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 nickel-rich cathodes

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Two types of commercial cylindrical cells in the format 18650 both with NCA cathode and anodes containing different silicon materials were electrochemically long-term cycled. Two voltage windows and their influence on the aging of the cells were investigated. Multiple CT images after a pre-determined number of completed cycles reveal volume expansion and jelly roll deformation during the aging of the cells. With an optical micrometer, the reversible and irreversible thickness change was studied in operando. Furthermore the electrode expansion was investigated with an electrochemical dilatometer. Depending on the type of silicon active material used, differences in the aging behavior of the cells were observed. These findings were linked to the rest of the non-destructive, destructive and post-mortem analyses. With the help of differential voltage analysis, pulse tests and different evaluations using the generated CT data, the inhomogeneous thickness change of the cells especially at critical spots could thus be revealed and reasons for capacity degradation or resistance increase can be directly seen. Due to post-mortem analysis a much better understanding of the underlying aging mechanisms was generated. With SEM/EDX analysis, ICP-OES and TGA, the formation of passivation layers on the electrodes was observed. By comparing the results with new cells, individual causes of failure were revealed. 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 pure Si instead of SiOx have exhibited 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, less resistance increase and overall less aging of silicon in all tested cells.