Impact of oxygen defects on electrochemical processes and charge compensation of Li-rich cathode material Li_1.2Mn_0.6Ni_0.2O_2−δ

Oxygen defects are closely related to the battery performance of oxide-based cathode active materials for lithium-ion batteries, and especially, positive influences have been reported in Li-rich cathode materials. However, the function of oxygen defects and its influence on electrochemical properties are poorly understood so far. Here, impacts of oxygen vacancy on electrochemical properties of a Li-rich cathode Li_1.2Mn_0.6Ni_0.2O₂−_δ are reported. Single-phase oxygen-deficient Li_1.2Mn_0.6Ni_0.2O_1.97 was prepared by using the solid electrolyte reactor composed of an oxide ion conductor, and its electrochemical properties, crystal, and electronic structures were compared with those of the oxygen-stoichiometric Li_1.2Mn_0.6Ni_0.2O₂. In the initial charge, the oxygen-deficient Li_1.2Mn_0.6Ni_0.2O_1.97 showed a larger oxidation current due to the oxygen release than the oxygen-stoichiometric Li_1.2Mn_0.6Ni_0.2O₂. This suggests that preliminary introduced oxygen vacancy promoted the further oxygen release during the initial charge possibly by accelerating the oxide ion diffusion in the active material via oxygen vacancy. Due to the oxygen release, heavily oxygen-deficient Li_1.2Mn_0.6Ni_0.2O₂−_δ phase (δ > 0.03) was formed at the near-surface region after the initial charge/discharge cycle. Vigorous Mn redox was observed in the heavily oxygen-deficient Li_1.2Mn_0.6Ni_0.2O₂−_δ phase during the charge/discharge in contrast to the oxygen-stoichiometric phase wherein Mn was almost inactive for the charge/discharge. The modulation of oxygen defects can be one of effective strategies to control redox species in battery active materials.