Orientation-Dependent Protection by LiF Interlayers at LATP Solid-Electrolyte Interfaces: A First-Principles Study

Solid-state lithium metal batteries require chemically stable and electronically insulating interfaces to suppress interfacial reduction reactions and enable long-term cycling stability. Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) exhibits high ionic conductivity but undergoes spontaneous Ti⁴⁺ reduction when placed in direct contact with a Li metal anode. Here, density functional theory and ab initio molecular dynamics are combined with a descriptor-based, physics-informed first-principles analysis to elucidate how crystallographic orientation governs the protective function of LiF coatings on LATP(012) surfaces. Three LiF orientations─(100), (110), and (111)─are systematically compared using formation energetics, layer-resolved projected density of states, Bader charge analysis, charge-density differences, electrostatic potential profiles, interface dipole moments, and interfacial electric-field distributions. Physically motivated descriptors extracted from first-principles outputs enable efficient, data-driven comparison of interfacial polarization and electronic blocking behavior across orientations. Among the orientations considered here, the LiF(100)/LATP(012) interface shows a favorable electronic-blocking response, with no pronounced interface-induced localized electronic states, reduced area-normalized charge redistribution, weaker interfacial polarization, and ordered Li coordination with comparatively low Li-ion diffusivity. In contrast, LiF(110) exhibits pronounced interfacial polarization, formation of interface-induced localized electronic states, enhanced Li-O coordination, and the highest Li-ion diffusivity, indicating stronger electronic and ionic coupling across the interface. Ab initio molecular dynamics simulations indicate structural stability within the simulated time scale at room temperature. These results demonstrate that crystallographic orientation is an important factor influencing the electronic and ionic behavior of ultrathin LiF-based protective interlayers on LATP surfaces.