Electrochemical Li⁺ Insertion/Extraction Reactions at LiPON/Epitaxial Graphene Interfaces

Redox reactions of the Li⁺ insertion/extraction from one to two interlayers of graphene (Gr) on area-defined single-crystalline SiC substrates are investigated using lithium phosphorus oxynitride glass (LiPON) as the solid-state electrolyte. Unlike an organic liquid electrolyte, this glassy electrolyte does not induce a reduction current and excludes the desolvation reaction of Li⁺. Gr electrodes with less than two Gr layers show a single reduction peak and one or two oxidation peaks below +0.21 V (vs Li⁺/Li), differing distinctly from those of graphite and multilayer Gr, which display multiple peaks (multiple stage transitions). However, this finding aligns with the conventional understanding that graphite stage structure transitions proceed with stepwise increases or decreases in the number of Gr layers between adjacent Li-inserted interlayers. Cyclic voltammetry measurements indicate the presence of surface capacity due to Li⁺ adsorption/desorption at the LiPON/Gr interface. Moreover, Li⁺ insertion and extraction induce different charge transfer resistances at the level of a single interlayer. These sensitive measurements are achieved using high-quality epitaxial Gr and LiPON electrolyte, which prevent the formation of a solid electrolyte interphase and the desolvation reaction of Li⁺. Similar measurements using bilayer Gr produced by chemical vapor deposition coupled with a Gr transfer method and an ethylene carbonate/dimethyl carbonate liquid electrolyte are not reliable. Thus, the proposed method is effective for electrochemical measurement of Gr electrodes with a controlled number of layers.