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

High-voltage spinel LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4obtained 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 LiNi0.5Mn1.5O4. In other words, the Mn3+ formation in LiNi0.5Mn1.5O4 spinel promotes Ni-Mn site disorder which, in turn, has been linked with faster Li+ transport.