Lithium plating (accumulation of metallic lithium) is a safety-critical phenomenon that occurs mainly during fast charging of lithium-ion batteries at cold temperatures. It is caused on the anode by the limited capability to absorb lithium ions at the desired rate. Reversible and irreversible plating can be accompanied by the formation of dendrites and lead to safety-critical short circuits. Irreversible plating additionally leads to a significant capacity loss due to the loss of cyclable lithium. To optimize fast charging, battery lifetime and safety for all requirement profiles, it is central to detect the formation of metallic lithium and dendrites at an early stage. In addition to the overall detection of lithium plating, the morphology of the plated structures is also of interest. Dendritic/rod-like lithium plating structures need to be evaluated differently from a safety standpoint than chunky/mossy plating.

For this purpose, commercial lithium-ion cells (850mAh pouch) were cycled under harsh conditions (low temperature and high C rates), opened in an argon filled glove box, and then examined with post-mortem analyses. A common method is the examination of the anode cell surface with a scanner or an optical/laser microscope. The advantage here is that you can see the typical silver shiny texture of fresh lithium plating. To study the plated structures in more detail, a scanning electron microscope (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX) was used.

In the submitted contribution, in addition to the well-known e.g., dendritic/mossy morphology, a different plating morphology will be shown – rhombic dodecahedral. The structure consists of well-defined polyhedra with 12 faces and is already described in the literature in laboratory cells as a new plating morphology that occurs at ultrafast charging rates [doi.org/10.1038/s41586-023-06235-w, doi.org/10.1021/acs.nanolett.2c02792].

Charging our commercial full cells at temperatures 2C with constant capacity charge (to avoid overcharging) leads to the formation of polyhedral structures on top of the graphite particles (visible in optical/laser- and scanning electron microscopy). With decreasing temperature, the polyhedral shape becomes more defined, so that lower temperatures (in addition/combination to high charge rates) support the growth of the polyhedron structures. With state-of-the-art EDX detectors, it is possible to detect the very weak lithium signal (ultra-soft Li Kα X-rays at around 54eV) directly in the EDX spectrum. Therefore, it was possible to validate the elemental compositions of the polyhedral structures and confirm that it is pure lithium metal.