Lithium-Intercalated Few Layer Graphene: Approaching the Limits of Transparency and Conductivity in Graphene-based Materials

We measure simultaneous \textit{in situ} optical transmittance spectra and electrical transport properties of few-layer graphene (FLG) nanostructures upon electrochemical lithiation/delithiation. Reversible Li-intercalation stages and a two-phase boundary are observed optically. Due to the unusual electronic structure of FLG, upon intercalation we observe a simultaneous increase of both optical transmittance and DC conductivity, strikingly different from other materials. Transmission as high as 91.7{\%} for sheet resistance of 3.0 $\Omega $/square is achieved for 19 layer LiC$_{6}$, corresponding to a figure of merit (FOM) $\sigma_{dc}$/$\sigma_{opt\, \, }=$ 1400, five times higher than any previously demonstrated for a continuous transparent electrode. The unconventional modification of FLG optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques can enable investigation of other aspects of intercalation in nanostructures, for example intercalation dynamics and solid-electrolyte interface formation.