The stability and redox mechanisms of Ni-rich NMC cathodes: Insights from first-principles many-body calculations

Ni-rich LiNi_aMn_bCo_cO_2 (NMC) cathodes undergo a series of degradation reactions, a prominent one being oxygen loss from the surface of the NMC particles, this process being more pronounced as Ni content is increased and at high voltages. Our first-principles study examines the redox behavior of transition metals 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 characterised by strong covalency between the transition metal and oxygen, DMFT calculations based on Wannier projections are essential 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 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 behaviour 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 LiNiO_2 in terms of O loss and evolution of singlet oxygen.