Method Of Electro-deoxidation Research Articles

C@CoSn2 composite prepared through one-step electro-deoxidation method as anode material of lithium-ion batteries with long-cycle-life and high capacity

Tin is widely studied as an alternative anode material of high-energy-density Li-ion batteries owing to its high theoretical capacity of 993 mAh g–1, low discharge potential, large electrical conductivity, and natural abundance. However, Sn-based anode materials still suffer from large changes in volume during lithium intercalation/de-intercalation processes, leading to cracking and pulverization of electrodes. Herein, carbon is in-situ coated on CoSn2 alloys (C@CoSn2) during the electro-deoxidation process of Co3O4 and SnO2 through the reduction of CO32− in the molten salt of LiCl–KCl–K2CO3. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectrometry all confirm CoSn2 alloys coated with a uniform carbon layer. The sizes of CoSn2 particles decrease to 0.2–1.0 µm due to the surface coating of carbon. The galvanostatic charge/discharge tests suggest discharge capacity and Coulombic efficiency of the optimized C@CoSn2 alloy film electrode reaching 991.5 mAh g–1 and 97.6 % after the 5th cycle, respectively. C@CoSn2 alloy also shows a good rate performance at 2 A g–1 along with a superior retained high capacity of 969.6 mAh g–1 after 300th cycles. Meanwhile, this proposed method looks efficient for the electrochemical fixation of CO2 in molten salts.

Preparation of Mg-Zr alloys through direct electro-deoxidation of MgO-ZrO2 in CaCl2-NaCl molten salt

The electro-deoxidation of MgO does not proceed efficiently due to the electrical insulation properties of MgO and large molar volume of metallic magnesium. In this experiment, an attempt was done to prepare Mg-Zr alloy from their oxides using the method of electro-deoxidation. The electrochemical mechanism of the deoxidation process of MgO, ZrO2 and MgO-ZrO2 composite was studied through cyclic voltammetry using metallic cavity electrode (MCE). There was an intermediate phase (Zr3O) in the reduction process of ZrO2. The electro-deoxidation of MgO occurred for MgO-ZrO2 composite in CaCl2-NaCl molten salt. Moreover, the electro-deoxidation of MgO was promoted by the addition of polyvinyl butyral (PVB). The formation of metal phase Mg was confirmed by XRD, EDS, Raman spectroscopy, and inductively-coupled plasma spectroscopy. The electro-deoxidation products showed significant differences with the concentration of PVB and the ratio between MgO and ZrO2. The electro-deoxidation of MgO and ZrO2 was favorable when the PVB concentration was 5.0% (wt.%). Pure metal phase of Mg-Zr alloys was formed when the molar ratio of MgO and ZrO2 was 1:2. The atomic fraction of Mg in the electrolytic product was ca. 28.0 mol.%. This work shows that MgO could be directly electro-deoxidized to Mg without the formation of intermediate phase oxide.

Electrochemical Preparation of Si Based Anode Material in Molten CaCl2

Graphite, which is commercially used as anode material in lithium-ion battery (LIB), has a low theoretical capacity of 372 mAh/g, and therefore should be replaced by alternative material with high capacity for the toward vehicles and other applications. Not only that, the cost and environmental issues should be considered in the production of alternative materials. In this regard, silicon materials have received the extensive research interest due to their high specific capacity of 3579 mAh/g, a low lithiation reaction voltage plateau (vs. Li/Li+) and abundant sources. Unfortunately, Si electrode typically suffers from irreversible capacity fading during cycling from their volume expansion and low intrinsic electronic conductivity. Recently, development of nanostructural design with various morphologies, Si nanowires, Si nanoparticles, and porous structures has received attention as a promising way to solve these problems. Nanostructural design largely improve cycle life by providing a sufficient free volume to accommodate silicon electrode expansion. However, commercially utilization of Si electrode is still insurmountable owing to some critical issues such as high costs, poor scalability and hazardous precursors. Furthermore, nanostructure Si includes severe side reactions due to a large surface area, low tap density. Here, we simply use the SiO2+Graphite mixture as the starting material and synthesize Si@SiC@Graphite composite material through electro-deoxidation method applied with the FFC Cambridge process, a simple approach for the high-purity extraction of metals from solid oxides via molten salt electrolysis that is known to be energy-saving and environmentally friendly. The electro-deoxidation was carried out in a molten CaCl2 at 1123 K in cell voltage of 2.6 V and 2.9 V. As an anode material, the prepared Si@SiC@graphite composite resolves all of the aforementioned critical issues and shows 900 mAh/g after 300 cycles without capacity fading. Furthermore, we fabricated high tap-density Si@SiC@graphite powder (~ 0.76 g/ml) using a mechanical pressing method, resulting in a high capacity (> 3.0 mAh/cm2) at the large loading of cathode, verifying the successful design of stable electron transfer and high dense structure.

Synthesis of La2MgNi9 hydrogen storage alloy in molten salt

La2MgNi9 alloy is synthesized directly from the sintered mixture of La2O3 + NiO + MgO in the molten CaCl2 electrolyte by the electro-deoxidation method at 740 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy are observed. Sintering (at 1200 °C for 2 h) converts the hygroscopic La2O3 (by the reaction with NiO) into the non-hygroscopic La2NiO4 and La3Ni2O7 phases. The X-ray diffraction peaks indicate that the electro-deoxidation causes LaOCl, Ni, LaNi5 and even target phase La2MgNi9 to form within 2 h process time. The molten salt synthesis process ends up with the final alloy structure of 79% La2MgNi9 and 21% retained LaNi5. The porous alloy structure (with approximately 31.66 m2g−1 specific surface area) is beneficial for higher hydrogen storage capacity and it is observed that La2MgNi9 alloy has promising discharge capacity which is approximately 280 mAhg−1. This work clearly indicates that the electro-deoxidation is a very effective method in the synthesis of the hydrogen storage materials.

Formation mechanism and preparation of YAl2 intermetallics by electro-deoxidation with different sintering conditions

A study was carried out on the preparation of YAl2 intermetallics from mixed oxide precursors using the method of electro-deoxidation. Y2O3 and Al2O3 mixed in molar proportions of 1:2 were sintered at temperatures ranging from 800 to 1200 °C for different hours. The sintered pellet was then electrolyzed in a molten CaCl2 at 850 °C using a graphite anode at a potential of 3.1 V. In this work, effects of sintering on the composition and the oxygen content in cathodic products were studied by X-ray diffraction (XRD) and oxygen–nitrogen analyzer. Sintering mainly affects the porosity of the pellets, and it is found that porosity decreases gradually with the increase in sintering temperature and the extension of sintering time. When sintered at 900 °C for 4 h, the oxygen content in the cathodic product decreases to 1.85 wt%, and the efficiency of electro-deoxidation is 94.48%. At the same time, the mechanism of electrolysis was speculated. The results suggest that the formation of YAl2 is a multi-step process. During the reduction process from mixed oxides Y2O3/Al2O3 to YAl2 included many intermediate materials, such as YAlO3, Y3Al5O12 and YAl3, YAl2 intermediate alloy was prepared finally.

Direct preparation of Al-base alloys from their oxides/metal precursors in the eutectic LiCl–KCl melt

A study was carried out on the preparation of Al–Cu–Li alloy from their oxides/metal precursors using the method of electro-deoxidation in the eutectic LiCl–KCl melt at 648 K. Cyclic voltammetry was used to characterize the system. The samples were prepared by potentiostatic electrolysis at −1.0 V to −2.0 V (vs. Ag+/Ag) for 5 h. XRD analysis shows that Li2O is not electrochemically reduced to Li at −1.0 V (vs. Ag+/Ag) or more negative potential. During the preparation process of Al–Cu–Li alloy, lithium peroxide is formed as an intermediate compound. Al–Cu–Li alloy is chemically prepared through the reaction between aluminum and lithium peroxide by heating of Al–Cu–Li2O precursors in KCl–LiCl–LiF melt at 1023 K. Eelectro-deoxidation in LiCl–KCl melt can increase the lithium content in the final alloy product. Al–Mg and Al–Nd alloy were also prepared by using the same method from their mixture of aluminum and corresponding oxide, respectively. Al–Nd alloy can only be obtained at the temperature above 773 K. Al–Li alloy could not be obtained in eutectic CaCl2–LiCl melt because of formation of calcium aluminates.

Molten salt synthesis of La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) type hydrogen storage alloys

La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) alloys were synthesized directly from sintered mixture of La2O3 + NiO + CoO in the molten CaCl2 electrolyte by the electro-deoxidation method at 850 °C and the electrochemical hydrogen storage characteristics of the synthesized alloys were observed. Sintering (at 1200 °C for 2 h) converted the hygroscopic La2O3 (by the reaction with NiO) into the non-hygroscopic La2NiO4, LaNiO3, La3Ni2O6.5, La3Ni2O6.84 and La4Ni3O9 depending on the Co content of the oxide mixture. The X-ray diffraction peaks indicated that La2NiO4 was the main La–Ni–O phase to initiate the LaNi5 phase formation. The increase in the Co content of the sample was observed to delay the La2NiO4 formation and thus the reactions initiated by La2NiO4 reduction. LaOCl formed chemically was the last oxygen including phase remained in the alloy before obtaining the final alloy structure. The porous alloy structure was beneficial for higher hydrogen storage capacity and it was observed that La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) alloys had promising discharge capacities changed between 223 mA h g−1 (LaNi5) and 325 mA h g−1 (La(Ni0.7Co0.3)5). This work clearly indicated that the electro-deoxidation was a very effective method in the synthesis of the hydrogen storage materials.

Synthesis of La2Ni7 hydrogen storage alloy by the electro-deoxidation technique

La2Ni7 alloy was synthesized in the molten CaCl2 electrolyte by the electro-deoxidation method at 850 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy was observed. Sintering at 1200 °C for 3 h was determined as the optimum condition to have the electrode pellet with enough porosity and mechanical strength. The hydroscopic La2O3 disappeared and the non-hydroscopic La2NiO4 formed during the sintering. The X-ray diffraction peaks indicated that the sinter product La2NiO4 and NiO reduced to LaNi5 and Ni, respectively, within 4 h electro-deoxidation process. The reduction kinetics of LaOCl was relatively slow. This sluggish reduction caused formation of the target La2Ni7 phase only after 6 h electro-deoxidation. The final La2Ni7 alloy structure with small amount of retained LaNi5 was obtained after 10 h electro-deoxidation and this structure did not change up to 25 h. It was observed that the synthesized alloy had maximum discharge capacity of 207 mA h g−1 and the alloy kept more than 90% of its discharge capacity within 20 charge/discharge cycles. According to the gathered EIS data La2Ni7 alloy was estimated to have no considerable surface barrier layer after discharging to hinder the atomic hydrogen diffusion on its surface.

Synthesis of La2(Ni1-xCox)7 (x = 0.05, 0.1, 0.2) Hydrogen Storage Alloys by the Electro-Deoxidation Technique

La2(Ni1-xCox)7 (x = 0.05, 0.1, 0.2) alloys were synthesized directly from sintered mixture of La2O3 + NiO + CoO in the molten CaCl2 electrolyte by the electro-deoxidation method at 850°C and the electrochemical hydrogen storage characteristics of the synthesized alloys were observed. Sintering (at 1200°C for 3 h) converted the hygroscopic La2O3 (by the reaction with NiO) into the non-hygroscopic LaNiO3, La3Ni2O6.5 and La4Ni3O9 depending on the Co content of the oxide mixture. The X-ray diffraction peaks indicated that La2NiO4 was the main La-Ni-O phase formed as a result of partial reduction or dissociation reaction to initiate the LaNi5 phase formation. The target La2Ni7 phase formed much later than LaNi5 phase probably due to the sluggish reduction of LaOCl. It was observed that La2(Ni1-xCox)7 (x = 0.05, 0.1, 0.2) alloys had promising discharge capacities changed between 227 mA h g−1 (La2(Ni0.95Co0.05)7) and 332 mA h g−1 (La2(Ni0.8Co0.2)7) depending on the alloy Co content. This work clearly indicated that the electro-deoxidation was very effective method in the synthesis of the hydrogen storage materials.

Synthesis of La2Ni7 Hydrogen Storage ALLOY By Electro-Deoxidation Method

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Hydrogen storage characteristics of Ti2Ni alloy synthesized by the electro-deoxidation technique

Ti2Ni alloy was synthesized in the molten CaCl2 electrolyte by the electro-deoxidation method at 900 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy was observed. The X-ray diffraction peaks indicated that stoichiometric oxides in TiO2–ZrO2–NiO mixture reduced to Ti3O5, CaTiO3, CaZrO3, Ni and Ti2O3 within 5 h electro-deoxidation process. Extension of the electro-deoxidation time to 10 h caused formations of TiO and equilibrium Ti2Ni phase. After 24 h electro-deoxidation the target alloy with the equilibrium Ti2Ni phase structure and the maximum amount of the dissolved Zr in it was obtained. It was observed that the synthesized alloy had maximum discharge capacity of 200 mA h g−1. Upon increase in the charge/discharge cycles, however, the discharge capacity decayed sharply. According to the gathered EIS data at various DODs, the rapid degradation in the electrode performance of Ti2Ni alloy was attributed to the developed barrier oxide layer on the electrode surface.

Direct synthesis of Mg–Ni compounds from their oxides

A study was carried out on the synthesis of Mg–Ni compounds as well as on the extraction of pure Ni and Mg from their oxides using the method of electro-deoxidation. The oxides sintered at 1200 °C were in the form of discrete phases NiO and MgO, suitably proportioned to yield Ni, MgNi 2, Mg 2Ni and Mg. The oxides were electrolyzed at 3.2 V in a eutectic mixture of CaCl 2–NaCl solution maintained at a constant temperature (900–600 °C), using a graphite anode. The study has shown that NiO rich mixture, MgO:NiO = 1:2, can be reduced successfully to metallic state. Some loss of Mg was apparent in the latter, with the result that the product was far from the target composition MgNi 2. The electroreduction of MgO rich mixtures was difficult to achieve. The mixture MgO:NiO = 2:1 when electrolyzed at 725 °C for 24 h, could be reduced to metallic phases only in small proportions (18 wt.%). In pure MgO, no trace of reduction was observed during the electrolysis at 600 °C. Difficulties in the electroreduction of MgO and MgO rich mixtures were partially attributed to low conductivity of MgO.

Synthesis of FeTi from mixed oxide precursors

A study was carried out on the synthesis of FeTi intermetallics from mixed oxide precursors using the method of electro-deoxidation. Fe 2O 3 and TiO 2 mixed in molar proportions of 1:2 were sintered at temperatures ranging from 900 °C to 1300 °C. The sintered pellet of mixed oxides, connected as cathode, was then electrolyzed in a molten CaCl 2 at 900 °C using a graphite anode at a potential of 3.2 V. The electrolysis yielded the target composition FeTi in substantial amounts only when the sintering temperature was close to or above 1100 °C. The process of deoxidation was followed with the use of interrupted experiments. This has shown that the two-phase structure of Fe 2TiO 5 and TiO 2 in the oxide pellet reacts with the molten salt even before the electrolysis forming CaTiO 3, transforming the rest into a mixture of ilmenite (FeTiO 3) and a spinel phase (Fe 2TiO 4). During electrolysis these complex oxides were converted into simpler ones. Fe is the first element to be produced followed by the intermetallic Fe 2Ti. FeTi evolves quite late in the electrolysis which seemed to have followed the reduction of CaTiO 3. It is shown that interrupted experiments and examination of partially reduced pellets yield considerable information with regard to the sequence of changes that occur during the deoxidation process.

Using electro-deoxidation to synthesize niobium sponge from solid Nb2O5 in alkali–alkaline-earth metal chloride melts

The method of electro-deoxidation was applied to reduce solid Nb2O5 directly to niobium metal, in the form of sponge, in the eutectic CaCl2–NaCl and BaCl2–NaCl systems, respectively. The direct electrochemical reduction of solid Nb2O5 was achieved by electrolysis in the chloride melts at temperatures between 850 and 950 °C and at a controlled potential of 3.1 V, well below the decomposition potentials of the melts used. The obtained results demonstrated that the method is applicable for preparing niobium sponges directly from solid Nb2O5 under the present experimental conditions. The niobium sponge prepared was proven to possess as remarkable superconducting properties. The parameters affecting the rate and the extent of electro-deoxidation were investigated experimentally, including initial particle sizes of the starting Nb2O5 powders, compaction pressure, sintering temperature and times, electrolysis temperature and duration, and electrolyte systems. The mechanism by which the Nb2O5 pellets were electro-deoxidized to the niobium sponges was also discussed on the basis of the present experimental observations.

Production of niobium powder by direct electrochemical reduction of solid Nb2O5 in a eutectic CaCl2-NaCl melt

The method of electro-deoxidation was used to reduce solid Nb2O5 to niobium metal in a CaCl2-NaCl eutectic melt. The direct electrochemical reduction of Nb2O5 was achieved by electrolysis in the eutectic melt at 1123 and 1173 K, respectively, at a controlled potential of 3.1 V, below the decomposition potential of the salts. Analysis of the anodic reaction gases carried by a flow of dry, high-purity argon confirmed that the preferred cathodic reaction is the oxygen ionization of the cathode to form oxygen ions that are successively dissolved in the chloride melt and then discharged at the graphite-rod anode. Chlorine ions are unlikely to be discharged at the anode. The niobium metal powders prepared contained as low as 2311 mass ppm oxygen, clearly demonstrating that the electrodeoxidation method is applicable to the reduction of solid Nb2O5 in chloride melts. The kinetics and mechanisms of reduction of the porous pellets of Nb2O5 are discussed in detail based on the measured current-time behavior, together with the microstructural analysis of the sintered Nb2O5 pellets and the phases present in the partially reduced samples obtained at various times of reduction.