Mechanistic Insights into Li+ Transport Enabled by Isolated Sulfur Species in Li3PS4 Glasses

Abstract

All-solid-state lithium-ion batteries have renewed interest in high-performance solid electrolytes. Li3PS4 (Li2S-P2S5) glasses are among the most studied due to their high ionic conductivity, traditionally ascribed to rotational motion of polyhedral units facilitating Li+ migration. Using ab initio molecular dynamics, we investigate Li-ion diffusion in Li3PS4 glass, demonstrating that our structural model reproduces experimental neutron and X-ray diffraction patterns and conductivity measurements. Importantly, we identify a previously unrecognized diffusion mechanism: Li+ ions near isolated sulfur species (Sn with n = 1, 3) display significantly enhanced mobility, with atomic displacements up to 1.7 greater than those associated with bulkier polyhedral units. These results highlight the critical role of free sulfur species in promoting fast ionic transport, providing insights for the rational design of glass compositions with optimized conductivity for solid-state battery applications

Figures (5)

• Figure 1: (A, B) Neutron (A) and X-ray (B) structure factors of the simulated Li3PS4 glass model (black) at 300 K compared with experimental data (red). (C) Arrhenius plot averaged over four independent molecular dynamics simulations at liquid-state temperatures, extrapolated to 300 K (green). The linear regression, $\ln(D) = 2640.2/T - 6.6078$ ($R^2 = 0.9988$), was used to determine the lithium ions diffusion coefficient and conductivity.
• Figure 2: Partial pair distribution functions (black) and their cumulative integrals (red) averaged over four independent molecular dynamics simulations. (A) P–P, P–S, and S–S correlations; the plateau of each integration is marked with a dashed line indicating the average coordination, and the cut-off radius rcut is shown in green. (B) Li–Li, Li–P, and Li–S correlations.
• Figure 3: Connectivity distributions and snapshots of Li3PS4 glass. (A, B) Phosphorus-(A) and sulfur-(B) resolved coordination environments. Key structural connections are color-coded: P–S (green), P–P–S (yellow), P–(P2)–S (red), S–P (light blue), S–S (dark blue), and S–S–P (black). (C) Snapshot of the glass at 300 K from MD-1, highlighting three types of sulfur ions (S1, blue; S2, green and S3, red).
• Figure 4: Li+ migration events over 12 ps at 500 K. (A) Identification of ions participating in migration events and event durations for MD-1. (B) Trajectories of cooperative Li ions (Li(16), green; Li(27), red; Li(55), blue) near a monosulfur species (S1, black); arrows indicate displacement directions. (C) Mean-squared displacement of Li ions involved in cooperative motion shown in (B). (D) Trajectory of Li(37) (pink) and the four sulfur atoms of a PS4 tetrahedron involved in a soft-cradle–like movement (S(2),blue; S(82), green; S(25), red and S(72), orange); arrows indicate movement directions. (E) Distances between Li(37) and surrounding sulfur atoms during soft-cradle–like movement.
• Figure 5: (A) Average mean squared displacement of lithium ions associated with species present in the glass namely S1 (blue), S2 (green), S3 (red) and P associated molecules (pink) compared to total lithium displacement (black). (B) Scheme representing Li-ions movement in proximity to the isolated sulfur species with high diffusion (Sn, n = 1,3) by the correlated transport of Li ions. (C) Scheme of Li+ near the PxSy species - soft-cradle–like mechanism. Lithium atoms are shown in pink, phosphorus in orange, P bonded sulfur in yellow and S bonded sulfur in black.