Our cover story of the Nature Nanotechnology 12 September issue reports on ultrafast lithium between two graphene layers.
Lithium dashes in bilayer graphene
It is hard to find materials that are simultaneously good conductors for both electrons and ions. A collaboration of scientists at the Max Planck Institute for Solid State Research now experimentally demonstrate bilayer graphene, consisting of only two atomically thin sheets of carbon, to be such a material. It allows lithium to diffuse faster in between its two layers than in bulk graphite, and even faster than sodium chloride in water. Apart from this materials science aspect, it is the deployed method that deserves attention: for the first time a single-crystalline graphene bilayer was operated as an electrode in a miniaturized electrochemical cell. In-situ magnetotransport techniques allowed to directly access the ultrafast lithium diffusion kinetics.
Materials that conduct both electrons and ions, so-called mixed conductors, are key elements for certain applications. An example is the lithium-ion battery, in which Li-ions are reversibly exchanged between anode and cathode. Each of these two electrodes needs to allow for hosting both electrons and Li-ions as well as for their diffusion into the respective bulk for storage. The mixed-conducting properties of each electrode are determined by their composition. One often combines pulverized, moderate ionic conductors with electron-conducting additives and binders. The exact composition has an influence on the final properties of the battery, such as its specific capacity or at which rate the battery may be discharged/charged. Precise knowledge about key materials properties or sensitive methods to quantify these properties are therefore not only interesting, they may in fact be crucial for the optimization of a given battery technology.
The negative electrode in most of today’s Li-ion batteries consists mainly of graphite, i.e., of stacked carbon sheets – so-called graphene layers. These are good electronic conductors with a number of exotic properties the discovery of which was awarded with the Nobel prize in physics in 2010. Graphene layers couple only weakly via so-called van der Waals-forces, leaving gaps between neighboring sheets sufficiently large to accommodate guest ionic species. For each Li-ion inserted in such a gallery, also one electron is added via an external electric circuit. The coupled motion of both the electron and the ion inside the host material can be described as chemical diffusion. The higher the chemical diffusion coefficient Dδ, the faster the lithium transport within the electrode.
Despite the widely established use of graphite as an active electrode material in Li-ion batteries, the value of Dδ for Li in graphite is not precisely known. Experimental values spread over many orders of magnitude: 10-12 – 10-5 cm²/s. Reasons are, among others, the anisotropic character of Li-diffusion in graphite, variations in the composition/preparation of the graphitic electrodes studied, as well as inherently imperfect assumptions that typically need to be made when extracting Dδ from electrochemical measurements.
Scientists at the Max Planck Institute for Solid State Research now deliberately extracted two graphene layers out of bulk graphite in order to operate them as an electrode in a novel, miniaturized electrochemical cell. The latter was implemented on top of a silicon chip using nanotechnology methods. Metallic lithium was used as a counter electrode, and the electrodes were connected both via an electrolyte as well as via an external electrical circuit. The decisive factor was to not cover the whole graphene bilayer with the electrolyte, but to position it at one end only. By doing so, Li-ions are forced to enter locally in-between the graphene layers followed by their diffusion into the uncovered area of the device. Combining different measurements and sample geometries, M. Kühne et al. demonstrated first of all that indeed there is only one efficient diffusion pathway: Li-ions diffuse neither on top nor below graphene sheets, only in-between. The samples were then exposed to very strong magnetic fields in order to measure the so-called Hall effect, from which the local charge carrier density can be extracted which is in turn linked to the local lithium concentration. From simultaneous, time-resolved measurements at different distances from the electrolyte, record values for the chemical diffusion coefficient Dδ for lithium could be revealed. The intrinsic value of Dδ for lithium in graphite could thus be higher than previously thought. However, the nature of thecoupled ionic-electronic transport in bilayer graphene differs from the one in graphite at least insofar as additional neighboring graphene or lithium layers are absent. Just as well is any of their impact on the chemical diffusion. Beyond that, it deserves mentioning that the device geometry realized is in principle transferrable to any other 2D material or thin film structure. Moreover, it allows the deployment of local probe techniques to probe the part of the microscopic electrode under study which was deliberately left uncovered from the electrolyte. Different in-situ studies to investigate for example local ordering phenomena and their dynamics can be envisaged.
J. H. Smet, J. Maier, M. Kühne, J. Popovich