Coverage-dependent ReaxFF molecular dynamics of O₂ adsorption and kinetic stability at pristine Li₂O₂/O₂ interfaces

The early interaction of molecular oxygen with Li₂O₂ surfaces plays a critical role in governing discharge-product growth, passivation, and reversibility in lithium-air batteries. However, the coverage-dependent atomistic dynamics of O₂ at pristine Li₂O₂ interfaces remain insufficiently resolved. Here, we use reactive molecular dynamics simulations based on ReaxFF to investigate the structural, dynamical, and interfacial response of Li₂O₂ surfaces exposed to increasing O₂ loadings of 8-24 molecules. The simulations show that crystalline Li₂O₂ remains structurally stable at 300 K, with negligible lattice reconstruction, preserved Li-O coordination, and no evidence of defect formation or peroxide decomposition within the 50 ps simulation window. Adsorbed O₂ molecules preferentially adopt orientations parallel to the surface and retain their molecular identity, indicating weak physisorption rather than spontaneous chemical activation under the investigated thermal conditions. Species-resolved mean-square displacement analysis further distinguishes the immobile lattice oxygen network from the more mobile adsorbed oxygen overlayer, showing that O₂ adsorption modifies interfacial mobility and ordering without triggering O-O bond cleavage or subsurface incorporation. These results support an adsorption-dominated kinetic-stability picture in which pristine Li₂O₂ acts as a structurally rigid interface toward molecular oxygen under the specific ReaxFF conditions considered here, rather than providing definitive evidence of complete chemical inertness on all timescales. Because reaction pathways, activation barriers, reactive-event kinetics, and electrochemical polarization are not directly calculated, the findings are interpreted primarily in terms of O₂ adsorption, interfacial mobility, and short-time kinetic stability of the idealized pristine Li₂O₂/O₂ interface. The findings provide an atomistic baseline for understanding early Li-air discharge processes and suggest that catalytic, defect-mediated, electrolyte-assisted, or electrochemically driven surface modification may be required to promote controlled O₂ activation and improve Li-air battery reversibility.