Microscopic Insights into the High-Temperature Interfacial Kinetics of Li 1+ x Al x Ti 2− x (PO ₄ ) ₃ (LATP) Solid Electrolytes: A ReaxFF-MD Study

NASICON-type Li 1+ x Al x Ti 2− x (PO 4 ) 3 (LATP) is considered as one of the most promising solid electrolytes for all-solid-state batteries (ASSBs) due to its high Li ionic conductivity, good chemical stability, high mechanical strength, excellent ambient air stability, and cost-effective raw materials. In LATP solid-state reaction synthesis, the interfacial structure evolution at particle contacts and their ion dynamics at elevated temperatures are still poorly understood. Here, we employ high-temperature ReaxFF-based molecular dynamics (MD) simulations to systematically study experimentally motivated morphological features such as in-grain two-phase coexistence domains (i.e., LTP( x Al = 0)|LATP( x Al = 0.30)→ LATP( x Al = 0.15) compositional homogenization) and interfaces related to LATP and its intermediate phases (LTP|LATP and LTP|AlPO 4 ). For the first time, we demonstrate that thermal activation enables Al incorporation from Al-rich into Al-poor domains, overcoming kinetic barriers for LATP formation. Additionally, polyhedral coordination analysis suggests the dynamic evolutions in PO 4, TiO 6, and AlO 4 coordination environments for the investigated LTP|LATP and LTP|AlPO 4 interfaces facilitated by the diffusion of the framework cations and correlating with the emergence of P−O−P−···, Ti−O−Ti−··· and P−O−Ti−··· linkages that can act as precursors to the initial stage of polyanion condensation and possible phase evolution during solid-state reactions. These insights establish a mechanistic framework linking interface structure kinetics to high-temperature reaction synthesis of LATP electrolytes for advancing ASSBs.