Electrochemical Growth of Very Long (∼80 μm) Crystalline Li 2 O 2 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 Li ₂ O ₂ , which is the discharged product at the positive electrode of a LAB, as well as the mechanism of Li ₂ O ₂ 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 Li ₂ O ₂ 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 Li ₂ O ₂ 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 Li ₂ O ₂ 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 ¹⁸ O ₂ to ¹⁶ O ₂ during the discharge revealed that the growth took place at the bottom of the NWs, i.e., the Li ₂ O ₂ /electrode interface, not the top of the NWs, i.e., the solution/Li ₂ O ₂ interface. This growth mechanism can explain why such long NWs can be grown despite the insulating nature of Li ₂ O ₂ .