  Various bandstructure engineering methods have been studied to improve the
performance of graphitic transparent conductors; however none demonstrated an
increase of optical transmittance in the visible range. Here we measure in situ
optical transmittance spectra and electrical transport properties of
ultrathin-graphite (3-60 graphene layers) simultaneously via electrochemical
lithiation/delithiation. Upon intercalation we observe an increase of both
optical transmittance (up to twofold) and electrical conductivity (up to two
orders of magnitude), strikingly different from other materials. Transmission
as high as 91.7% with a sheet resistance of 3.0 {\Omega} per square is achieved
for 19-layer LiC6, which corresponds to a figure of merit
{\sigma}_dc/{\sigma}_opt = 1400, significantly higher than any other continuous
transparent electrodes. The unconventional modification of ultrathin-graphite
optoelectronic properties is explained by the suppression of interband optical
transitions and a small intraband Drude conductivity near the interband edge.
Our techniques enable the investigation of other aspects of intercalation in
nanostructures.
