Degradation Mechanism of Highly Ni-Rich Li[NixCoyMn1–x–y]O2 Cathodes with x > 0.9

A series of Ni-rich Li[Ni_xCo_(1-x)/2Mn_(1-x)/2]O₂ (x = 0.9, 0.92, 0.94, 0.96, 0.98, and 1.0) (NCM) cathodes are prepared to study their capacity fading behaviors. The intrinsic trade-off between the capacity gain and compromised cycling stability is observed for layered cathodes with x ≥ 0.9. The initial specific capacities of LiNiO₂ and Li[Ni_0.9Co_0.05Mn_0.05]O₂ are 245 mAh g^-1 (91% of the theoretical capacity) and 230 mAh g^-1, and their corresponding capacity retentions are 72.5% and 88.4%. However, the capacity retention characteristic deteriorates at an increasingly faster rate for x > 0.95, in contrast with the nearly linear increase of specific capacity. The fast capacity fading stems from the chemical attack of the cathode by the electrolyte infiltrated through the microcracks, resulting from the mechanical instability inflicted by the anisotropic internal strain caused by the H2 ⇆ H3 phase transition. Thus, the capacity fading of the NCM cathodes for x > 0.9 critically depends on the extent of the H2 → H3 phase transition. Retardation or protraction of the H2 ⇆ H3 phase transition by engineering the microstructure should improve the cycle life of these highly Ni-enriched NCM cathodes.