Unveiling the Reaction Mechanism during Li Uptake and Release of Nanosized "NiFeMnO₄": Operando X-ray Absorption,...

Here, we report that the trimetallic nanosized oxide NiFeMnO₄ consists of a mixture of NiO and a strained cubic spinel phase, which is clearly demonstrated by analysis of the pair distribution function (PDF) and synchrotron X-ray data. Such a finding can easily be overlooked by using only inhouse X-ray powder diffraction, leading to inaccurate assumption of the stoichiometry and oxidation states. Such advanced characterization is essential because a homogeneous distribution of the elements is observed in energy-dispersive X-ray spectroscopy maps, giving no hints for a phase separation. Cycling of the sample against Li delivers a high reversible capacity of ≈840 mAh/g in the 50th cycle. Operando X-ray absorption spectroscopy (XAS) experiments indicate that ≈0.8 Li/fu is consumed without detectable changes of the electronic structure. Increasing amounts of Li, Mn³⁺, and Fe³⁺ are simultaneously reduced. The disappearance of the pre-edge features in X-ray absorption near-edge spectroscopy indicates movement of these cations from tetrahedral sites to octahedral sites. PDF analysis of the pattern after an uptake of 2 Li/fu evidences that the principal structure can be sufficiently well modeled assuming coexisting NiO, a mixed monoxide, and a small amount of residual spinel phase. Thus, the majority of cations is located on octahedral sites. Furthermore, an improvement of the PDF model is achieved taking into account small amounts of LiOH. The ⁷Li MAS NMR spectrum of this sample clearly shows the signal of Li in a diamagnetic environment, excluding Li-O-TM bonds. A further increase of the Li content leads to a successive conversion of the cations to nanosized metal particles embedded in a LiOH/Li₂O matrix. Ex situ XAS results indicate that Fe can be reversibly reoxidized to Fe³⁺ during charge whereas Mn does not reach the oxidation state observed in the pristine material. After excessive cycling, reoxidation of metallic Ni is suppressed and contributes to a capacity loss compared with the early discharge/charge cycles.