Haphazardly arranged graphite flakes in a conventional anode (above left and center): lithium ions attempting to dock or return to the cathode are forced to take detours (above right). But if the graphite is subjected to a rotating magnetic field (below), the flakes in the suspension align themselves vertically in parallel formation. They keep this orientation after they have been dried (below centre). The ions have shorter paths (below right). (Graphics: Juliette Billaud, Florian Bouville, Tommaso Magrini/Paul Scherrer Institute, ETH Zurich). Click to enlarge.

One of the main limitations of existing batteries lies in the transport of Li ions, especially at high rates, in highly loaded electrodes. This shortcoming does not arise from the insertion/extraction kinetics within the bulk active particles but from the slow diffusion of Li ions across the electrode. In particular,reduction of the tortuosity would accelerate such a rate-limiting transfer process. Minimizing the ion and electron path tortuosity within thick electrodes would lead to batteries capable of sustaining higher cycling rates and power densities.

… Graphite is the most advanced commercially available anode material, due mainly to its relatively high energy density, good reversibility, non-toxicity, safety and low cost. However, the diffusion of charge carriers across thick graphite electrodes is often reduced by the high tortuosity of the porous anode structure, particularly when anisotropic flake-like particles are used. Moreover, the insertion/extraction kinetics of lithium in graphite particles is extremely anisotropic, as it occurs only in the crystallographic basal plane within the stacked graphene layers held together by van der Waals forces. Aligning graphite flakes perpendicularly to the current collector could ease the transport of the charge carriers within the anode by creating short diffusion paths and exposing preferential insertion/extraction sites

Here, we propose a simple, up-scalable and inexpensive technique to orient graphite particles perpendicularly to the current collector by applying a low magnetic field during the electrode fabrication. Despite initial work aimed at applying the magnetic alignment technology to battery anodes, the use of this approach to improve the high-rate performance of batteries has, to the best of our knowledge, not yet been reported in the literature. The alignment of graphite in architectured electrodes allowed us to cycle at a fast rate of up to 2C with a specific charge that is three times higher than in similarly loaded anodes fabricated using a conventional route.

—Billaud et al.

Simply by optimizing the graphite anode on a conventional Li-ion battery, the researchers were able—under laboratory conditions—to enhance storage capacity by a factor of up to 3. Owing to their complex construction, commercial batteries will not be able to fully replicate these results, the researchers noted. But performance will definitely be enhanced, perhaps by as much as 30 – 50%—further experiments should yield more accurate prognoses.

The researchers point out that in terms of industrial implementation, improving existing components has the great advantage of requiring less developmental input than a new battery design using new materials.

We already have everything we need. If a manufacturer were willing to take on production, enhanced batteries could be ready for the market within one or two years. The procedure is simple, cost-effective and scalable for use on rechargeable batteries in all areas of application, from wristwatch to smartphone, from laptop to car. And it has the additional bonus of being transferable to other anode-cathode batteries such as those based on sodium.

—Claire Villevieille, head of the battery materials research group at the Paul Scherrer Institute PSI

The method to align the graphite flakes involves coating the graphite flakes with nanoparticles of iron oxide sensitive to a magnetic field and suspending them in ethanol. The suspended and already magnetized flakes are subsequently subjected to a magnetic field of 100 millitesla—about the strength of a fridge magnet. By rotating the magnet during this process, the platelets not only align vertically but in parallel formation to one another, like books on a shelf. As a result, they are perfectly ordered, reducing the diffusion distances covered by the lithium ions to a minimum.

Microscopic images show that if the magnet remains turned on during the ensuing drying process, the platelets keep their new orientation even when removed from the ethanol suspension. Instead of their formerly haphazard arrangement, the flakes in the compressed graphite bar are now parallel, enabling the lithium ions to flow much more easily and quickly, while also increasing storage capacity by allowing more ions to dock during the charging process.

The remaining iron oxide nanoparticles are negligible in quantity and do not influence battery function.

The chemical composition of batteries remains the same. All we did was optimise the anode structure.

—Claire Villevieille

Resources

• J. Billaud, F. Bouville, T. Magrini, C. Villevieille, A.R. Studart (2016) “Magnetically aligned graphite electrodes for high rate performance Li-ion batteries” Nature Energy doi: 10.1038/nenergy.2016.97