Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, ...we present a rigorous study about Na storage properties of ultra-small Fe3S4 nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of Fe3S4 into nanocrystalline intermediates occurs accompanied by reduction of Fe3+ to Fe2+ cations. Discharge to 0.1 V leads to formation of strongly disordered Fe0 finely dispersed in a nanosized Na2S matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0–0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of Na2S is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention, e.g., 486 mAh g−1 after 300 cycles applying 0.5 A g−1 and superior coulombic efficiencies >99.9%. During charge to 3.0 V, Fe3S4 with smaller domains are reversibly generated in the 1st cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of Fe3S4 in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.
The influence of low‐level metal cation substitution in the thermoelectric material NiCr2S4, treated via field‐assisted sintering, is investigated in X‐ray diffraction (XRD) and transmission electron ...microscopy (TEM) studies. NiCr2S4 and Mn0.1Ni0.9Cr2S4 can be synthesized and compacted as phase‐pure pellets, while In0.1Ni0.9Cr2S4 appears as a mixture of different phases. XRD investigations reveal that Mn can be incorporated into the host material's Ni lattice sites, while In is mainly incorporated into additional phases. Both NiCr2S4 and Mn0.1Ni0.9Cr2S4 form a structure of chemically segregated, nanoscale domains, which appear significantly more pronounced for Mn0.1Ni0.9Cr2S4. All materials exhibit similar, promising thermal conductivities around 2.0 W m−1 K−1, with Seebeck coefficients ranging from −55 to −65 μV K−1. Only the electrical conductivity is noticeably influenced by the substitutions, with the highest value of 504 S cm−1 obtained for the pristine material, and subsequently declining for both substituted phases.
Pristine NiCr2S4 and cation substitutions M0.1Ni0.9Cr2S4 (M = Mn, In) are synthesized and compacted via field‐assisted sintering technique (FAST). NiCr2S4 and Mn0.1Ni0.9Cr2S4 are obtained as phase‐pure pellets and exhibit the formation of nanoscale domains. In0.1Ni0.9Cr2S4 remains homogeneous and forms additional phases. Cation substitution mainly affects the electrical conductivity, which is lowered by the addition of Mn or In.
The effects of low-level partial cation substitution in Cr
2−
x
M
x
S
3
with M = Ti, V or Sn and
x
= 0.05 and 0.1 have been investigated regarding the long- and short-range crystal structures and ...thermoelectric properties. All substituted compounds crystallized in the equilibrium phase of Cr
2
S
3
, adopting the space group
R
3
¯
. Electron beam irradiation led to a phase transformation from space group
R
3
¯
to
P
3
¯
1
c
with a subsequent appearance of diffuse scattering, indicating short-range ordering of cations in the partially occupied cation layers. Substitution of Cr by V led to a reduction in electrical conductivity and subsequently to a lower thermoelectric performance in comparison to the pristine material. In contrast, substitution with Ti yielded an improvement of the performance due to a higher electrical conductivity and a reasonably high Seebeck coefficient. Both Sn-substituted compounds contained only traces of Sn. Surprisingly, a significant improvement of the electrical conductivities could be observed in comparison to the pristine material as well as the other Cr
2−
x
M
x
S
3
materials.
The influence of sintering parameters on the physical properties and the chemical structure of rhombohedral Cr2S3 (rh‐Cr2S3) is investigated using high pressures and high temperatures. The ...densification of the powder is performed by applying the high‐pressure field‐assisted sintering technique/spark plasma sintering. Using a titanium–zirconium–molybdenum (TZM) alloy as sintering tool, it is possible to increase the magnitude of the applied pressure to several hundred MPa at temperatures as high as 1223 K. A relative density of up to 99.9% is achieved at a sintering temperature of 1223 K and a pressure of 395 MPa. The presence of phase‐pure rh‐Cr2S3 is proven by X‐ray diffraction analysis and transmission electron microscopy. The Seebeck coefficients of the self‐doped samples change drastically with the sintering temperatures ranging between −650 and −350 μV K−1. The densities and the thermal conductivities of the sintered samples increase with increasing sintering temperatures. The electrical conductivity is largely increased compared with the thermal conductivity potentially due to the current‐assisted high‐pressure sintering.
Highly densified rhombohedral Cr2S3 is successfully prepared by high‐pressure field‐assisted sintering technique using titanium–zirconium–molybdenum alloy sintering tools. A theoretical density of 99.9% is achieved by sintering at 1223 K and 395 MPa. The investigation of the microstructure proves that the initial structure is preserved. The electrical and the thermal conductivities indicate a strong dependency of the microstructure.
Here, we report on the time dependence of a synthesis procedure for generation of both n- and p-type bismuth telluride-based materials. To initiate the reaction, the starting materials were first ...mechanical pre-reacted. The Rietveld refinements of X-ray diffraction (XRD) data collected after different milling times demonstrate that Bi
Te
was formed after only 10 min, and longer milling times do not alter the composition. To complete the phase formation, the powders were treated by field-assisted sintering and heat treatment afterwards. The effect of this fast procedure on the structural and thermoelectric properties was investigated. Samples were obtained with relative densities above 99%. A clear preferred orientation of the crystallites in the samples is evidenced by Rietveld refinements of XRD data. The thermoelectric characteristics demonstrate a good performance despite the short milling time. Further, it was demonstrated for this fast synthesis that the physical transport properties can be varied with well-known n- and p-type dopants like CHI
or Pb. For these non-optimized materials, a
value of 0.7 (n-type) and 0.9 (p-type) between 400 and 450 K was achieved. The long-term stability is demonstrated by repeated measurements up to 523 K showing no significant alteration of the thermoelectric performance.
Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, ...we present a rigorous study about Na storage properties of ultra-small Fe
S
nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of Fe
S
into nanocrystalline intermediates occurs accompanied by reduction of Fe
to Fe
cations. Discharge to 0.1 V leads to formation of strongly disordered Fe
finely dispersed in a nanosized Na
S matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0-0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of Na
S is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention,
, 486 mAh g
after 300 cycles applying 0.5 A g
and superior coulombic efficiencies >99.9%. During charge to 3.0 V, Fe
S
with smaller domains are reversibly generated in the 1
cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of Fe
S
in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.
Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, ...we present a rigorous study about Na storage properties of ultra-small Fe
3
S
4
nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of Fe
3
S
4
into nanocrystalline intermediates occurs accompanied by reduction of Fe
3+
to Fe
2+
cations. Discharge to 0.1 V leads to formation of strongly disordered Fe
0
finely dispersed in a nanosized Na
2
S matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0-0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of Na
2
S is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention,
e.g.
, 486 mAh g
−1
after 300 cycles applying 0.5 A g
−1
and superior coulombic efficiencies >99.9%. During charge to 3.0 V, Fe
3
S
4
with smaller domains are reversibly generated in the 1
st
cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of Fe
3
S
4
in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.
The Na storage mechanism of Fe
3
S
4
nanoparticles is studied
via
electrochemical techniques, synchrotron-based X-ray diffraction and absorption methods. The results explain the relation of the electrodes cycle life and cut-off potentials.
The influence of low‐level metal cation substitution in the thermoelectric material NiCr
2
S
4
, treated via field‐assisted sintering, is investigated in X‐ray diffraction (XRD) and transmission ...electron microscopy (TEM) studies. NiCr
2
S
4
and Mn
0.1
Ni
0.9
Cr
2
S
4
can be synthesized and compacted as phase‐pure pellets, while In
0.1
Ni
0.9
Cr
2
S
4
appears as a mixture of different phases. XRD investigations reveal that Mn can be incorporated into the host material's Ni lattice sites, while In is mainly incorporated into additional phases. Both NiCr
2
S
4
and Mn
0.1
Ni
0.9
Cr
2
S
4
form a structure of chemically segregated, nanoscale domains, which appear significantly more pronounced for Mn
0.1
Ni
0.9
Cr
2
S
4
. All materials exhibit similar, promising thermal conductivities around 2.0 W m
−1
K
−1
, with Seebeck coefficients ranging from −55 to −65 μV K
−1
. Only the electrical conductivity is noticeably influenced by the substitutions, with the highest value of 504 S cm
−1
obtained for the pristine material, and subsequently declining for both substituted phases.
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