Size-Dependent Phase Transformations in Bismuth Oxide Nanoparticles. II. Melting and Stability Diagram

Melting of nanocrystalline bismuth oxide particles between 6 and 50 nm was investigated in situ in the transmission electron microscope (TEM). It revealed a size-dependent melting behavior with a strong melting point reduction (−55% at 6 nm). One reason is a −230 K offset in the bulk melting temperature which is apparently caused by the β-phase in which the nanomaterial resides. As a second reason, a strong size dependency was observed from which an approximate solid-surface energy of 0.3 J/m² was determined. Yet, the conditions in the TEM could cause a lowering of the transition temperatures compared to chemically neutral conditions, although theoretical considerations predict reduction in the solid state to be negligible. Everything indicates that no stable, liquid surface layer forms prior to melting. In spite of the covalent-ionic bonds in this oxide material the qualitatively same size dependence shows like in metals. Combined with size-dependent evaporation examined in a companion study [Guenther et al. <i>J. Phys. Chem. C</i> <b>2014</b>, 10.1021/jp412531t], a size-dependent phase diagram is proposed for this oxide material

Size-Dependent Phase Transformations in Bismuth Oxide Nanoparticles. I. Synthesis and Evaporation

At the nanoscale material properties can be tuned by altering the size and shape of the specimen. Such effects are quite well investigated for metallic materials. On the other hand inorganic compounds have received relatively little interest due to the more demanding experimental procedures. While the size effects are similar for any kind of inorganic material, the degree of size-dependent changes depends on the bond strength and bond nature of the material at the surface: the higher the surface energy, the stronger the size dependence. These thoughts are demonstrated in this contribution by investigating the size-dependent thermodynamic properties of monodisperse, size-selected bismuth oxide (Bi₂O₃) nanoparticles in the range between 6 and 50 nm. This first part is mainly concerned with evaporation, while the second part (<i>J. Phys. Chem. C</i> <b>2014</b>, 10.1021/jp509841s) covers size-dependent melting. Heating experiments up to the evaporation of the particles were performed with a new, custom method based on loss of matter caused by evaporation. The results in this part show the validity of the Kelvin equation and a size-dependent evaporation behavior of this oxide

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High Capacity Garnet-Based All-Solid-State Lithium Batteries: Fabrication and 3D-Microstructure Resolved Modeling

The development of high-capacity, high-performance all-solid-state batteries requires the specific design and optimization of its components, especially on the positive electrode side. For the first time, we were able to produce a completely inorganic mixed positive electrode consisting only of LiCoO₂ and Ta-substituted Li₇La₃Zr₂O₁₂ (LLZ:Ta) without the use of additional sintering aids or conducting additives, which has a high theoretical capacity density of 1 mAh/cm². A true all-solid-state cell composed of a Li metal negative electrode, a LLZ:Ta garnet electrolyte, and a 25 μm thick LLZ:Ta + LiCoO₂ mixed positive electrode was manufactured and characterized. The cell shows 81% utilization of theoretical capacity upon discharging at elevated temperatures and rather high discharge rates of 0.1 mA (0.1 C). However, even though the room temperature performance is also among the highest reported so far for similar cells, it still falls far short of the theoretical values. Therefore, a 3D reconstruction of the manufactured mixed positive electrode was used for the first time as input for microstructure-resolved continuum simulations. The simulations are able to reproduce the electrochemical behavior at elevated temperature favorably, however fail completely to predict the performance loss at room temperature. Extensive parameter studies were performed to identify the limiting processes, and as a result, interface phenomena occurring at the cathode active material/solid–electrolyte interface were found to be the most probable cause for the low performance at room temperature. Furthermore, the simulations are used for a sound estimation of the optimization potential that can be realized with this type of cell, which provides important guidelines for future oxide based all-solid-state battery research and fabrication

Scandium-Substituted Na₃Zr₂(SiO₄)₂(PO₄) Prepared by a Solution-Assisted Solid-State Reaction Method as Sodium-Ion Conductors

As possible electrolyte materials for all-solid-state Na-ion batteries (NIBs), scandium-substituted Na₃Zr₂­(SiO₄)₂(PO₄) in the structure of NASICONs (Na superionic conductors) has received hardly any attention so far, although among all the trivalent cations, Sc³⁺ might be the most suitable substitution ion for Na₃Zr₂(SiO₄)₂(PO₄) because the ionic radius of Sc³⁺ (74.5 pm) is the closest to that of Zr⁴⁺ (72.0 pm). In this study, a solution-assisted solid-state reaction (SASSR) method is described, and a series of scandium-substituted Na₃Zr₂(SiO₄)₂(PO₄) with the formula of Na_3+<i>x</i>Sc_<i>x</i>Zr_2‑<i>x</i>(SiO₄)₂(PO₄) (NSZSP<i>x</i>, 0 ≤ <i>x</i> ≤ 0.6) have been prepared. This synthesis route can be applied for powder preparation on a large scale and at low cost. With increasing degrees of scandium substitution, the total conductivity of the samples also increases. An optimum total Na-ion conductivity of 4.0 × 10^–3 S cm^–1 at 25 °C is achieved by Na_3.4Sc_0.4Zr_1.6­(SiO₄)₂(PO₄) (NSZSP0.4), which is the best value of all reported polycrystalline Na-ion conductors. The possible reasons for such high conductivity are discussed

Fast Na⁺ Ion Conduction in NASICON-Type Na_3.4Sc₂(SiO₄)_0.4(PO₄)_2.6 Observed by ²³Na NMR Relaxometry

The bulk diffusion of Na in Na_3.4Sc₂(SiO₄)_0.4(PO₄)_2.6 was investigated by ²³Na NMR relaxometry in the temperature range from 250 to 670 K. These measurements reveal fast Na diffusion with hopping rates of 3 × 10⁸ s^–1 for the Na⁺ ions at 350 K and activation barriers for single Na⁺ ion jumps of (0.20 ± 0.01) eV. From these values a diffusion coefficient of <i>D</i> = 6.4 × 10^–12 m²/s and a Na ion conductivity of σ_Na = 4 mS/cm (both at 350 K) can be estimated. Measurements on two samples, one stored in air and one stored in Ar, do not show significant differences, which reveals that these NMR measurements are probing the bulk diffusion while conductivity measurements usually are also influenced by grain boundaries that can be affected by the moisture level during storage

Li₇La₃Zr₂O₁₂ Interface Modification for Li Dendrite Prevention

Al-contaminated Ta-substituted Li₇La₃Zr₂O₁₂ (LLZ:Ta), synthesized via solid-state reaction, and Al-free Ta-substituted Li₇La₃Zr₂O₁₂, fabricated by hot-press sintering (HP-LLZ:Ta), have relative densities of 92.7% and 99.0%, respectively. Impedance spectra show the total conductivity of LLZ:Ta to be 0.71 mS cm^–1 at 30 °C and that of HP-LLZ:Ta to be 1.18 mS cm^–1. The lower total conductivity for LLZ:Ta than HP-LLZ:Ta was attributed to the higher grain boundary resistance and lower relative density of LLZ:Ta, as confirmed by their microstructures. Constant direct current measurements of HP-LLZ:Ta with a current density of 0.5 mA cm^–2 suggest that the short circuit formation was neither due to the low relative density of the samples nor the reduction of Li–Al glassy phase at grain boundaries. TEM, EELS, and MAS NMR were used to prove that the short circuit was from Li dendrite formation inside HP-LLZ:Ta, which took place along the grain boundaries. The Li dendrite formation was found to be mostly due to the inhomogeneous contact between LLZ solid electrolyte and Li electrodes. By flatting the surface of the LLZ:Ta pellets and using thin layers of Au buffer to improve the contact between LLZ:Ta and Li electrodes, the interface resistance could be dramatically reduced, which results in short-circuit-free cells when running a current density of 0.5 mA cm^–2 through the pellets. Temperature-dependent stepped current density galvanostatic cyclings were also carried out to determine the critical current densities for the short circuit formation. The short circuit that still occurred at higher current density is due to the inhomogeneous dissolution and deposition of metallic Li at the interfaces of Li electrodes and LLZ solid electrolyte when cycling the cell at large current densities

Impact of Ni–Mn–Co–Al-Based Cathode Material Composition on the Sintering with Garnet Solid Electrolytes for All-Solid-State Batteries

A systematic and comprehensive study of the thermal stability of the cathode active materials LiNi1/3Mn1/3Co1/3O2 (NMC111), LiNi0.6Mn0.2Co0.2O2 (NMC622), LiNi0.8Mn0.1Co0.1O2 (NMC811), and LiNi0.8Co0.15Al0.05O2 (NCA) in combination with the garnet solid electrolyte Li6.45La3Zr1.6Ta0.4Al0.05O12 was performed, and the respective thermal stability limits in air were assessed. Compared to prior studies on such material mixtures, additional Zr-containing secondary phases were detected, which had not been taken into consideration in a previously published work. Here, these phases were successfully identified for the first time by a combination of X-ray diffraction, Raman spectroscopy, and microstructural analysis

Characterization and Optimization of La_0.97Ni_0.5Co_0.5O_3−δ-Based Air-Electrodes for Solid Oxide Cells

On the basis of previous studies of perovskites in the quasi-ternary system LaFeO₃–LaCoO₃–LaNiO₃, LaNi_0.5Co_0.5O₃ (LNC) is chosen as the most promising air-electrode material in the series for solid oxide cells (SOCs). In the present study, A-site deficiency of LNC is discussed and La_0.97Ni_0.5Co_0.5O₃ (LNC97) is selected as the optimal composition. Compatibility of LNC97 with 8 mol % Y₂O₃ stabilized ZrO₂ (8YSZ) is analyzed and compared with that of the state-of-the-art air-electrode La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and 8YSZ. Targeting to the requirements of high-performance SOC air-electrodes (high electronic and ionic conductivity and high catalytic activity for the oxygen reduction reaction), LNC97-based air-electrodes are tailored, characterized and optimized by symmetric-cell tests. Principles of air-electrode design for SOCs are revealed accordingly. Long-term measurement of the symmetric cells over a period of 1000 h is performed and possible degradation mechanisms are discussed. Full cells based on optimized LNC97 air-electrodes are also tested. Lower reactivity with 8YSZ in comparison to LSCF and a similar performance render LNC97 a very competitive candidate to substitute LSCF as air-electrode material of choice for SOCs