Surface segregation-the enrichment of one element at the surface, relative to the bulk-is ubiquitous to multi-component materials. Using the example of a Cu-Au solid solution, we demonstrate that compositional variations induced by surface segregation are accompanied by misfit strain and the formation of dislocations in the subsurface region via a surface diffusion and trapping process. The resulting chemically ordered surface regions acts as an effective barrier that inhibits subsequent dislocation annihilation at free surfaces. Using dynamic, atomic-scale resolution electron microscopy observations and theory modelling, we show that the dislocations are highly active, and we delineate the specific atomic-scale mechanisms associated with their nucleation, glide, climb, and annihilation at elevated temperatures. These observations provide mechanistic detail of how dislocations nucleate and migrate at heterointerfaces in dissimilar-material systems.
The atomic-scale reduction mechanism of α-Fe2O3 nanowires by H2 was followed using transmission electron microscopy to reveal the evolution of atomic structures and the associated transformation pathways for different iron oxides. The reduction commences with the generation of oxygen vacancies that order onto every 10th
(303false¯0) plane. This vacancy ordering is followed by an allotropic transformation of α-Fe2O3 → γ-Fe2O3 along with the formation of Fe3O4 nanoparticles on the surface of the γ-Fe2O3 nanowire by a topotactic transformation process, which shows 3D correspondence between the structures of the product and its host. These observations demonstrate that the partial reduction of α-Fe2O3 nanowires results in the formation of a unique hierarchical structure of hybrid oxides consisting of the parent oxide phase, γ-Fe2O3, as the one-dimensional wire and the Fe3O4 in the form of nanoparticles decorated on the parent oxide skeleton. We show that the proposed mechanism is consistent with previously published and our density functional theory results on the thermodynamics of surface termination and oxygen vacancy formation in α-Fe2O3. Compared to previous reports of α-Fe2O3 directly transformed to Fe3O4, our work provides a more in-depth understanding with substeps of reduction, i.e., the whole reduction process follows: α-Fe2O3 → α-Fe2O3 superlattice → γ-Fe2O3 + Fe3O4→ Fe3O4.
ε-LiVOPO
4
is a promising multielectron cathode
material for Li-ion batteries that can accommodate two electrons per
vanadium, leading to higher energy densities. However, poor electronic
conductivity and low lithium ion diffusivity currently result in low
rate capability and poor cycle life. To enhance the electrochemical
performance of ε-LiVOPO
4
, in this work, we optimized
its solid-state synthesis route using in situ synchrotron X-ray diffraction
and applied a combination of high-energy ball-milling with electronically
and ionically conductive coatings aiming to improve bulk and surface
Li diffusion. We show that high-energy ball-milling, while reducing
the particle size also introduces structural disorder, as evidenced
by
7
Li and
31
P NMR and X-ray absorption spectroscopy.
We also show that a combination of electronically and ionically conductive
coatings helps to utilize close to theoretical capacity for ε-LiVOPO
4
at C/50 (1 C = 153 mA h g
–1
) and to enhance
rate performance and capacity retention. The optimized ε-LiVOPO
4
/Li
3
VO
4
/acetylene black composite yields
the high cycling capacity of 250 mA h g
–1
at C/5
for over 70 cycles.
Olivine MnPO4 is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO4, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO4. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO4, which causes amorphization of olivine MnPO4. The properties of crystalline MnPO4 obtained from carbon-coated LiMnPO4 and of the amorphous product resulting from delithiation of pure LiMnPO4 were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP2O7 and MnH2P2O7 from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO4 is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal.
Sn-based alloy materials are strong
candidates to replace graphitic
carbon as the anode for the next generation lithium-ion batteries
because of their much higher gravimetric and volumetric capacity.
A series of nanosize Sn
y
Fe alloys derived
from the chemical transformation of preformed Sn nanoparticles as
templates have been synthesized and characterized. An optimized Sn
5
Fe/Sn
2
Fe anode with a core–shell structure
delivered 541 mAh·g
–1
after 200 cycles at the
C/2 rate, retaining close to 100% of the initial capacity. Its volumetric
capacity is double that of commercial graphitic carbon. It also has
an excellent rate performance, delivering 94.8, 84.3, 72.1, and 58.2%
of the 0.1 C capacity (679.8 mAh/g) at 0.2, 0.5, 1 and 2 C, respectively.
The capacity is recovered upon lowering the rate. The exceptional
cycling/rate capability and higher gravimetric/volumetric capacity
make the Sn
y
Fe alloy a potential candidate
as the anode in lithium-ion batteries. The understanding of Sn
y
Fe alloys from this work also provides insight
for designing other Sn–M (M = Co, Ni, Cu, Mn, etc.) system.
We report in situ atomic-scale transmission electron microscopy observations of the surface dynamics during Cu2O reduction. We show inhomogeneous oxide reduction caused by the preferential adsorption of hydrogen at step edges that induces oxygen loss and destabilizes Cu atoms within the step edge, thereby resulting in the retraction motion of atomic steps at the oxide surface.
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