Long-term cycling induced jelly roll deformation in commercial 18650 cells

The effects of extended charge/discharge cycling on the morphology of the jelly roll of commercial 18650 lithium-ion battery cells (Sanyo UR18650E) are shown and discussed. Using micro X-ray computed tomography combined with post-mortem analysis it is shown that the jelly roll exhibits significant deformations after charge/discharge cycling. This effect appears despite the presence of a solid inner pin, which had been suggested in literature as prohibitive for jelly roll deformation. The effect is related to an inhomogeneous architecture of the cells caused mainly by the cathode tab in the vicinity of which deformations were observed most often. While it is shown here that such deformations eventually lead to delamination of the otherwise rather stable cathode coating and cause a rapid capacity fade, jelly roll deformation has also been observed after more than 1700 equivalent full cycles on cells which show a typical, non-accelerated ageing behavior for this cell type. A correlation between occurrence of delamination and occurrence of deformation was clearly identified by combining post-mortem and computed tomography analysis. It is discussed, if the well-documented deformation is caused by thickness variations of the anode and cathode during charge/discharge cycles. Furthermore, an in-depth characterization of the cell design is documented.