Highly
crystalline CuFeS2 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 23Na 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/CuFeS2, nanocrystalline
NaCuFeS2 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
Na2S and nanosized elemental Fe are formed in the discharged
state. After charging, the main crystalline phase is NaCuFeS2. At the 150th cycle, the mechanisms clearly changed, and in the
charged state, nanocrystalline Cu
x
S phases
are observed. At later stages of cycling, the mechanisms are altered
again: NaF, Cu2S, and Cu7.2S4 appeared
in the discharged state, while NaF and Cu5FeS4 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.
Bulk
isocubanite (CuFe2S3) was synthesized
via a multistep high-temperature synthesis and was investigated as
an anode material for sodium-ion batteries. CuFe2S3 exhibits an excellent electrochemical performance with a
capacity retention of 422 mA h g–1 for more than
1000 cycles at a current rate of 0.5 A g–1 (0.85
C). The complex reaction mechanism of the first cycle was investigated
via PXRD and X-ray absorption spectroscopy. At the early stages of
Na uptake, CuFe2S3 is converted to form crystalline
CuFeS2 and nanocrystalline NaFe1.5S2 simultaneously. By increasing the Na content, Cu+ is
reduced to nanocrystalline Cu, followed by the reduction of Fe2+ to amorphous Fe0 while reflections of nanocrystalline
Na2S appear. During charging up to −5 Na/f.u., the
intermediate NaFe1.5S2 appears again, which
transforms in the last step of charging to a new unknown phase. This
unknown phase together with NaFe1.5S2 plays
a key role in the mechanism for the following cycles, evidenced by
the PXRD investigation of the second cycle. Even after 400 cycles,
the occurrence of nanocrystalline phases made it possible to gain
insights into the alteration of the mechanism, which shows that Cu
x
S phases play an important role in the region
of constant specific capacity.
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.
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