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The mechanism of direct electrolytic reduction of SiO 2 in molten CaCl 2 was studied. Morphological and crystallographic investigations were conducted on Si prepared by potentiostatic electrolysis of SiO 2 contacting electrode at 1.10 V ͑vs. Ca 2+ /Ca͒ for 1 h at 1123 K. X-ray diffraction confirmed that amorphous SiO 2 was reduced to crystalline Si. From scanning electron microscopy ͑SEM͒ and energy-dispersive X-ray results, it was proved that Si columns were formed perpendicular to the reaction interface between Si and SiO 2 and that vacant spaces were formed between the columns. It was found from field emission SEM observation that the Si column had basically a hexagonal prismatic and stacking structure. Transmission electron microscopy and electron diffraction results revealed that the Si column was a single crystal having ͕111͖ twin planes perpendicular to the axis of the column. It is explained that amorphous Si is first formed by electrochemical reduction and then thermally transformed to crystalline Si. The rate-determining step of the reduction was found to be O 2− diffusion in the vacant space between the columns.Recently, electrolysis of metal oxides in molten chlorides such as CaCl 2 has come to attract attention as a substitute for the traditional metal production methods. Electrochemical reduction of metal oxide to metal in molten CaCl 2 was first reported by Okabe et al. 1 and recently also reported by Chen et al. 2,3 However, their works were restricted to metal oxides, which generally have nonstoichiometric oxide phases, and there was no report for oxides of nonmetallic elements.The present authors tried to electrochemically reduce SiO 2 to Si, where Si is classified as a nonmetal element, and found that direct electrolytic reduction of solid SiO 2 to Si was possible in molten CaCl 2 . 4 Despite the good insulating ability of SiO 2 , successful reduction was achieved by using a contacting electrode method in which a SiO 2 plate was wound by Mo wire. In the initial stage of the reduction, electrons are supplied directly to the SiO 2 through the Mo wire. The total reaction is written asThe reduction rate is extremely large in spite of it being a solid-state reaction, and the whole reduction of a SiO 2 plate ͑1 mm thick͒ has been achieved. 4 Recently, it was found that the reaction reaches a thickness of 200 m inside of the plate at 1.00 V ͑vs. Ca 2+ /Ca͒ for 1 h. 5 Applying this new electrochemical reaction, solar grade Si is expected to be produced by removing only oxygen from high-purity SiO 2 , called "solar grade SiO 2 " hereafter. This method can contribute to further widespread use of Si solar cells from the standpoints of both cost and productivity, because solar grade SiO 2 is not expensive and is easily purified. 6 With reference to our report, 4 Jin et al. recently reported that porous pellets of SiO 2 powder can be electrochemically reduced to Si in molten CaCl 2 . 7 Furthermore, by using this method, pinpoint reduction is possible at the neighborhood of the contacting point between the c...

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Fluoride ion batteries (FIBs) are regarded as promising energy storage devices, and it is important and urgent to develop cathode materials with high energy densities for use in FIBs.

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Protective coatings on cathode active materials have become paramount for the implementation of solid-state batteries; however, the development of coatings lacks the understanding of the necessary coating properties. In this study, guidelines for the design of solid electrolytes and electrode coatings in all-solid-state batteries are proposed from the viewpoint of the steady-state Li chemical potential profile across the battery cell. The model calculation of the (electro)chemical potential profile in all-solid-state batteries is established by considering the steady-state mixed ionic and electronic conduction in the solid electrolyte under the assumption of local equilibrium. For quantitative discussion, the potential profiles within oxygen ion conductors are calculated instead of Li/Na ion conductors as their partial electronic conductivities have not been reported so far in sufficient detail. Based on the calculated chemical potential profile, two main conclusions are obtained: (1) the decisive factor for the formation of the chemical potential profile of the neutral mobile component (e.g., oxygen or lithium) in the solid electrolyte is its electronic conductivity (and the activity dependence), and (2) a particularly large potential drop is formed in a region where the electronic conductivity becomes small. While these conclusions are valid and general for any solid electrolyte device, they are particularly important for the design of protective coatings and the understanding of the functionality of selfassembled solid electrolyte interphases in all-solid-state batteries. To protect the solid electrolyte from decomposition by reduction/oxidation at the anode/cathode interfaces, a sufficient chemical potential drop is necessary within the coating layer or directly at the interphase layer. To achieve this situation, the coating/interphase materials need to have a lower electronic conductivity than the solid electrolyte.

The fundamental factors that influence the hydration of BaZrO 3 (BZO) doped with trivalent cation M 3+ (Al, Sc, Ga, Y, In, and Lu) for proton conductors were investigated by means of density functional theory calculations which take the configuration of complex defects into account. The creation of oxygen vacancies is favored for Aland Ga-doped BZOs and leads to small hydration energies with stable proton sites at the nearest neighbor (1NN). Meanwhile, Y-, In-, and Ludoped BZOs prefer protons at the second nearest neighbor (2NN). The stability of those defects can be formulated in the context of the energies of oxygen vacancy formation and hydration. BZOs with larger dopants gain more hydration energy by structural relaxation with protons located at 2NN. By isolating the associated complex defects, it is possible to increase the negative hydration energy, which in effect improves the degree of hydration of BZOs.

Measurements of the thermoelectric power were applied to investigate the electrical conductivity in 5 mol % Sr-substituted LaPO4 as a function of temperature (600 to 900°C), water vapor partial pressure (0.2 to 6 kPa), and oxygen partial pressure (4 to 100 kPa). Expressions of the thermoelectric power for materials conducting protons, oxide ions, and electronic defects were derived, based on a thermodynamic treatment of entropy production by heat and charge transfer. The experimental data for 5 mol % Sr-substituted LaPO4 were interpreted in terms of protonic conduction, and some additional conduction, possibly by electron holes. In air, the proton transport number is unity at 600°C when PH1O> 1 kPa, and at 700°C when PH,o> 3 kPa. In wet atmospheres, the transported entropy of protons in this material was found to be nearly constant, 112 2 J moY1 K' at 600 to 800°C in wet atmospheres.