Unraveling the Role of Al Substitution in Anionic Oxygen Activity of Ni-Rich Layered Oxide Cathodes

Abstract

Ni-based layered oxides have attracted great attention for the development of high energy and high capacity Li-ion cathodes. The most popular Ni-based cathodes, LiNi x Mn y Co1-x-y O2 (NMC) and LiNi x Co y Al1-x-y O2 (NMC), are indeed derivatives of LiNiO2 by partially substituting Ni with Co, Mn or Al. The current trend to develop high-Ni and low-Co or Co-free layered Ni-based oxide cathodes, makes a strong push to the limit of Ni content in such layered oxides, leading to many common properties with their parent compound, LiNiO2. In fact, lithium nickel oxide often exists off-stoichiometry because of the presence of Ni2+ in the Li layer, which blocks the Li+ diffusion pathway upon lithiation/delithiation. Therefore, a second metal of a minimal content, such as Co or Al, which can somehow facilitate the formation of layered structure is still needed, although they are not strongly favored due to cost and sacrifice upon the use of inactive Al. In this work, a number of Al substituted lithium nickel oxides, LiAl x Ni1-x O2 (0 ≤ x ≤ 0.20), have been synthesized via a solid state reaction, showing good R m layered structure. Along with their parent LiNiO2, the oxygen activity in these compounds have been studied using combined synchrotron X-ray absorption spectroscopy (XAS), resonant inelastic X-ray scattering (RIXS) and differential electrochemical mass spectrometry (DEMS). The goal of this research is to elucidate the role of Al on the anionic oxygen activity in Ni-based layered oxides, including both reversible anionic oxygen redox and irreversible oxygen release during the electrochemical process. The electronic structure of transition metal and O was investigated at different states of charge to obtain a complete picture of cationic and anionic redox during the charge/discharge process. Beyond the redox chemistry, the effect of Al substitution on the phase transformation at highly delithiated states, dominated by H2 to H3 phase transformation for LiNiO2, as well as thermal stability was carefully studied. We hope this work helps provide a comprehensive picture of the role of Al substitution on the overall performance of Ni-based layered oxide cathodes.