Degradation Mechanism of Ni-Enriched NCA Cathode for Lithium Batteries: Are Microcracks Really Critical?

A series of Ni-enriched LiNixCoyAlzO2 cathodes (x = 0.80–0.95) were synthesized and evaluated comprehensively to investigate the capacity fading mechanism. Capacity retention was shown to be strongly related to the extent of microcracking within the secondary particles. Moreover, the range and limit of the depth of discharge (DOD), which determined the extent of microcracking, critically affected the cycling stability such that the extremely Ni-rich LiNi0.95Co0.04Al0.01O2 cathode cycled at an upper DOD of 60% exhibited the poorest capacity retention. The anisotropic strain produced by the H2–H3 phase transition was not fully relieved, and persistent microcracks in the discharged state (3.76 V) allowed the electrolyte to penetrate the particle interior. Resultant extended exposure of the interior primary particles within secondary particle to electrolyte attack accelerated structural damage and eventually undermined the mechanical integrity of the cathode particles.