  Solid mixed conductors with significant ionic as well as electronic
conduction play a pivotal role for mass transfer and storage as required in
battery electrodes. Single-phase materials with simultaneously high electronic
and ionic conductivity at room temperature are hard to come by and therefore
multi-phase systems with separate ion and electron channels have been put
forward instead. Here, we explore bilayer graphene as a true single phase mixed
conductor and demonstrate ultrafast lithium diffusion exceeding diffusion in
bulk graphite by an order of magnitude and even surpassing diffusion of sodium
chloride in liquid water. To this end, an innovative electrochemical cell
architecture has been developed where the redox-reaction forcing lithium
intercalation is localized at a protrusion of the device only. Its remainder
consists of pristine bilayer graphene unperturbed by an electrolyte. The
geometry lends itself to the use of magnetotransport machinery known from
mesoscopic low-dimensional physics. Time dependent Hall measurements across
spatially displaced Hall probes deliver a direct view on the in-plane diffusion
kinetics. The device layout with a perimeterial electrochemical cell is
transferable to other 2D materials as well as thin films and may promote a
paradigm shift on the use of electrolytes in on-chip experiments.
