation.
The study, published in Nature Communications, examined argyrodite sulfide electrolytes with the formula Li6PS5X, where X represents chlorine, bromine, or iodine. The team trained lightweight moment tensor potentials using density-functional theory calculations in an active-learning loop. They then ran molecular dynamics simulations across bicrystal, columnar, and random polycrystalline models before passing local diffusivities into continuum finite element simulations.
Grain boundary energetics and diffusion
The atomistic calculations showed that grain boundary morphology dictates defect energetics. In anion-ordered Li6PS5Cl, the Σ3 tilt grain boundary at 109 degrees registered the lowest formation energy at approximately 5 meV per square angstrom. Other grain boundary configurations, including low-angle boundaries, ranged from 12 to 24 meV per square angstrom, driven by heavy distortion of lithium cages and ortho-thiophosphate units.
Molecular dynamics simulations at 600 K showed that grain boundaries in anion-ordered Li6PS5Cl exhibited self-diffusivities roughly threefold higher than the ordered bulk lattice. Diffusion within the grain boundary plane was slightly faster than transport perpendicular to the boundary plane, with perpendicular diffusivity reaching about 0.87 times the parallel rate.
Anion disorder and halide scaling
The impact of grain boundaries diverged sharply depending on the ordering of the bulk material. In 50% anion-disordered Li6PS5Cl, grain boundaries increased the diffusion barrier by roughly 10 meV relative to the bulk and reduced room-temperature self-diffusivity by about 60%. However, characteristic frequencies remained near 100 MHz, matching experimental measurements of low grain boundary resistance.
In anion-ordered bulk lattices, substituting chlorine with larger halides raised the lithium-ion diffusion barrier linearly with anion radius. Each 0.1 angstrom increase in anion radius added approximately 80 meV to the migration barrier, making iodine-based bulk argyrodites roughly 0.3 eV more resistive than chlorine variants.
Continuum finite element simulations of anion-ordered Li6PS5I revealed non-Arrhenius transport behavior with a transition temperature around 250 K. Bulk diffusion dominated high-temperature transport with an activation energy near 0.69 eV, while grain boundaries took over at low temperatures with an activation energy near 0.59 eV.
