Some cells seem to regenerate their capacity (i.e. negative Cell capacity fade) at the beginning of the tests if ageing stress factors are low (T = 45 or 30 ° C, SoC = 65 or 30%). This phenomenon is probably due to an initial decrease of impedance in the firsts stages of SEI formation. By performing capacity measurements at low rates, for example C/5 or lower, distortion due to this initial decrease of impedance is minimised. [2]
In other words, this capacity recovery is not fundamental in the sense that there are more Lithium ions to cycle or accessible slots in the electrodes to store the ions. The capacity appears to recover when the cell impedance decreases because the discharge protocol with a fixed cutoff cell voltage stops earlier when the Cell internal resistance is higher.
Related:
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[2] Cell degradation in commercial LiFePO4 cells with high-power and high-energy designs (2014)