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.