On the microscopic origin of reversible and irreversible reactions of LiNixCoyMnx cathode materials: Ni-O hybrid bond formation vs. cationic and anionic redox

TL;DR

This study reveals that redox reactions in Ni-rich layered cathodes involve reversible Ni oxidation and Ni-O hybridization, with hybridization degree affecting capacity and potential, challenging previous assumptions of pure cationic or anionic redox processes.

Contribution

It provides the first experimental evidence that Ni-O hybridization, not pure cationic or anionic redox, governs redox reactions in layered Ni-based cathodes.

Findings

Ni-O hybridization is reversible and crucial for redox processes.

Irreversible reactions cause structural collapse limiting capacity.

Ni content influences hybridization degree, electronic structure, and voltage.

Abstract

Energy density limitations of layered oxides with different Ni contents, i.e., of the conventional cathode materials in Li-ion batteries, are investigated across the first discharge cycle using advanced spectroscopy and state-of-the-art diffraction. For the first time unambiguous experimental evidence is provided, that redox reactions in NCMs proceed via a reversible oxidation of Ni and a hybridization with O, and not, as widely assumed, via pure cationic or more recently discussed, pure anionic redox processes. Once Ni-O hybrid states are formed, the sites cannot be further oxidized. Instead, irreversible reactions set in which lead to a structural collapse and thus, the lack of ionic Ni limits the reversible capacity. Moreover, the degree of hybridization, which varies with the Ni content, triggers the electronic structure and the operation potential of the cathodes. With an increasing amount of Ni, the covalent character of the materials increases and the potential decreases.