Nico Gräßler

Van der Waals layered rhodium chalcochlorides demonstrate a unique interplay between the structure of the layers and the size of the anions. While RhSeCl has a 2D Janus structure with Se and Cl on opposite sides of the layers, RhTeCl forms a non-Janus structure with alternating stripes of Cl and Te. Here, we study the lattice dynamics of these semiconductors by a combination of far-infrared and Raman spectroscopy and DFT phonon calculations. Polarization-dependent measurements enabled the complete assignment of all Raman-active modes of both compounds. The resonance effects in Raman scattering were addressed by using multiple excitation wavelengths and a tunable dye laser between 457 and 785 nm. Raman excitation profiles appeared to be more sensitive to optical excitations than direct measurements of excitation spectra by diffuse reflectance spectroscopy. Finally, variable-temperature Raman measurements between 10 and 300 K revealed that the optical phonons of RhSeCl are weakly susceptible to temperature variations, whereas the temperature dependence of RhTeCl is more pronounced.

Lithium-rich antiperovskites are promising high-capacity cathode materials for next-generation Li-ion batteries. In this study, partially Mn-substituted (Li₂Fe_1–y Mn _y )­SeO (y = 0.1–0.4) cathode materials were synthesized via solid-state reaction. Structural analysis confirmed the cubic antiperovskite phase with lattice expansion proportional to the Mn content, while thermal analysis revealed congruent melting behavior with improved thermal stability. Electrochemical characterization demonstrated good cycling stability across all Mn-substituted compositions. The (Li₂Fe_0.9Mn_0.1)­SeO cathode exhibited an initial discharge capacity of 123 mAh/g and excellent cycling stability, but higher Mn content led to a performance decline. Battery optimization for (Li₂Fe_0.9Mn_0.1)­SeO, including electrolyte selection and electrode formulation, had a pronounced impact on performance. Notably, increasing the active material ratio from 70 to 85 wt % in the electrode mixture resulted in a significantly enhanced discharge capacity, achieving 203 mAh/g in the second cycle and maintaining 85% of this value (173 mAh/g) after 100 cycles. These results underscore the importance of composition and electrode design in maximizing the electrochemical performance of Li-rich antiperovskites.

Lithium-rich antiperovskites promise to be a compelling class of high-capacity cathode materials due to the existence of both cationic and anionic redox activity. Little is however known about the effect of separating the electrochemical cationic process from the anionic process and the associated implications on the electrochemical performance. In this context, we report the electrochemical properties of the illustrative example of three different (Li₂Fe)SO materials with a focus on separating cationic from anionic effects. With the high-voltage anionic process, an astonishing electrochemical capacity of around 400 mAh g^-1 can initially be reached. Our results however identify the anionic process as the cause of poor cycling stability and demonstrate that the fading reported in previous literature is avoided by restricting to only the cationic processes. Following this path, our (Li₂Fe)SO-BM500 shows strongly improved performance as indicated by constant electrochemical cycling over 100 cycles at a capacity of around 175 mAh g^-1 at 1 C. Our approach also allows us to investigate the electrochemical performance of the bare antiperovskite phase excluding extrinsic activity from initial or cycling-induced impurity phases. Our results underscore that synthesis conditions are a critical determinant of electrochemical performance in lithium-rich antiperovskites, especially with regard to the amount of electrochemical secondary phases, while the particle size has not been found to be a crucial parameter. Overall, separating and understanding the effects of the cationic from the anionic redox activity in lithium-rich antiperovskites provides the route to further improve their performance in electrochemical energy storage.

Heteroanionic materials show potential as 2D semiconductors due to their tunable band gaps, making them excellent candidates for photocatalytic water splitting applications.

Exciton bandwidths and exciton transport are difficult to control by material design. We showcase the intriguing excitonic properties in an organic semiconductor material with specifically tailored functional groups, in which extremely broad exciton bands in the near-infrared-visible part of the electromagnetic spectrum are observed by electron energy loss spectroscopy and theoretically explained by a close contact between tightly packing molecules and by their strong interactions. This is induced by the donor–acceptor type molecular structure and its resulting crystal packing, which induces a remarkable anisotropy that should lead to a strongly directed transport of excitons. The observations and detailed understanding of the results yield blueprints for the design of molecular structures in which similar molecular features might be used to further explore the tunability of excitonic bands and pave a way for organic materials with strongly enhanced transport and built-in control of the propagation direction.

We report in the context of lithium-rich antiperovskite cathode materials outstanding electrochemical properties of (Li₂Fe)SeO, which for the first time was synthesized via direct ball-milling. The unique structured material displays an electrochemical cycling performance of 250 mA h g⁻¹ at 0.1C when used as a cathode in lithium-ion batteries. Comprehensive electrochemical analysis combined with detailed transmission electron microscopy studies reveal that, above 2.5 V, the multi electron storage mechanism involves conversion of (Li₂Fe)SeO to Fe_1−xSe_x. Our results furthermore demonstrate the general relevance of our findings to the whole class of antiperovskite cathode materials and present a route to strongly enhance their cell performance by avoiding the degradation path deciphered by our studies.

In the search for novel 2D materials with potentially valuable properties, such as a tunable band gap for optoelectronic or catalytic applications, solid solutions hold the potential to significantly expand the inventory of available 2D nanomaterials. In this study, we present for the first time the synthesis of such 2D rhodium trihalide solid solutions: RhBr _x Cl_3−x and RhBr _x I_3−x . We use thermodynamic simulations and simultaneous thermal analysis to predict conditions for their rational synthesis and to investigate suitable chemical vapor transport (CVT) parameters for these solid solutions. The evolution of the lattice parameters was investigated by powder x-ray diffraction, showing an isostructural relationship of the synthesized compounds and only minor deviation from Vegard’s law. The optical band gap of these materials can be tuned in an energy range from 1.5 eV (RhCl₃) to 1.2 eV (RhI₃) by choosing the composition of the solid solution, while the samples also exhibit photoluminescence in similar energy ranges. Ultimately, the successful deposition of bulk as well as ultrathin 2D nanocrystals of RhBr _x Cl_3−x by CVT from 925 °C to 850 °C is shown, where the composition of the deposited crystals is precisely controlled by the choice of the starting composition and the initial amount of material. The high quality of the obtained nanocrystals is confirmed by atomic force microscopy, high resolution transmission electron microscopy and selected area electron diffraction. For RhBr _x I_3−x , the CVT from 900 °C to 825 °C is more difficult and has only been practically demonstrated for an exemplary case. According to the observed properties, these novel solid solutions and nanocrystals show a great potential for an application in optoelectronic devices.