Negative Electrode In Ni-MH Batteries Research Articles

Improvement of reversible H storage capacity by fine tuning of the composition in the pseudo-binary systems A2-xLaxNi7 (A = Gd, Sm, Y, Mg)

A2B7 compounds (A = rare earth or Mg, B = transition metal) are widely studied as active materials for negative electrode in Ni-MH batteries. By playing on the substitution rate of both A and B elements, it is possible to prepare various compositions. This strategy will help to improve the properties, for example get a higher reversible hydrogen capacity and a well-adapted hydrogen sorption pressure. Indeed, A can be almost all light rare earths (La to Gd), yttrium and alkaline earth metals (Mg or Ca), whereas B can contain various late transition metals (Mn to Ni). To understand the effects of composition on the physicochemical properties of La2Ni7-based compound, various pseudo-binary systems have been investigated: Gd2-xLaxNi7 (x = 0, 0.6, 1, 1.5), Sm2-xLaxNi7 (x = 0, 0.5, 1, 1.5), Y2-xLaxNi7 (x = 0, 0.4, 0.5, 0.6, 0.8, 1, 1.5, 1.75) and A0.5La1.1Mg0.4Ni7 (A = Sm, Gd and Y). To determine their crystallographic properties, X-ray diffraction analysis was performed followed by Rietveld refinement. Thermodynamic properties regarding reversible hydrogen sorption were investigated at room temperature. Quaternary compounds present drastically improved sorption properties in the frame of practical energy storage applications with reversible capacity equivalent to 400 mAh/g for the compound La1.1Sm0.5Mg0.4Ni7.

Influence of Milling Conditions on the Behavior of AB5-Type Materials as Metal Hydride Electrodes

A series of AB5-type alloys with different particle and grain size were characterized electrochemically as materials for negative electrodes in Ni-MH batteries. Besides, the effect of alloy composition on the discharge capacity and cycle life was studied. Ball milling under hydrogen atmosphere was applied to reduce significantly the grain size of the alloys and to avoid the initial activation at the electrochemical charge/discharge cycling. The alloy morphology and microstructure after different mechanical treatments were characterized by XRD and SEM. The best effect in respect of discharge capacity and cycle life was achieved for the materials milled for 5 h under hydrogen atmosphere.

Relationship between H2 sorption, electrochemical cycling and aqueous corrosion properties in A5Ni19 hydride-forming alloys (A = Gd, Sm)

A5B19 compounds (A = rare earth, B = transition metal) are promising materials as negative electrodes in Ni-MH batteries. They are built from stacking along the c axis of one [A2B4] and three [AB5] subunits. The former one exhibits a good ability to absorb hydrogen, and the later one a good cycling stability, thus A5B19 compounds are expected to benefit the large capacity and the cycling stability of both units. In this paper, Gd5Ni19 and Sm5Ni19 were synthesized and their hydrogen sorption properties were investigated using the Sieverts' method. Calendar corrosion was also studied and X-ray diffraction, magnetic measurements and thermal gravimetric analysis were made to determine the amount of corroded alloy. Electrochemical cycling was performed for both compounds. After 100 cycles, the electrochemical capacity of Gd5Ni19 remains constant whereas the Sm5Ni19 one decreases sharply after five cycles. This good capacity retention is linked to the low corrosion rate of Gd5Ni19 which corrodes 3 times less than Sm5Ni19 after 18 weeks in KOH. For Sm5Ni19, we conclude that the capacity decrease was due for 19% to crystallinity loss and 53% to corrosion. Finally, a comparison is made between A5Ni19 and A2Ni7-type compounds and the structure influence on the hydrogenation and corrosion properties is discussed.

Systematic investigation of mechanically alloyed Ti-Mg-Ni used as negative electrode in Ni-MH battery

In this study, a series of Ti-Mg-Ni alloys have been synthesized by mechanical alloying using a planetary high-energy ball mill. The effect of varied composition on the microstructure and electrochemical hydrogenation properties of Ti-Mg-Ni system have been investigated through the compared analysis of Ti1-xMgNix and TiMg1-xNix alloys. The structural transformation and discharge capacities were characterized by XRD and electrochemical measurements at galvanostatic conditions, respectively. Additionally, a separate milling process has been employed for improving the electrochemical properties of Ti-Mg-Ni alloys. It is shown that the separated milling process can extremely enhance the discharge capacities of Ti-Mg-Ni composite materials.

Optimization of the La substitution by Mg in the La2Ni7 hydride-forming system for use as negative electrode in Ni-MH battery

The crystallographic, thermodynamic and electrochemical properties of the hydrogen-sorbing system La2−xMgxNi7 has been examined as a function of x. X-ray diffraction shows the coexistence of two polymorphic structures, hexagonal (P63/mmc) and rhombohedral (R-3m), for the same compositions as already observed for other systems La–Mg–Ni. Thermodynamic properties regarding hydrogen uptake have been determined by means of solid–gas reaction. Capacity and equilibrium pressure have been measured and electrochemical properties in alkaline medium have been determined as a function of the magnesium content. Mg-free compound exhibits much lower reversible electrochemical capacity than the Mg-containing one and an optimum Mg composition has been obtained around 25 at.% Mg sitting on the lanthanum site.

Quinary icosahedral quasicrystalline Ti–V–Ni–Mn–Cr alloy: A novel anode material for Ni-MH rechargeable batteries

The substitution of manganese and chromium for 6 at.% nickel in Ti 1.6V 0.4Ni leads the rapid quenching synthesis of quinary icosahedral phase (i-phase) evidenced by the observations of 2-, 3- and 5-fold symmetries. As negative electrode in Ni-MH battery, the quinary Ti–V–Ni–Mn–Cr i-phase can deliver a maximum discharge capacity of 278 mAh g −1 at 30 mA g −1, larger than that of Ti 1.6V 0.4Ni master alloy anode owing to Mn and/or Cr doping. After a preliminary test of 30 consecutive cycles the cycling capacity retention rate (CR%) is 80%. The strong chemisorption of hydrogen shown in cyclic voltammetric (CV) response indicates that the electrocatalytic activity improvement for the i-phase negative electrode is highly demanded.

Hydrogenation properties of shape memory Ti(Ni,Pd) compounds

The effect of Pd substitution on the crystal structure and hydrogenation properties of TiNi compound has been investigated. Ti(Ni,Pd) are pseudo-binary compounds. The unit-cell volumes of B2 (Austenite) and B19 (Martensite) structures linearly increase with Pd substitution in Ti 1.04Ni 0.96− x Pd x samples. The hydrogenation properties of Pd-substituting Ti(Ni,Pd) compounds are not affected by the crystal structure of the parent compounds. For all samples, hydrogen absorption occurs without showing any clear plateau pressure in Pressure-Composition-Isotherm (PCI) curves. All hydrided samples crystallise in the tetragonal I 4/ mmm space group. At 6 MPa of hydrogen pressure and T = 423 K, the hydrogenation capacity of Ti(Ni,Pd) compounds reaches 1.52 hydrogen atoms per formula unit (H/f.u.) for x = 0.1 and then gradually decreases with Pd content down to 0.93 H/f.u. for x = 0.5. Ti 1.04Ni 0.86Pd 0.10 sample yields a discharge capacity of 148 mAh/g at C/5 regime when used as negative electrode in Ni-MH battery. The hydrogenation properties of Pd-substituting Ti(Ni,Pd) compounds are discussed in detail by comparison with previous studies on Zr substituting, (Ti,Zr)Ni compounds.

Elaboration and characterization of magnesium-substituted La 5Ni 19 hydride forming alloys as active materials for negative electrode in Ni-MH battery

The crystallographic structure as well as the thermodynamic and electrochemical properties of the hydrogen absorbing compounds La 5Ni 19 and La 4MgNi 19 have been determined. X-ray diffraction and microprobe analysis have shown the presence of a single hexagonal ( P6 3/ mmc) phase for the Mg-free binary compound whereas only one composition but two different crystallographic structures, hexagonal ( P6 3/ mmc) and rhombohedral ( R-3 m), are observed for La 4MgNi 19. Each phase can be described as the stacking along the c-axis of n[A 2B 4]/ m[AB 5] sub-units with n = 1 and m = 3 and A = La, Mg; B = Ni. Depending on the stacking sequence, either the hexagonal or the rhombohedral phase is obtained. Thermodynamic properties toward hydrogen uptake (capacity and equilibrium pressure) and electrochemical properties (cycling behavior) in alkaline medium have been measured and are compared for these compounds with and without magnesium. It is observed that the Mg-free compound exhibits a much lower reversible hydrogen capacity than the Mg-containing one. However, despite larger capacity, this latter compound remains to be optimized in term of cycle life.

A study of the Al content impact on the properties of MmNi 4.4− xCo 0.6Al x alloys as precursors for negative electrodes in NiMH batteries

AB 5-type hydrogen storage alloys with MmNi 4.4− x Co 0.6Al x (Mm-mischmetal) composition are synthesized, structurally and thermodynamically characterized, and electrochemically tested in 6 M KOH electrolyte. It is shown that an increase of the Al content in the alloy results in expansion of both the alloy lattice cell size and the unit cell volume. These structural changes lead to decrease of the plateau pressure and increase of the plateau width in the pressure-composition-temperature desorption isotherms. Improvement of the specific electrode capacity is also registered with the increase of the cell parameters. In addition to that the higher Al content is found to enhance the stability of the alloy components’ hydrides. Maximum discharge capacity of 278 mAh g −1 is measured with an electrode made from a MmNi 3.6Co 0.6Al 0.8 alloy. Cycle life tests of the accordingly prepared electrodes suggest a stability that is comparable to the stability of commercially available hydrogen storage electrodes.

Study of the structural, thermodynamic and electrochemical properties of LaNi 3.55Mn 0.4Al 0.3(Co 1− xFe x) 0.75 (0 ≤ x ≤ 1) compounds used as negative electrode in Ni-MH batteries

This study concerns the influence of iron for cobalt substitution on the structural, thermodynamic and electrochemical properties of the hydrides of poly-substituted LaNi 3.55Mn 0.4Al 0.3(Co 1− x Fe x ) 0.75 (0 ≤ x ≤ 1) alloys used as material for negative electrode in Ni-MH batteries. The Fe substitution leads to an increase of the cell parameter, this increase is linear according to the rate of substitution, and a decrease of the equilibrium pressure in agreement with the geometric law. Nevertheless, it is observed that the Fe substitution leads to a deviation from the linear variation between the logarithm of the pressure and the cell volume observed for Co, Mn and Al for Ni substitution. The Fe for Co substitution leads also to a decrease of the solid–gas and electrochemical capacity.

Optimization of the Ball-Milling Parameters for the Synthesis of Amorphous MgNi Alloy Used as Negative Electrode in Ni-MH Batteries

Optimization of the Ball-Milling Parameters for the Synthesis of Amorphous MgNi Alloy Used as Negative Electrode in Ni-MH Batteries

Optimization of the Ball-Milling Parameters for the Synthesis of Amorphous MgNi Alloy Used as Negative Electrode in Ni-MH Batteries

The optimization of the ball milling parameters resulted in the synthesis with a milling duration equal to 10 hours of amorphous MgNi having an initial discharge capacity over 520 mAh/g. Further milling results in a partial crystallization of amorphous MgNi into nanocrystalline MgNi 2 and Mg2Ni, which decreases significantly the electrode performance. This study also shows that it is possible to obtain an amorphous and electroactive material in large scale (1 kg of alloy per batch) using an industrial high-energy attritor. In addition, it was demonstrated that the carbon added at the beginning of the milling to avoid powder welding, in spite of its small proportion (1 wt.%), has a notable influence on the performance of the material. This study also confirms the major loss of activity during cycles. However, the partial substitution of Ti for Mg leads to a remarkable improvement of the cycle lifetime of the alloy.

Surface Structure and Electrochemical Characteristics of Ti-V-Cr BCC-Type Solid Solution Alloys Sintered with NI

ABSTRACTTi-V-Cr bcc-type solid solution alloys can absorb a large amount of hydrogen and be applied to active materials of the negative electrode in Ni-MH batteries. However, because of the insolubility of Ni into these alloys, the electrochemical characteristics like discharge capacity and cycle life were poor. In order to increase the discharge capacity of hydrogen absorbing alloy electrodes, Ti-V-Cr bcc-type alloy powders were sintered with Ni in order to form Ni contained surface layer on the alloy surface. As sintering temperature rose up, the surface composition changed from TiNi to Ti2Ni. TiNi surface layer showed better electrochemical characteristics. For the Ni adding method, Ni electroless plating was preferred because of good adhesion. As a result of optimized conditions, a discharge capacity of 570 mAh/g and an improvement of cycle life were achieved.

Effect of microstructure, composition and non-stoichiometry on electrochemical properties of low-Co rare-earth nickel hydrogen storage alloys

A series of low-Co stoichiometric and non-stoichiometric rare-earth nickel hydrogen storage alloys were examined for the possible application as negative electrodes in NiMH batteries. Partial substitution of Co by one transitional metal such as Cu, Fe, Cr or Zn did not effectively improve the electrochemical cycling stability, while a combined replacement of Co by Cr, Cu and Si was very effective in improving the cycling durability though the initial capacity was slightly low as compared with the parent alloy. In this work, the low-Co non-stoichiometric alloys did not appear superior in the capacity, rate capability and the charge–discharge cycling stability. The discharge capacity seems to be determined by the alloying components and compositional homogeneity. The cycling durability was dominated by the microstructure of alloys. The microstructure and chemical composition of alloys were examined and analyzed by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The relationship between the microstructure and electrochemical properties of alloys was discussed.

Characteristics of the Alkaline-pretreated AB5 Type Hydrogen Storage Alloys as the Negative Electrode in Ni-MH Battery.

Study of the surface structures reformed by the alkaline treatment of the rare-earth metal based AB 5 type alloys