Low-temperature performance and high-rate discharge capability of AB₅-type non-toichiometric hydrogen storage alloy

LU Yan-jing(陆延静), ZHU Lei(朱 磊), CHENG Yan(成 艳),

CHEN Hui(陈 晖), JIAN Xu-yu(简旭宇), WANG Zhong(王 忠)

Energy Material and Technology Research Institute, General Research Institute for Nonferrous Metals,Beijing 100088, China

Received 15 July 2007; accepted 10 September 2007

Abstract: Low-temperature performance and high-rate discharge capability of AB₅-type non-stoichiometric hydrogen storage are studied. X-ray diffraction(XRD), pressure-composition-temperature(PCT) curves and electrochemical impedance spectroscopy(EIS) are applied to characterize the electrochemical properties of AB_x (x=4.8, 4.9, 5.0, 5.1, 5.2) alloys. The results show that the non-stoichiometric alloys exhibit better electrochemical properties compared with that of the AB₅ alloy.

Key words: AB₅-type hydrogen storage alloy; non-stoichiometry; low-temperature performance; high-rate discharge capability

1 Introduction

In recent years, nickel-metal hydride(Ni-MH) secondary batteries have been widely used in various portable electronic devices, electric tools and vehicles because of their excellent electrochemical performance and environmental compatibility[1]. However, the low-temperature performance of Ni-MH batteries is unsatisfied. As we known, hydrogen storage alloys play a key role in the discharging process at low temperature for Ni-MH battery. At present, the commercialized negative electrode materials are mainly AB₅-type alloys due to its long-term cycling stability, high-rate capacity, and good charge-discharge kinetics[2-5]. Therefore, it is necessary to improve the low-temperature electro- chemical performances of hydrogen storage alloy.

To our knowledge, no systematically investigation has been done for the AB₅-type non-stoichiometric alloys under high rate and low temperature. In this work, the performance of the AB₅-type non-stoichiometric alloys are investigated.

Based upon the AB₅-type (LaCePr)(NiCoMnAl)₅ hydrogen alloy, (LaCePr)(NiCoMnAl)_x (x=4.8, 4.9, 5.0, 5.1, 5.2)  non-stoichiometric alloys were designed and prepared by induction melting under argon shield of pure elements in a water-cooled copper crucible. The ingots were melted and turned over three times for homogeneity Then the ingots were crushed and mechanically grounded to a particle size of less than 74 μm.

The crystal structures of (LaCePr)(NiCoMnAl)_x (replaced by AB_x in the following) were analyzed by X-ray diffractogram.

2.1 Preparation of the MH electrode

0.1 g alloy powder was mixed uniformly with 0.3 g nickel powder in proportion of 1?3, and then they were pressed by 20 MPa into the negative plate 13 mm in diameter and 0.5 mm in thickness. The counter electrode was formed by the NiOOH/Ni(OH)₂, whereas the reference electrode was Hg/HgO electrode filled with 6 mol/L KOH solution.

2.2 Electrochemical measurements

After activation at room temperature (20 ℃), the electrodes were charged for 7 h at 60 mA/g (0.2C), then some were kept for 30 min at room temperature and the others were placed in the low temperature cabinets for  4 h and 6 h at the temperature of -20 ℃ and -40 ℃,respectively, then the samples were discharged at various rates (0.2C, 0.5C, 1C and 3C) to -0.6, -0.6, -0.5, -0.4 voltage with respect to Hg/HgO reference electrode.

Pressure-composition-temperature(PCT) curves of the alloys were determined by electrochemical methods[6]. The thermodynamics parameters including the equilibrium pressure of the hydrogen absorption or desorption and the change of enthalpy were calculated.

The electrochemical impedance spectroscopies of the 50%DOD (depth of discharge) electrodes were measured and the scan frequency was from 100 kHz to 1 mHz.

3.1 Discharge capacity of electrode

Fig.1 shows the variation of the discharge capacity, versus the value of x of the ABx alloy. It can be seen that with the increase of x, the discharge capacities curves of various rates (0.2C, 0.5C, 1C, 3C) at 20 ℃ and -20 ℃, as well as that of 0.2C at -40 ℃, present the shape of character M. Over-stoichiometric alloys and under- stoichiometric alloys exhibit better high-rate discharge capability and low-temperature performance compared with that of the stoichiometric alloy (x=5.0). The alloy with x=5.1 shows the excellent electrochemical capability, of which the 3C discharge capacity reaches 284 mA?h/g at 20 ℃, and 0.2C also reaches 233 mA?h/g at -40 ℃.

3.2. Structural analysis

The XRD patterns of AB_x metal-hydride alloys are given in Fig.2. The results show that all alloys keep the typical single phase of CaCu₅.The lattice parameters and lattice volume are calculated and compiled in Table 1. It can be seen that with the increase of the value of x, The lattice parameters c first increase and then decrease, which is similar to the behaviour of the low-temperature discharge capacity. On the contrary, the lattice parameters a exhibit the opposite phenomenon. The variation of lattice parameters and volume as a function of x is showed in Fig.3. The alloys with smaller lattice parameter a and larger lattice parameter c show better low-temperature performances and high-rate discharge capabilities.

Table 1 Smaller (a) and bigger (b) lattice parameters and lattice volume(V) of AB_x metal- hydride alloys

Fig.1 Variation of different rate discharge capacities with x for AB_x alloys at different temperatures: (a) 20 ℃; (b) -20 ℃;    (c) -40 ℃

3.3 Thermodynamic properties

The pressure-composition-temperature(P—C—T) curves measured at 20, -20 and -40 ℃ are shown in Fig.4.

The equilibrium pressure of hydrogen absorption or desorption (P_eq) are defined as following equation[7]:

                               (1)

where  P₂ and P₁ are the pressures of inflection points of curves in Fig.4. The variation of P_eq versus the value of x is given in Fig.5. According to P_eq, Van’t Hoff curves of AB_x alloys is presented in Fig.6, and the variation of the change of enthalpy(ΔH) of AB_x alloys as a function of x is given in Fig.7.

Fig.2 XRD patterns of of AB_x metal-hydride alloys

Fig.3 Variation of lattice parameters (a, c) and lattice volume (V) of AB_x metal-hydride alloys as function of x: (a) a; (b) c;  (c) V

The results show that over-stoichiometric alloys and under-stoichiometric alloys present higher equilibrium pressure (P_eq) and less absolute value of the change of enthalpy (|ΔH|). The alloy with x=5.0 exhibit the lowest P_eq and the highest |ΔH|, while the alloy with x=5.1 shows the higher P_eq and the least |ΔH|, and its |ΔH| reaches 28.9 kJ/mol. With the increase of the value of x, the change of enthalpy (ΔH) shows the same tendency as that of the discharge capacities, which indicates that there is a close relationship between the electrochemical property and the thermodynamic performance for non-stoichiometric alloys. The alloys with higher equilibrium pressure and less absolute value of enthalpy display higher high-rate discharge capacities and low- temperature capabilities, which may be ascribed to the poor stability of metal hydrides materials.

3.4 Electrochemical impedance spectroscopy

Electrochemical impedance is a powerful tool for characterization of metal-hydride electrodes, and has been widely used[8-10]. According to the mathematical model developed by WANG et al[11], the charge-transfer resistance, R_t, could be calculated from the electro- chemical impedance spectroscopy(EIS) by constructing the equivalent circuit, and the exchange current density (I₀) could be defined as following equation[12]:

                                 (2)

where  R, T, m and F are the gas constant, the absolute temperature, the effective mass of material and the Faraday constant, respectively.

Fig.8 shows the Nyquist plot of AB_x metal-hydride alloys with 50%DOD, and the calculated R_t and I₀ are listed in Table 2 and Table 3.

Fig.9 shows the variation of the exchange current density (I₀) of AB_x metal-hydride alloys (50%DOD) as a function of x. With the increase of x, the exchange current densities of AB_x alloys present the same tendency as that of the discharge capacities, and the exchange current densities of the non-stoichiometric alloys are higher than that of AB₅ alloy. The high-rate discharge capacities increase gradually with the increase of the exchange current density[13-15]. It is well known that the high exchange current density indicates not only the high reaction rate of the electrode, i.e. high rate charge-discharge capability, but also a low degradation rate of electrode performance[16]. Thus, we can conclude that the non-stoichiometry is beneficial to the increase of electrodes exchange current density, which can improve the low-temperature performance and high-rate discharge ability.

Fig.4 P—C—T curves measured at different temperatures of AB_x metal- hydride alloys: (a) AB_4.8; (b) AB_4.9;    (c) AB_5.0; (d) AB_5.1; (e) AB_5.2

Table 2 Charge-transfer resistance (R_t) of AB_x metal-hydride alloys (50%DOD) at different temperatures

Table 3 Exchange current density (I₀) of AB_x metal-hydride alloys (50%DOD) at different temperatures

Fig.5 Variation of equilibrium pressures of AB_x metal-hydride alloys as function of x: (a) 20 ℃; (b) -20 ℃; (c) -40 ℃

Fig.6 Van’t Hoff curves of AB_x metal-hydride alloys

Fig.7 Variation of enthalpy (ΔH) of AB_x metal-hydride alloys as function of x

Fig.8 Nyquist plots of AB_x metal-hydride alloys (50%DOD)

Fig.9 Variation of exchange current density (I₀) of AB_x metal-hydride alloys as function of x: (a) 20 ℃; (b) -20 ℃;  (c) -40 ℃

  4 Conclusions

1) With the increase of x, the discharge capacities of AB_x electrodes present the shape of “M”. Under- stoichiometric and over-stoichiometric alloys present the better low-temperature performance and the higher high-rate discharge ability compared with that of the stoichiometric alloy (x=5.0).

2) Non-stoichiometric alloys possess smaller lattice parameter a and bigger lattice parameter c, which may be beneficial to their low-temperature performance and high-rate discharge capability.

3) With the increase of x, the change of enthalpy (ΔH) and the exchange current density (I₀) of AB_x alloys show the same tendency as that of the discharge capacities, which indicate that there exists a close relationship between the ΔH, I₀, low-temperature performance and high-rate discharge ability.

4) The AB_5.1 alloy, with the least |ΔH| and highest I₀, shows the best discharge capability. Its 3C discharge capacity reaches 284 mA?h/g at 20 ℃, while 0.2C reaches 233 mA?h/g at -40 ℃.

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(Edited by LAI Hai-hui)

Foundation item: Project(2006AA11A151) supported by the National Hi-Tech Research and Development Program of China

Corresponding author: LU Yan-jing; Tel: +86-10-82241241; E-mail: luyanjing82@126.com