Geometrical Inhomogeneities as Cause of Mechanical Failure in Commercial 18650 Lithium Ion Cells

The mechanical degradation of 18650 lithium ion cells is studied by X-ray computed tomography and is correlated to the electrochemical performance. A method for the geometrical analysis of electrodes by computer tomography is developed and applied to charged and discharged cells. As shown in earlier studies, the geometry of the jelly roll is inhomogeneous leading to mechanical stress during charge/discharge cycles. This effect leads to significant deformations of the jelly roll, which can be analyzed by computed tomography. The detailed analysis reveals that expansion of the anode takes place as expected during charging, but the degree of expansion depends on the position within the battery cell: the largest expansion during charging was found within the area of strongest deformations, whereas other areas without any expansion of the jelly roll were also observed. It is reasoned that the observed inhomogeneous expansion/contraction contribute significantly to cell degradation. The strong expansion within the deformed areas leads to sharp bending of the electrodes resulting in delamination of active layers. On the other hand, the absence of anode expansion reflected by a lack of increase in thickness when charging may indicate pore clogging assuming that the additional volume of graphite with intercalated lithium has to be accommodated within the pore structure.