2 research outputs found
Multi‐Method Characterization of the High‐Entropy Spinel Oxide MnCoNiCuZnFeO: Entropy Evidence, Microstructure, and Magnetic Properties
The novel spinel Cu0.2Co0.2Mn0.2Ni0.2Zn0.2Fe2O4 comprising six transition metal cations was successfully prepared by a solution-combustion method followed by distinct thermal treatments. The entropic stabilization of this hexa-metallic material is demonstrated using in situ high temperature powder X-ray diffraction (PXRD) and directed removal of some of the constituting elements. Thorough evaluation of the PXRD data yields sizes of coherently scattering domains in the nanometre-range. Transmission electron microscopy based methods support this finding and indicate a homogeneous distribution of the elements in the samples. The combination of 57Fe Mössbauer spectroscopy with X-ray absorption near edge spectroscopy allowed determination of the cation occupancy on the tetrahedral and octahedral sites in the cubic spinel structure. Magnetic studies show long-range magnetic exchange interactions which are of ferri- or ferromagnetic nature with an exceptionally high saturation magnetization in the range of 92–108 emu g−1 at low temperature, but also an anomaly in the hysteresis of a sample calcined at 500 °C
CuFeS as a Very Stable High-Capacity Anode Material for Sodium-Ion Batteries: A Multimethod Approach for Elucidation of the Complex Reaction Mechanisms during Discharge and Charge Processes
Highly crystalline CuFeS containing earth-abundant and environmentally friendly elements prepared via a high-temperature synthesis exhibits an excellent electrochemical performance as an anode material in sodium-ion batteries. The initial specific capacity of 460 mAh g increases to 512 mAh g in the 150th cycle and then decreases to a still very high value of 444 mAh g at 0.5 A g in the remaining 550 cycles. Even for a large current density, a pronounced cycling stability is observed. Here, we demonstrate that combining the results of X-ray powder diffraction experiments, pair distribution function analysis, and Na NMR and Mössbauer spectroscopy investigations performed at different stages of discharging and charging processes allows elucidation of very complex reaction mechanisms. In the first step after uptake of 1 Na/CuFeS, nanocrystalline NaCuFeS is formed as an intermediate phase, which surprisingly could be recovered during charging. On increasing the Na content, Cu is reduced to nanocrystalline Cu, while nanocrystalline NaS and nanosized elemental Fe are formed in the discharged state. After charging, the main crystalline phase is NaCuFeS. At the 150th cycle, the mechanisms clearly changed, and in the charged state, nanocrystalline CuxS phases are observed. At later stages of cycling, the mechanisms are altered again: NaF, CuS, and CuS appeared in the discharged state, while NaF and CuFeS are observed in the charged state. In contrast to a typical conversion reaction, nanocrystalline phases play the dominant role, which are responsible for the high reversible capacity and long-term stability