Ni-rich layered oxides LiNi_1-xM_xO₂ (x = 0.1-0.2, M = Ni, Co, Mn, Al, etc.) have been widely viewed as the next generation cathode materials for Li ion battery, due to their high capacities (>200 Ah/kg), high energy densities (>800 Wh/kg), high ionic diffusivity (10^-8 cm²/s) and high electronic conductivity (10^-1 S/cm).[1] However, these oxides have faced a big challenge of severe structural instability. Depending on different particle sizes, morphologies and doping/alloying treatments, the materials normally have 40%-80% capacity degradation after ~100 charging/discharging cycles. Based on traditional understanding of layered oxides, three main hypothesis[2-3] have been proposed within battery research community: 1) surface phase transition and interaction, 2) bulk phase transformation, and 3) mechanical degradation involving inter-grain crack formation. However, it is still not clear which mechanism is playing the dominating role during degradation of Ni-rich layered oxides. Unfortunately, due to lack of systematic theoretical study on the thermodynamic and kinetic process of degradation upon delithiation, the fundamental driving forces and the relative kinetics are poorly understood. Therefore, in this work, we report our recent results from atomic scale Density Functional Theory modeling, which carefully simulates the processes relevant to degradation of Ni-rich layered oxides, from which we have delivered significant theoretical understanding of the above mentioned three hypothesis.

Furthermore, based on these theoretical understandings, we have performed experiments to validate the proposed surface modifications through both conventional wet chemistry coating and advanced particle Atomic Layer Deposition (ALD). These computational and experimental combined studies could provide us deeper understanding of degradation of Ni-rich layered oxides, and shed light on the future direction towards stabilized high energy cathode materials.

[1] J. Xu, F. Lin, M. M. Doeff, W. Tong, J. Mater. Chem. A, 2017, 5, 874

[2] M. D. Radin, Y. S. Meng, A. Van der Ven, etc., Adv. Energy Mater. 2017, 1602888

[3] W. Li, B. Song, A. Manthiram, Chem. Soc. Rev., 2017, 46, 3006