Unveiling the Reaction Mechanism during Li Uptake and Release of Nanosized “NiFeMnO₄”: Operando X-ray Absorption, X-ray Diffraction, and Pair Distribution Function Investigations

Abstract

Here, we report that the trimetallic nanosized oxide NiFeMnO 4 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, Mn 3+ , and Fe 3+ 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 7 Li 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/Li 2 O matrix. Ex situ XAS results indicate that Fe can be reversibly reoxidized to Fe 3+ 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.

In-situ/operando characterization techniques in lithium-ion batteries and beyond

“…The results of CoFe 2 O 4 and NiFeMnO 4 shown that with the lithium consumption, the initial spinel phase transformed into NaCl-like structure, which accounted for the cation arrangement with value state changing. The capacity degradation was found in both materials owing to formation of inactive metal particles (Co, Ni) [141,142]. A similar migration of site occupation was found in ZnFe 2 O 4 with the lithiation process, leading to three critical reduction products Li x ZnFe 2 O 4 (x = 0.5, 1, 2) (Fig.…”

“…A number of works were carried out on the conversion reactions of spinel materials, including ZnFe 2 O 4 , CoFe 2 O 4 , NiFeMnO 4 [140][141][142]. Through in-situ XAS combined with other operando techniques, the evolutions of crystalline structure and valent state were studied during the lithiation in detail.…”

Long-Term Stable, High-Capacity Anode Material for Sodium-Ion Batteries: Taking a Closer Look at CrPS4 from an Electrochemical and Mechanistic Point of View

“…Simultaneously, the pre-edge signal slightly shifts toward 5988.7 eV gaining intensity and reaching maximum intensity at the discharged state, resembling that of nanocrystalline elemental Cr. The differences between the spectrum of the discharged sample and the Cr foil is a phenomenon known for metal nanoparticles and amorphous alloys ,− caused by strong disorder and large amounts of surface atoms/defects altering scattering paths, compared to well-crystalline phases. At this point of data analysis, the formation of CrP instead of elemental Cr cannot be ruled out, because the present spectrum shows similarities with that of literature data for crystalline CrP .…”

Synthesis of Metastable Inorganic Solids with Extended Structures

“…[109,110] PDF and difference PDF were also used to examine other solid state reactions. [111][112][113] While these experiments show the promise of using total scattering investigations to follow solid state reactions, there remain significant experimental challenges. The most significant are the subtraction of background scattering and the lack of a method to analyze total scattering from samples that are textured.…”