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

Spinel LiNi0.5Mn1.5O4 (LNMO) exhibits two different Ni/Mn arrangements, i.e., the Ni/Mn ordered P4(3)32 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 P4(3)32 phase. However, in oxygen deficient LiNi0.5Mn1.5O4 delta, the oxygen vacancies tend to diminish the valence discrepancy between the Ni aggregated and the ordered P4(3)32 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 P4(3)32 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. (C) 2016 Elsevier B.V. All rights reserved.