Multi‐Method Characterization of the High‐Entropy Spinel Oxide Mn0.2_{0.2}Co0.2_{0.2}Ni0.2_{0.2}Cu0.2_{0.2}Zn0.2_{0.2}Fe2_{2}O4_{4}: 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

CuFeS2_{2} 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$_2$ 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$^{–1}$ increases to 512 mAh g$^{–1}$ in the 150th cycle and then decreases to a still very high value of 444 mAh g$^{–1}$ at 0.5 A g$^{–1}$ 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 $^{23}$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$_2$, nanocrystalline NaCuFeS$_2$ 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 Na$_2$S and nanosized elemental Fe are formed in the discharged state. After charging, the main crystalline phase is NaCuFeS$_2$. 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, Cu$_2$S, and Cu$_{7.2}$S$_4$ appeared in the discharged state, while NaF and Cu$_5$FeS$_4$ 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