From LiNiO2 to Li2NiO3: Synthesis, Structures and Electrochemical Mechanisms in Li-Rich Nickel Oxides

The Li-Ni-O phase diagram contains a variety of compounds, most of which are electrochemically active in Li-ion batteries. Other than the well-known LiNiO2, here we report a facile solid-state method to prepare Li2NiO3 and other Li-rich Ni oxides of composition Li1+xNi1-xO2, (0 <= x <= 0.33). We characterize their crystal and electronic structure, exhibiting a highly oxidized Ni state and defects of various nature (Li-Ni disorder, stacking faults, oxygen vacancies). We then investigate the use of Li2NiO3 as a cathode active material and show its remarkably high specific capacity, which however fades quickly. While we demonstrate that the initial capacity is due to irreversible O-2 release, such process stops quickly in favor of more classical reversible redox mechanisms that allow cycling the material for >100 cycles. After the severe oxygen loss (similar to 15-20%) and prolonged cycling, the Bragg reflections of Li2NiO3 disappear. Analysis of the diffracted intensities suggests the resulting phase is a disordered rock salt-type material with high Li content, close to Li0.5Ni0.5O, never reported to date and capable of Li diffusion. Our findings demonstrate that the Li-Ni-O phase diagram has not been fully investigated yet, especially concerning the preparation of new promising materials by out-of-equilibrium methods.