On the RE2TiAl3 (RE = Y, Gd–Tm, Lu) Series : The First Aluminum Representatives of the Rhombohedral Mg2Ni3Si Type Structure

Several ternary rare-earth metals containing titanium aluminum intermetallics in the RE2TiAl3 series (RE = Y, Gd–Lu) have been synthesized from the elements using arc-melting techniques. All compounds crystallize in the trigonal crystal system with rhombohedral space group R3m (Z = 3) and lattice parameters ranging between a = 582–570 and c = 1353–1358 pm. They adopt the Mg2Ni3Si-type structure, which is an ordered superstructure of the cubic Laves phase MgCu2 and has been observed for Al intermetallics for the first time. Tetrahedral [TiAl3 ] entities that are connected over all corners form a network where the empty [TiAl3 ] tetrahedra exhibit a full Ti/Al ordering based on the single crystal results. The Al atoms are arranged into 63 Kagomé nets, while the Ti atoms connect these nets over the triangular units. In the cavities of this three-dimensional arrangement, the RE cations can be found forming a distorted diamond-type substructure. Magnetic measurements revealed that Y2TiAl3 and Lu2TiAl3 are Pauli paramagnetic substances, in line with the metallic character. The other compounds exhibit paramagnetism with antiferromagnetic ordering at a maximum Néel temperature of TN = 26.1(1) K for Gd2TiAl3

Understanding Lithium-Ion Transport in Selenophosphate-Based Lithium Argyrodites and Their Limitations in Solid-State Batteries

To develop solid-state batteries with high power and energy densities, solid electrolytes with fast Li+ transport are required. Superionic lithium argyrodites have proven to be a versatile system, in which superior ionic conductivities can be achieved by elemental substitutions. Herein, we report the novel selenophosphate-based lithium argyrodites Li6–xPSe5–xBr1+x (0 ≤ x ≤ 0.2) exhibiting ionic conductivities up to 8.5 mS·cm–1 and uncover the origin of their fast Li+ transport. Rietveld refinement of neutron powder diffraction data reveals a better interconnection of the Li+ cages compared to the thiophosphate analogue Li6PS5Br, by the occupation of two additional Li+ sites, facilitating fast Li+ transport. Additionally, a larger unit cell volume, lattice softening, and higher structural disorder between halide and chalcogenide are unveiled. The application of Li5.85PSe4.85Br1.15 as the catholyte in In/LiIn|Li6PS5Br|LiNi0.83Co0.11Mn0.06O2:Li5.85PSe4.85Br1.15 solid-state batteries leads to severe degradation upon charging of the cell, revealing that selenophosphate-based lithium argyrodites are not suitable for applications in lithium nickel cobalt manganese oxide-based solid-state batteries from a performance perspective. This work further expands on the understanding of the structure–transport relationship in Li+ conducting argyrodites and re-emphasizes the necessity to consider chemical and electrochemical stability of solid electrolytes against the active materials when developing fast Li+ conductors

On the Discrepancy between Local and Average Structure in the Fast Na+Na^+ Ionic Conductor Na2.9Sb0.9W0.1S4Na_{2.9}Sb_{0.9}W_{0.1}S_{4}

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS43– by WS42– in Na2.9Sb0.9W0.1S4 leads to a very high ionic conductivity of 41 mS cm–1 at room temperature. While pristine Na3SbS4 crystallizes in a tetragonal structure, the substituted Na2.9Sb0.9W0.1S4 crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse 121Sb NMR experiments that the short-range order of Na2.9Sb0.9W0.1S4 remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na+ substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine Na3SbS4, quasi-elastic neutron scattering and solid-state 23Na nuclear magnetic resonance measurements revealed drastically improved Na+ diffusivity and decreased activation energies for Na2.9Sb0.9W0.1S4. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors