Insights into Fast-Charge-Induced Cracking and Bulk Structural Deterioration of Ni-Rich Layered Cathodes for Lithium-Ion Batteries.

Ni-rich NMCs (lithium nickel manganese cobalt oxides) have been extensively utilized as a type of cathode material for Li-ion batteries due to their high energy density and cost efficiency. Meeting the increasing consumer demand for fast charging has become an urgent priority within the industry. Studies on structural degradation caused by fast charging, however, remain limited, especially concerning the understanding of the inter-related failure phenomena and mechanisms. In this study, our results reveal that prolonged fast-charge cycling of an NMC811 cathode leads to significant crack growth at both nano- and microscales. The rapid propagation of cracks from the particle interior to the surface significantly accelerates electrolyte infiltration, leading to the formation of the cathode electrolyte interphase inside NMC811 particles. A fatigue-cracking mechanism based on Li concentration gradients is proposed to be the root cause of crack formation. Additionally, electron energy loss spectroscopy and X-ray diffraction analysis provide direct evidence of irreversible Li loss and layered structure distortion in the bulk material of fast-charged NMC811, which contributes to the significant capacity loss of the NMC811 cathode after fast-charge cycling. The intragranular transition metal/Li cation mixing is also observed within the particle interior of fast-charged NMC811, which further deteriorates the material. This study offers valuable insights into the structural challenges encountered by Ni-rich NMC cathodes under fast-charge conditions, providing a foundational framework for designing strategies to enhance their structural integrity and electrochemical performance in demanding applications.