Rare Earth Stoichiometric Ab, <sub> 5 + X </ Sub> Cobalt Hydrogen Storage Alloy Electrode Materials

For the commercial ABs-type electrode alloys, cobalt is a key element in improving the cyclic stability of the alloys electrodes, however the cobalt is very expensive and shares the 40-50% of the total cost of alloys, so the research and development of low-Co or Co-free alloy has been the focus of the study on ABs-type alloys and the focus of industrialization. It is regretful that the electrochemical properties of low-Co or Co-free ABs-type alloys studied previously, especial for the cyclic stability, haven't been improved satisfactorily unit now. In this paper, the electrochemical properties and effect factors of over-stoichiometric La(NiMnM)5.6⁶.0 (M=A1, Fe, Cu, Sn, Cr) alloys have been investigated by means of composition design, process of the alloys preparation, crystal and microstructural characterization and electrochemical experiments. It is our aim to develop the Co-free hydrogen storage alloy with favorable overall electrochemical properties through the optimum of alloy composition and preparation. Based on the above experiments, the influence of crystal microstructure on the cyclic stability of electrode alloys has been analyzed. The detailed results are as follows:1. Miedma' s semi-experienced model was adopted to calculate and forecast the forming enthalpy of binary and ternary LaNi_5-xM_x(M=Cu, Fe, Co, Mn, Al, Cr) hydrogen storage alloys. It is shown that the addition of Cu, Fe, Mn, Cr and Co decreases the forming enthalpy of LaNi_5-xM_x alloy whereas Al increases the forming enthalpy. The forming enthalpy of alloys has an important influence on the hydrogenation behavior under high temperature condition.2. Based on the La(NiMn)6 alloys, over-stoichiometric La(NiMn)5.6 and La(NiMnM)6.0 (M=Mn, Cu, Fe, Cr Sn) alloys were prepared by arc melting followed by annealing at 1000℃ for 7d. The crystal structure and electrochemical properties of the annealed alloys have been investigated. Combined with refinement of XRD and ND, the crystal charactaristics of La(NiMnM)6.o alloys were analyzed, and then the distribution and occupation of La, Ni, Mn and M atoms at la, 2c, 3g, 61 and 2e sites in lattice were obtained. It is shown that the annealed over-stoichiometric La(NiMnM)6.0 (M=Cu, Fe, Cr, Sn) alloys all exhibited good cyclic stability under proper alloying condition. When the Ni in La(NiMn)6 alloy was substituted by M element, alloys exhibited good cyclic stability. While the Mn in La(NiMn)6 alloy was replaced, the cyclic stability deteriorated. The proper occupancy of Mn atoms at the 2e site (double-atom structure) is necessary to keep the cyclic stability of La(NiMnM)6.0 alloy;Under the same over-stoichiometry, the discharge capacity of alloys decreased as the content of M increased;proper decrease of over-stoichiometry could enhance the hydrogen storage capacity and electrochemical capacity of alloys.3. The phase structure, microstructure and electrochemical properties of rapidlyquenched La(NiMnM)s.6 (M=A1? Fe) alloys under different content of Al and Fe, and different quenching rate have been investigated. It is shown that over-stoichiometric La(NiMnM)s.6 alloys could exhibit metastable single phase with CaCu5-type structure after rapidly quenching treatment. The quenched alloys contained nanocrystalline, microcrystalline and amorphous phase in part. With the different quenching rate, the microstructure tissue varied, and the morphologies of quenched alloys were mainly composed of columnar crystal, equiaxial crystal and dendritic crystal. Under quenching rate of lOm/s and 15m/s, the alloys could mainly form tiny columnar grains;Al element could show a unique and important role in improving the cyclic stability of quenched La(NiMnM)s.6 alloys. The addition of Al element could improve the cyclic stability of alloy electrode dramatically, as the concentration of Al element increase in alloys, resulting in a good cyclic stability, but the decrease in discharge capacity slightly;The addition of Fe element is also effective to improve the cyclic stability of quenched alloys. When compared with Al element, the quenched alloys containing Fe element showed higher discharge capacity. Based on the above study of as-quenched La(NiMnM) (M=Al,Fe) alloys, the Co-free hydrogen storage alloys with high capacity, good cyclic stability and satisfying overall electrochemical properties have been obtained. The electrochemical properties of typical alloys are as follows: for alloy containing Al element, the maximum capacity can reach 334mAh/g, the capacity retention ration after 150 charge/discharge cycles is 95.2%, HRD at 1C rate discharge is 96.7%. For the alloys containing Fe element, the maximum discharge capacity 341mAh/g, the capacity retention ratio after 200 cycles 96.2%, HRD at 1C rate discharge is 95.2%. Based on the broadening of x ray diffraction files and the refinement analysis of single peak, the crystal size and lattice strain of typical five alloys and hydrides were calculated and the effect of microstructure parameters on pulverization and cyclic stability of electrode alloys were investigated undergoing hydrogen absorption/ desorption cycling . The broadening of x-ray diffraction peak is closely related to the crystal grain size with different preparation conditions and accumulative lattice strain during hydrogen absorption/desorption. The stronger the broadening of XRD peak effect is, the smaller the grain size and the larger the strain deformation is. In dependence of the broadening of diffraction peak, the lattice strain for alloys, hydride and dehydrided alloys were calculated respectively. It is shown that the lattice strain of hydrides showed a perfectly corresponding relation to the pulverization of alloy and the cyclic stability of electrodes. The larger the strain is, the more serious the pulverization of alloys and the more poor the cyclic stability of alloy electrodes is. Based on the analysis of effect factors about pulverization of rapid cooling alloys, a corresponding hypothesis that very fine grains tissue of as quenched alloys could preferentially suffers from crack and pulverization along the crystall grain boundary were brought forward. According to the hypothesis, the effect of different composition and microstructure on the electrochemical stability of over-stoichiometric rapidly quenched alloys were discussed and interprated. Through the systematical investigation, the cycling stability of Co-free AB5-typeelectrode alloys has been resolved preferably. The developed over-stoichiometric quenched alloys electrode can exhibit higher discharge capacity, good cyclic stability and better overall electrochemical properties.