Structures of Hydrocarbon Hydrates during Formation with and without Inhibitors

The formation of hydrates from a methane–ethane-propane mixture is more complex than with single gases. Using nuclear magnetic resonance (NMR) and high-pressure powder X-ray diffraction (PXRD), we have investigated the structural properties of natural gas hydrates crystallized in the presence of kinetic hydrate inhibitors (KHIs), two commercial inhibitors and two biological ice inhibitors, or antifreeze proteins (AFPs). NMR analyses indicated that hydrate cage occupancy was at near saturation for controls and most inhibitor types. Some exceptions were found in systems containing a new commercial KHI (HIW85281) and a recombinant plant AFP, suggesting that these two inhibitors could impact the kinetics of cavity formation. NMR analysis confirmed that the hydrate composition varies during crystal growth by kinetic effects. Strikingly, the coexistence of both structures I (sI) and II (sII) were observed in NMR spectra and PXRD profiles. It is suggested that sI phases may form more readily from liquid water. Real time PXRD monitoring showed that sI hydrates were less stable than sII crystals, and there was a conversion to the stable phase over time. Both commercial KHIs and AFPs had an impact on hydrate metastability, but transient sI PXRD intensity profiles indicated significantly different modes of interaction with the various inhibitors and the natural gas hydrate system

Enhancing Ionic Conductivity by in Situ Formation of Li₇SiPS₈/Argyrodite Hybrid Solid Electrolytes

Solid electrolytes (SEs) with high ionic conductivities are prerequisites to establish solid state batteries on a broad basis. Here we report a novel approach to thiophosphate SEs with improved ionic conductivities based on the in situ formation of LGPS-type tetra-Li7SiPS8/lithium argyrodite Li6PS5X (X= Cl, Br, I) hybrid SEs. Quantitative phase analysis reveals the formation of halogen-poor argyrodites Li6+yPS5+yX1–y next to the tetra-Li7SiPS8 majority phase and an amorphous side phase. EIS measurements indicate ionic conductivities of up to 7 mS cm–1, which exceed those of the parent tetra-Li7SiPS8 and Li6PS5X (X = Cl, Br, I) phases as well as those of simple physical mixtures whose conductivities are well described by the effective medium approximation. In contrast to previous reports, no evidence for halide substitution of the PS43– anions in tetra-Li7SiPS8 was found. Instead, the observed increase in ionic conductivity along with reduced grain boundary resistance is attributed to the directed growth of the tetra-Li7SiPS8 majority phase in the presence of an argyrodite side phase. As a result, a substantially increased isotropic Li diffusion radius is observed by PFG NMR, consistent with both more bulk-like Li transport within secondary particles and with reduced grain boundary resistance through more benign argyrodite interphases as compared to pristine tetra-Li7SiPS8. The microstructural changes induced by hybridization thus provide access to the bulk properties of tetra-Li7SiPS8

Copper Selenidophosphates Cu₄P₂Se₆, Cu₄P₃Se₄, Cu₄P₄Se₃, and CuP₂Se, Featuring Zero‑, One‑, and Two-Dimensional Anions

Five new compounds in the Cu/P/Se phase diagram have been synthesized, and their crystal structures have been determined. The crystal structures of these compounds comprise four previously unreported zero-, one-, and two-dimensional selenidophosphate anions containing low-valent phosphorus. In addition to two new modifications of Cu₄P₂Se₆ featuring the well-known hexa­selenido­hypodiphosphate­(IV) ion, there are three copper selenidophosphates with low-valent P: Cu₄P₃Se₄ contains two different new anions, (i) a monomeric (zero-dimensional) selenidophosphate anion [P₂Se₄]^4– and (ii) a one-dimensional selenidophosphate anion [P(−Se)]−∞1, which is related to the well-known gray-Se-like [P]−∞1 Zintl anion. Cu₄P₄Se₃ contains one-dimensional [P4(−Se)2]2−∞1 polyanions, whereas CuP₂Se contains the 2D selenidophosphate [P2(−Se)]−∞2 polyanion. It consists of charge-neutral CuP₂Se layers separated by a van der Waals gap which is very rare for a Zintl-type phase. Hence, besides black P, CuP₂Se constitutes a new possible source of 2D oxidized phosphorus containing layers for intercalation or exfoliation experiments. Additionally, the electronic structures and some fundamental physical properties of the new compounds are reported. All compounds are semiconducting with indirect band gaps of the orders of around 1 eV. The phases reported here add to the structural diversity of chalcogenido phosphates. The structural variety of this family of compounds may translate into a variety of tunable physical properties

Reversible Li Intercalation in Layered Cathodes Enabled by Dopant-Induced Medium-Range Orders

Doping could effectively tune the electrochemical performance of layered oxide cathodes in Li-ion batteries, whereas the working mechanism is usually oversimplified (i.e., a “pillar” effect). Although the Jahn–Teller effect is generally regarded as the fundamental origin of structural instability in some oxides, more polyhedral distortions are associated with pseudo-JTE (PJTE), which involves vibronic couplings. In this work, the atomic structures of doped LiCoO2 by Mg cations, F anions, and both were investigated thoroughly to reveal the atomic environments of these dopants and their influence on electrochemical performance. The function of these dopants as pillars is well discussed from the view of PJTE manipulation. Briefly, the MgO4 tetrahedra in Mg-doped LiCoO2 could suppress the charge transfer from the ligand to Co in neighboring octahedra, thus depressing PJTE. Although F doping does increase the ligand-field strength, the induced octahedral distortion reduces the structural stability dramatically. Comparatively, Mg/F co-doping generates the CoO5F–MgO4F2–CoO5F medium-range orders (MROs), which could depress both structural distortion and charge transfer in Co-centered octahedra for reduced PJTE. The reduced PJTE accounts for the improved electrochemical performance, making the co-doped LiCoO2 offer the best performance: a 70% capacity retention after 200 cycles within the potential range of 2.8–4.6 V, followed by Mg-doped LiCoO2. In contrast, although F-doping could induce an extra rock salt-like surface layer for higher capacity, its cycling improvement is rather limited. These results highlight the importance of structural modulation in enhancing the material performance and propose that the manipulation of PJTE would be an effective strategy in developing novel high-performance oxide cathodes