Batteries based on lithium now power everything from our watches to our cars, and we've made major strides towards stuffing more energy into them more quickly over the last several years. But there are limits to how quickly a battery can charge, and pushing past them can cause the lithium to form metallic microstructures within the battery. These can do ugly things like creating a short between the electrodes or puncturing the membranes that contain the battery's electrolyte.

Most techniques that could image these miscrostructures involved taking the battery apart, meaning that we could only take static images of the impact of charge/discharge cycles on the battery. One of the best techniques for non-invasive imaging, NMR, relies on radiofrequency signals that simply don't penetrate beyond the surface of a battery. Now, some researchers have figured out that there are conditions that enable the use of NMR to peek inside a battery—and they happen to be the formation of the microstructures we care about.

NMR relies on the fact that the nuclei of atoms have spins. Place a material in a strong magnetic field, and the spins will all align along the field, lining up either parallel or antiparallel to the field lines. Once they're lined up, a small jolt of energy—corresponding to radiofrequency radiation—is enough to get them to flip their alignment. We can then read the changes in the absorption of these frequencies to determine things about a material's composition and structure.

Lithium batteries are, in some ways, an excellent candidate for NMR imaging, since there's an isotope (7Li) that produces an NMR signal that's easy to distinguish. Unfortunately, the radiofrequency radiation portion of the technique fails miserably, as radio waves barely get past the surface of a battery.

However, the authors of the new work noted a study that came out in 2010, which showed that the formation of lithium microstructures allowed radiofrequencies to penetrate into the battery. This is probably because these structures are fairly porous, with many branches and connections (they're referred to as looking either "mossy" or "dendritic"). For the purpose of understanding battery performance, the NMR doesn't have to actually detect the details of the structures themselves. Instead, it just has to be able to determine when signal starts appearing from deep within the battery, and it can identify the where and when of microstructure formation.

The authors built a simplified battery with pure lithium electrodes, and showed that NMR was able to detect the formation of microstructures at resolutions well below a millimeter. It wasn't nearly as good as electron microscopy, which showed lots of details of the structures, but it had one key advantage: the battery was intact afterwards, and could be sent through additional charge cycles.

The authors indicate that their technique should work with electrodes that are currently in use, although they don't actually show any data to indicate it does. But the more intriguing possibility they mention is that their technique should allow them to explore what happens after the microstructures are present. This raises the possibility that they could empirically develop a charge/discharge pattern that actually reverses the process by which the structures formed. If we could identify a charging program that worked, it could potentially extend the life of existing batteries.

Nature Materials, 2012. DOI: 10.1038/NMAT3246 (About DOIs).