A Li-rich polyanionic compound based on V³⁺ with a previously unknown structure, Li₅VF₄(SO₄)₂, has been developed as a high-voltage cathode material for Li-ion batteries. The solvothermal preparation of this material, crystal structure solution, and initial electrochemical characterization are presented. An analysis based on density functional theory electronic structure calculations suggests that a high voltage close to 5 V is required to extract two Li ions and to reach the oxidation state of V⁵⁺. However, the use of conventional carbonate-based electrolytes, which exhibit increasing degradation above a potential of 4.3 V, does not permit the full capacity of this compound to be achieved at this time.

Three compositions of lithium copper phosphates: Li2CuPO4, Li2Cu5(PO4)4, and Li2CuP2O7have been studied as potential high-voltage cathode materials for Li-ion batteries, following computational predictions of high operating voltages. An assisted-microwave preparation of Li2CuPO4, which is otherwise difficult to prepare in nearly pure form, has been developed. The electrochemical performance of all three compounds has been investigated. The cyclability of these materials is found to be poor due to structural changes, irreversible reduction to metallic copper at potentials as high as 2.5 V, and the possibility of dissolution into the electrolyte. Some general understanding in regard to the use of Cu compounds in redox electrodes is presented.

High-entropy alloy (HEA) nanoparticles are promising catalyst candidates for the acidic oxygen evolution reaction (OER). Herein, we report the synthesis of IrFeCoNiCu-HEA nanoparticles on a carbon paper substrate via a microwave-assisted shock synthesis method. Under OER conditions in 0.1 M HClO₄, the HEA nanoparticles exhibit excellent activity with an overpotential of ∼302 mV measured at 10 mA cm^-2 and improved stability over 12 h of operation compared to the monometallic Ir counterpart. Importantly, an active Ir-rich shell layer with nanodomain features was observed to form on the surface of IrFeCoNiCu-HEA nanoparticles immediately after undergoing electrochemical activation, mainly due to the dissolution of the constituent 3d metals. The core of the particles was able to preserve the characteristic homogeneous single-phase HEA structure without significant phase separation or elemental segregation. This work illustrates that under acidic operating conditions, the near-surface structure of HEA nanoparticles is susceptible to a certain degree of structural dynamics.

The soft, dynamic lattice of inorganic lead halide perovskite CsPbX3 (X = Cl-, Br-, I-) leads to the emergence of many interesting photophysical and optoelectronic phenomena. However, probing their lattice dynamics with vibrational spectroscopy remains challenging. The influence of the fundamental octahedral building block in the perovskite lattice can be better resolved in zero-dimensional (0D) vacancy-ordered double perovskites of form A2BX6. Here we study Cs2TeX6 (X = Cl-, Br-, I-) single crystals to yield detailed insight into the fundamental octahedral building block and to explore the effect that its isolation in the crystal structure has on structural and electronic properties. The isolated [TeX6]2- octahedral units serve as the vibrational, absorbing, and emitting centers within the crystal. Serving as the vibrational centers, the isolated octahedra inform the likelihood of a random distribution of 10 octahedral symmetries within the mixed-halide spaces, as well as the presence of strong exciton-phonon coupling and anharmonic lattice dynamics. Serving as the absorbing and emitting centers, the isolated octahedra exhibit compositionally tunable absorption (1.50-3.15 eV) and emission (1.31-2.11 eV) energies. Due to greater molecular orbital overlap between neighboring octahedra with increasing halide anion size, there is a transition from a more molecule-like electronic structure in Cs2TeCl6 and Cs2TeBr6-as expected from the effective 0D nature of these single crystals-to a dispersive electronic structure in Cs2TeI6, typical of three-dimensional (3D) bulk single crystals.

Traditional covalent semiconductors require complex processing methods for device fabrication due to their high cohesive energies. Here, we develop a stable, ligand-free perovskite semiconductor ink that can be used to make patterned semiconductor-based optoelectronics in one step. The perovskite ink is formed via the dissolution of crystals of vacancy-ordered double perovskite Cs₂TeX₆ (X = Cl^-, Br^-, I^-) in polar aprotic solvents, leading to the stabilization of isolated [TeX₆]^2- octahedral anions and free Cs⁺ cations without the presence of ligands. The stabilization of the fundamental perovskite ionic octahedral building blocks in solution creates multifunctional inks with the ability to reversibly transform between the liquid ink and the solid-state perovskite crystalline system in air within minutes. These easily processable inks can be patterned onto various materials via dropcasting, spraying or painting, and stamping, highlighting the crucial role of solvated octahedral complexes toward the rapid formation of phase-pure perovskite structures in ambient conditions.

Because of the toxicity of lead, searching for a lead-free halide perovskite semiconducting material with comparable optical and electronic properties is of great interest. Rare-earth-based halide perovskite represents a promising class of materials for this purpose. In this work, we demonstrate the solution-phase synthesis of single-crystalline CsEuCl₃ nanocrystals with a uniform size distribution centered around 15 nm. The CsEuCl₃ nanocrystals have photoluminescence emission centered at 435 nm, with a full width at half-maximum of 19 nm. Furthermore, CsEuCl₃ nanocrystals can be embedded in a polymer matrix that provides enhanced stability under continuous laser irradiation. Lead-free rare-earth cesium europium halide perovskite nanocrystals represent a promising candidate to replace lead halide perovskites.

The metal halide ionic octahedron, [MX₆] (M = metal cation, X = halide anion), is considered to be the fundamental building block and functional unit of metal halide perovskites. By representing the metal halide ionic octahedron in halide perovskites as a super ion/atom, the halide perovskite can be described as an extended ionic octahedron network (ION) charge balanced by selected cations. This new perspective of halide perovskites based on ION enables the prediction of different packing and connectivity of the metal halide octahedra based on different solid-state lattices. In this work, a new halide perovskite Cs₈Au_3.5In_1.5Cl₂₃ was discovered on the basis of a BaTiO₃-lattice ION {[InCl₆][AuCl₅][Au/InCl₄]₃}^8-, which is assembled from three different ionic octahedra [InCl₆], [AuCl₆], and [Au/InCl₆] and balanced by positively charged Cs cations. The success of this ION design concept in the discovery of Cs₈Au_3.5In_1.5Cl₂₃ opens up a new venue for the rational design of new halide perovskite materials.

Vacancy ordered double perovskites Cs2Te(IV)X6 have been found to exhibit molecule-like electronic behavior when X is Cl- or Br- due to the zero-dimensional (0D) nature of their octahedral units. Electronically isolated building blocks, the [TeBr6]2- ionic octahedron, serve as the fundamental electronic unit of the Cs2TeBr6 solid. Herein, a detailed understanding of the Cs2TeBr6 electronic structure and its photoexcitation is presented with consideration of individual molecular orbitals from these isolated octahedral building blocks. Two optical absorption features correspond to two unique electronic transitions, (1) a highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition under 455 nm excitation and (2) mixed transitions including lower HOMO states to LUMO transition and HOMO to higher LUMO states transition under 365 nm excitation. With this in mind, we examined the excitation wavelength-dependent photo-oxidation of benzyl alcohol using Cs2TeBr6 as the photocatalyst. Significant differences in photocatalytic performance were observed, and different forms of activated alcohol radicals were detected under the two excitation wavelengths. As a case study, this work highlights the application of molecule-like halide perovskites in photocatalysis. The highly tunable energy band structures and catalytic centers in perovskites can offer a valuable platform for photocatalytic mechanistic studies and catalyst development in the foreseeable future.