paper

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

arXiv:2205.10462 · doi:10.1016/j.joule.2023.06.017

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 O loss from the (012) surface of delithiated LiNiO, 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. For LiNiO, NiO, 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) and -1 (NiO). Predicted XAS Ni and O -edge spectra are in excellent agreement with experimental XAS spectra, confirming the predicted charge states. The calculations also show that a high-voltage O -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 surface loss route observed here consists of 2 surface O radicals combining to form a peroxide ion, which is oxidised to O, leaving behind 2 O vacancies and 2 O ions: effectively 4 O radicals disproportionate to O and 2 O ions. The reaction liberates ca. 3 eV per O molecule. Singlet oxygen formation is caused by the singlet ground state of the peroxide ion, with spin conservation dictating the preferential release of O, the strongly exergonic reaction providing the free energy required for the formation of O in its excited state.

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