High-performance LiNi0.5Mn1.5O4 spinel controlled by Mn3+ concentration and site disorder 

High-voltage spinel LiNi_0.5Mn_1.5O₄ is considered to be one of the most promising cathode materials for Li-ion batteries that can be deployed in hybrid and plug-in hybrid electric vehicles. To improve the cyclability and eliminate the impurities in the spinel, one of the commonly adopted approaches is to partially substitute Ni and/or Mn with elements, such as Ti, Fe, Cr, Ru, and Mg.

In this work, we demonstrate that the electrochemical properties displayed by LiNi_0.5Mn_1.5O₄obtained using several preparation approaches, are intrinsically related to the Ni-Mn site disorder and the amount of residual Mn³⁺, which can be tuned through post-synthesis annealing and element substitution.

In particular, we show that the relative content of Mn³⁺ can be increased by Cr substitution and decreased by annealing, thus providing an approach to tune it. In the Cr-free samples, the content of Mn³⁺ can be controlled by selecting the annealing temperature. In this case, fully stoichiometric samples show signs of additional lattice ordering, which is not observed in samples containing Mn³⁺. Moreover, x-ray and electron diffraction (top Figure) patterns indicate that the amount of Mn³⁺ ions changes concurrently with that of disordered phase caused by randomisation of Ni²⁺and Mn⁴⁺ occupancies of the octahedral lattice sites. This is the first direct observation of the effect of post-synthesis annealing and substitution on the relative ratio between ordered/disordered phases in spinel.

Computational modelling results suggest that in stoichiometric samples, where Ni and Mn species are in 2+ and 4+ oxidation states, respectively, only one type of Ni²⁺-Mn⁴⁺ ordering dominates. However, in oxygen-deficient and Cr-doped samples, which give rise to Mn³⁺, all possible Ni-Mn arrangements become statistically significant, thus, lifting the Ni-Mn ordering. This agrees well with the absence of the additional reflections in the as-prepared LiNi_0.5Mn_1.5O₄. In other words, the Mn³⁺ formation in LiNi_0.5Mn_1.5O₄ spinel promotes Ni-Mn site disorder which, in turn, has been linked with faster Li⁺ transport.