Origin of the Ni/Mn ordering in high-voltage spinel LiNi0.5Mn1.5O4: The role of oxygen vacancies and cation doping

[Display omitted] Spinel LiNi0.5Mn1.5O4 (LNMO) exhibits two different Ni/Mn arrangements, i.e., the Ni/Mn ordered P4332 phase and disordered Fd-3m phase. It has been found that the Ni/Mn disorder is correlated with the formation of oxygen vacancies, nevertheless the underlying mechanism remains unclear. Density functional theory (DFT) calculations show that formation of 1:3 ordered Ni2+ and Mn4+ ions is energetically favorable compared to the disordered Ni3+ and Mn3+ ions caused by Ni aggregation in the stoichiometric P4332 phase. However, in oxygen deficient LiNi0.5Mn1.5O4−δ, the oxygen vacancies tend to diminish the valence discrepancy between the Ni aggregated and the ordered P4332 phases, making the former energetically competitive and consequently resulting in the disordered Ni/Mn distribution. Understanding the origin of Ni/Mn disorder also sheds light on the cation doping effect. Calculations show that Mg2+ ion tends to replace Ni2+ ion in ordered P4332 phase, and maintaining the Ni/Mn order. By contrast, Al doping promotes the Ni/Mn disorder, as Al3+ ion prefers to substitute for Ni3+ and Mn3+ ions emerged in Ni/Mn disordered structure. Our findings rationalize the experimental observations, and further reveal that Ni/Mn arrangement could be controlled by adjusting the electronic structure of spinel LNMO system.