Identifying the origin of inhomogeneous degradation and irreversible swelling inside cylindrical 21700 high-energy lithium-ion cells

Abstract

Cylindrical cells are gaining market volume in the production of batteries for electric vehicles among others due to high flexibility with regard to space utilization. However, the internal winding structure can be prone for inhomogeneities regarding current distribution, electrode degradation, and mechanical stress. In this study, 27 cylindrical SDI INR21700 50E cells with a nickel-rich NCA cathode and a Si/C composite anode were subjected to five individual aging procedures. Computed tomography scans were conducted at 100%, 90% and 80% capacity retention to investigate internal swelling and deformation of the jelly roll non-destructively. The cells were disassembled at 80% capacity retention for post-mortem analysis on coin half-cells harvested from different locations of the jelly roll to quantify inhomogeneous aging and electrode swelling.Increased cycle depth and discharge current exacerbated inhomogeneous degradation across the electrode length. In comparison to 860 equivalent full cycles (EFC) at 0.5C and 100% cycle depth, limiting the SoC range between 20% to 80% at 25°C increased cycle life to 4150EFC despite a high discharge current of 2C. Minimizing utilization of the silicon content and avoiding lithium plating, the cathode and the anode retained around 4% and 2% increased capacity at the inner winding area in comparison to the outer area, presumably due to increased mechanical pressure at the inner area. At the same time, the inner area displayed reduced swelling, further suggesting increased mechanical pressure at the inner area. Overall, the irreversible swelling measured 6% to 16% and 8% to 20% for the cathodes and the anodes after disassembly, depending on the operating conditions and the location on the electrode. To prevent non-linear capacity fade and deformation of the jelly roll, the results suggest to restrict utilization of the silicon content and to avoid high charging currents at low temperature.