The Effect of Anode Slippage on Cathode Cutoff Potential and Degradation Mechanisms in Ni-Rich Li-Ion Batteries

摘要

High energy-density Li-ion batteries based on Ni-rich layered cathodes are the state-of-the-art technologies for electric vehicles. However, batteries using these advanced cathode materials suffer from rapid performance fading and the underlying mechanisms are not fully understood. In this work, we report a critical turning point during the aging of graphite/LiNi_(0.8)Mn_(0.1)Co_(0.1)O_2 (NMC811) full cells after which the degradation is significantly accelerated. This turning point is identified using differential voltage analysis (DVA) applied to standard two-electrode full cell data, which shows a decreased capacity usage of the graphite with cycle number. The graphite becomes progressively less lithiated, as confirmed by operando long-duration X-ray diffraction, and therefore has higher electrochemical potential when the cell has reached the end of charge. As the upper cutoff voltage of the full cell is fixed, this increase leads to a proportional raise of the cathode potential and an accelerated impedance increase is observed from this point. This mechanism is expected to be universal for the vast majority of today's Li-ion battery chemistries, particularly for Ni-rich cathodes whose degradation is shown to be extremely sensitive to the upper cutoff voltage, and our work provides fundamental guidelines for developing effective countermeasures.