Researchers have used magnetic resonance imaging (MRI) to track the growth of lithium deposits from a lithium metal anode in three-dimensions (3D) and in real time. The non-destructive approach could be used to diagnose other changes that lead to battery failure without having to destroy the battery to examine it, the researchers say.

Lithium metal is a promising material for Li-ion battery anodes because Li has the highest theoretical specific capacity among possible anode materials. However, lithium dendrites typically grow from the Li anode during charging cycles, eventually penetrating the separator and short-circuiting the battery.

To control dendrite formation, researchers want to know more about how they form. To observe the dendrite structure, they typically open a battery after a certain number of charging cycles and examine the contents using a microscope. Magnetic resonance imaging (MRI) of lithium ions can yield images of the dendrites noninvasively. However, the inherently low sensitivity of ⁶Li to an external magnetic field means that each image requires lengthy scans.

Alexej Jerschow, of New York University, and his colleagues wanted a faster way to image Li dendrites using MRI. They built a plastic cell capped by two lithium electrodes that contained an electrolyte of 1 M LiPF₆ dissolved in 1:1 ethylene carbonate and dimethyl carbonate.

They charged the cell and monitored dendrite growth indirectly using 3D proton MRI. Dendrites influence the volume and magnetic environment of the electrolyte in their immediate surroundings. This means the signal from protons near a dendrite is lower than in pure solution.

The researchers traced the outline of dendrites by identifying 180 × 180 × 180 µm³ volumes, or voxels, that showed at least an 80% decrease in signal during each 17-minute scan. By taking several scans while charging the cell for 26 hours, they followed the dendrite growth until it connected the two electrodes.

Jerschow thinks this approach could be used to image other material changes inside a battery such as electrode cracking or swelling. And because this technique monitors signals from atoms common in electrolytes, he thinks it could also be used for 3D imaging of sodium and magnesium batteries.

This technique is useful because it is a non-intrusive way to image a battery’s interior, says Ji-Guang Zhang, at the Pacific Northwest National Laboratory. He thinks it would be interesting to use this technique to image a cell containing an electrolyte that reduces dendrite growth, such as lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane (read the abstract in Nature Communications).

However, the images produced by this technique suggest that the dendrites are larger than their known diameter of 1–3 µm and estimated volume fraction of up to 10% per voxel, says Gunther Brunklaus, of the University of Münster. To improve the resolution of the image while exploiting superior signal sensitivity, he suggests performing MRI with ions that reflect dendrite growth more directly, such as fluorine anions in the electrolyte accumulating near the lithium electrode surface.

Read the abstract in the Proceedings of the National Academy of Sciences.