The origin of the aggressive degradation of Ni-rich transition metal oxide cathodes for high-energy density lithium-ion batteries

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Ni-rich transition metal oxide cathodes (Ni-rich NMC), which can be represented as the formular of LiNixCoyMn1-x-y (x ​≧ ​0.5, y ​< ​0.5), have emerged as the promising electrode material for high-energy density lithium-ion batteries. This battery type can achieve high capacities due to the high amount of Ni as it can provide a large number of electrons by two-stage redox reaction between Ni2+/Ni3+ and Ni3+/Ni4+. However, the increase of the Ni content in the NMC cathodes often results in a capacity fading and poor cycle life of the batteries because of the cathode degradation. Herein, the origin of the active involvements of the Ni element in the degradation process has been studied. The degradation of the Ni-rich NMC is likely to be triggered by NiO on the surface with the cathode-electrolyte interphase (CEI) formation, followed by the disorder in the layered structure both at surface and bulk after the initial cycle. The difference in the Ni content of the active material leads to the discrepancy of the surface environment. To gain deep insights into the aggressive deterioration of the Ni-rich NMC material, a density functional theory (DFT) simulation has been adopted for the cathodes with different Ni concentrations (Ni: 10%, 33%, 50%, 80%, and 100%). Also, the electronic structure of LiNi0.5Co0.2Mn0.3O2, and LiNi1/3Co1/3Mn1/3O2 are examined at 50% Li+ (de)intercalation. The findings deduced from the DFT study suggest that the Ni(d)-O(p) character with high oxidation Ni (Ni3+ and/or Ni4+) is the key to understand the (electro)chemical process involved with the degradation mechanism of the cathode.