According to literature a shoulder appears in the relaxation voltage curve if Lithium plating occurred in the charge step due to the coexistence of Lithium metal (0 V vs. Li/Li+) and graphite (0.08 V vs Li/Li+) on the anode side [1].

[2] confirms that Lithium deposition overtakes the SEI growth as the leading capacity fade mechanism at 1C rate and 25 °C:

References

[1] Voltage relaxation and impedance spectroscopy as in-operando methods for the detection of lithium plating on graphitic anodes in commercial lithium-ion cells (2016)

[2] Investigation of capacity recovery effects due to rest periods during high current cyclic ageing tests in Li-ion cells and their influence on lifetime