Clear Representation of Surface Pathway Reactions at Ag Nanowire Cathodes in All-Solid Li-O₂ Batteries

All-solid Li-O₂ batteries have been constructed with Ag nanowire (AgNW) cathodes coated on Au-buffered garnet ceramic electrolytes and Li anodes on the other sides. Benefiting from the clean contacts of Li⁺, e^-, and O₂ on the AgNWs, the surface pathway reactions are demonstrated. Upon discharge, two types of Li₂O₂ morphologies appear. The film-like Li₂O₂ forms around the smooth surfaces of AgNWs, and hollow disk-like Li₂O₂ forms at the joints in between the AgNWs as well as at the garnet/AgNW interfaces. The formation of films and hollow disks is in accordance with the process of O₂ + Li⁺ + e^- → LiO₂ and 2LiO₂ → Li₂O₂ + O₂, indicating that the disproportionation of LiO₂ occurs at the solid interfaces. During the initial charge, decomposition occurs below the potential of 3.5 V, indicating the process of Li₂O₂ → LiO₂ + Li⁺ + e^- and LiO₂ → Li⁺ + e^- + O₂ rather than Li₂O₂ → 2Li⁺ + 2e^- + O₂. The Li₂O₂ decomposition starts at the AgNWs/Li₂O₂ interfaces, causing the film-like Li₂O₂ to shrink and the gas to release, followed by the collapse of hollow disk-like Li₂O₂. The results here clearly disclose the Li-O₂ reaction mechanism at the all-solid interfaces, facilitating a deep understanding of key factors influencing the electrochemical performance of the solid-state Li-O₂ batteries.