Chemistry Colloquium - Singlet oxygen formation in Ni-rich cathodes for Li-ion batteries

Dr. Annalena Genreith-Schriever

Title: Singlet oxygen formation in Ni-rich cathodes for 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. Using density functional theory calculations and dynamical mean-field theory calculations based on maximally localised Wannier functions, we find that the origin of the material’s instability lies in the pronounced oxidation of O during delithiation, i.e., O plays a central role in Ni-O redox in LiNiO₂. 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 O^2– ions: effectively 4 O^•– radicals disproportionate to O₂ and 2 O^2– 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.