Better lithium ion batteries, how do they work? Magnets!

Specific charge can be increased by a factor of three.

Shalini Saxena

Battery research focuses on balancing three competing factors: performance, lifetime, and safety. Typically, you have to sacrifice one of these factors to get gains in the other two. But for applications like electric vehicles, we'd really like to see all three improved.

In an investigation recently published in Nature Energy, scientists demonstrated the ability to use a magnetic field to align graphite flakes within electrodes as they're manufactured. The alignment gives lithium ions a clearer path to transit the battery, leading to improved performance.

The electrodes of Lithium-ion batteries are often composed of graphite, which balances attributes such as a high energy density with non-toxicity, safety, and low cost. Graphite, composed of stacked sheets of carbon atoms, is often incorporated into these electrodes in the form of flake-like particles.

While graphite has many advantages, it has a downside: it limits the movement of lithium ions, which is a fundamental part of charging and discharging. The lithium ions are only able to move within the planes between stacked graphene sheets and often have to navigate a highly torturous path as they move around during charge and discharge. This slow movement through the electrodes remains a critical challenge in the development of batteries with improved performance.

The authors of the new paper reasoned that it should be possible to align the graphite flakes so that they provide a more linear path for ions to move within the battery. To accomplish this, they decided to use magnetic fields. There was just one problem: graphite doesn't respond to magnetic fields.

To work around this, the scientists coated the flakes with superparamagnetic iron oxide nanoparticles. The coated graphite flakes were then suspended in ethanol. They homogenized the suspension and added a small amount of a chemical binder (2 percent by weight poly(vinyl pyrrolidone) that helped ease the alignment process. A relatively dilute suspension was needed to give the flakes enough room to move during alignment.

During fabrication of the electrodes, the graphite particles were oriented using a rotating magnetic field aligned perpendicular to the part of the battery that would exchange charges with the graphite (called a current collector). The scientists found that a magnetic field as low as 100 mT was capable of aligning the flakes. For comparison, this magnetic strength is larger than the average fridge magnet (1 mT), but significantly smaller than an MRI magnet (1.5 T). As a control, they also prepared reference electrodes in the absence of a magnetic field.

After fabrication, the team evaluated the alignment of the graphite flakes deposited under both conditions. Visual analysis revealed a clear orientation of flakes in electrodes fabricated under the influence of the magnetic field. The flakes were tilted at an angle of 60 degrees above the plane of the current collector. By contrast, the graphite flakes in the reference electrodes fell mostly parallel to the current collector.

Next, the scientists carried out a series of experiments to evaluate the change in the path the lithium ions needed to navigate. Overall, they saw that the magnetic field decreased the tortuosity of the paths through the electrode by a factor of 4 compared to the reference electrodes.

Finally, they evaluated how this impacted the battery performance by testing the electrode in a half-cell configuration (meaning they didn't build a full battery). At practical charging rates, alignment of the graphite flakes increased the lithium storage capacity of the electrode by a factor of between 1.6 and 3.

This investigation demonstrates that chemistry isn’t the only important factor at play in battery design—optimization of the electrode architecture can help boost battery performance as well. Future studies will need to determine the scalability of this technique.

Nature Energy, 2016. DOI: 10.1038/NENERGY.2016.97 (About DOIs).

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