  Ni-rich cathode materials achieve both high voltages and capacities in Li-ion
batteries but are prone to structural instabilities and oxygen loss via the
formation of singlet oxygen. Using ab initio molecular dynamics simulations, we
observe spontaneous O$_2$ loss from the (012) surface of delithiated LiNiO$_2$,
singlet oxygen forming in the process. We find that the origin of the
instability lies in the pronounced oxidation of O during delithiation, i.e., O
plays a central role in Ni O redox in LiNiO$_2$. For LiNiO$_2$, NiO$_2$, and
the prototype rock salt NiO, density-functional theory and dynamical mean-field
theory calculations based on maximally localised Wannier functions yield a Ni
charge state of ca. +2, with O varying between -2 (NiO), -1.5 (LiNiO$_2$) and
-1 (NiO$_2$). Predicted XAS Ni $K$ and O $K$-edge spectra are in excellent
agreement with experimental XAS spectra, confirming the predicted charge
states. The calculations also show that a high-voltage O $K$-edge feature at
531 eV previously assigned to lattice O-redox processes could alternatively
arise from O-redox induced water intercalation and O-O dimer formation with
lattice O at high states of charge. The O$_2$ surface loss route observed here
consists of 2 surface O$^{.-}$ radicals combining to form a peroxide ion, which
is oxidised to O$_2$, leaving behind 2 O vacancies and 2 O$^{2-}$ ions:
effectively 4 O$^{.-}$ radicals disproportionate to O$_2$ and 2 O$^{2-}$ ions.
The reaction liberates ca. 3 eV per O$_2$ molecule. Singlet oxygen formation is
caused by the singlet ground state of the peroxide ion, with spin conservation
dictating the preferential release of $^1$O$_2$, the strongly exergonic
reaction providing the free energy required for the formation of $^1$O$_2$ in
its excited state.
