Oxygen hole formation controls stability in LiNiO 2 cathodes

Ni-rich lithium-ion cathode materials achieve both high voltages and capacities but are prone to structural instabilities and oxygen loss. The origin of the instability lies in the pronounced oxidation of O during delithiation: for LiNiO 2, NiO 2, and the rock salt NiO, density functional theory and dynamical mean-field theory calculations based on maximally localized Wannier functions yield a Ni charge state of ca. +2, with O varying between −2 (NiO), −1.5 (LiNiO 2 ), and −1 (NiO 2 ). Calculated X-ray spectroscopy Ni K and O K -edge spectra agree well with experimental spectra. Using ab initio molecular dynamics simulations, we observe loss of oxygen from the (012) surface of delithiated LiNiO 2, two surface O ⋅− radicals combining to form a peroxide ion, and the peroxide ion being oxidized to form O 2, leaving behind two O vacancies and two O 2− ions. Preferential release of 1 O 2 is dictated via the singlet ground state of the peroxide ion and spin conservation.