- Email: [email protected]
Effects of particle size on the electrochemical properties of Mm(NiCoMnAl)5 alloy
Journal of Alloys and Compounds 270 (1998) L7–L9
L
Letter
Effects of particle size on the electrochemical properties of Mm(NiCoMnAl) 5 alloy a, b Zhang Zhaoliang *, Sun Dongsheng b
a Department of Chemistry, Peking University, Beijing 100871, China Department of Materials Science and Engineering, ShangDong University of Technology, Jinan 250061, China
Received 17 November 1997; received in revised form 26 February 1998
Abstract The effects of particle size on the electrochemical properties of the La 0.65 Nd 0.2 Pr 0.15 Ni 3.55 Co 0.75 Mn 0.4 Al 0.3 alloy have been investigated. The electrode composed of particles with size of 54–74 mm (200–300 mesh) shows the best discharge capacity, high-rate dischargeability and long-term stability. From the application point of view, the electrode can be composed of a mixture of particles of 54–74 mm and particles smaller than 30 mm (500 mesh), the latter also showing a high discharge capacity and long cycle life. 1998 Elsevier Science S.A. Keywords: Particle size; Hydrogen storage alloy; Nickel–metal hydride battery
1. Introduction Among the various types of hydrogen storage alloys, the LaNi 5 -type has recently proven to be very attractive as negative electrode material in rechargeable nickel–metal hydride (Ni–MH) batteries [1], and the effects of partial substitution of La by mischmetal (Mm) and Ni by Co, Mn and / or Al were deeply discussed in previous works [1,2]. In general, the properties of negative electrodes are affected by the composition and preparation conditions of the electrode. Naito et al. [3] showed that the electrochemical properties of the MmNi 3.31 Mn 0.37 Al 0.28 Co 0.64 alloy, such as discharge capacity and high-rate dischargeability, were better for particle sizes of 106–125 mm than for 20–25 mm. In this work, a AB 5 -type compound, i.e. La 0.65 Nd 0.2 Pr 0.15 Ni 3.55 Co 0.75 Mn 0.4 Al 0.3 (for brevity, the alloy is written as MmB 5 ), was employed as negative electrode material. The effects of the particle size in finer distributions from below 30 mm to 147 mm (100 mesh) on electrochemical properties, such as discharge capacity, high-rate dischargeability and cycle life were investigated.
2. Experimental details The MmB 5 alloy was prepared from high purity ele*Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00498-8
ments, .99.9%, by arc melting under an argon atmosphere. After the first melting the ingot was inverted and re-melted twice. The ingot was separated into various fractions, 74–147 mm (100–200 mesh), 54–74 mm (200– 300 mesh), 38–54 mm (300–400 mesh), 30–38 mm (400– 500 mesh) and a fraction below 30 mm (500 mesh) by passing through sieves after mechanical pulverization. Then the alloy powders were mixed with fine Cu powder in a weight ratio of 1:4 and then pressed at about 8310 6 N m 22 to produce pellets 12 mm in diameter. Each pellet contained 200 mg of hydride-forming material without microencapsulations. Electrochemical measurements were carried out at room temperature. The discharge capacity was measured by charging the pellets at a current density of 200 mA g 21 for 2 h, resting for 1 min, and discharging at 200 mA g 21 to 20.65 V. For determining the high-rate dischargeability, charging was done at the same current density, resting for 1 min, but discharging at different current density according to different particle sizes to 20.65 V with respect to a Hg / HgO reference electrode. The long-term stability was measured in a MH electrode-limited open cell, by charging at 150 mA g 21 for 2.33 h, resting for 1 min, and discharging at 150 mA g 21 to 20.9 V. NiOOH plates and 6 M KOH solution were used as the counterelectrodes and electrolyte, respectively. The pressure–composition isotherm for the MmB 5 alloy–H system was measured by using Sieverts’ method.
Z. Zhaoliang, S. Dongsheng / Journal of Alloys and Compounds 270 (1998) L7 –L9
L8
particle sizes below 30 mm has a higher discharge capacity (286 mAh g 21 ) than those for particle sizes of 38–54 mm and 30–38 mm. This indicates that the discharge capacity is not always improved by increasing the alloy particle size. But on the whole, a higher discharge capacity is obtained by employing the larger particles. The electrode containing particles with sizes of 74–147 mm nearly has the same high discharge capability as for sizes of 54–74 mm. Naito et al. [3] mention that a new surface having high catalytic activity is produced in an activation process without increasing the contact resistance.
Fig. 1. Pressure–composition isotherms of the MmB 5 alloy.
The amount of hydrogen stored (CH ) was evaluated from the P–C–T data by the following equation [3]: CH (mAh /g) 5 6 3 1000F 3 [(H /M) 0.5 2 (H /M) 0.01 ] / 3600M Where H /M is the atomic hydrogen-to-metal ratio, F is the Faraday constant and M is the molecular weight of the alloy.
3. Results and discussion
3.1. Discharge capacity Fig. 1 shows the pressure–composition isotherm at room temperature. The calculated capacity has a maximum value of about 325 mA h g 21 . Fig. 2 shows the discharge capacity as a function of particle sizes of the MmB 5 alloy. It can be seen that there is a low discharge capacity (246 mA h g 21 ) for particle sizes of 38–54 mm and 30–38 mm. The highest discharge capacity (300 mA h g 21 ) is obtained for particle sizes of 54–74 mm, while the alloy with
Fig. 2. Discharge capacity of the MmB 5 alloy vs. particle sizes.
3.2. High-rate dischargeability The high-rate dischargeability of an alloy is influenced by the temperature, by the grain and surface morphology (surface area per unit of weight, current densities on the grain surface, length of diffusion paths for hydrogen atoms in the grains and for OH 2 ions to the surface), by the hydrogen equilibrium pressure (concentration of adsorbed hydrogen atoms at the oxidation state) [4]. Fig. 3 shows the relationship between the discharge current density and discharge capacity for five different particle sizes. To consider the dependence of the discharge capacity on discharge rate properties, the relationship between the capacity ratio Cn /C100 (Cn refers to the discharge capacity at a discharge current density of n mA g 21 ) and discharge current density is given in Fig. 4. The electrode containing particle sizes of 54–74 mm shows the highest rate capability. For sizes of 74–147 mm, still a good rate capability is obtained. The electrodes containing particle sizes of below 30 mm, of 38–54 mm and 30–38 mm have a much lower rate capability than those of 54–74 mm. The discharge capacity obtained at 1000 mA g 21 is approximately 83% of that obtained at 100 mA g 21 for particle sizes of 54–74 mm. For particle sizes of 30–38 mm, the discharge capacity obtained at 1000 mA g 21 was only 54% of that obtained at 100 mA g 21 .
Fig. 3. Discharge current density dependence of discharge capacity for the MmB 5 alloy.
Z. Zhaoliang, S. Dongsheng / Journal of Alloys and Compounds 270 (1998) L7 –L9
L9
Table 1 2dC /dn for various particle sizes of the MmB 5 alloy
Fig. 4. Relationship between capacity ratio and discharge current density.
3.3. Cycle life Because particle sizes larger than approximately 100 mm were not suitable for electrode fabrication [5], particles with sizes of 74–147 mm were not included in discussion of cycle life. The discharge capacities vs. cycle number curves for various particle sizes are shown in Fig. 5. There is an initial steep increase in capacity in the first few cycles. All electrodes containing various particle sizes reach their highest capacity Cmax nearly at the same time after about fifty cycles. This indicates that particle size has no effect on activation. Then a nearly linear decrease in capacity as a function of cycle number for all particle sizes is seen. It can be characterized by the slope of the capacity vs. cycle number curve, i.e., 2dC /dn, and determined via a least square fit of the data [6]. The results are shown in Table 1. It can be seen that the capacity decay for particles of 54–74 mm and below 30 mm is significantly less than that for 30–38 mm and 38–54 mm particles. The capacities after 220 cycles for 54–74 mm particles and for those below 30 mm are both about 80% of Cmax .
particle size (mm)
2dC /dn
54–74 38–54 30–38 below 30
0.35 0.45 0.60 0.36
From the above consideration we have concluded that particle size has great effects on the electrochemical properties of the MmB 5 alloy. In conclusion, the electrode composed of particles with sizes of 54–74 mm shows the best discharge capacity, high-rate dischargeability and long-term stability. But from the application point of view, if only 54–74 mm particles were used, particles below 54 mm would be wasted. A solution to this problem is that the electrode can be composed of a mixture of 54–74 mm particles and particles smaller than 30 mm, because particles smaller than 30 mm also show a high discharge capacity and long cycle life according to the above discussions. The method also has the following advantages, particles below 30 mm are so small that they can be firmly pressed into the holes of a porous nickel substrate. The alloy powder will then not easily drop down from the electrode. Besides, if the alloy powder on the electrode is oxidized when the battery was over-charged, 54–74 mm particles can produce new active surfaces by pulverization. Further works on details are underway.
Acknowledgements All experiments of this work were carried out at the Research Center of Physical Chemistry of Metallurgy and Materials, Beijing General Research for Non-ferrous Metals. We thank Professor Li Guoxun and Dr. Jin Hongmei for their instructions in the course of the experiments.
References [1] H. Ogawa, M. Ikoma, H. Kawano, I. Matrumoto, Proc. 16th International Power Sources Symposium, Bournemouth, UK, 1988, p. 393 [2] T. Sakai, T. Hazama, H. Ishikawa, N. Kuriyama, A. Kato, H. Ishikawa, J. Less-Common Met. 172 / 174 (1991) 1175. [3] K. Naito, T. Matsunami, K. Okuno, M. Matsuoka, C. Iwakura, J. Appl. Electrochem. 23 (1993) 1051. ¨ [4] F. Meli, A. Zuttel, L. Schlapbach, J. Alloys Comp. 202 (1993) 81. [5] H.S. Lim, G.R. Zelter, D.U. Allison, R.E. Haun, J. Power Sources 66 (1997) 101. [6] G.D. Adzic, J.R. Johnson, J.J. Reilly, J. McBreen, S. Mukerjee, M.P. Sridhar Kumar, W. Zhang, S. Srinivasan, J. Electrochem. Soc. 142 (1995) 3429.
Fig. 5. Discharge capacity vs. cycle number (n) for the MmB 5 alloy.
L
Letter
Effects of particle size on the electrochemical properties of Mm(NiCoMnAl) 5 alloy a, b Zhang Zhaoliang *, Sun Dongsheng b
a Department of Chemistry, Peking University, Beijing 100871, China Department of Materials Science and Engineering, ShangDong University of Technology, Jinan 250061, China
Received 17 November 1997; received in revised form 26 February 1998
Abstract The effects of particle size on the electrochemical properties of the La 0.65 Nd 0.2 Pr 0.15 Ni 3.55 Co 0.75 Mn 0.4 Al 0.3 alloy have been investigated. The electrode composed of particles with size of 54–74 mm (200–300 mesh) shows the best discharge capacity, high-rate dischargeability and long-term stability. From the application point of view, the electrode can be composed of a mixture of particles of 54–74 mm and particles smaller than 30 mm (500 mesh), the latter also showing a high discharge capacity and long cycle life. 1998 Elsevier Science S.A. Keywords: Particle size; Hydrogen storage alloy; Nickel–metal hydride battery
1. Introduction Among the various types of hydrogen storage alloys, the LaNi 5 -type has recently proven to be very attractive as negative electrode material in rechargeable nickel–metal hydride (Ni–MH) batteries [1], and the effects of partial substitution of La by mischmetal (Mm) and Ni by Co, Mn and / or Al were deeply discussed in previous works [1,2]. In general, the properties of negative electrodes are affected by the composition and preparation conditions of the electrode. Naito et al. [3] showed that the electrochemical properties of the MmNi 3.31 Mn 0.37 Al 0.28 Co 0.64 alloy, such as discharge capacity and high-rate dischargeability, were better for particle sizes of 106–125 mm than for 20–25 mm. In this work, a AB 5 -type compound, i.e. La 0.65 Nd 0.2 Pr 0.15 Ni 3.55 Co 0.75 Mn 0.4 Al 0.3 (for brevity, the alloy is written as MmB 5 ), was employed as negative electrode material. The effects of the particle size in finer distributions from below 30 mm to 147 mm (100 mesh) on electrochemical properties, such as discharge capacity, high-rate dischargeability and cycle life were investigated.
2. Experimental details The MmB 5 alloy was prepared from high purity ele*Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00498-8
ments, .99.9%, by arc melting under an argon atmosphere. After the first melting the ingot was inverted and re-melted twice. The ingot was separated into various fractions, 74–147 mm (100–200 mesh), 54–74 mm (200– 300 mesh), 38–54 mm (300–400 mesh), 30–38 mm (400– 500 mesh) and a fraction below 30 mm (500 mesh) by passing through sieves after mechanical pulverization. Then the alloy powders were mixed with fine Cu powder in a weight ratio of 1:4 and then pressed at about 8310 6 N m 22 to produce pellets 12 mm in diameter. Each pellet contained 200 mg of hydride-forming material without microencapsulations. Electrochemical measurements were carried out at room temperature. The discharge capacity was measured by charging the pellets at a current density of 200 mA g 21 for 2 h, resting for 1 min, and discharging at 200 mA g 21 to 20.65 V. For determining the high-rate dischargeability, charging was done at the same current density, resting for 1 min, but discharging at different current density according to different particle sizes to 20.65 V with respect to a Hg / HgO reference electrode. The long-term stability was measured in a MH electrode-limited open cell, by charging at 150 mA g 21 for 2.33 h, resting for 1 min, and discharging at 150 mA g 21 to 20.9 V. NiOOH plates and 6 M KOH solution were used as the counterelectrodes and electrolyte, respectively. The pressure–composition isotherm for the MmB 5 alloy–H system was measured by using Sieverts’ method.
Z. Zhaoliang, S. Dongsheng / Journal of Alloys and Compounds 270 (1998) L7 –L9
L8
particle sizes below 30 mm has a higher discharge capacity (286 mAh g 21 ) than those for particle sizes of 38–54 mm and 30–38 mm. This indicates that the discharge capacity is not always improved by increasing the alloy particle size. But on the whole, a higher discharge capacity is obtained by employing the larger particles. The electrode containing particles with sizes of 74–147 mm nearly has the same high discharge capability as for sizes of 54–74 mm. Naito et al. [3] mention that a new surface having high catalytic activity is produced in an activation process without increasing the contact resistance.
Fig. 1. Pressure–composition isotherms of the MmB 5 alloy.
The amount of hydrogen stored (CH ) was evaluated from the P–C–T data by the following equation [3]: CH (mAh /g) 5 6 3 1000F 3 [(H /M) 0.5 2 (H /M) 0.01 ] / 3600M Where H /M is the atomic hydrogen-to-metal ratio, F is the Faraday constant and M is the molecular weight of the alloy.
3. Results and discussion
3.1. Discharge capacity Fig. 1 shows the pressure–composition isotherm at room temperature. The calculated capacity has a maximum value of about 325 mA h g 21 . Fig. 2 shows the discharge capacity as a function of particle sizes of the MmB 5 alloy. It can be seen that there is a low discharge capacity (246 mA h g 21 ) for particle sizes of 38–54 mm and 30–38 mm. The highest discharge capacity (300 mA h g 21 ) is obtained for particle sizes of 54–74 mm, while the alloy with
Fig. 2. Discharge capacity of the MmB 5 alloy vs. particle sizes.
3.2. High-rate dischargeability The high-rate dischargeability of an alloy is influenced by the temperature, by the grain and surface morphology (surface area per unit of weight, current densities on the grain surface, length of diffusion paths for hydrogen atoms in the grains and for OH 2 ions to the surface), by the hydrogen equilibrium pressure (concentration of adsorbed hydrogen atoms at the oxidation state) [4]. Fig. 3 shows the relationship between the discharge current density and discharge capacity for five different particle sizes. To consider the dependence of the discharge capacity on discharge rate properties, the relationship between the capacity ratio Cn /C100 (Cn refers to the discharge capacity at a discharge current density of n mA g 21 ) and discharge current density is given in Fig. 4. The electrode containing particle sizes of 54–74 mm shows the highest rate capability. For sizes of 74–147 mm, still a good rate capability is obtained. The electrodes containing particle sizes of below 30 mm, of 38–54 mm and 30–38 mm have a much lower rate capability than those of 54–74 mm. The discharge capacity obtained at 1000 mA g 21 is approximately 83% of that obtained at 100 mA g 21 for particle sizes of 54–74 mm. For particle sizes of 30–38 mm, the discharge capacity obtained at 1000 mA g 21 was only 54% of that obtained at 100 mA g 21 .
Fig. 3. Discharge current density dependence of discharge capacity for the MmB 5 alloy.
Z. Zhaoliang, S. Dongsheng / Journal of Alloys and Compounds 270 (1998) L7 –L9
L9
Table 1 2dC /dn for various particle sizes of the MmB 5 alloy
Fig. 4. Relationship between capacity ratio and discharge current density.
3.3. Cycle life Because particle sizes larger than approximately 100 mm were not suitable for electrode fabrication [5], particles with sizes of 74–147 mm were not included in discussion of cycle life. The discharge capacities vs. cycle number curves for various particle sizes are shown in Fig. 5. There is an initial steep increase in capacity in the first few cycles. All electrodes containing various particle sizes reach their highest capacity Cmax nearly at the same time after about fifty cycles. This indicates that particle size has no effect on activation. Then a nearly linear decrease in capacity as a function of cycle number for all particle sizes is seen. It can be characterized by the slope of the capacity vs. cycle number curve, i.e., 2dC /dn, and determined via a least square fit of the data [6]. The results are shown in Table 1. It can be seen that the capacity decay for particles of 54–74 mm and below 30 mm is significantly less than that for 30–38 mm and 38–54 mm particles. The capacities after 220 cycles for 54–74 mm particles and for those below 30 mm are both about 80% of Cmax .
particle size (mm)
2dC /dn
54–74 38–54 30–38 below 30
0.35 0.45 0.60 0.36
From the above consideration we have concluded that particle size has great effects on the electrochemical properties of the MmB 5 alloy. In conclusion, the electrode composed of particles with sizes of 54–74 mm shows the best discharge capacity, high-rate dischargeability and long-term stability. But from the application point of view, if only 54–74 mm particles were used, particles below 54 mm would be wasted. A solution to this problem is that the electrode can be composed of a mixture of 54–74 mm particles and particles smaller than 30 mm, because particles smaller than 30 mm also show a high discharge capacity and long cycle life according to the above discussions. The method also has the following advantages, particles below 30 mm are so small that they can be firmly pressed into the holes of a porous nickel substrate. The alloy powder will then not easily drop down from the electrode. Besides, if the alloy powder on the electrode is oxidized when the battery was over-charged, 54–74 mm particles can produce new active surfaces by pulverization. Further works on details are underway.
Acknowledgements All experiments of this work were carried out at the Research Center of Physical Chemistry of Metallurgy and Materials, Beijing General Research for Non-ferrous Metals. We thank Professor Li Guoxun and Dr. Jin Hongmei for their instructions in the course of the experiments.
References [1] H. Ogawa, M. Ikoma, H. Kawano, I. Matrumoto, Proc. 16th International Power Sources Symposium, Bournemouth, UK, 1988, p. 393 [2] T. Sakai, T. Hazama, H. Ishikawa, N. Kuriyama, A. Kato, H. Ishikawa, J. Less-Common Met. 172 / 174 (1991) 1175. [3] K. Naito, T. Matsunami, K. Okuno, M. Matsuoka, C. Iwakura, J. Appl. Electrochem. 23 (1993) 1051. ¨ [4] F. Meli, A. Zuttel, L. Schlapbach, J. Alloys Comp. 202 (1993) 81. [5] H.S. Lim, G.R. Zelter, D.U. Allison, R.E. Haun, J. Power Sources 66 (1997) 101. [6] G.D. Adzic, J.R. Johnson, J.J. Reilly, J. McBreen, S. Mukerjee, M.P. Sridhar Kumar, W. Zhang, S. Srinivasan, J. Electrochem. Soc. 142 (1995) 3429.
Fig. 5. Discharge capacity vs. cycle number (n) for the MmB 5 alloy.