Anomalous calendar aging of Ni-rich cathode batteries: Focusing on structural degradation

Ni-rich layered oxide cathode exhibits significant potential for enhancing battery energy density. This study aims to investigate the long-term calendar aging behavior of Ni-rich batteries. Surprisingly, we uncover a non-monotonic anomaly in aging rate with state-of-charge (SOC) increasing, reaching its peak at approximately 70 % SOC. To comprehend the underlying mechanism, in-depth characterizations and calculations fully proves that the non-monotonic anisotropic stress accumulation (ASA) due to special M-H2-H3 phase transformation of Ni-rich lattice is the critical factor. When the battery at approximately 70 % SOC, it enters M-H2 phase transition region and experiences maximal ASA, which leads to the most severe structure degradation and the lowest capacity retention. As SOC further increases, it enters H2-H3 phase transition region, ASA rapidly decreases and capacity retention inversely improves, this is also why the capacity retention under 100 % SOC is higher than 70 % SOC or even 35 % SOC. Correlation analysis, exceeds 0.94, also reveals that the structural damage caused by ASA is the key factor to the anomalous degradation. This research provides valuable insights that could support future breakthroughs of high-energy density and long-life Ni-rich batteries suitable for long-range electric vehicles.