Ultrafast lithium diffusion in bilayer graphene

Abstract

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. Singlephase 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 redoxreaction 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.The electrodes of state-of-the-art Li-ion batteries consist primarily of mixed conducting compounds, in which, by ambipolar motion, Li-ions diffuse coupled to electron transport. The higher both ionic and electronic conductivities of these electrodes are, the faster one may insert (and remove) Li-ions up to full storage capacity. However, room-temperature values for the chemical diffusion coefficient of Li, D δ , are typically rather low in common Li insertion compounds, with higher values potentially conceivable in artificially designed heterogeneous mixed conductors. 1 In practical electrodes, which are composites containing the insertion compound (typically as a powder) and additives such as binders, Li diffusion is a complex process with D δ being an effective parameter. For insertion compounds with a highly anisotropic ionic conductivity such as bulk graphitic carbons, 2,3 reported values spread over many orders of magnitude as D δ ≈ 10 −12 − 10 −5 cm 2 /s, 4-13 partially also reflecting difficulties to reliably extract this parameter from electrochemical measurements. 8,11,12 Nanoscale derivatives of bulk insertion compounds might therefore offer a better platform both to determine and to exploit their likely similar if not better mixed conducting properties. As such, graphene -a single sheet of carbon atoms -has been used in various electrodes as helpful current-collecting admixture, and a possibly increased capacity over graphite by Li adsorption on both sides is being discussed. 14,15 Here, we study bilayer graphene, which features a single van der Waals-gap between its two carbon sheets suitable for Li intercalation. With current nanofabrication methods and characterization tools, ...

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