Capacity Fading of Ni-Rich NCA Cathodes: Effect of Microcracking Extent

Ni-rich Li[Ni1-x-yCoxAly]O-2 (NCA) cathodes (1 - x - y = 0.8, 0.88, and 0.95) are synthesized to investigate the capacity fading mechanism of Ni-rich NCA cathodes. The capacity retention and thermal property of the cathodes deteriorate as their discharge capacity increases when the Ni fraction is increased. The capacity fading correlates well with the anisotropic volume variations caused by the H2-H3 phase transition and the resulting extent of microcracking. Although all three cathodes start to develop microcracks after being charged to 3.9 V, the potential at which microcracks propagated to the outer surface of the particle decreases with increasing Ni content. These microcracks undermine the mechanical integrity of the cathode and facilitate electrolyte penetration into the particle core, which accelerates surface degradation of the internal primary particles. Therefore, mitigating or delaying the H2-H3 phase transition is key to improving the cycling performance of Ni-rich NCA cathodes.