*Article* **Non-Uniform Circumferential Expansion of Cylindrical Li-Ion Cells—The Potato Effect**

**Jessica Hemmerling 1,\*, Jajnabalkya Guhathakurta 2, Falk Dettinger 1, Alexander Fill <sup>1</sup> and Kai Peter Birke <sup>1</sup>**


**Abstract:** This paper presents the non-uniform change in cell thickness of cylindrical Lithium (Li)-ion cells due to the change of State of Charge (SoC). Using optical measurement methods, with the aid of a laser light band micrometer, the expansion and contraction are determined over a complete charge and discharge cycle. The cell is rotated around its own axis by an angle of α = 10◦ in each step, so that the different positions can be compared with each other over the circumference. The experimental data show that, contrary to the assumption based on the physical properties of electrode growth due to lithium intercalation in the graphite, the cell does not expand uniformly. Depending on the position and orientation of the cell coil, there are different zones of expansion and contraction. In order to confirm the non-uniform expansion around the circumference of the cell in 3D, X-ray computed tomography (CT) scans of the cells are performed at low and at high SoC. Comparison of the high resolution 3D reconstructed volumes clearly visualizes a sinusoidal pattern for non-uniform expansion. From the 3D volume, it can be confirmed that the thickness variation does not vary significantly over the height of the battery cell due to the observed mechanisms. However, a slight decrease in the volume change towards the poles of the battery cells due to the higher stiffness can be monitored.

**Keywords:** lithium-ion battery cell; volumetric expansion; mechanical degradation; state of charge dependency; cell thickness; mechanical aging; non-uniform volume change
