Microscopic insights into interfaces formed by Li₁₊ₓAlₓTi₂₋ₓ(PO₄)₃ (LATP) electrolyte and its intermediate phases by ReaxFF Molecular Dynamics simulations

Li₁₊ₓAlₓTi₂₋ₓ(PO₄)₃ (LATP) is a promising solid electrolyte (SE) for next-generation all-solid-state Li-ion batteries (ASSBs) due to its high Li ionic conductivity and structural stability. Understanding interfacial ion transport and reactivity during high-temperature solid-state synthesis of LATP is crucial towards minimizing the composition/phase variability of the synthesized product, which can then be strongly linked to SE performance. In this study, ReaxFF-based molecular dynamics (MD) simulations are employed to investigate interface kinetics between LATP and its intermediate phases, focusing on particle contact geometries such as LATP|LTP and LTP|AlPO₄ with varying crystallographic orientations and terminations. The MD simulations are performed in the NVT ensemble with an MD time step of 0.25 fs. The interface structure models are initially heated from 300 K up to 1600 K for 100 ps, followed by a 500-ps production run at 1600 K. Results show that oxygen-terminated, high-index interfaces ((O₂)LTP–(O₂)LATP and (O₂)LTP–(Al₂P₂O₃)AlPO₄) exhibit the highest interfacial ion diffusivity compared to cation-terminated or low-index interfaces. At higher thermal activation (2400 K), Al migration across the LATP|LTP interface, crucial for forming the high-conductivity LATP phase, is successfully observed. Local structural analysis revealed signs of amorphization, indicated by the broadening of radial distribution functions, and temperature-induced polyhedral transformations such as PO₄ → PO₃/PO₅, TiO₆ → TiO₅/TiO₄/TiO₃, and AlO₄ → AlO₅/AlO₃. Additionally, high-temperature simulations uncovered Ti–O–Ti clustering (TiO₂-like local structure) and the formation of P₂O₇-like units through P–O–P bridging, suggesting the condensation of polyanion groups. These findings demonstrate that interface orientation, termination, and temperature play key roles in controlling ion mobility and interfacial reactivity during LATP synthesis. The insights provide valuable guidance for tailoring synthesis conditions and microstructural design strategies to enhance solid electrolyte performance in next-generation solid-state batteries.