Effect of Co substitution for Ni on the structural and electrochemical properties of La2Mg(Ni1−xCox)9 (x = 0.1-0.5) hydrogen storage electrode alloys

The effect of partial substitution of Co for Ni on the structure and electrochemical properties of the thus formed La₂Mg(Ni_1−xCo_x)₉ (x = 0.1-0.5) quaternary alloys was investigated. All alloys are consisted of a main phase with hexagonal PuNi₃-type structure and a few impurity phases (mainly La₂Ni₇ and LaNi). The increase of Co content in the alloys leads to an increase in both the cell volume and the hydride stability, and leads to a noticeable decrease in cell volume expansion rate (ΔV/V) on hydriding. The discharge capacity of the alloys at 50 mA/g increases slightly with the increase of Co content and passes though a maximum of 404.5 mAh/g at x = 0.2. As the Co content increases, the high-rate dischargeability of the alloy electrodes at 800 mA/g (HRD₈₀₀) decreases sharply from 72.8 (x = 0) to 24.5% (x = 0.5), yet the decrease of HRD₈₀₀ of the alloy electrodes with lower Co substitution (with x ≤ 0.2) is much milder. The slower decrease of HRD₈₀₀ (from 72.8 to 64.2%) of the alloys with x from 0 to 0.2 is mainly attributed to the decrease of eletrocatalytic activity for charge-transfer reaction, the more rapid decrease of the alloys with x > 0.2 is mainly attributed to the lowering of the hydrogen diffusion rate in the bulk of alloy. The cycling capacity retention rate (S₁₀₀) of the alloys increase greatly with increasing of Co content, increasing from 60.2% for the alloy with x = 0 to a much higher value of 87.9% for the alloy with x = 0.5. The improvement in cycling stability is attributed to the lower cell volume expansion on hydriding.     

A novel method to synthesize whisker-like Co(OH)2 and its electrochemical properties as an electrochemical capacitor electrode

A loose whisker-like Co(OH)₂ was synthesized by means of polyethylene glycol 4000 as soft template under ultrasonic condition, and investigated as an active electrode material for electrochemical capacitors. The composition and microstructure of the as-prepared Co(OH)₂ were investigated by X-ray diffraction spectroscopy and transmission electron microscopy. The formation mechanism of the whisker-like Co(OH)₂ was attentively proposed based on Fourier transform infrared spectroscopy analysis. Electrochemical studies revealed that the whisker-like Co(OH)₂ delivered a specific capacitance of 325 F/g at a current density of 20 mA/cm² (ca. 1.3 A/g) and even 279 F/g at 80 mA/cm² (ca. 5.3 A/g) due to its special nanostructure, indicating its fast electrochemical response property. A capacitance attenuation of ca. 7% over 1000 cycles meant the good cyclic stability of the whisker-like Co(OH)₂ for electrochemical capacitors application.     

Preparation and electrochemical properties of Cr-doped Li9V3(P2O7)3(PO4)2 as cathode materials for lithium-ion batteries

Cr-doped Li₉V_3−xCr_x(P₂O₇)₃(PO₄)₂ (x = 0.0–0.5) compounds have been prepared using sol–gel method. The Rietveld refinement results indicate that single-phase Li₉V_3−xCr_x(P₂O₇)₃(PO₄)₂ (x = 0.0–0.5) with trigonal structure can be obtained. Although the initial specific capacity decreased with Cr content at a lower current rate, both cycle performance and rate capability have excited improvement with moderate Cr-doping content. Li₉V_2.8Cr_0.2(P₂O₇)₃(PO₄)₂ compound presents the good electrochemical rate and cyclic ability. The enhancement of rate and cyclic capability may be attributed to the optimizing particle size, morphologies, and structural stability during the proper amount of Cr-doping (x = 0.2) in V sites.     

Electrochemical hydrogen storage in (Ti1−xVx)2Ni (x = 0.05-0.3) alloys comprising icosahedral quasicrystalline phase

For (Ti_1−xV_x)₂Ni (x = 0.05, 0.1, 0.15, 0.2 and 0.3) ribbons, synthesized by arc-melting and subsequent melt-spinning techniques, an icosahedral quasicrystalline phase was present, either in the amorphous matrix or together with the stable Ti₂Ni-type phase. With increasing x values, the maximum discharge capacity of the alloy electrodes increased until reached 271.3 mAh/g when x = 0.3. The cycling capacity retention rates for these electrodes were approximately 80% after a preliminary test of 30 consecutive cycles of charging and discharging. Ti_1.7V_0.3Ni alloy electrode displayed the best high-rate discharge ability of 82.7% at the discharge current density of 240 mA/g.     

Study on phase structure and electrochemical properties of Ml1−xMgxNi2.80Co0.50Mn0.10Al0.10 (x = 0.08, 0.12, 0.20, 0.24, 0.28) hydrogen storage alloys

Phase structure and electrochemical properties of the Ml_1−xMg_xNi_2.80Co_0.50Mn_0.10Al_0.10 (x = 0.08, 0.12, 0.20, 0.24, 0.28) (Ml = La-rich mixed lanthanide) alloys were studied. X-ray diffraction (XRD) analysis and Rietveld refinement show that the alloys consist mainly of LaNi₅ and (La,Mg)Ni₃ phase. Due to variation in phases of the alloys, the maximum discharge capacity, the high rate dischargeability (HRD), and the low temperature dischargeability increase first and then decrease. The maximum discharge capacity increases from 322 mAh g⁻¹ (x = 0.08) to 375 mAh g⁻¹ (x = 0.12), and then decreases to 351 mAh g⁻¹ (x = 0.28) with increasing x. As the case of x = 0.20, HRD at 1200 mA g⁻¹ and discharge capacity at 233 K reaches 41.7% and 256 mAh g⁻¹, respectively. The cycling stability is improved by substituting La with Ml and B-site multi-alloying, and the capacity retention of Ml_0.72Mg_0.28Ni_2.80Co_0.50Mn_0.10Al_0.10 at the 200th cycle is 71%.     

Structure and electrochemical properties of nanocarbon-coated Li3V2(PO4)3 prepared by sol-gel method

A liquid-based sol-gel method was developed to synthesize nanocarbon-coated Li₃V₂(PO₄)₃. The products were characterized by XRD, SEM and electrochemical measurements. The results of Rietveld refinement analysis indicate that single-phase Li₃V₂(PO₄)₃ with monoclinic structure can be obtained in our experimental process. The discharge capacity of carbon-coated Li₃V₂(PO₄)₃ was 152.6 mAh/g at the 50th cycle under 1C rate, with 95.4% retention rate of initial capacity. A high discharge capacity of 184.1 mAh/g can be obtained under 0.12C rate, and a capacity of 140.0 mAh/g can still be held at 3C rate. The cyclic voltammetric measurements indicate that the electrode reaction reversibility is enhanced due to the carbon-coating. SEM images show that the reduced particle size and well-dispersed carbon-coating can be responsible for the good electrochemical performance obtained in our experiments.     

Electrochemical properties of MmNi3.03Si0.85Co0.60Mn0.31Al0.08 hydrogen storage alloys in alkaline electrolytes—A cyclic voltammetric study at different temperatures

The cyclic voltammetric behavior of MmNi_3.03Si_0.85Co_0.60Mn_0.31Al_0.08-based metal hydride electrode was studied in alkaline electrolytes at various temperatures (303, 308, 318 and 328 K). Electrochemical parameters such as limiting current density and corrosion potential were determined at these temperatures. The corrosion potential became more negative with increasing temperature. Hydrogen diffusivity was also found to increase with increasing temperature. From electrochemical discharge experiments, it was concluded that the charge transfer process was the rate-determining step.     

Light energy storage and photoelectrochemical behavior of the titanate nanotube array/Ni(OH)2 electrode

A new kind of TiO₂ nanotube array/Ni(OH)₂ (TiO₂/Ni(OH)₂) composite electrode with the storage ability of light energy was prepared by the deposition of Ni(OH)₂ on the TiO₂ nanotube array, which was synthesized by anodizing Ti foils in an HF aqueous solution. SEM and XRD results showed that Ni(OH)₂ particles were well distributed on high density, well-ordered and uniform TiO₂ nanotube arrays. The photoelectrochemical properties of the TiO₂/Ni(OH)₂ electrode were investigated in NaHCO₃/NaOH buffer solution (pH 10) by means of UV-vis absorption spectra, cyclic voltammogram (CV) and photocurrent measurements. It was found that the TiO₂/Ni(OH)₂ electrode was highly sensitive to light and exhibited excellent photoelectrochromic properties. Upon UV irradiation, the photogenerated holes by TiO₂ nanotube arrays can oxidize Ni(OH)₂ to NiOOH, and thus the TiO₂/Ni(OH)₂ electrode can be photo-charged by light.     

Microstructure and electrochemical characteristics of Mm(Ni,Co,Mn,Al)5Bx (x=0-0.4) hydrogen storage alloys prepared by cast and rapid quenching

Low Co AB₅-type MmNi_3.8Co_0.4Mn_0.6Al_0.2B_x (x=0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by cast and rapid quenching. The microstructures and electrochemical performances of the as-cast and quenched alloys were analysed and measured. The effects of boron additive and rapid quenching technique on the microstructures and electrochemical properties of as-cast and quenched alloys were investigated comprehensively. The experimental results showed that the microstructure of as-cast MmNi_3.8Co_0.4Mn_0.6Al_0.2B_x (x=0, 0.1, 0.2, 0.3, 0.4) alloys was composed of CaCu₅-type main phase and a small amount of CeCo₄B-type secondary phase. The abundance of the secondary phase increases with the increase of boron context x. The rapid quenching techniques were used in the preparation of the alloys. The amount of secondary phase in the alloys decreased with the increase of quenching rate. Rapid quenching made lattice constants increase slightly. The effects of rapid quenching on the electrochemical performances of the alloys are very significant. The discharge capacity of the alloys decreased obviously and the cycle stability increased dramatically with the increase of quenching rate. Rapid quenching made the activation capability of the alloys lowered. However, the activate performance and high rate discharge capability as well as discharge voltage characteristic of the alloys were modified obviously with the increase of boron content x.     

Synthesis and electrochemical properties of Co-doped Li3V2(PO4)3 cathode materials for lithium-ion batteries

Co-doped Li₃V_2−xCo_x(PO₄)₃/C (x = 0.00, 0.03, 0.05, 0.10, 0.13 or 0.15) compounds were prepared via a solid-state reaction. The Rietveld refinement results indicated that single-phase Li₃V_2−xCo_x(PO₄)₃/C (0 ≤ x ≤ 0.15) with a monoclinic structure was obtained. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the cobalt is present in the +2 oxidation state in Li₃V_2−xCo_x(PO₄)₃. XPS studies also revealed that V⁴⁺ and V³⁺ ions were present in the Co²⁺-doped system. The initial specific capacity decreased as the Co-doping content increased, increasing monotonically with Co content for x > 0.10. Differential capacity curves of Li₃V_2−xCo_x(PO₄)₃/C compounds showed that the voltage peaks associated with the extraction of three Li⁺ ions shifted to higher voltages with an increase in Co content, and when the Co²⁺-doping content reached 0.15, the peak positions returned to those of the unsubstituted Li₃V₂(PO₄)₃ phase. For the Li₃V_1.85Co_0.15(PO₄)₃/C compound, the initial capacity was 163.3 mAh/g (109.4% of the initial capacity of the undoped Li₃V₂(PO₄)₃) and 73.4% capacity retention was observed after 50 cycles at a 0.1 C charge/discharge rate. The doping of Co²⁺into V sites should be favorable for the structural stability of Li₃V_2−xCo_x(PO₄)₃/C compounds and so moderate the volume changes (expansion/contraction) seen during the reversible Li⁺ extraction/insertion, thus resulting in the improvement of cell cycling ability.     

Synthesis and electrochemical properties of CeO2 nanoparticle modified TiO2 nanotube arrays

In this paper, a cerium dioxide (CeO₂) modified titanium dioxide (TiO₂) nanotube array film was fabricated by electrodeposition of CeO₂ nanoparticles onto an anodized TiO₂ nanotube array. The structural investigation by X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated that the CeO₂ nanoparticles grew uniformly on the walls of the TiO₂ nanotubes. The composite was composed of cubic-phase CeO₂ crystallites and anatase-phase TiO₂ after annealing at 450 °C. The cyclic voltammetry and chronoamperometric charge/discharge measurement results indicated that the CeO₂ modification obviously increased the charge storage capacity of the TiO₂ nanotubes. The charge transfer process at the surface, that is, the pseudocapacitance, was the dominate mechanism of the charge storage in CeO₂-modified TiO₂ nanotubes. The greater number of surface active sites resulting from uniform application of the CeO₂ nanoparticles to the well-aligned TiO₂ nanotubes contributed to the enhancement of the charge storage density.     

Structural, thermal and mechanical properties of transparent Yb:(Y0.97Zr0.03)2O3 ceramic

Yb doped (Y_0.97Zr_0.03)₂O₃ transparent ceramics were fabricated by solid state reaction and vacuum sintering. The microstructure, thermal and mechanical properties of Y₂O₃ ceramic, as well as the effect of Yb doping concentration on these properties were investigated in detail. The lattice parameter and unit cell volume decrease with the increasing of Yb content, whereas thermal expansive coefficient increases. With Yb content increasing from 0 to 8 at.%, the mean grain size increases from 15.82 μm to 26.54 μm, and the thermal conductivity at room temperature (RT) decreases from 11.97 to 6.39 W/m/K. The microhardness decreases with Yb content, and the microhardness and fracture toughness of (Y_0.97Zr_0.03)₂O₃ transparent ceramic is 11.11 GPa and 1.29 MPa m^1/2, respectively.     

Effects of metal oxides on electrochemical hydrogen storage of nanocrystalline LaMg12-Ni composites

Nanocrystalline LaMg₁₂-Ni composites were prepared by ball-milling a LaMg₁₂ alloy and Ni powders with additions of small amounts of metal oxides (TiO₂, Fe₃O₄, La₂O₃ and CuO). The composites with additions of small amounts of metal oxides were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the effects of the addition of the metal oxides on the electrochemical hydrogen storage were investigated. It is demonstrated that the initial discharge capacities of the composites with additions of small amounts of metal oxides were significantly higher than that of the original composite. The additions of TiO₂ and Fe₃O₄ as catalysts improved the electrochemical hydrogen storage properties more effective than additions of La₂O₃ and CuO. Analysis of the electrochemical impedance spectra (EIS) showed that the function of the metal oxides was considered to reduce the electrochemical reaction resistance as catalysts and to increase the specific surface area as impurities. However, more extensive investigation is still necessary in order to improve the cyclic stability of these materials for practical application in Ni/MH batteries.     

Study on the phase structure and electrochemical properties of RE0.93Mg0.07Ni2.96Co0.60Mn0.37Al0.17 hydrogen storage alloy

In our endeavor to improve overall properties of the La-Mg-Ni-Co type alloys, RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 hydrogen storage electrode alloy with low magnesium content was obtained by inductive melting. The phase structure and electrochemical characteristics of the alloy were investigated by XRD and electrochemical measurement. The results indicate that RE_0.93Mg_0.07Ni_2.96Co_0.60 Mn_0.37Al_0.17 alloy has multi-phase microstructure containing the CaCu₅ structure of LaNi₅ phase as matrix phase and a little of LaNi₃ phase as the secondary phase. The maximum discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy reaches 359 mAh/g, which is 7.2% higher than that of commercial AB₅ alloy electrode. The discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy electrode at 233 K is up to 147 mAh/g, which is 308.3% higher than that of commercial AB₅ alloy electrode. Meanwhile, the discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy can reach 92.7% of commercial AB₅ alloy after 100 charge/discharge cycles.     

Mn influence on the electrochemical behaviour of Li3V2(PO4)3 cathode material

The role played by the substitution of Mn on the electrochemical behaviour of Li₃V₂(PO₄)₃ has been investigated. Independently of the synthesis route, the Mn doping improves the electrochemical features with respect to the undoped samples. Different reasons can be taken into consideration to explain the electrochemical enhancement. In the sol–gel synthesis the capacity slightly enhances due to the Mn substitution on both the V sites, within the solubility limit x = 0.124 in Li₃V_2−xMn_x(PO₄)₃. In the solid state synthesis the significant capacity enhancement is preferentially due to the microstructural features of the crystallites and to the LiMnPO₄ phase formation.     

Preparation and electrochemical performance studies on Cr-doped Li3V2(PO4)3 as cathode materials for lithium-ion batteries

Cr-doped Li₃V_2−xCr_x(PO₄)₃/C (x = 0, 0.05, 0.1, 0.2, 0.5, 1) compounds have been prepared using sol–gel method. The Rietveld refinement results indicate that single-phase Li₃V_2−xCr_x(PO₄)₃/C with monoclinic structure can be obtained. Although the initial specific capacity decreased with Cr content at a lower current rate, both cycle performance and rate capability have excited improvement with moderate Cr-doping content in Li₃V_2−xCr_x(PO₄)₃/C. Li₃V_1.9Cr_0.1(PO₄)₃/C compound presents an initial capacity of 171.4 mAh g⁻¹ and 78.6% capacity retention after 100 cycles at 0.2C rate. At 4C rate, the Li₃V_1.9Cr_0.1(PO₄)₃/C can give an initial capacity of 130.2 mAh g⁻¹ and 10.8% capacity loss after 100 cycles where the Li₃V₂(PO₄)₃/C presents the initial capacity of 127.4 mAh g⁻¹ and capacity loss of 14.9%. Enhanced rate and cyclic capability may be attributed to the optimizing particle size, carbon coating quality, and structural stability during the proper amount of Cr-doping (x = 0.1) in V sites.     

Study on synthesis routes and their influences on chemical and electrochemical performances of Li3V2(PO4)3/carbon

In this study, Li₃V₂(PO₄)₃/carbon samples were synthesized by two different synthesis routes. Their influence on chemical and electrochemical performances of Li₃V₂(PO₄)₃/carbon as cathode materials for lithium-ion batteries was investigated. The structure and morphology of Li₃V₂(PO₄)₃/carbon were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM) measurements. TEM revealed that the Li₃V₂(PO₄)₃ grains synthesized through the sol-gel route had a depressed grain size. Electrochemical behaviors were characterized by galvanostatic charge/discharge, cyclic voltammetry and AC impedance measurements. Li₃V₂(PO₄)₃/carbon with smaller grain size showed better performances in terms of the discharge capacity and cycle stability. The improved electrochemical properties of the Li₃V₂(PO₄)₃/carbon were attributed to the depressed grain size and enhanced electrical contacts produced via the sol-gel route. AC impedance measurements also showed that the sol-gel route significantly decreased the charge-transfer resistance and shortened the migration distance of lithium ion.     

A delithiated LiNi0.65Co0.25Mn0.10O2 electrode material: A structural, magnetic and electrochemical study

A crystalline LiNi_0.65Co_0.25Mn_0.10O₂ electrode material was synthesized by the combustion method at 900 °C for 1 h. Rietveld refinement shows less than 3% of Li/Ni disorder in the structure. Lithium extraction involves only the Ni²⁺/Ni⁴⁺ redox couple while Co³⁺ and Mn⁴⁺ remain electrochemically inactive. No structural transition was detected during cycling in the whole composition range 0 < x < 1.0. Furthermore, the hexagonal cell volume changes by only 3% when all lithium was removed indicating a good mechanical stability of the studied compound. LiNi_0.65Co_0.25Mn_0.10O₂ has a discharge capacity of 150 mAh/g in the voltage range 2.5-4.5 V, but the best electrochemical performance was obtained with an upper cut-off potential of 4.3 V. Magnetic measurements reveal competing antiferromagnetic and ferromagnetic interactions - varying in strength as a function of lithium content - yielding a low temperature magnetically frustrated state. The evolution of the magnetic properties with lithium content confirms the preferential oxidation of Ni ions compared to Co³⁺ and Mn⁴⁺ during the delithiation process.