Thickness change and jelly-roll deformation and its impact on the aging and lifetime of commercial 18650 cylindrical li-ion cells with silicon containing anodes and NCA cathodes

Summary:

In this work, different commercial available cylindrical cells in the format 18650 with each NCA cathode and anodes containing different silicon materials were electrochemically long-term cycled. Multiple Computed Tomography (CT) images were carried out after a settled number of completed cycles. Two voltage windows and their influence on the aging of the cells were investigated. With the CT-images, the influence of the aging of the cells in terms of volume expansion and jelly-roll deformation could be made visible. With an optical micrometer, the reversible and irreversible thickness change of the cells was studied. Furthermore the electrode expansion was investigated with an electrochemical dilatometer showing that the Ni-rich cathode has also an impact on the volume change of the cells. Depending on the silicon active material used, differences in the cells´ aging behavior were observed. These findings were linked to the rest of the (non-) destructive and post-mortem analyzes. The aging behavior of the cells could be tracked during the aging experiments. With the help of non-destructive analyzes such as differential voltage analysis (DVA), pulse tests to determine the internal cell resistance and different evaluations using the generated CT data, the inhomogeneous thickness change of the cells especially at critical spots such as the anode and cathode tabs could thus be revealed. With direct visual of the jelly-roll deformation and the remaining space in the winding core, reasons for capacity degradation or resistance increase can be directly seen.

From cell opening and post-mortem analyzes a much better understanding of the cells´ aging mechanisms was generated. With surface electron microscopy with energy dispersive X-ray analysis (SEM/EDX), inductively coupled plasma – optical emission spectrometry (ICP-OES) and thermogravimetric analysis (TGA), the formation of passivation layers on the electrodes because of decomposition products from electrolytes or conductive salts was observed. By comparing the results with fresh, non-cycled cells of the same type, individual causes of failure were revealed. Together with DVA it was found that the main aging mechanism is the loss of lithium from the cathode together with the volume change of the anode. Cells with higher silicon content and pure silicon instead of SiOx have furthermore a faster degradation because of the additionally higher thickness change during charging and discharging. Delamination of the electrode coating, because of particle swelling, binder failure and jelly-roll deformation is seen after unrolling the aged cells.
Limiting the voltage range leads to longer lifetime and less resistance increase in all types of tested cells. This is primarily attributed to less volume expansion of the silicon in the anode resulting in less electrolyte degradation, cathode aging and jelly-roll deformation.