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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.
Findings from two surveys of all physicians in Pima County, Arizona, in 1985 and 1990 to determine the trajectory of hospital/physician relationships suggest that conflicts have increased in almost all areas. Physicians report the greatest number of and increase in conflicts in traditional problem areas such as the quality of nursing and response to equipment requests. They report a smaller increase in conflicts relating to the new competitive marketplace, including hospital efforts to influence medical practice. Both types of problems are most prevalent among younger physicians. The new marketplace thus appears to exacerbate traditional hospital/physician conflicts more than it generates new areas of contention.
