Ternary lithium halide-based
solid electrolytes have attracted
broad scientific interest due to their high ionic conductivities in
conjunction with good electrochemical stabilities against high-voltage
cathode materials. Here, we analyze the structure of the rare earth
halide solid solution series Li3HoBr6–x
I
x
and quantify structural
defects such as intralayer cation disorder and stacking faults. Almost
all members of the solid solution series show strong stacking fault
disorder, whereas the intralayer cation disorder systematically increases
with increasing iodide content. The substitution of bromide by iodide
in Li3HoBr6–x
I
x
leads to an increasing lattice softness and therefore
to a higher ionic conductivity and to a lower activation energy. This
is counteracted by the increased cation disorder, which also has a
strong influence on the ionic conductivity of the solid solution series.
Thus, a decrease in ionic conductivity by one order of magnitude is
observed when the iodine content exceeds an optimum value. Stacking
faults on the other hand do not have a significant impact on the ionic
conductivity as the connectivity between neighboring lithium halide
octahedra and hence the energy landscape of their migration pathways
is not affected for equivalent stacking vectors. The competing factors
above give rise to an optimum ionic conductivity at x ≈ 3 with 2.7 × 10–3 S cm–1 at 20 °C and an activation energy of 0.21 eV. While thermal
annealing reduces the structural disorder of Li3HoBr6–x
I
x
,
the self-healing properties are significantly impeded by mechanical
sample treatment, demonstrating the complex interplay between thermal
and mechanical effects on the microstructure and, hence, ionic conductivity
of layered superionic conductors.
Lithium rare earth metal halides have emerged as attractive candidates for solid electrolytes in all- solid-state batteries due to their high ionic conductivities and stability against oxidation. Here, we study...
Hard carbons are promising candidates for high-capacity anode materials in alkali metal-ion batteries, such as lithium- and sodium-ion batteries. High reversible capacities are often coming along with high irreversible capacity losses during the first cycles, limiting commercial viability. The trade-off to maximize the reversible capacities and simultaneously minimizing irreversible losses can be achieved by tuning the exact architecture of the subnanometric pore system inside the carbon particles. Since the characterization of small pores is nontrivial, we herein employ Kr, N2 and CO2 gas sorption porosimetry, as well as H2O vapor sorption porosimetry, to investigate eight hard carbons. Electrochemical lithium as well as sodium storage tests are compared to the obtained apparent surface areas and pore volumes. H2O, and more importantly CO2, sorption porosimetry turned out to be the preferred methods to evaluate the likelihood for excessive irreversible capacities. The methods are also useful to select the relatively most promising active materials within chemically similar materials. A quantitative relation of porosity descriptors to the obtained capacities remains a scientific challenge.
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