Toward Establishing Electronic and Phononic Signatures of Reversible Lattice Oxygen Oxidation in Lithium Transition Metal Oxides For Li-Ion Batteries

The prospect of accessing anionic (oxygen) oxidation and reduction reversibly in lithium transition metal oxides for the positive electrode offers exciting opportunities to greatly boost the energy density of Li-ion batteries. Unfortunately, the physical mechanisms governing oxygen redox in these oxides remain under debate. In this article, density functional theory studies using maximally localized Wannier functions revealed that deintercalation of both lithium ions from Li_2–xRuO₃ as well as Li_1–xNiO₂ (Li_3/2–xNi_3/2O₃) was dominated by the oxidation of nonbonding states of oxygen or bonding states of oxygen from metal–oxygen bonds, which was accompanied by moderate Ru/Ni oxidation and reduction in the O–O bond distance, facilitated by high metal–oxygen covalency in oxides. In contrast, deintercalation of lithium ions from Li_2–xMnO₃ as well as Li_2–xTiO₃ and Li_2–xSnO₃ was dominated by the oxidation of nonbonding states of oxygen to form O–O p sigma (σ) and π states with accompanied distinct O–O peroxo-like bond formation but without Mn oxidation, which is facilitated by relatively low metal–oxygen covalency. Remarkably, the average oxygen phonon density of states (phonon DOS) of oxides with high metal–oxygen covalency like Li_2–xRuO₃, Li_2–xIrO₃, and Li_1–xNiO₂ was moved to higher frequencies while that of those with low covalency like Li_2–xMnO₃, Li_2–xTiO₃, and Li_2–xSnO₃ was moved to lower frequencies, which could promote oxygen and metal migration and structural instability, leading to irreversible oxygen redox. It is postulated that high metal–oxygen covalency is essential to enable reversible access of oxygen redox along with metal redox in transition metal oxides, which bridges different schools of thoughts for oxygen redox and provide new insights into design of new oxygen-redox capable positive electrodes.