“Soft Chemistry” synthesis of superfine powder alloys AB5 for Ni-MH batteries

241 Journal of Chemistry, Vol. 42 (2), P. 241 - 249, 2004 “Soft Chemistry” synthesis of superfine powder alloys AB5 for Ni-MH batteries Received 25-12-2003 Ngo Quoc Quyen, Nguyen Quynh Anh, Phan Thi Binh Lab. for Appl. Electrochemistry, Vietnamese Academy of Science and Technology Summary The oxide reduction diffusion (ORD) procedure has recently been applied in synthesizing hydrogen storage materials AB5 for Ni-MH batteries. Starting from metal hydroxides and La oxalat precursor, superfine powder alloys LaNi5, LaNi4.5Co0.5 and LaNi3.87Mn1.13 were obtained by this “soft-chemistry” route. Chemical composition, structure and morphology of alloy phases were examined by different analysis techniques such as AAS, EPMA, X-ray and TEM. The H2-absorption and desorption behavior of crystalline products was determined by Sieverts’ method. Electrochemical properties of alloy samples were characterized by CV, EIS and Battery Test method. I - Introduction Soft-Chemistry synthesis of superfine powder alloys AB5 is based on the reduction of oxides by calciothermic reaction, which was carried out by R. E. Cech [1] many years ago, however there are only a few reports [2 - 5] dealing with this synthesis route for hydrogen storage electrode materials although nickel- metal hydrid batteries (Ni-MH) have especially been directed towards practical use recently. This procedure can, in case of known LaNi5, be represented by: La2O3 + 10 NiO + 13 Ca 2 LaNi5 + 13CaO (1) The formation mechanism of LaNi5, according to T. Tanabe and Z. Asaki [6], includes two stages: • The reduction of La2O3 and NiO by Ca: La2O3 La (2a) NiO Ni (2b) • The simultaneous diffusion of the just- formed rare earth and transition metal (Ni) in molten calcium leads to initial formation of CaNi5 (3), following the substitution of Ca by La to form the more thermodyna- mically stable alloy LaNi5 (4): Ca + 5 Ni CaNi5 (3) La + CaNi5 LaNi5 + Ca (4) Single phase crystals of LaNi5 growth in the CaO – Ca slurry as micron-size loose particle of angular shape, whose hexagonal structures are closely related to that of CaCu5. Particle can be easily recovered after washing in weak acidic solution. The purpose of our work is on the ORD-route to produce some non-stochiometric phases of 1300 K Argon Ca Ca 242 well definite composition, such as LaNi4.5Co0.5, LaNi3.87Mn1.13, used for Ni-MH batteries. II - experimental procedure Schematic drawing of the ORD procedure is shown in figure 1 and includes two main stages: - The preparation of precussors. - The calciothermic synthesis. One of the advantages of the ORD method is the ability of using metal oxides as starting materials. In the synthesis procedure used here, however, superfine powder mixture of transition metal and rare earth oxides were prepared first of all by sol-gel process. The composition of constituent oxides can be tailored by varying the concentration of metal ion in the starting salt solution. The preparation conditions to the formation of superfine precussors are very important for the following ORD synthesis. It is known that, employment of superfine precussors in the ORD-reaction can significantly reduce the reaction tempera- ture and reaction time which relate to the short diffusion length and large diffusion coefficients of the small particle size. Nitrates of Ni, Co, Mn, Nitrate of La Oxalate of La Fine sols of hydroxide of Ni, Co, Mn, Mixture of oxides Microware Heating Complex oxides of spinel phase Calcining Fine Powder AB5 LaNi5 LaNi4.5Co0.5 LaNi3.87Mn1.13 ORD-Reaction with Ca (T=1300K, Argon) Test • Stoichiometry by EPMA, ASS • Structure and morphology analysis by X-ray and SEM • H2-absorption/- desorption isotherms (by Sieverts method) • Electrochemical characterization by CV, EIS and Modelling • Battery tester Pr ep ar at io n of pr ec ur so rs C al ci ot he rm ic Sy nt he si zi ng Figure 1: Flow chart of the synthesis procedure 243 Among many others, some main conditions are summarized as followed: - The mixture of fine hydroxide sol of transition metal (Ni, Co, Mn... and oxalate of La was first converted into oxides by microware decomposition and then into complex oxides of spinel phase by intensive calcining (at 800oC for 2 h). - The main ORD-reaction with excess calcium was carried out in the stainless steel reactor (Fig. 2) at ~1000OC for ~4 h under purified argon. After quenching to room temperature the black fine crystalline powder of AB5 was recovered by thorough washing with dilute acetic acid up to complete eliminating of Ca(OH)2 by-product. The chemical composition of alloy samples was determined by AAS and EPMA. Phase structure and morphology were examined by X-ray diffractometry (Siemens D-5000) and TEM (EM-125K). The behavior of the hydrogen absorption as well as desorption of obtained alloy powder was determined by Sieverts’ methode. The electrochemical proper- ties of samples were measured by Cyclic Voltammetry and Electrochemical Impedance Spectroscopy (Zahner-IM6). Some storage characteristics were estimated by Battery- Tester method (ZSW-Basytec). In this work, we mainly described the research results on compounds LaNi5 and LaNi4.5Co0.5. Figure 2: Reactor of ORD processing 1, 2, 6 Electrical Furnace 3 Stainless steel crucible 4 Reactor - Chamber 5, 10 Thermocouple and Thermocontrol unit 7 Cooling top cap 8 Argon flux 9 Outgas 244 III - results and discussion 1. Structure and morphology analysis Figures 3a, 3b and 3c represent the X-ray patterns of some obtained AB5 compounds. Despite these rather rough growth conditions of Figure 3a: X-ray pattern of LaNi5 Figure 3b: X-ray patte of LaNi3.87Mn1.13 . l l i Figures 3a, 3b and 3c represent the X-ray patterns of some obtained AB5 compounds. Despite these rather rough growth conditions of the ORD procedure, one always observes a remarkable crystal quality with sharp X-ray diffraction line. All the samples were pure phase and their X-ray patterns were refined in the CaCu5 – type hexagonal structure of LaNi5. Figures 4 represents particle morphology of a LaNi4.5Co0.5 alloy observed by TEM with selected area of electron diffraction. In general the particles of the AB5 alloys, formed during the ORD process, consists a mixture of crystalline (~70%) and amorphous phase (~30%) and are narrowly distributed with a typical size of a few micrometers. 30 35 40 45 50 55 60 65 30 35 40 45 50 55 60 65 70 75 245 Figure 3c: X-ray pattern of LaNi4.5Co0.5 Figure 4: Particle morphology of crystalline LaNi4.5Co0.5 observed by TEM 25 30 35 40 45 50 55 60 70 75 246 0006.00507.0 2 = HpN The crystallite appear in angular shapes and, in many cases, the rectangular- or hexagonal- shaped crystals are identified (as in Fig. 4). The amorphous phase can be crystallized after annealing, but it is not necessary for using as electrode materials in field of the battery technology. 2. Hydrogen absorption behavior of the obtained AB5 at 30 oC It was found at room temperature that the AB5 compounds can be reversibly absorbed up to six atoms of hydrogen per formula unit at equilibrium hydrogen pressure. Therefore, the hydrogen absorption properties of obtained alloy powders such as LaNi5 and LaNi4.5Co0.5 were measured by means of the Sieverts’ method. Details of the experimental apparatus used in this study are described in a previous article [7]. Figure 5 shows the change in the absorption properties in LaNi5 resulting from the partial replacement of Ni by Co in the form LaNi4.5Co0.5. The relationship between the concentration of hydrogen loading in the -phase of AB5 (N  ][AB [H] 5 ) and the equilibrium hydrogen pressure (PH 2 ) can be represented by linear Sieverts’ equation (5): N = KS . pH 2 1/2 + K0 (5) The calculated Sieverts’ parameter KS, K0 are represented in table 1 Figure 5 Table 1: Sieverts’ parameter KS, K0 and PH 2 -range at 30oC AB5 N = KS . pH 2 1/2 + K0 pH 2 - range, atm Storage capacity*, mAh/g LaNi5 1 ÷4 ~ 80 LaNi4.5Co0.5 0 ÷1 ~ 320 * estimated by battery-tester method at 30oC and 1 atm. 0.0005p0.0023 N 2H = 1/2 0.0006p0.0507 N 2H = 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.0005 0.001 0.0015 0.002 0.0025 N=[H] / [AB5] pH2 [atm] LaNi5 LaNi4.5Co0.5 pH2 [atm] N = [H] / [AB5] 247 The effect of partical cobalt substitution for nickel shows clairly in the H2-absorption behavior. Sieverts’ constante KS indicating the plateau slope decreases remarkably so that the H2-absorption shifts to direction of the high hydrogen concentration even by lowering PH 2 - range in the vicinity of internal gas pressure of battery ~1 atm. The comparison measurements of initial storage capacity determined by battery- tester method are also given in table 1. The high storage capacity of LaNi4.5Co0.5 in comparison with LaNi5 again results from this fact. 3. EIS measurements and modeling based on Zahner-IM6 Messtechnik The performance of the MH-electrode is mainly controlled by kinetics of the charge transfer on the surface as well as by the mass transfer of hydrogen within the bulk of the storage alloys. In order to obtain more insight into functioning of MH-electrode, modelling by EIS method according to Gohrs’ concept was carried out. Gohr model is suitable for electrode materials having porosity, roughness distribution as well as polycrystallinity, particle-size effects such as hydrogen insertion AB5-electrode [8] Figure 6 shows typical Nyquist impedance spectra of electrode material LaNi4.5Co0.5 at different potentials in the whole frequency range (103 to 10-3Hz). The Nyquist plot in the vicinity of equilibrium potential (-1.0 V vs Hg/HgO) consist of two distinct semicircles, whereas the plots in the discharge range (-0.8 V to -0.4 V vs Hg/HgO) consist of only a depressed semicircle and a diffusional region, which is described not by a 45o line but by a line at increasing angle in depend on applied potential. They show a restricted diffusion behavior. Curves fitting of Nyquist plots were made by the complex non-linear least square method to determine the electrochemical components of equivalent circuit (Fig. 7) for the MH-electrode containing the finite-length diffusion response. Figure 6: Nyquist diagram of electrode material LaNi4.5Co0.5 at different potentials (vs. Hg/HgO) Im ag yn ar y Pa rt (O hm ) 50 40 30 20 10 0 -10 0 10 20 30 40 50 60 70 Real Part (Ohm) 248 Figure 7: Equivalent circuit for MH-electrode, according to Gohrs’ model Table 2 shows in details of the so-called stack interface impedance Z(), which may be expressed in terms of the impedance element such as ZT (electrode top interface consisting of R1 and C1), ZP (pore ground interface consisting of C3 and R4) and ZR (a modified Randles circuit consisting of double layer capacitance Cdl, charge transfer resistance Rct, Cin insertion capacitance and Re electrolyt resistance). The shape of the impedance diagram with restricted diffusion behavior depends on the ratio of resistances Rct and Rin and the ratio of capacitances Cdl and Cin. The limiting shapes of Figure 6 were obtained as a consequence Cdl < Cin (and probably Rct  Rin when low - frequency impedance date were carried out in extended frequency range to  < 10-3Hz in order to separate the semi-infinite diffusion process (the Warburg impedance) from the finite-length diffusion effect) according to J. S. Chen [9]. However, this is not discussed further, as not enough data are available to characterize the phenomenon at present. Table 2: Parameters values obtained by equivalent-circuit analysis according to Gohrs’ model ZT ZP ZRPotential [V vs Hg/ HgO] R1[ ] C2 [mF] C3 [µF] R4 [ ] Cdl [µF] Cin [mF] Rct [ ] Re [ ] Remark -1.0 -0.8 -0.6 -0.4 48.8 108.7 75.5 73.9 918.6 5.42 8.18 11.06 6.97 4.74 5.68 6.43 1.74 4.95 3.25 2.91 51.1 12.81 12.63 11.69 1.36 1.85 2.07 3.51 11.33 49.4 38.0 37.8 6.95 8.75 9.23 9.31 H2-evolution reaction H2 oxydation reaction End of decharge At potential in the vicinity of equilibrium potential  -1.0V vs. Hg/HgO, the exchange current density i0 is expressed by: ct 0 FR RT A 1 i = (6) where R is 8.314 J mol-1 K-1, T = 303 K, F = 96500 As, Rct = 11.33 , A = 0.054 cm2, then i0 of obtained LaNi4.5Co0.5 powder is  42.7 mAcm-2. In addition, the high reaction resistance Rct in the range of the discharge process (at –0.8 ÷ -0.4V vs Hg/HgO) results from depth discharge (DOD) dependence. 1 : R1 5 : Cdl 2 : C2 6 : Cin 3 : C3 7 : Rct 4 : R4 8 : Re 249 IV - conclusion This study has shown that the ORD method seems to be a very attractive way to produce the AB5 compounds for Ni-MH batteries because of its simple procedure (using inexpensive oxide precursors to avoid starting from high cost pure metals; low expense for equipment; under mild synthesizing conditions in comparison with traditional arc-melting or HF methods and finely crystalline powder as endproduct without use of mechanical milling). The properties of ORD powder can be easily controlled by varying the composition of metal ions and synthesizing conditions based on the sol-gel chemistry. Results of different analysis techniques show that no significant differences of properties of obtained ORD powder were found in comparison with the same products of other metallurgy methods. Finally, EIS – modelling according to Gohrs’ concept allows to obtain interesting insight into functioning of the MH-electrode. Acknowledgement: This work was supported by a Grant-in-Aid for Basic Research No. 5.31.301 from the Ministry of Science and Technology of Vietnam. The authors wish also to express their thank to Humboldt fellowship and BMF of Germany for the important support of research equipments in the course of this work. References 1. R. E. Cech. J. Met., Vol. 26, P. 32 (1974). 2. Z. Li, K. Yasuda, et al.. J. Alloys & Comp., Vol. 193, P. 26 - 28 (1993). 3. D. Y. Kim, M. Ohtsuka, et al.. Metall. Review of MMIJ, Vol. 10, P. 2 - 45 (1993). 4. 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