Application of Nanocrystalline LaNi_5-type Hydrogen Absorbing Alloys in Ni-MH_x Batteries

The structure and electrochemical properties of nanocrystalline LaNi_5-type alloys were studied. These materials were prepared by mechanical alloying (MA) followed by annealing. The properties of hydrogen host materials can be modified substantially by alloying to obtain the desired storage characteristics. It was found that the partial substitution of Ni by Al or Mn in LaNi_(5-x)M_x alloy leads to an increase in discharge capacity. The alloying elements such as Al, Mn and Co greatly improved the cycle life of LaNi_5 material. For example, in the nanocrystalline LaNi_(3.75)Mn_(0.75)Al_(0.25)Co_(0.25) powder, discharge capacity up to 258 mAh·g^(-1) was measured (at 40 mA·g^(-1) discharge current). Furthermore, the effect of the graphite coating on the structure of some nanocrystalline alloys and the electrodes characteristics were investigated. The mechanical coating with graphite effectively reduced the degradation rate of the studied electrode materials. The combination of a nanocrystalline LaNi_5-type hydride electrodes and a nickel positive electrode to form a Ni-MH battery, was successful.

R & D OF HYDROGEN ABSORBING ALLOYS IN JAPAN

The R & D of hydrogen absorbing alloys in Japan started in the early 1970s.Many alloys such as TiMm1.5 based alloys, Fe-Ti-O alloys (e.g. FeTi1.15 O0.024) andthe(mischmetal)Ni5 based alloys (e.g. MmNi4.5 Cr0.46 Mn0.04) were developed by the early 1980s. The application of these alloys to hydrogen storage, heat storage, heat pump, hydrogen purification and motor vehicles has been tried in many iaboratories,and the various techniques for using hydrogen absorbing alloys have been developed.The standarkization of evaluation methods for hydrogen absorbing alloys has been promoted by the Ministry of International Trade and Industry (MITI), and four of them were established as Japanese Industrial Standard (JIS).Alloys for Ni-Metal Hydride batteries have been extensively investigated since 1987in Japun. Mm-Ni-Co-Al-Mn alloys (e.g. MmNi3.55 Co0.75Al0.9Mn0.4) have been devel-oped and commereialized since 1990. The amount of production of small-size Ni-MH batteries in 1995 was about three hundred milliion in number and about one hundred billion yen. The R & D for higher enerpy-density Ni-MH batteries is intensively in progress.MITI and STA (Science and Technology Agency) have promoted the R & D of hydro-gen absorbing alloys in Japan by carrying out the national projects such as Sunshine Program (MITI: 1974-1993) and Utilization of Wind Engeray (STA 1980-1985). The New Sunshine Program (MITI 1993-2020) have started in 1993. This program con-tains the application of hydrogen absorbing alloys to Economical- Enerpy- City System and to We-NET (International Clean Energy System of Technology Utilizing Hydro-gen: World Energy Network.

Study on Diffusion of Hydrogen in AB_5 Non-Stoichiometric Hydrogen Absorbing Alloys

The diffusion coefficient of hydrogen atoms in the studied alloys was electrochemically measured by chronoamperometry when the metallic hydride microelectrodes were completely discharged. It is found that the diffusion coefficient of hydrogen atoms decreases when cobalt is substituted for a minority of nickel. On the contrary, the diffusion coefficient increases with the partial substitution of manganese or aluminum for nickel, which is related to the lattice constant and cell volumes of hydrogen absorbing alloys. The lattice constant c is obviously affected by the substitute elements greatly. The result shows that the expansion of cell volume resulted from the increase of value c causes the increase of diffusion coefficient which is especially obvious when the lattice constant is relatively low. However, this relationship is not clear if the lattice constant increases to some extent. It is suggested that this phenomenon is related to the interaction of hydrogen atoms with metallic atoms.

Enhancing magnetic properties of anisotropic NdDyFeCoNbCuB powder by applying magnetic field at high temperature during hydrogen desorption

Anisotropic powder was prepared with precursor （NdDy）-（FeCoNbCu）-B sintered magnets by hydrogen decrepitation, desorption, and subsequent annealing treatment. The hydrogen desorption was performed in magnetic fields of 0, 1, 3, and 5 T. The orientation of tetragonal phase grains of the powder was evaluated from the hysteresis loops measured by extraction magnetometer. Residual hydrogen content of the powder was evaluated by thermal-magnetic analysis. The powder with Hcj, Br, and （BH）max of 1138 kA.m^-1, 1.029 T, and 172.5 kJ.m^-3, respectively, was achieved under the condition of the magnetic field of 3 T. Magnetic properties of the powder, especially, the remanence of the powder, are enhanced upon magnetic fields, which is due to better orientation of powder particles and less residual hydrogen in the powder resulted from the magnetic field during the hydrogen desorption process.

Electrochemical Properties of Sn Doped AB_5 Non-Stoichiometric Hydrogen Absorbing Alloys-Ⅱ

The effects of some substituting elements on the specific discharge capacity, activation and cycling life of AB 5 non stoichiometric hydrogen absorbing alloys were studied electrochemically. Results show that the characteristics of hydrogen absorbing alloys under low current density are greatly improved through the substitution of Ni by Sn, Co, and Mn. Three hydrogen storage alloys, i.e., La(NiSn) 5.14 , La(NiSnCo) 5.12 and La(NiSnMn) 5.12 have the higher specific capacity and easy activation. However, Al in the alloy shows the opposite effect on the electrochemical properties. Under high current density, the discharge capacity and potential of La(NiSn) 5.14 is superior to La(NiSnCo) 5.12 and La(NiSnMn) 5.12 , but the cycling life of La(NiSn) 5.14 is very short. The addition of Mn, Co and Al to the alloys improves the cycling characteristics. In all these alloys, La(NiSnCo) 5.12 is considered as an ideal hydrogen storage alloy.

Structure of Sn Doped AB_5 Non-Stoichiometric Hydrogen-Absorbing Alloys-Ⅰ

Five non stoichiometric alloys, i.e., LaNi 5.15 , La(NiSn) 5.14 , La(NiSnCo) 5.12 , La(NiSnMn) 5.12 , and La(NiSnCoMnAl) 5.10 were studied. It is shown that the second minor phase does not exist in the major phase through the X ray diffraction analysis. By the measurements of the crystal lattice constant, the volume of crystal lattice decreases with the increasing of the amount of atom B in AB 5. It is more obvious when the element nickel in B is partially subsituted by other elements. For non stoichiometric alloys, the lattice volume greatly increases when Ni is partially substituted by Sn. The lattice constant is also effected by the addition of Mn, Co, and Al. By means of electrochemical measurements, the plateau pressure of hydrogen absorption/desorption is measured with the results that Sn, Co, Mn and Al decrease the plateau pressure.

Electrochemical Hydrogen Absorbing/Desorbing Behavior of Double Phase Mg-Ni Alloy

The electrochemical performance of double phase Mg Ni alloy was characterized at 25°C and 70°C, in order to evaluate briefly its utility as negative electrode materials in nickel metal hydride batteries. The results show that the electrochemical capacity of double phase Mg Ni alloy is rarely low at 25°C, but increased rapidly when the temperature is enhanced, and the double phase Mg Ni alloy has its maximum capacity at the first discharge cycle, but the capacity degrades rapidly with cycling number.

Synthesis of Intermetallic Compounds by Using Lithium Hudride

Intermetallic compounds of AB 5 type are promising materials for M-H batteries. In this report we present the results about the influence of quality of intermetallic compounds prepared by a new method of synthesis on their electrochemical properties. The well-known intermetallic Ln 1-xM xNi 5-yMe y (Ln=La, Mm; M=Zr; Me=Mn, Ge, Sn, Al, Co+Sn, Co+Ge) compounds were synthesized by using mixtures of oxides, chlorides and carbonates of metals by interaction with lithium hydride at 700～1000 ℃. Prepared samples have the uniform microstructure with average dimension of particles about 20～30 μm. Electrochemical tests show that kinetic behaviour of compositions are satisfactory under current up to 200 mA·g -1. These compositions require practically no activation and limiting values of the discharge capacity were reached at 2～3 cycle up to 300 mAh·g -1, which can be considered as most promising for practice.