Electrochemical Growth of Very Long (∼80 μm) Crystalline Li2O2 Nanowires on Single-Layer Graphene Covered Gold and Their Growth Mechanism.

For the development of lithium-air battery (LAB), which is one of the most promising next generation batteries, it is essential to understand the structure and properties of Li2O2, which is the discharged product at the positive electrode of a LAB, as well as the mechanism of Li2O2 growth because its deposition limits the discharge capacity and is the origin of the high charging overpotential of LAB. Characterization of the structure and properties of the Li2O2 formed in LABs is, however, difficult because it is usually in the form of poorly ordered small particles. In this study, we successfully grew well-aligned very long (∼80 μm) crystalline Li2O2 nanowires (NWs: average diameter of 22 nm) electrochemically at a gold electrode covered with single-layer graphene (SLG/Au). Preferential growth of the NWs along c-axis was confirmed by X-ray diffraction, transmission electron microscopy with electron diffraction, and Raman scattering. Raman imaging indicated that the sites of NW growth were the grain boundaries of single-layer graphene. The long, crystalline Li2O2 NWs provided the opportunity to investigate not only their structure and properties but also their growth mechanism during discharge. Raman measurements in the O-O stretching frequency region of the SLG/Au electrode at various depths of the discharge combined with exchange of oxygen in the solution from 18O2 to 16O2 during the discharge revealed that the growth took place at the bottom of the NWs, i.e., the Li2O2/electrode interface, not the top of the NWs, i.e., the solution/Li2O2 interface. This growth mechanism can explain why such long NWs can be grown despite the insulating nature of Li2O2.