On the Composition of LiNi0.5Mn1.5O4 Cathode Active Materials

LiNi0.5Mn1.5O4 (LNMO) cathode active materials for Li-ion batteries have been investigated for over 20 years. Despite all effort, it has not been possible to transfer their favourable properties into applicable, stable battery cells. In order to make noteworthy progress, we believe that the research perspective on these spinel type materials has to change and a number of persisting misconceptions on LNMO have to be overcome. Therefore, an in-situ high temperature XRD study was executed and four different types of LNMO materials are investigated, including Mn(III) rich ordered LNMO, and disordered LNMO with very low Mn(III) content. We systematically examine how the actual LNMO phase compositions deviate from the ideal composition LiNi0.5Mn1.5O4. Therefore, it becomes obvious that the importance of cation order is limited to a small composition range. Moreover, multiple evidence contradicting the idea of oxygen defects in LNMO are presented. To comply with these results, we propose an enhanced classification of LNMO based on the Ni content of the spinel phase. Moreover, a balanced chemical equation for the formation of LNMO is presented, which allows for comprehensive calculations of key properties of LNMO materials. Finally, we suggest suitable target compositions and appropriate calcination programs to obtain better LNMO materials.