Site-selective ion beam synthesis and optical properties of individual CdSe nanocrystal quantum dots in a SiO2 matrix

Cadmium selenide nanocrystal quantum dots (NC-QDs) are site-selectively synthesized by sequential ion beam implantation of selenium and cadmium ions in a SiO2 matrix through sub-micron apertures followed by a rapid thermal annealing step. The size, areal density and optical emission energy of the NC-QDs are controlled by the ion fluence during implantation and the diameter of implantation aperture. For low fluences and small apertures the emission of these optically active emitters is blue-shifted compared to that of the bulk material by $>100\,{\rm meV}$ due to quantum confinement. The emission exhibits spectral diffusion and blinking on a second timescales as established also for solution synthesized NC-QDs

Direct Observation of Reductive Coupling Mechanism between Oxygen and Iron/Nickel in Cobalt-Free Li-Rich Cathode Material: An in Operando X-Ray Absorption Spectroscopy Study

Li-rich cathodes possess high capacity and are promising candidates in next-generation high-energy density Li-ion batteries. This high capacity is partly attributed to its poorly understood oxygen-redox activity. The present Li-rich cathodes contain expensive and environmentally-incompatible cobalt as a main transition metal. In this work, cobalt-free, iron-containing Li-rich cathode material (nominal composition Li$_{1.2}$Mn$_{0.56}$Ni$_{0.16}$Fe$_{0.08}$O$_{2}$) is synthesized, which exhibits excellent discharge capacity (≈250 mAh g$^{-1}$ and cycling stability. In operando, X-ray absorption spectroscopy at Mn, Fe, and Ni K edges reveals its electrochemical mechanism. X-ray absorption near edge structure (XANES) features of Fe and Ni K edges show unusual behavior: when an electrode is charged to 4.5 V, Fe and Ni K edges’ XANES features shift to higher energies, evidence for Fe$^{3+}$→Fe$^{4+}$ and Ni$^{2+}$→Ni$^{4+}$ oxidation. However, when charged above 4.5 V, XANES features of Fe and Ni K edges shift back to lower energies, indicating Fe$^{4+}$→Fe$^{3+}$ and Ni$^{4+}$→Ni$^{3+}$ reduction. This behavior can be linked to a reductive coupling mechanism between oxygen and Fe/Ni. Though this mechanism is observed in Fe-containing Li-rich materials, the only electrochemically active metal in such cases is Fe. Li$_{1.2}$Mn$_{0.56}$Ni$_{0.16}$Fe$_{0.08}$O$_{2}$ has multiple electrochemically active metal ions; Fe and Ni, which are investigated simultaneously and the obtained results will assist tailoring of cost-effective Li-rich materials

Investigation of Structural and Electronic Changes Induced by Postsynthesis Thermal Treatment of LiNiO2_{2}

Postsynthesis thermal treatments at various temperatures in air have been applied to LiNiO2, and the induced structural and electronic changes have been uncovered. Except for the familiar decomposition process at higher temperatures, a series of transformations also take place under mild conditions. To identify such subtle changes, ex situ and in situ synchrotron radiation diffraction, ex situ7Li nuclear magnetic resonance spectroscopy, and ex situ measurements of magnetic properties have been performed. We show that the reaction between LiNiO2 and CO2 starts already at a temperature of 200 °C, forming Li1–zNi1+zO2 layers. If the thickness of this layer is well adjusted, the electrochemical performance of LiNO2 can be improved. A cation off-stoichiometry of [Li0.90Ni0.10]NiO2 is identified at 600 °C even before the decomposition occurs. We also investigate the interplay of the reaction between LiNiO2 and CO2 with the decomposition at 700 °C. The changes in the Ni oxidation state and local Li environments are also monitored during the whole process

Phosphoric acid and thermal treatments reveal the peculiar role of surface oxygen anions in lithium and manganese-rich layered oxides

The interplay between cationic and anionic redox activity during electrochemical cycling makes layered Li-rich oxides appealing cathodes for state-of-the-art lithium-ion batteries. However, it remains challenging as the origin of lattice oxygen activity is not yet fully understood. Here we report on the effects of a lithium-deficient layer in the near-surface region of Co-free Li-rich Li[Li0.2Ni0.2Mn0.6]O-2 (LLNMO) achieved via a phosphoric acid surface treatment. Our results show that oxidized On- (0 < n < 2) species are formed on the surface of H3PO4-treated LLNMO resulting from Li ion deficiency and lattice distortion. The metastable On- could be easily released from the oxygen surface lattice forming O-2 via thermal treatment, accompanied by a surface reconstruction and a layered-to-rock-salt/spinel transition. The presented results demonstrate that the surface lattice structure plays a critical role in the electrochemical performance of LLNMO. This information provides new insights into the oxygen redox in LLNMO and opens up an opportunity for Li-rich cathodes to achieve long cycle life toward a broad range of applications in electrical energy storage devices

Einleitung und Vorgehensweise

Das deutsche Stiftungswesen hat sich in den vergangenen drei Jahrzehnten beachtlich entwickelt. Der vorliegende dritte Band der Serie „Stiftungen in Deutschland“ diskutiert anhand vier zentraler gesellschaftlicher Felder – Bildung, Wissenschaft, Soziales, und Kultur – welchen Beitrag Stiftungen in Deutschland leisten