Charge Compensation Mechanisms and Oxygen Vacancy Formations in LiNi_1/3Co_1/3Mn_1/3O₂: First-Principles Calculations

Charge compensation mechanisms in the delithiation processes of LiNi1/3Co1/3Mn1/3O2 (NCM111) are compared in detail by the first-principles calculations with GGA and GGA+U methods under different U values reported in the literature. The calculations suggested that different sets of U values lead to different charge compensation mechanisms in the delithiation process. Co3+/Co4+ couples were shown to dominate the redox reaction for 1 ≥ x ≥ 2/3 by using the GGA+U1 method (U1 = 6.0 3.4 3.9 for Ni, Co, and Mn, respectively). However, by using the GGA+U2 (U2 = 6.0 5.5 4.2) method, the results indicated that the redox reaction of Ni2+/Ni3+ took place in the range of 1 ≥ x ≥ 2/3. Therefore, according to our study, experimental charge compensation processes during delithiation are of great importance to evaluate the theoretical calculations. The results also indicated that all the GGA+Ui (i = 1, 2, 3) schemes predicted better voltage platforms than the GGA method. The oxygen anionic redox reactions during delithiation are also compared with GGA+U calculations under different U values. The electronic density of states and magnetic moments of transition metals have been employed to illustrate the redox reactions during the lithium extractions in NCM111. We have also investigated the formation energies of an oxygen vacancy in NCM111 under different values of U, which is important in understanding the possible occurrence of oxygen release. The formation energy of an O vacancy is essentially dependent on the experimental conditions. As expected, the decreased temperature and increased oxygen partial pressure can suppress the formation of the oxygen vacancy. The calculations can help improve the stability of the lattice oxygen.