Oxygen Hole Formation Controls Stability in LiNiO2 Cathodes: DFT Studies of Oxygen Loss and Singlet Oxygen Formation in Li-Ion Batteries

Abstract

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 O2 loss from the (012) surface of delithiated LiNiO2, 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 LiNiO2. For LiNiO2, NiO2, 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 (LiNiO2) and –1 (NiO2). 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 O2 surface loss route observed here consists of 2 surface O•– radicals combining to form a peroxide ion, which is oxidised to O2, leaving behind 2 O vacancies and 2 O2– ions: effectively 4 O•– radicals disproportionate to O2 and 2 O2– ions. The reaction liberates ca. 3 eV per O2 molecule. Singlet oxygen formation is caused by the singlet ground state of the peroxide ion, with spin conservation dictating the preferential release of 1O2, the strongly exergonic reaction providing the free energy required for the formation of 1O2 in its excited state.