Hinrich-Wilhelm Meyer

The mechanism of Li-ion transport in the garnet Li5La3Nb2O12.

We present a detailed study on the exact location and dynamics of Li ions in the garnet-type material Li(5)La(3)Nb(2)O(12) employing advanced solid state NMR strategies. Applying temperature-dependent (7)Li-NMR, (6)Li-MAS-NMR, (6)Li-{(7)Li}-CPMAS-NMR, (6)Li-{(7)Li}-CPMAS-REDOR-NMR as well as 2D-(6)Li-{(7)Li}-CPMAS-Exchange-NMR spectroscopy, we were able to quantify the distribution of the Li cations among the various possible sites within the garnet-type structure and to identify intrinsic details of Li migration. The results indicate a sensitive dependence of the distribution of Li cations among the tetrahedral and octahedral sites on the temperature of the final annealing process. This distribution profoundly affects the mobility of the Li cations within the garnet-type framework structure. Extended Li mobility at ambient temperature is only possible if the majority of the Li cations is accommodated in the octahedral sites, as observed for the sample annealed at 900 degrees C. Octahedrally-coordinated Li cations could be identified as the mobile Li species, whereas the tetrahedral sites seem to act as a trap for the Li cations, rendering the tetrahedrally-coordinated Li cations immobile on the time scale of the NMR experiments.

Dual-ion Cells Based on Anion Intercalation into Graphite from Ionic Liquid-Based Electrolytes

Abstract Electrochemical energy storage systems using graphite as both the negative and the positive electrode have been proposed as “dual-graphite cells”. In this kind of electrochemical system, the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode, both based on graphite, during the charging process. On discharge, cations and anions are released back into the electrolyte. So far, the systems proposed in literature are primarily based on Li+ and PF6- intercalation/de-intercalation into/from graphite from non-aqueous organic solvent based electrolytes. As the positive electrode potential during charging always exceeds 4.2 V vs. Li/Li+, the organic electrolyte starts to decompose at these highly oxidizing conditions resulting in insufficient discharge/charge efficiencies. The replacement of organic solvent by ionic liquids (ILs) leads an increased stability of the electrolyte towards oxidation and thus to remarkably higher efficiencies as well as an increased cycling stability. In fact, ionic liquids provide extended anodic electrochemical stability and in addition, no solvent co-intercalation occurs in parallel to anion intercalation at high potentials. Here, we present highly promising results for “dual-ion cells” based on a graphite cathode and an ionic liquid based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI). As the compatibility of this IL with graphite anodes is poor, alternative anodes such as metallic lithium or lithium titanate (Li4Ti5O12, LTO) are used. Consequently, the “dual-graphite” cell is renamed to “dual-ion” cell. In addition, the calculation of the specific energy of these systems will be in the focus of the discussion.

Synthesis and electrochemical performance of surface-modified nano-sized core/shell tin particles for lithium ion batteries

Tin is able to lithiate and delithiate reversibly with a high theoretical specific capacity, which makes it a promising candidate to supersede graphite as the state-of-the-art negative electrode material in lithium ion battery technology. Nevertheless, it still suffers from poor cycling stability and high irreversible capacities. In this contribution, we show the synthesis of three different nano-sized core/shell-type particles with crystalline tin cores and different amorphous surface shells consisting of SnOx and organic polymers. The spherical size and the surface shell can be tailored by adjusting the synthesis temperature and the polymer reagents in the synthesis, respectively. We determine the influence of the surface modifications with respect to the electrochemical performance and characterize the morphology, structure, and thermal properties of the nano-sized tin particles by means of high-resolution transmission electron microscopy, x-ray diffraction, and thermogravimetric analysis. The electrochemical performance is investigated by constant current charge/discharge cycling as well as cyclic voltammetry.

Crystalline Cation Conductors with Rotational Anion Disorder: Results of Quasielastic Neutron Scattering Experiments on Orthophosphates

Abstract Results and interpretations of several quasielastic neutron scattering measurements on samples of the isostructural high-temperature phases of Ag3PO4 and Na3PO4, prominent members of the group of fast cation conducting plastic phases are reported.Time-of-flight experiments reveal details on the phosphate reoriention in Ag3PO4: We find isotropic rotational diffusion. A consistent interpretation of the data suggests that silver ions are involved in the fast anion dynamics, a behaviour which is in line with the displacement of phosphorus detected in earlier neutron diffraction experiments. The reorientation activation energy of 0.17 eV is in the typical range found also in other ion conducting rotor phases.Na3PO4 was examined in time-of-flight and backscattering experiments, differing considerably in the time scale of the detected molecular dynamics. Anion reorientation, observed on the time scale of picoseconds and activated with 0.18 eV, appears as a predominantly circular diffusion with one P–O axis fixed. Our results suggest the involvement of sodium ions in the anion reorientation, i.e., a correlated motion of both types of ions in the picosecond regime.The much slower sodium jump diffusion in Na3PO4 was found to be thermally activated with 0.74 eV. Moreover, we find clear evidence for the predominance of jumps between tetrahedrally co-ordinated cation sites.Quasielastic neutron scattering reveals a wealth of dynamic details on the two high-temperature phases of Na3PO4 and Ag3PO4 which exhibit considerable differences (e.g., in the geometric details of anion dynamics) in spite of their structural similarities. Both plastic phases, however, give evidence of the involvement of cations in the anion dynamics, i.e., in a correlated reorientational motion of cations and anions on the picosecond time scale.