In high nickel lithium ion batteries, a counterintuitive “spoon shaped” capacity fade has been reported during calendar aging, where capacity loss increases with state of charge (SOC) up to approximately 80 % before decreasing again toward 100 % SOC (Zülke et al., 2021, Majherova et al., 2025). This behavior challenges the conventional expectation that higher electrode potentials accelerate degradation and complicates capacity based diagnostics. Previous studies on nickel rich 21700 cells have documented this effect and attributed it to cathode lithiation phenomena that increase the available lithium inventory (Hartmann et al., 2024; Keil & Jossen, 2017). However, it remains unclear whether the spoon shaped behavior persists once cells transition from linear to nonlinear aging, particularly as they approach the knee or elbow point where degradation accelerates sharply.

This study investigates the persistence of the spoon shaped capacity fade beyond the linear aging regime through a comprehensive calendar aging study on cylindrical 21700 NCA/C Si cells across the full SOC range, with particular emphasis on high SOC conditions. Cells were stored for 455 days with recurrent check up tests conducted every 35 days. Capacity evolution at C/20 and C/1, as well as DC internal resistance, were monitored throughout the aging period.

The results confirm that spoon shaped capacity behavior is present during the linear aging phase but diminishes as cells stored at SOCs close to 100 % transition earlier into nonlinear aging. In this regime, the apparent advantage in capacity retention at very high SOC disappears, and a monotonic SOC dependent degradation trend is restored. This demonstrates that while capacity measurements may temporarily suggest reduced degradation at very high SOC, the underlying degradation processes remain driven by elevated electrode potentials and are merely masked during early aging by cathode lithiation effects. For the investigated cells, the observed loss of rate capability further indicates electrolyte degradation as a critical aging mechanism at high potentials. In this case, DC internal resistance consistently reflects the actual degradation state throughout the complete test duration, in contrast to capacity based indicators.

Overall, these findings highlight the limitations of capacity based health indicators and emphasize the importance of mechanistically informed diagnostics for accurate lifetime assessment, particularly for high nickel lithium ion batteries subjected to prolonged high SOC storage.