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Li-ion batteries based on Ni-rich layered cathodes are the state-of-the-art technology for electric vehicles; however, batteries using these advanced materials suffer from rapid performance fading. In this work, we report a critical turning point during the aging of graphite/LiNi0.8Mn0.1Co0.1O2 (NMC811) full cells, after which the degradation is significantly accelerated. This turning point was identified using differential voltage analysis (DVA) applied to standard two-electrode data, which shows that graphite becomes progressively less lithiated, as confirmed by operando long-duration X-ray diffraction, and therefore has a higher electrochemical potential at the end of charge. This increase leads to a proportional increase in 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 Li-ion battery chemistries, particularly for Ni-rich cathodes, whose degradation is extremely sensitive to the upper cutoff voltage, and our work provides fundamental guidelines for developing effective countermeasures.
Dose et al. (Sun,) studied this question.