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At least 631 records · Page 35

An Enantiotropic Disorder–Partial Order Solid-State Transformation in a Molecular Solid Involving a Phase with Z ' = 12

A structural study of the complex aminated stereotriads, [( tert -butyldimethylsilyl)oxy]-4-ethynyl-4-(1-fluorohexyl)-1,2,3-oxathiazocane-2,2-dione (1) and its reduced analogue, [( tert -butyldimethylsilyl)oxy]-4-ethenyl-4-(1-fluorohexyl)-1,2,3-oxathiazocane-2,2-dione (2), revealed a k 12-type second-order phase transition in 1 and phase stability of 2 over the temperature range 100–400 K. The observed phase change in 1 has been linked to the partial ordering of the extensive positional disorder of the high temperature phase upon cooling as revealed by single-crystal X-ray diffraction. The ordering breaks the translational symmetry, introduces differences in the molecular volumes of the packed species, and results in a low-temperature crystal structure with Z '=12. The in situ powder X-ray diffraction studies suggest that the transition occurs between 237 K and 180 K upon cooling, but complete amorphization in the range of 244–274 K during heating complicated the assignment of the transition temperature. Lastly, the observed loss of crystallinity has been linked to the phase transition and implies that the loss of long range order is due to the crystallites undergoing the transformation independently from one another.

36 MATERIALS SCIENCE↗

Unravelling Solid-State Redox Chemistry in Li 1.3 Nb 0.3 Mn 0.4 O 2 Single-Crystal Cathode Material

Recent reports on high capacities delivered by Li-excess transition-metal oxide cathodes have triggered intense interest in utilizing reversible oxygen redox for high-energy battery applications. To control oxygen electrochemical activities, fundamental understanding of redox chemistry is essential yet has so far proven challenging. In the present study, micrometer-sized Li 1.3 Nb 0.3 Mn 0.4 O 2 single crystals were synthesized for the first time and used as a platform to understand the charge compensation mechanism during Li extraction and insertion. We explicitly demonstrate that the oxidation of O 2- to O n - (0 < n < 2) and O 2 loss from the lattice dominates at 4.5 and 4.7 V, respectively. While both processes occur in the first cycle, only the redox of O 2- /O n - participates in the following cycles. The lattice anion redox process triggers irreversible changes in Mn redox, which likely causes the voltage and capacity fade observed on this oxide. Two drastically different redox activity regions, a single-phase behavior involving only Mn 3+/4+ and a two-phase behavior involving O 2- /O n - (0 ≤ n < 2), were found in Li x Nb 0.3 Mn 0.4 O 2 (0 < x < 1.3). Morphological damage with particle cracking and fracturing was broadly observed when O redox is active, revealing additional challenges in utilizing O redox for high-energy cathode development.

25 ENERGY STORAGE↗