Cycling at different C-rates, as well as storage aging is performed with five different types of commercial 18650-type Lithium-ion cells. X-ray computed tomography measurements show strong deformations of the inner part of the jelly rolls for three different cell types without center pin after cycling at rates in the range of 3.6-16.6C. For cells cycled at 1C, this deformation is less pronounced, whereas it is totally absent for stored cells. The consequence on capacity loss of the cells was investigated by Post-Mortem analysis with unrolled electrodes, as well as scanning electron microscopy imaging with cross-sections of 18650 cells. In order to investigate the reason for the jelly roll deformation, we conducted in-operando temperature measurements in the middle of the jelly roll and at the cell surface during discharge with 16C for one selected cell type. Finally, the effect of a center pin on jelly roll deformation is tested by X-ray computed tomography imaging for two different cell types after cycling.

Li-ion batteries are a key technology for both electro-mobility and stationary energy storage systems. In order to be able to represent and improve their service life in these applications, a better understanding of the processes which lead to the degradation of the individual cells is essential. The work presented in this article focuses on the comparative post mortem analysis of type 18650 commercially available cells containing the state of the art active materials (Cathode: LiMn2O4 (LMO) and Li(Ni1/3Mn1/3Co1/3)O2 (NMC), Anode: Graphite). These cells were subjected to various different ageing procedures. Amongst other effects, the cells investigated revealed signs of crack formation in the LMO- and NMC-particles, a loss in the mechanical integrity of the cathode active mass and plastic deformation of cell structure together with pronounced delamination between the active mass layers, the separator and the current collector.

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