Stability and Redox Mechanisms of Ni-Rich NMC Cathodes: Insights from First-Principles Many-Body Calculations

Ni-rich LiNiaMnbCocO2 (NMC)
cathodes
undergo a series of degradation reactions, a prominent one being oxygen
loss from the surface of the NMC particles; this process is more pronounced
as Ni content is increased and at high voltages. Our first-principles
study examines the redox behavior of transition metals (TMs) and O
in Ni-rich NMC cathodes as a function of (de)lithiation. We use ab
initio multiple scattering, density-functional theory (DFT)-based
core-loss spectroscopy, and dynamical mean-field theory (DMFT) to
give a many-body treatment of both dynamic and static correlations.
Despite Ni, Mn, and Co K-edges calculated using ab initio multiple
scattering based on Green’s functions showing an excellent
match with experimentally obtained X-ray absorption near-edge spectra
(XANES), we demonstrate that the ionic model of ascribing shifts in
the XANES spectra to changes in metal oxidation states is inappropriate.
We show that in these cases, which are characterized by strong covalency
between the TM and oxygen, DMFT calculations based on Wannier projections
are to date to the best of our knowledge the most accurate as well
as computationally accessible method to calculate charges and hence
assign oxidation states accurately. Due to the corresponding charge
transfer from O p to Ni d, a ligand hole forms on O in Ni-rich regions.
The individual Ni charge remains fairly constant throughout the charging/discharging
process, particularly in Ni-rich environments in the material. In
contrast, O has dual redox behavior, showing greater involvement in
redox in Ni-rich regions while showing negligible redox involvement
in Ni-poor regions. The Ni–O covalent system starts participating
in redox around a state of delithiation of ∼17%, which represents,
in our system, the beginning of the charge. Contrary to previous DFT
calculations, we show that Co oxidation does not occur at the very
end of charge but rather starts at an earlier state of delithiation
of ∼67%. The dual behavior of O in terms of participation in
the redox process helps explain the overall higher relative stability
of lower Ni content NMCs compared to Ni-rich NMCs or LiNiO2 in terms of O loss and evolution of singlet oxygen.