Understanding Surface Densified Phases in Ni-Rich Layered Compounds

Understanding structural transformation at the surface of Ni-rich layered compounds is of particular importance for improving the performance of these cathode materials. In this Letter, we identify the surface phases using first-principles-based kinetic Monte Carlo simulations. We show that slow kinetics precludes the conventional Li<sub>0.5</sub>NiO<sub>2</sub> spinel to form from its layered parent phase at room temperature. Instead, we suggest that densified phases of the types Ni<sub>0.25</sub>NiO<sub>2</sub> and Ni<sub>0.5</sub>NiO<sub>2</sub> can form by Ni back diffusion from the surface owing to oxygen loss at highly charged states. Our conclusion is supported by the good agreement between the simulated STEM images and diffraction patterns and previously reported experimental data. While these phases can be mistaken for spinel and rock salt structures in STEM, they are noticeably different from these common structure types. We believe that these results clarify a long-standing puzzle about the nature of surface phases on this important class of battery materials.