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

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.

Hierarchical porous MnCo2O4 yolk-shell microspheres from MOFs as secondary nanomaterials for high power lithium ion batteries

Hierarchical porous MnCo2O4 yolk-shell microspheres have been synthesized via a facile chemical precipitation method with subsequent calcination treatment. The hierarchical porous MnCo2O4 yolk-shell microspheres as secondary nanomaterials can improve the effective contact area between the MnCo2O4 electrode and electrolyte, accommodate the volume variations during cycling, and shorten the Li+ diffusion path in the nanoparticles. Benefiting from their particular structure and interconnected pores, as anodes for lithium ion batteries, the hierarchical porous MnCo2O4 yolk-shell microspheres show high reversible lithium storage capacity, excellent cycling performance and enhanced rate capability. More importantly, they also exhibit long-life and high-rate lithium storage as high as 691.3 mA h g-1 after 500 cycles even at 1 C.

CuV2S4: A High Rate Capacity and Stable Anode Material for Sodium Ion Batteries

The ternary compound CuV₂S₄ exhibits an excellent performance as anode material for sodium ion batteries with a high reversible capacity of 580 mAh g^-1 at 0.7 A g^-1 after 300 cycles. A Coulombic efficiency of ≈99% is achieved after the third cycle. Increase of the C-rate leads to a drop of the capacity, but a full recovery is observed after switching back to the initial C-rate. In the early stages of Na uptake first Cu⁺ is reduced and expelled from the electrode as nanocrystalline metallic Cu. An increase of the Na content leads to a full conversion of the material with nanocrystalline Cu particles and elemental V embedded in a Na₂S matrix. The formation of Na₂S is evidenced by ²³Na MAS NMR spectra and X-ray powder diffraction. During the charge process the nanocrystalline Cu particles are retained, but no crystalline materials are formed. At later stages of cycling the reaction mechanism changes which is accompanied by the formation of copper(I) sulfide. The presence of nanocrystalline metallic Cu and/or Cu₂S improves the electrical conductivity, leading to superior cycling and rate capability.

Controlled synthesis of CoxMn3−xO4 nanoparticles with a tunable composition and size for high performance lithium-ion batteries

Preparation of Co_xMn_3−xO₄ nanoparticles using bimetallic coordination-polymer precursors, and a case study of MnCo₂O₄ for LIBs, suggesting an optimal size for improved capacity retention.