Metal–organic
frameworks (MOFs), owing to their unique architecture,
attract consistent attention in the design of high-performance Li
battery materials. Here, we report a new category of ion-conducting
crystalline materials for all-solid-state electrolytes based on an
MIL53(Al) framework featuring a superchaotropic metallacarborane (Li+CoD–) salt and present the first quantitative
data on Li+ ion sites, local dynamics, chemical exchange,
and the formation of charge-transfer pathways. We used multinuclear
solid-state nuclear magnetic resonance (ss-NMR) spectroscopy to examine
the mechanism of ionic conductivity at atomic resolution and to elucidate
order–disorder processes, framework–ion interactions,
and framework breathing during the loading of Li+CoD– species and transfer of Li+ ions. In this
way, the MIL53(Al)@LiCoD framework was found to adopt an open-pore
conformation accompanied by a minor fraction of narrow-pore channels.
The inserted Li+ ions have two states (free and bound),
which both exhibit extensive motions. Both types of Li+ ions form mutually communicating chains, which are large enough
to enable efficient long-range charge transfer and macroscopic conductivity.
The superchaotropic anions undergo high-amplitude uniaxial rotation
motions supporting the transfer of Li+ cations along them,
while the fluctuations of MOF aromatic linkers support the penetration
of Li+ through the channel walls. Our findings provide
a detailed atomic-resolution insight into the mechanism of ionic conductivity
and thus have significant implications for the design of the next
generation of energy-related materials.
The development of intrinsic vacancies in SnSe single crystals was investigated as a function of annealing temperature by means of positron annihilation spectroscopy accompanied by transport measurements. It has been demonstrated that two types of vacancies are present in single-crystalline SnSe. While Sn vacancies dominate in the low-temperature region, Se vacancies and vacancy clusters govern the high-temperature region. These findings are supported by theoretical calculations enabling direct detection and quantification of the most favorable type of vacancies. The experiments show that Sn vacancies couple with one or more Se vacancies with increasing temperature to form vacancy clusters. Interestingly, the clusters survive the α→β transition at ≈800 K and even grow in size with temperature. The concentration of both Se vacancies and vacancy clusters increases with temperature, similar to thermoelectric performance. This indicates that the extraordinary thermoelectric properties of SnSe are related to point defects. We suggest that either these defects vary the band structure in favor of high thermoelectric performance or introduce an energy-dependent scattering of free carriers realizing, in fact, energy filtering of the free carriers. Cluster defects account for the glasslike thermal conductivity of SnSe at elevated temperatures.
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