Limitation of discharge capacity and mechanisms of air-electrode deactivation in silicon-air batteries

The electrocatalytical process at the air cathode in novel silicon-air batteries using the room-temperature ionic liquid hydrophilic 1-ethyl-3-methylimidazolium oligofluorohydrogenate [EMI⋅2.3 HF⋅F] as electrolyte and highly doped silicon wafers as anodes is investigated by electrochemical means, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy. The results obtained by XPS and EPR provide a model to describe the limited discharge capacity by means of a mechanism of air-electrode deactivation. In that respect, upon discharge the silicon-air battery's cathode is not only blocked by silicon oxide reduction products, but also experiences a major modification in the MnO₂ catalyst nature. The proposed modification of the MnO₂ catalyst by means of a MnF₂ surface layer greatly impacts the Si-air performance and describes a mechanism relevant for other metal-air batteries, such as the lithium-air. Moreover, the ability for this deactivation layer to form is greatly impacted by water in the electrolyte.

Polymorphism of Li2Zn3

Crystal structures of low- and high-temperature modifications of the binary phase Li2Zn3 were determined by single-crystal X-ray diffraction techniques. The low-temperature modification is a disordered variant of Li5Sn2, space group R\bar 3m (No. 166). The high-temperature modification crystallizes as an anti-type to Li5Ga4, space group P\bar 3m1 (No. 164). Two polymorphs can be described as derivative structures to binary Li5Ga4, Li5Sn2, Li13Sn5, Li8Pb3, CeCd2 and CdI2 phases which belong to class 2 with the parent W-type in Krypyakevich's classification. All atoms in both polymorphs are coordinated by rhombic dodecahedra (coordination number CN = 14) like atoms in related structures. The Li2Zn2.76 (for the low-temperature phase) and Li2Zn2.82 (for the high-temperature phase) compositions were obtained after structure refinements. According to electronic structure calculations using the tight-binding–linear muffin-tin orbital–atomic spheres approximations (TB–LMTO–ASA) method, strong covalent Sn—Sn and Ga—Ga interactions were established in Li5Sn2 and Li5Ga4, but no similar Zn—Zn interactions were observed in Li2Zn3.

La(5)Zn(2)Sn

A single crystal of penta-lanthanum dizinc stannide, La(5)Zn(2)Sn, was obtained from the elements in a resistance furnace. It belongs to the Mo(5)SiB(2) structure type, which is a ternary ordered variant of the Cr(5)B(3) structure type. The space is filled by bicapped tetra-gonal anti-prisms from lanthanum atoms around tin atoms sharing their vertices. Zinc atoms fill voids between these bicapped tetra-gonal anti-prisms. All four atoms in the asymmetric unit reside on special positions with the following site symmetries: La1 (..m); La2 (4/m..); Zn (m.2m); Sn (422).

A tetragonal form of dysprosium orthomolybdate at room temperature

In the present tetragonal modification of dysprosium orthomolybdate, Dy(2)(MoO(4))(3), the Dy, one Mo and one O atom are located on a mirror plane with Wyckoff symbol 4e, while another Mo atom is located on a fourfold inverse axis, Wyckoff symbol 2a. A single crystal was selected from a polycrystalline mixture of the Dy(2)O(3)-ZrO(2)-MoO(3) system and was stable at room temperature for at least three months. The structure refinement does not indicate the presence of Zr on the Dy sites (to within 1% accuracy). Thus, the stabilization of the tetragonal form is due to disordered positions for a second O atom and split positions for a third O atom that also maintain the DyO(7) coordination, which is not expected for short Dy-O distances [2.243 (6)-2.393 (5) Å].

Thermal structural properties of calcium tungstate

The results ofin situtemperature-resolved powder diffraction studies of CaWO4scheelite using both synchrotron radiation and neutron scattering are reported. The studies performed over a broad temperature range of 5–1773 K confirm the scheelite type of structure for calcium tungstate over the whole temperature range. The anisotropy of thermal expansion in calcium tungstate as well as the rigidity of WO4complexes have been analysed in terms of bond distances, interatomic angles and anisotropic displacement parameters. The WO42−complex anions showed a remarkable robustness in the whole studied temperature range, thus pointing out that the layered structure formed by two-dimensional CsCl-type arrangements of Ca cations and WO4complexes is the primary reason for the anisotropy of thermal expansion in calcium tungstate.

Field-induced phase transition in Bi1/2Na1/2TiO3-based lead-free piezoelectric ceramics

The origin of the electric field-induced strain in the polycrystalline ceramic 0.92Bi1/2Na1/2TiO3–0.06BaTiO3–0.02K1/2Na1/2NbO3was investigated usingin situhigh-resolution X-ray and neutron diffraction techniques. The initially existing tetragonal phase with pseudocubic lattice undergoes a reversible phase transition to a significantly distorted rhombohedral phase under electric field, accompanied by a change in the oxygen octahedral tilting froma0a0c+toa−a−a−and in the tilting angle. The polarization values for the tetragonal and rhombohedral phases were calculated based on the structural information from Rietveld refinements. The large recoverable electric field-induced strain is a consequence of a reversible electric field-induced phase transition from an almost nonpolar tetragonal phase to a ferroelectrically active rhombohedral phase.