Redox Chemistry in Conventional Layered Lithium Metal Oxide Cathodes

Abstract

Li-ion battery technology is one of the best performing energy storage devices and holds the best promise to power the upcoming electrical vehicles (EV). Extensive efforts have been devoted to further advancing conventional layered oxide cathodes, such as LiNixMnyCozO2 and LiNixCoyAlzO2 (0 < x, y, z <1), for Li-ion batteries of even better performance and lower cost ($/Wh). Strategies that are largely pursued include increasing Ni content to achieve a higher practical capacity at a given charge cutoff voltage and/or increasing charge cutoff voltage for a given composition. Alongside with these research efforts, questions regarding the stability of electrode, especially at highly delithiated states, electrolyte and electrode/electrolyte interface arise. In this work, of particular interest is the redox chemistry upon electrochemical (de)lithiation of LiNiO2, the parent compound of Ni-rich layered oxides, with a key focus on highly delithiated states in the high voltage region. Electrochemistry of LiNiO2 seems straightforward, in principle, Ni3+/Ni4+ redox can perfectly accommodate 1 Li+ transport during (de)lithiation process. However, the precise state at full oxidation of Ni3+ to Ni4+ and/or full removal of 1 Li+ are difficult to achieve because of the possible side reactions between highly charged electrode and electrolyte, for example, surface Ni reduction and gas evolution upon charge often occur at high voltages. Here, the electronic states of Ni and O are probed at different states of charge through combined soft X-ray absorption (XAS), resonant inelastic X-ray spectroscopy (RIXS) and in situ differential electrochemical mass spectrometry (DEMS) in the high voltage region to elucidate the explicit redox chemistry in LiNiO2. This work is anticipated to provide additional insights into the redox chemistry of conventional Ni-rich layered oxides, especially its utilization in the high voltage region.