Lithium-ion cells deliver very high energy densities compared to many other battery technologies, but they bring with them a danger of fire or explosion if they are misused. We’re mostly aware of the battery conditioning requirements to ensure cells stay in a safe condition, but how much do we know about the construction of the cells as a factor? [Lumafield] is an industrial imaging company, and to demonstrate their expertise, they’ve subjected a large number of 18650 cells from different brands to a CT scan.
The construction of an 18650 sees the various layers of the cell rolled up in a spiral inside the metal tube that makes up the cell body. The construction of this “jellyroll” is key to the quality of the cell. [Lumafield’s] conclusions go into detail over the various inconsistencies in this spiral, which can result in cell failure. It’s important that the edges of the spiral be straight and that there is no electrode overhang. Perhaps unsurprisingly, they find that cheap no-name cells are poorly constructed and more likely to fail, but it’s also interesting to note that these low-quality cells also have fewer layers in their spiral.
We hope that none of you see more of the inside of a cell in real life than you have to, as they’re best left alone, but this report certainly sheds some light as to what’s going on inside a cell. Of course, even the best cells can still be dangerous without protection.
