The distinct role of transition metal doping for LiFePO<Subscript>4</Subscript> cathode material

This study addresses the controversial issue of the effect of metal ion doping on the electrochemical performance of LiFePO₄. Metal doping is claimed to be a possible cause for the capacity improvement of LiFePO₄ as carbon coating. Results obtained inthis study show that dry-milled LiFePO₄ and LiFe_0.9Cr_0.1PO₄ deliver 119 mAh g⁻¹ and 101 mAh g⁻¹, while wet-milled LiFePO₄ and LiFe_0.9Cr_0.1PO₄ deliver 149 mAh g⁻¹ and 138 mAh g⁻¹, respectively. This indicates that the capacity improvement by metal doping is due to the carbonaceous materials produced during fabrication and not by the enhancement of ion diffusion. On the other hand, cycle test results show that metal doping enhances the rate capability at high C-rates by accelerating lithium ion diffusion.     

Electrochemical performance of LiFe1−xVxPO4/carbon composites prepared by solid-state reaction

LiFe_1−xV_xPO₄/C cathode materials (x = 0, 0.1, 02, 0.3, 0.4) were synthesized by solid-state reaction using polypropylene as the reducing agent and carbon precursor. XRD results show that Li₉Fe₃P₈O₂₉ and Li₃V₂(PO₄)₃ occur when vanadium was added. TEM images show that most of LiFe_1−xV_xPO₄/C particles take on a spherical or quadrate shape with a size less than 200 nm. Electrochemical tests indicate that LiFe_0.9V_0.1PO₄/C and LiFe_0.8V_0.2PO₄/C have a flat discharge plateau at about 3.45 V versus Li⁺/Li and an initial discharge capacity higher than 150 mAh/g at 0.1 C. LiFe_0.8V_0.2PO₄/C also performed relatively good cycle stability which is attributed to their high electronic conductivity as proved by the electrochemical impedance spectroscopy (EIS). Cyclic voltammogram (CV) curves demonstrate that as increase of content of vanadium, LiFe_1−xV_xPO₄/C presents several couples of redox peaks.     

Effect of Mg and Co co-doping on electrochemical properties of LiFePO4

LiFePO₄ co-doped with Mg²⁺ and Co⁴⁺ ions was synthesized by a solid state reaction method. The structure and electrochemical properties of the prepared LiFe_0.99Mg_0.005Co_0.005PO₄ were investigated by X-ray diffraction (XRD), galvanostatic charge-discharge experiment and cyclic voltammograms (CV). Specific discharge capacity of LiFePO₄ co-doped with Mg and Co ions reach 147.2 mA·h/g at 0.1C and 133.3 mA·h/g at 1C. The results of CV show that the reversibility of lithium extraction/insertion in LiFePO₄ can be promoted by (Mg²⁺, Co⁴⁺) multiple-ion doping.     

Preparation,characterization and electrochemical properties of mesoporous LiFe<Subscript>0.99</Subscript>Mo<Subscript>0.01</Subscript>PO<Subscript>4</Subscript>/C

A mesoporous LiFe_0.99Mo_0.01PO₄/C composite was synthesized by the sol-gel method using (NH₄)₂MoO₄ as a doping starting material. The formation of conductive carbon, metal doping and mesopores was achieved simultaneously in the prepared material. The characterizations of crystal structures and microstructures were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), extended X-ray-absorption fine-structure (EXAFS) and X-ray-absorption near-structure spectroscopy (XANES), while the surface area was determined using N₂ adsorption techniques. Cyclic voltammetry (CV) and charge-discharge cycling performance were used to characterize its electrochemical properties. The sample possessed uniformly distributed mesopores with an average pore size of 4 nm, and the specific surface area was about 69.368 m²/g. The results show that the reversible capacity of mesoporous LiFe_0.99Mo_0.01PO₄/C is about 160 mAh/g at 0.1C, 135 mAh/g at 1C and 90 mAh/g at 5C, respectively. The capacity fading is neglectable.     

Preparation and electrochemical performance of 2LiFe1–xCoxPO4–Li3V2(PO4)3/C cathode material for lithium-ion batteries

2LiFe_1–xCo_xPO₄–Li₃V₂(PO₄)₃/C was synthesized using Fe_1–2xCo_2xVO₄ as precursor which was prepared by a simple co-precipitation method. 2LiFe_1–xCo_xPO₄–Li₃V₂(PO₄)₃/C samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. All 2LiFe_1–xCo_xPO₄–Li₃V₂(PO₄)₃/C composites are of the similar crystal structure. The XRD analysis and SEM images show that 2LiFe_0.96Co_0.04PO₄–Li₃V₂(PO₄)₃/C sample has the best-ordered structure and the smallest particle size. The charge–discharge tests demonstrate that these powders have the best electrochemical properties with an initial discharge capacity of 144.1 mA·h/g and capacity retention of 95.6% after 100 cycles when cycled at a current density of 0.1C between 2.5 and 4.5 V.     

锂离子电池用LiVPO4F/C基正极材料的制备和性能（英文）

将H2C2O4·2H2O,NH4H2PO4,NH4VO3和LiF通过球磨反应、烧结,合成了LiVPO4F/C基正极材料。在这个过程中,草酸起还原剂和碳源的作用,利用热重、X射线衍射、扫描电镜、透射电镜和碳-硫分析等手段对合成的前驱体和材料进行检测和分析。XRD分析表明,球磨反应后所得到的前驱体为无定形态,而烧结后的材料中除了LiVPO4F的衍射峰外,还存在Li3V2(PO4)3和V2O3衍射峰。材料颗粒均匀,尺寸约2μm。透射电镜分析表明,合成的材料颗粒表面包裹着一层约2nm厚的无定形碳。在截止电压3.0～4.4V时,合成的材料在0.1C和10C倍率下的放电比容量分别为151.3和102.5mA·h/g。在10C倍率下循环50次后容量保持率为90.4%。在LiVPO4F和Li3V2(PO4)3的循环伏安曲线中可以明显看到V3+/V4+的氧化还原峰。     

Electrochemical properties of carbon-coated LiFePO<Subscript>4</Subscript> and LiFe<Subscript>0.98</Subscript>Mn<Subscript>0.02</Subscript>PO<Subscript>4</Subscript> cathode materials synthesized by solid-state reaction

Olivine structured LiFePO4/C (lithium iron phosphate) and Mn2+-doped LiFe0. 98Mn0. 024/C powders were synthesized by the solid-state reaction. The effects of manganese partial substitution and different carbon content coating on the surface of LiFePO4 were considered. The structures and electrochemical properties of the samples were measured by X-ray diffraction (XRD), cyclic voltammetry (CV), charge/discharge tests at different current densities, and electrochemical impedance spectroscopy (EIS). The electrochemical properties of LiFePO4 cathodes with x wt. ％ carbon coating (x=3, 7, 11, 15) at γ=0. 2C, 2C (1C=170 mAh·g-1) between 2. 5 and 4. 3 V were investigated. The measured results mean that the LiFePO4 with 7 wt. ％ carbon coating shows the best rate performance. The discharge capacity of LiFe0. 98Mn0. 02PO4/C composite is found to be 165 mAh·g 1 at a discharge rate, γ=0. 2C, and 105 mAh·g-1 at γ=2C, respectively. After 10cycles, the discharge capacity has rarely fallen, while that of the pristine LiFePO4/C cathode is 150 mAh·g-1 and 98 mAh·g-1 at γ=0. 2 and 2C, respectively. Compared to the discharge capacities of both electrodes above, the evident improvement of the electrochemical performance is observed, which is ascribed to the enhancement of the electronic conductivity and diffusion kinetics by carbon coating and Mn2+-substitution.     

An environmentally friendly molybdate/phosphate black film on Mg-Zn-Y-Zr alloy

An environmentally friendly molybdate/phosphate black film for light absorption application was developed as an alternative to the chromate system on Mg-Zn-Y-Zr alloy. The microstructure, surface morphology, chemical composition, light absorbance and protection property of the black film were investigated. The initiation and growth mechanisms of the black film were also discussed. The black film consists of MoO₂ and a fraction of Mg₃(PO₄)₂. The MoO₂ contributes to the black color and the Mg₃(PO₄)₂ aims to improve the protection property. The experimental results indicate that the black film can provide not only high light absorbance but also corrosion protection to the Mg-Zn-Y-Zr substrate. The formation mechanism of the black film is associated with the electrochemical heterogeneity of the Mg-Zn-Y-Zr substrate. At the cathodic sites, MoO₄²⁻ is reduced to MoO₂ accompanying with the hydrogen evolution reaction. At the anodic sites, the dissolution of Mg²⁺ bonds with PO₄³⁻ to form Mg₃(PO₄)_2.     

High pressure synthesis of Mg0.90Al0.08Ni0.94V0.08H1.6

The effect of high-pressure (6 gigapascal) and heating (600°C) the hydrogen source of LiAlHL₄ on the structural and hydrogenation properties of 0.9MgH₂+0.1Al+0.9Ni+0.1VH_0.9 was investigated. After recovery to ambient conditions, the sample was analyzed using X-ray diffraction, differential scanning calorimetry, thermogravimetry and differential thermal analyses. The results reveal that a nominal hydride phase with the composition of Mg_0.90Al_0.08Ni_0.94V_0.08H_1.6 can be obtained, in which a maximum H₂-uptake can reach 1.9wt.%, with hydride decomposition occurring between 160°C and 250°C.     

Synthesis and performance of Li3V2(PO4)3/C composites as cathode materials

Carbon-coated Li₃V₂(PO₄)₃ cathode materials for lithium-ion batteries were prepared by a carbon-thermal reduction (CTR) method using sucrose as carbon source. The Li₃V₂(PO₄)₃/C composite cathode materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical measurement. The results show that the Li₃V₂(PO₄)₃ samples synthesized using sucrose as carbon source have the same monoclinic structure as the Li₃V₂(PO₄)₃ sample synthesized using acetylene black as carbon source. SEM image exhibits that the particle size is about 1 μm together with homogenous distribution. Electrochemical test shows that the initial discharge capacity of Li₃V₂(PO₄)₃ powders is 122 mAh·g⁻¹ at the rate of 0.2C, and the capacity retains 111 mAh−g⁻¹ after 50 cycles.     

Surface Characteristics and Electrochemical Impedance Investigation of Spark-Anodized Ti-6Al-4V Alloy

In this study, the surface characteristic of oxide films on Ti-6Al-4V alloy formed by an anodic oxidation treatment in H₂SO₄/H₃PO₄ electrolyte at potentials higher than the breakdown voltage was evaluated. Morphology of the surface layers was studied by scanning electron microscope. The results indicated that the diameter of pores and porosity of oxide layer increase by increasing the anodizing voltage. The thickness measurement of the oxide layers showed a linear increase of thickness with increasing the anodizing voltage. The EDS analysis of oxide films formed in H₂SO₄/H₃PO₄ at potentials higher than breakdown voltage demonstrated precipitation of sulfur and phosphor elements from electrolyte into the oxide layer. X-ray diffraction was employed to exhibit the effect of anodizing voltage on the oxide layer structure. Roughness measurements of oxide layer showed that in spark anodizing, the Ra and Rz parameters would increase by increasing the anodizing voltage. The structure and Corrosion properties of oxide layers were studied using electrochemical impedance spectroscopy (EIS) techniques, in 0.9 wt.% NaCl solution. The obtained EIS spectra and their interpretation in terms of an equivalent circuit with the circuit elements indicated that the detailed impedance behavior is affected by three regions of the interface: the space charge region, the inner compact layer, and outer porous layer.     

介孔Li3V2（PO4）3正极材料的研磨溶胶凝胶法合成和电化学性能（英文）

以V2O5·nH2O、LiOH·H2O、NH4H2PO4和蔗糖为原料,采用研磨溶胶凝胶技术制备了无定形Li3V2(PO4)3前驱体,再经过焙烧获得具有单斜结构的介孔Li3V2(PO4)3正极材料,并用XRD、SEM、TEM、比表面积和电化学性能测试来表征材料的性能。研究表明,在700°C下焙烧的样品具有良好的介孔结构、最大的比表面积(188cm2/g)和最小的孔径(9.3nm)。在0.2C倍率下,该介孔样品的首次放电容量达155.9mA·h/g,经过50次循环后其容量仍然可达154mA·h/g,表现出非常稳定的放电性能。     

Analysis of the electrochemical reaction behavior of alloy AZ91 by EIS technique in H3PO4/KOH buffered K2SO4 solutions

The EIS technique was used to analyze the electrochemical reaction behavior of Alloy AZ91 in H₃PO₄/KOH buffered K₂SO₄ solution at pH 7. The corrosion resistance of Alloy AZ91 was directly related with the stability of Al₂O₃ · xH₂O rich part of the composite oxide/hydroxide layer on the alloy surface. The break down of the oxide layer was estimated to occur mainly on the matrix solid solution phase in Alloy AZ91. The mf capacitive loop arose from the relaxation of mass transport in the solid oxide phase in the presence of Al₂O₃ · xH₂O rich part and from Mg⁺ ion concentration within the broken area in the absence of Al₂O₃ · xH₂O rich part in the composite oxide structure on the alloy surface. The lf inductive loop had tendency of disappear when the dissolution rate of the alloy decreased as a result of the formation of the protective oxide layer.     

Gel-like layer development during formation of thin anodic films on titanium in phosphoric acid solutions

Anodising of commercial pure titanium in phosphoric acid solutions at different concentrations (0.5-4 M) have been investigated using galvanostatic and potentiodynamic polarisation techniques. Under galvanostatic conditions at low current densities (0.1-0.6 A m⁻²), the chrono-potentiometric curves always show a linear section in the early stage of the process. The slope of the linear section, dE/dt, decreases initially and then increases with increasing H₃PO₄ concentration, with the minimum slope at a concentration, ∼2 M. Under potentiodynamic conditions at this concentration the samples exhibit a different anodic behaviour. The development of gel-like layer during formation of thin anodic films on titanium in H₃PO₄ solutions is proposed to account for experimental observations.     

Spontaneous and anodic oxidation of Tantalum and Zirconium in phosphate solutions

Spontaneous and anodic oxidation of Ta and Zr have been studied in 0.1 M solutions of H₃PO₄, NaH₂PO₄, Na₂HPO₄ and Na₃PO₄. Below 10 A/cm² the dependence of the spontaneous-oxidation rate on potential follows a Tafel relation. Te results indicate that the partial anodic reaction of film formation is balanced by the cathodic reduction of dissolved oxygen. The kinetics of anodic oxidation are discussed on the basis of a one phase oxide file liable to contamination by phosphate anions. Oxide growth follows the high field ionic conduction even below the oxygen evolution potential. The simple exponential relation of Güntherschulze and Betz is valid. This is supported by the linearity obtained between (1) reciprocal capacity and the logarithm of current density i and (2) the potential E and log i for oxide formation at constant charge. Approximate calculations of the effective activation distance show that the degree of contamination by anions is the same in the four equimolar solutions indepent of PH.     

Mössbauer spectroscopy study of MgAl2O4-matrix nanocomposite powders containing carbon nanotubes and iron-based nanoparticles

Materials involved in the catalytic formation of carbon nanotubes are for the first time systematically studied by Mössbauer spectroscopy between 11 K and room temperature. Mg_1−xFe_xAl₂O₄ (x=0.1, 0.2, 0.3, 0.4) solid solutions are transformed into carbon nanotubes–Fe/Fe₃C–MgAl₂O₄ composite powders by reduction in a H₂–CH₄ gas mixture. The oxides are defective spinels of general formulae (Mg_1−x²⁺Fe_x−3α²⁺Fe_2α³⁺□_αAl₂³⁺)O₄²⁻. Ferromagnetic α-Fe, ferromagnetic Fe₃C and a γ-Fe form, the latter possibly corresponding to a γ-Fe–C alloy, are detected in the composite powders. An attempt is made to correlate these results with the microstructure of the powder. It seems that the nanoparticles, which catalyze the formation of the carbon nanotubes, are detected as Fe₃C in the post-reaction Mössbauer spectroscopy analysis.     

The effects of niobium and nickel on the corrosion resistance of the zinc phosphate layers

In this investigation the viability of nickel substitution by niobium in zinc phosphate (PZn) baths has been studied. Samples of carbon steel (SAE 1010) were phosphated in two baths, one containing nickel (PZn + Ni) and the other with niobium substituting nickel (PZn + Nb). Potentiodynamic polarization curves (anodic and cathodic, separately) and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion resistance of the phosphated carbon steels in a 0.5 mol L^− 1 NaCl electrolyte. The phosphate layers obtained were analysed by X-ray diffraction and it was found that they are composed of Zn₃(PO₄)₂.4H₂O (hopeite) and Zn₂Fe(PO₄)₂.4H₂O (phosphophylite). Surface observation by scanning electron microscopy (SEM) showed that the PZn + Ni layer is deposited as needle-like crystals, whereas the PZn + Nb layer shows a granular morphology. The electrochemical results showed that the PZn + Nb coating was more effective in the corrosion protection of the carbon steel substrate than the PZn + Ni layer. The results also suggested that nickel can be replaced by niobium in zinc phosphate baths with advantageous corrosion properties of the layer formed.     

Investigation of corrosion behavior of silver coated brass in acidic solutions

The corrosion behavior of uncoated brass and electrolytically silver coated brass in cyanide solutions such as 0.1 M HCl, 0.1 M H₂SO₄ and 0.1 M H₃PO₄ was investigated by Tafel polarization curves and cyclic voltammetry curves at 1., 24., 48., 72., 96. and 168. hours. The coating efficiency was calculated by current density of corrosion determined from Tafel polarization curves. Surface analysis of coated brass immersed into acidic solution by 168 hours was done. It is concluded that silver coating is very effective to protect the corrosion of brass for a long time.     

Characterization and corrosion behavior of ceramic coating on magnesium by micro-arc oxidation

In this study, the commercial pure magnesium was coated in different aqueous solutions of Na₂SiO₃ and Na₃PO₄ by the micro-arc oxidation method (MAO). Coating thickness, phase composition, surface and cross sectional morphology and corrosion resistance of coatings were analyzed by eddy current method, X-ray diffraction (XRD), scanning electron microscope (SEM) and tafel extrapolation method, respectively. The average thickness of the coatings ranged from 52 to 74 μm for sodium silicate solution and from 64 to 88 μm for sodium phosphate solution. The dominant phases on the coatings were detected as spinal Mg₂SiO₄ (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg₃(PO₄)₂ (Farringtonite) and MgO (Periclase) for sodium phosphate solution. SEM images reveal that the coating is composed of two layers as of a porous outer layer and a dense inner layer. The corrosion results show the coating consisting Mg₂SiO₄ is more resistant to corrosion than that containing Mg₃(PO₄)₂.     

An XPS study of passive films on Nickel and alloy 600 in acids

Compositions of surface films formed on nickel and Alloy 600 in I M HCI, 0.5 M H₂SO₄ and I/3 M H₃PO₁ solutions were investigated as a function of polarization potential. The main constituent of surface films formed on Ni in 0.5 M H₂SO₄ or 1/3 M H₃PO₄ solution was hydrated nickel oxyhydroxide, in which the ratio of O₂ to OH⁻ increased when passivation occurred. The surface films formed on Ni and Alloy 600 at lower potentials in 0.5 M H₂SO₄ solution contain S² ions other than SOP_4²⁻ ions, whereas S²⁻ ions were not incorporated in the passive film. Passivation of Alloy 600 took place by the formation of hydrated chromium oxyhydroxide. Pitting led to no substantial change in the average composition of the film.